Deep Sea Drilling Project Initial Reports Volume 54 · This paper presents petrographie and...

27. PETROLOGY AND GEOCHEMISTRY OF BASALTS FROMDEEP SEA DRILLING PROJECT LEG 54

R.K. Srivastava, Department of Geology, Rajasthan University, Udaipur, Indiaand

R. Emmermann and H. Puchelt, Institut für Petrographie und Geochemie,Universitàt Karlsruhe, Karlsruhe, Federal Republic of Germany

ABSTRACTA suite of 89 basalt samples recovered at nine sites on DSDP Leg

54 from the East Pacific Rise (EPR) and Galapagos Rift (GR) havebeen analyzed for major oxides and trace elements. Basalts eruptedat the EPR crest near 9°N, north of the Siqueiros fracture zone, arehigh-Fe and Ti tholeiites, whereas basalts ponded near an east-westtrending ridge in the area are similar to more typical less fractionatedocean-floor basalts. The most fractionated EPR basalts with highestFe and Ti are from the Siqueiros fracture zone.

The GR basalts range in chemistry from normal mid-oceantholeiite to Fe- and Ti-enriched tholeiite. Both EPR and GR basaltsare LIL-element and LREE-depleted. Galapagos Rift basalts haveremarkably low concentrations of REE, K2O, Sr, and Zr and appearto have been formed from a source material that has undergonegreater partial melting than that encountered by EPR basalts.

There is a remarkable positive correlation between the elementpairs Ti-Zr and Ti-Y for all sites. These, and variations in MgO,CaO, alkalies, AI2O3, Fe3θ, and TiO2, are consistent with thebasalts having experienced variable and, in some cases, extensivefractional crystallization from LIL-depleted melts. The dominatingphases were plagioclase and clinopyroxene with lesser olivine. Basaltsfrom many of the sites, notably GR basalts from Site 424, and Si-queiros fracture zone basalts, are more fractionated than typicalMid-Atlantic Ridge basalts.

INTRODUCTION

This paper presents petrographie and chemical dataon 89 basalt samples from six sites on young East Pacif-ic Rise (EPR) crust near 9°N, one site in the Siqueirosfracture zone (SFZ), and two sites from the flanks of theGalapagos Rift (GR), drilled during Leg 54 of the DeepSea Drilling Project. The purpose of this study was (1)to characterize chemically and petrographically basaltsfrom oceanic crust at the drilling sites of Leg 54, (2) toinvestigate the differences, if any, between the EPR,SFZ, and GR basalts, (3) to study the spatial variationamongst EPR basalts as a function of their distancefrom the EPR, (4) to determine the relationship, if any,between the rate of spreading and the nature of basalts,and finally (5) to evaluate the role of petrogenetic proc-cesses in the development of these basalts. Site locationsand prominent sea-floor physiographic features men-tioned in the text are shown on Figure 1. In this study,we distinguish sites drilled on abyssal hill topography onthe flanks of the EPR (normal fabric topography) fromthose drilled in ponds in the moat-like structure adja-cent to the OCP Ridge (Figure 1) and in the SFZ.

All 89 samples were renumbered in our laboratory.Table 1 lists our number and corresponding DSDP sam-ple designation. For the sake of brevity, our numberswill be used throughout this report. The samples werethoroughly cleaned and submerged in double-distilledwater for 24 hours. They were then dried at 60°C, andpulverized in an agate mill. The major oxide data wereobtained by a combined atomic absorption and spectro-photometric scheme, as described by Srivastava (1977).FeO was measured titrimetrically by the classical dichro-mate method. H2O was determined by the modifiedPenfield method. Ni, Gu, Zn, Y, Sr, Zr, and Rb wereobtained by X-ray fluorescence analysis (XRF) on pow-der discs prepared by mixing in with a few drops of 2 percent Moviol solution. A large number of Internationalreference samples were used as standards (Flanagan,1973). Rare earth elements Sc, Hf, Cr, and Co were de-termined in selected samples by instrumental neutronactivation analysis (INAA), following the method ofPuchelt and Kramar (1974).

Major oxide analyses for all 89 samples along withtheir C.I.P.W. norm are given in Table 2. Table 3 showsthe trace element contents for the same samples and also

671

ON

10°N

9°N

T 1 1 1 1 1 1 r

SIQUEIROS FRACTURE ZONE

PT•4 SITES(419423. 426429)

105°W 104°W

Figure 1. Location of sites drilled on DSDP Leg 54.

PETROLOGY AND GEOCHEMISTRY OF BASALTS

TABLE 1Correlation of Our Sample and DSDP Sample Designations

for the Samples of this Study

Our SampleDesignation

RK 201202203204205

206207208209210

211212213214215

216217218219220

221222223224225

226227228229230

231232233234235

236237238239240

241242243244245

DSDP Sample Designation(Hole-Core-Section-Piece)

420-13, CC 1420-13, CC 4420-14-1,4420-15-1,6420-17-1, 1421-2-1, 1421-3-1,4421-3-1,16421-3-1,20421-3-2,2

421-4-1,5421-Bit, 3422-7-1, 6c422-7-1,13422-8-5,3422-9-1, 4d422-9-2, 5c422-9-3, 6a422-9-4, 13a422-9-5,2

422-10-1,3423-5, CC 4423-6-1,4423-7-1,7423-8-1, 1423-8-1,7424-4-6,4424-5-1, 1424-5-1,8424-5-1, 14a424-5-2, lc424-5-3, 1424-5-3,9424-5-4, 3424-6-1,2424-6-1,7424-6-1, 10b424-6-2, lb424-6-3, lc424-7-1, 1424A-4-1, 2424A^t-l,6424B-5-1.8424B-5-1, 12424B-5-2, 2

Our SampleDesignation

RK246247248249250

251252253254255

256257258259260

261262263264265

266267268269270

271272273274275

276277278279280

281282283

285

286287288289

DSDP Sample Designation(H ole-Core-S ection-Piece)

424B-6-l,2424B-6-1.9424B-6-1, 16424C-2-1, 3424C-3-l,4425-7-1,6425-7-1,13425-7-1, 17a425-7-2, 7b425-7-2,10425-8-1,6425-8-1,11425-8-1,16425-9-1, 11425-9-1,14425-9-1, 18425-9-2, 3425-9-2,13425-9-3, 10427-9-1,2427-9-3,6427-9-5, 7427-10-3, 4b428-5-4, 1428-6-1,4428-6-2, 7428A-1-1, 1428A-1-1, 15428A-1-2, 7428A-1-2, 10428A-1-2, 12428A-1-3, 4428A-1-4, 7428A-2-1, 10428A-3-1, 6428A-3-1, 10b428A-4-3, 4428A-5-3, 11428A-6-1, 11428A-7-2, la429A-1-1.2429A-2-1.3429A-2-1J4429A-3-l,4

their FeOVMgO and SI values.1 The rare-earth and ad-ditional trace element data of some selected samples aregiven in Table 4. Tables 5a and b show the average com-position of the magmas found at the EPR, SFZ, and GRsites.

Petrographic Summary of Basalts from the West Flankof the EPR—Holes 420, 421, 422, 423, 428, 428A, and429A

Salient petrographic features of the basalts from thedifferent holes are given in the appendix. We here sum-marize these features:

1) Basalts from Holes 420, 421, bottom of Holes 422,423, and from the lower part of Hole 429A, are essen-tially glassy-cryptocrystalline to fine-grained aphyric tosparsely microphyric plagioclase-pyroxene basalts.

2) Depending upon the degree of crystallinity, thegroundmass shows various textures such as hyalopilitic,spherulitic, variolitic, intersertal, and subophitic.

3) The glass present in these rocks is generally fresh.In some of the glassy basalts, the glass shows zonation;

1 FeO* expresses total iron.

that is, (/) an outer light yellowish glass zone, («) a glob-ular intermediate zone, and (///) an aphanitic inner zone.This gradual textural variation is typical of pillow lavas.

4) In general the basalts are finely vesicular; the ves-icles are either empty or filled with smectites.

5) At Holes 422, 428, 428A, and in the upper part ofHole 429A, there are two types of basaltic rocks: a fine-grained olivine-bearing basalt, and a fine- to medium-grained doleritic rock. The former is found at Hole 428and the upper part of Holes 428A and 429A; the latter isfound at Hole 422 and in the lower part of Hole 428A.

6) The olivine-bearing basalts from these holes aresimilar to the aphyric plagioclase-pyroxene basalts, ex-cept that the latter do not contain oli vine. The amountof olivine in these rocks does not exceed one to two percent.

7) At Hole 429A, the olivine-bearing basalts areyounger than the magnetic anomaly age for the site, andthey may not therefore have formed at the EPR proper.

8) The doleritic rocks at Holes 422 and 428A aremesocratic, fine- to medium-grained, and almost holo-crystalline rocks, showing subophitic to ophitic texture.The chief mineral constituents are plagioclase, clinopy-roxene, and titanomagnetite.

9) On the basis of the petrographic studies it can beconcluded that the nature of basalts ponded near theOCP Ridge (Figure 1) — that is, basalts from Holes 422,428, and 428A — is entirely different from that of thenormal fabric basalt, that is, basalts from Holes 420,421, 423, and the lower part of Hole 429A. Primarily,they are coarse-grained and apparently representedthick flows, sills, or even former lava lakes.

Petrographic Summary of Basalts from the SFZ

Petrographically, the basalts from Hole 427 are akinto the doleritic rocks from Holes 422 and 428A, exceptthat they do not contain olivine which is occasionallypresent in the doleritic rocks. Furthermore, clinopyrox-ene microphenocrysts are much more common in theserocks, indicating that these have crystallized when themagma composition had left the olivine-plagioclase-pyroxene cotectic.

Petrographic Summary of Basalts from the GR—Holes424, A, B, and C, and Hole 425

1) Megascopically, the basalts recovered from theseholes can be described as fine- to medium-grained, ves-icular aphyric basalts with occasional microphenocrystsof plagioclase. Microscopically, they show various tex-tures. The quenched samples show hyalopilitic, vario-litic, and intersertal texture, while relatively coarsersamples show intergranular texture.

2) There are no significant petrographic differencesin basalts from Holes 424, A, B, and C.

3) The minerals constituting these basalts are plagio-clase, clinopyroxene, titanomagnetite, and variableamounts of glass.

4) Contrary to Site 424 basalts, the basalts fromHole 425 show large variation in their color, grain size,vesiculation, and mineral assemblages.

673

R. K. SRIVASTAVA, R. EMMERMANN, H. PUCHELT

TABLE 2Chemical Analyses and C.I.P.W. Norm of Basaltic Rocks, Leg 54

Site/Hole

Sample

SiO2

Tiθ2A12O3

F e 2 O 3

FeO

MnOMgOCaO

Na2OK 2 O

P 2 θ 5 +H 2 O

Total

C.I.P.W.

Qorabandi

hyolmtilap

RK201

50.731.91

14.314.966.20

0.186.42

11.012.550.45

0.151.22

100.09

Norm

1.372.70

21.9226.6822.85

18.20

2.063.690.36

202

50.961.91

13.914.936.50

0.186.52

10.952.430.47

0.161.04

99.96

2.092.82

20.8726.0423.11

18.89_

2.123.680.36

420

203

51.252.05

14.254.955.15

0.196.63

11.212.560.47

0.121.14

99.87

2.062.83

22.0126.4223.94

16.63_

1.863.950.29

204

50.241.91

14.545.235.55

0.196.65

10.952.590.50

0.131.36

99.84

0.393.01

22.3727.1222.51

18.58_

2.003.710.31

205

50.011.87

13.904.547.85

0.197.00

11.212.520.28

0.160.86

100.39

_

1.6621.5026.0123.90

19.511.152.303.580.37

206

50.252.68

13.706.426.60

0.226.019.942.660.44

0.191.10

100.21

2.092.64

22.9024.5520.10

19.77_

2.405.180.45

207

49.692.58

13.326.826.60

0.215.959.872.550.67

0.201.38

99.75

1.254.04

22.0423.4120.93

20.35_

2.495.000.47

208

51.252.13

13.175.405.50

0.206.52

11.012.660.43

0.171.10

99.54

2.352.59

22.9723.2025.77

16.58_

2.014.130.40

421

209

51.072.21

13.126.015.25

0.216.63

11.402.630.48

0.161.24

100.41

1.412.88

22.5622.8827.38

16.20_

2.064.260.37

210

51.392.12

13.006.804.75

0.196.47

10.662.550.62

0.171.24

99.97

2.623.73

22.0122.6424.80

17.67_

2.114.110.40

211

51.502.18

13.125.216.00

0.226.56

10.732.660.43

0.191.28

100.08

2.692.58

22.8822.9724.54

17.61_

2.064.210.45

212

50.252.00

13.436.995.65

0.216.87

10.732.600.55

0.161.12

100.56

_

3.2922.2623.6224.04

19.231.042.313.840.37

213

50.251.52

14.253.556.70

0.188.02

11.682.690.15

0.130.96

100.08

_

0.9023.0326.6825.35

14.274.631.902.920.30

422

214

50.551.63

14.003.496.75

0.197.90

11.532.690.11

0.150.78

99.77

-0.66

23.0626.1425.13

17.741.871.903.140.35

215

50.551.57

13.934.096.20

0.197.90

11.532.720.11

0.130.78

99.70

-0.66

23.3525.8425.54

17.012.351.913.020.30

TABLE 2 - Continued

Site/Hole

Sample

SiO2

TiO2

A12O3

F e 2 O 3

FeO

MnOMgOCaO

Na2OK 2 O

P 2 θ 5 +H 2 O

Total

232

50.741.74

12.894.059.10

0.216.25

10.192.330.11

0.151.04

98.80

C.I.P.W. Norm

Qorabandi

hy1

oimtilap

4.020.66

20.2425.0321.43

22.37

2.483.390.36

233

50.251.83

13.184.428.80

0.206.48

10.502.240.06

0.170.76

98.89

3.320.36

19.3926.3221.39

22.77

2.503.560.40

234

50.101.78

13.182.76

10.30

0.216.70

10.502.330.06

0.160.90

98.98

2.320.36

20.1525.8821.74

23.34

2.473.450.38

235

50.641.78

12.742.72

10.35

0.216.70

10.302.510.06

0.170.84

99.02

2.480.36

21.6823.8122.52

22.82

2.493.450.40

424

236

50.641.76

13.182.82

10.25

0.226.48

10.192.460.08

0.180.72

98.98

2.860.48

21.2225.1720.84

23.03

2.493.470.43

237

50.801.78

13.332.45

10.60

0.206.15

10.202.370.05

0.161.38

99.47

3.970.31

20.4926.1320.31

22.48

2.493.450.38

238

51.401.83

12.483.09

10.00

0.206.36

10.712.370.05

0.191.26

99.94

4.260.30

20.3723.6324.08

20.89

2.483.530.45

239

51.851.77

12.563.00

10.10

0.206.36

10.512.440.05

0.201.24

100.30

4.500.30

20.9023.4523.20

21.81

2.473.400.47

240

52.001.77

12.713.10

10.00

0.226.08

10.712.250.16

0.161.14

100.30

5.260.96

19.2524.2723.48

20.46

2.453.400.37

424A

241

51.851.82

12.782.22

10.80

0.235.92

10.712.310.10

0.201.44

100.38

5.300.60

19.7924.5123.22

20.15

2.453.500.47

242

51.851.71

12.882.42

10.60

0.216.03

10.332.370.12

0.171.54

100.19

5.030.72

20.3524.5121.90

21.31

2.483.200.40

243

50.631.70

12.943.279.85

0.206.03

10.142.440.06

0.171.96

99.39

3.870.36

21.2524.8821.37

22.02

2.513.320.40

244

50.801.85

13.113.609.50

0.206.159.952.440.09

0.191.86

99.74

4.030.54

21.1625.1520.03

22.54

2.483.600.45

424B

245

51.181.74

13.312.61

10.45

0.206.639.582.370.03

0.200.88

99.18

4.390.18

20.4526.0817.4Q

25.17

2.493.360.47

5) Three to four distinct petrographic types, namely(/) aphyric to sparsely phyric plagioclase-pyroxene ba-salts, (//) phyric-glomeroporphyritic basalts, (Hi) glom-eroporphyritic ± olivine, (/v) plagioclase-pyroxene ±olivine sparsely phyric basalts, constitute the entire col-umn drilled at Hole 425.

CHEMISTRY OF BASALTS

The major, minor and trace element data of 89samples are given in Tables 2, 3, and 4. Before at-tempting to classify and interpret the mode of origin ofthese basalts, it would be useful to discuss the salient

674


TABLE 2 - Continued

216

49.701.59

14.183.646.700.187.74

11.532.720.100.120.80

99.00

0.6023.5126.7525.2014.104.521.943.080.29

217

49.331.39

15.353.745.450.208.22

11.532.800.19

0.150.96

99.31

1.1424.1729.3422.75

8.599.251.702.690.36

218

49.361.50

15.063.875.400.188.56

11.532.720.21

0.150.74

99.28

1.2623.4228.7523.06

9.359.161.732.900.36

422

219

49.081.30

15.843.544.800.308.06

12.032.780.17

0.131.36

99.39

1.0224.0730.9523.71

5.8110.031.552.530.31

220

49.271.43

15.283.016.100.178.56

11.782.720.21

0.130.60

99.28

1.2623.9929.3223.66

7.2110.371.712.760.30

221

50.272.03

13.652.948.00

0.207.25

11.282.630.31

0.170.96

99.69

1.8622.6024.9025.19

18.890.182.063.920.40

222

50.572.13

13.754.756.600.226.63

10.172.700.35

0.251.32

99.44

1.852.12

23.3824.9220.55

20.32_

2.114.140.59

223

51.002.14

13.545.166.60

0.206.73

10.552.690.38

0.200.78

99.97

1.502.28

23.0424.0422.70

19.67_

2.184.110.47

423

224

51.682.21

13.345.007.20

0.196.33

10.312.690.38

0.200.22

99.75

3.042.28

23.0123.4522.19

19.09_

2.214.250.47

225

49.532.02

13.696.166.20

0.196.48

10.412.740.40

0.191.38

99.39

2.4223.7924.4922.45

19.290.872.293.940.45

226

49.942.04

13.775.706.200.196.04

10.412.890.32

0.211.06

98.79

0.581.94

25.1624.3322.60

18.69-

2.223.980.50

227

50.441.87

12.862.88

10.20

0.216.48

10.292.480.08

0.181.14

99.11

2.660.48

21.5124.3022.19

22.53_

2.243.640.43

228

50.351.85

12.833.00

10.10

0.216.50

10.292.480.18

0.181.18

99.15

2.211.09

21.5123.9322.51

22.47_

2.243.600.43

424

229

51.101.81

12.822.98

10.10

0.216.48

10.192.480.11

0.201.16

99.64

3.340.66

21.4023.9921.72

22.68_

2.213.510.47

230

51.101.82

12.963.10

10.00

0.216.63

10.292.550.11

0.180.56

99.51

2.550.66

21.9023.9322.07

22.73_

2.223.510.43

231

50.791.81

12.963.339.80

0.216.55

10.192.280.10

0.150.74

98.91

3.860.60

19.7025.3620.92

23.19_

2.493.510.36

TABLE 2 - Continued

246

51.331.73

13.472.67

10.400.216.399.952.310.160.170.98

99.77

4.200.96

19.8326.2818.7823.74

2.483.340.40

247

51.181.80

13.332.68

10.350.216.63

10.142.310.160.200.90

99.89

3.630.96

19.7825.8419.6523.75

2.453.460.47

424B

248

51.181.74

13.753.10

10.000.206.63

10.252.310.160.200.90

100.42

3.230.96

19.6926.8719.0623.94

2.463.320.46

424C

249

50.991.84

12.802.28

10.750.236.63

10.832.310.160.160.60

99.58

2.990.96

19.7824.3823.9421.57

2.473.530.37

250

50.841.91

12.861.98

10.900.236.63

10.562.270.100.160.80

99.24

3.630.60

19.5425.0322.49

22.18

2.453.690.38

251

50.901.34

13.753.268.90

0.197.67

11.432.200.08

0.120.76

100.60

1.190.47

18.7027.5323.56

23.45

2.262.550.28

252

51.181.36

13.802.779.20

0.197.52

11.432.140.05

0.120.74

100.50

2.130.30

18.1928.0323.15

23.10

2.232.590.28

253

51.181.31

13.802.728.90

0.198.00

11.522.000.03

0.120.74

100.51

2.260.18

16.9928.7122.88

24.02

2.172.500.28

254

51.001.09

14.203.557.70

0.178.16

11.952.00tr.

0.080.74

100.64

2.19_

17.1630.1923.98

22.08

2.102.100.19

255

49.931.14

14.353.857.60

0.188.00

12.171.870.05

0.120.44

99.70

0.600.30

15.9930.9524.12

23.48

2.122.190.23

256

50.381.24

13.923.477.50

0.168.16

11.751.870.05

0.120.86

99.48

1.940.31

16.0829.9323.34

23.66

2.052.380.28

425

257

50.231.02

14.863.147.15

0.168.30

12.171.730.03

0.080.74

99.61

1.540.18

14.8433.1522.48

23.72

1.931.970.19

258

50.530.90

13.912.867.10

0.158.50

12.171.610.03

0.121.36

99.24

2.920.18

13.9531.3724.17

23.49

1.881.750.29

259

50.530.96

14.203.187.30

0.178.42

12.001.870.03

0.120.46

99.24

1.430.18

16.0630.7123.61

23.91

1.961.850.28

260

50.690.92

14.284.676.75

0.178.16

11.751.730.18

0.080.60

99.97

1.761.07

14.7930.9822.44

24.89

2.111.770.19

261

50.580.97

14.803.477.60

0.188.42

11.951.750.03

0.080.70

100.53

1.320.18

14.8632.5621.61

25.28

2.141.840.15

features of their chemistry and to see if these could berelated to the mineralogical variations encounteredwithin these basalts.

Site 420

Five analyzed samples of basalts show very little orno chemical variation with respect to their major andtrace element chemistry, which is in keeping with limitedor no variation in petrography. Their relatively high

TiO2(l 87 to 2.05%), K2O (about 0.5%, except for onesample which has 0.28%), Zr (154 to 164 ppm), and Rb(6 to 12 ppm), coupled with relatively high FeO/MgO,and low A12O3 and Ni, imply that these rocks are highlyfractionated.

Hole 421Seven chemical analyses of basalts are given in Tables

2 and 3. The data reveal that there are two types or

675


TABLE 2 - Continued

Site/Hole

Sample

SiO2

TiO2A 1 2°3Fe 2 O 3

FeO

MnOMgOCaO

Na2OK 2 O

P 2 θ 5 +H 2 O

Total

262

50.451.01

14.523.647.35

0.178.00

11.612.120.01

0.110.90

99.89

C.I.P.W. Norm

Qorabandi

hyolmtilap

0.750.06

18.1830.4822.24

24.05_

2.041.940.25

263

49.801.00

14.063.777.20

0.178.35

11.892.170.01

0.100.86

99.38

_

0.0618.6829.1224.76

21.201.962.061.940.23

425

264

50.551.02

14.213.537.40

0.178.43

12.002.200.01

0.110.60

100.22

_

0.0618.8229.0824.68

22.390.722.041.960.25

265

49.952.57

13.753.958.70

0.236.359.943.060.06

0.291.00

99.85

0.500.36

22.2823.9619.94

20.96_

2.374.950.68

266

50.402.58

13.753.419.40

0.206.639.573.060.10

0.300.98

100.38

0.710.60

26.1223.7018.32

22.53_

2.384.940.71

427

267

49.552.64

13.754.169.00

0.206.639.832.880.10

0.301.08

100.12

0.300.60

24.6924.6218.82

22.73_

2.455.080.71

268

50.302.53

13.914.388.40

0.186.809.732.880.08

0.201.06

100.54

1.090.47

24.5825.0017.87

23.05_

2.374.850.71

269

49.501.50

14.523.387.05

0.187.66

11.892.650.12

0.190.42

99.06

_

0.7222.8027.8325.24

12.425.701.942.900.45

428

270

50.101.57

14.454.816.05

0.187.31

11.452.650.19

0.190.30

99.25

_

1.1422.7627.3823.75

17.242.252.003.030.45

271

50.101.61

14.833.876.60

0.207.48

11.722.500.12

0.181.06

100.27

_

0.7221.4029.2123.21

18.831.181.933.090.42

272

49.801.55

14.833.616.50

0.207.58

11.722.650.12

0.190.56

99.31

_

0.7222.7828.6723.96

14.643.971.892.970.38

42 8A

273

50.051.64

15.054.066.40

0.267.48

11.892.650.12

0.190.86

100.65

_

0.7222.5528.9823.78

14.364.121.923.130.44

274

50.201.58

14.673.327.20

0.217.66

11.722.650.06

0.180.64

100.09

_

0.3522.6128.1923.91

16.433.111.963.020.42

275

49.951.60

15.282.917.30

0.207.29

11.852.500.25

0.180.88

99.99

groups of basalts. Two samples from Core 2 and the up-per part of Core 3 (RK 206 and 207) represent a magmatype richer in TiO2 and Zr, and poorer in Ca as com-pared with the remaining five basalts. The FeO*/MgOratio in the group is also much higher than in the others.

These two distinct chemical types correspond withthe two petrographic types encountered within the hole.The high TiO2, Zr, and high FeO*/MgO type representsthe glassy basalts, and the other group represents theaphyric to sparsely microphyric basalts.

Hole 422

The analyzed nine samples from this hole, except forone sample from the bottom, differ significantly fromthe basalts from Holes 420 and 421. All samples are sig-nificantly richer in MgO, and poorer in TiO2 as com-pared with the basalts from Holes 420 and 421. Besidesthese differences in major oxides, there are also char-acteristic differences in trace elements. For example, Zrand Sr contents of these rocks range from 98 to 122 ppmand 136 to 172 ppm, respectively, whereas at Holes 420and 421 the same elements vary from 154 to 212 ppmand 104 to 132 ppm, respectively.

The basalts from Hole 422 can be grouped into threedistinct chemical groups, corresponding to three dif-ferent petrographic units in the hole, namely the upperdoleritic unit, the lower doleritic unit, and the aphyricbasalt in the bottom of the hole. The chemical analysesin Table 4a reveal that the upper doleritic unit (magmagroup, Core 422-1) is richer in Ti and Fe and poorer inMg, Ca, and Al than the lower doleritic unit (magmagroup, Core 422-2). Amongst the trace elements in thesetwo groups also, there are significant differences, par-ticularly in Ni, Sr, and Zr. These differences in thechemistry can be attributed to the presence of olivine

and larger numbers of plagioclase phenocrysts in thelower doleritic unit.

The third magma group at this hole (Core 422-3,Table 5 a), represented by the single aphyric basaltfound at the bottom of the hole below the doleriticrocks, is characterized by high Ti (TiO2 = 2.03%), Zr(144 ppm), and FeOVMgO (1.51), and low Al (A12O3

= 13.65%), Sr (109 ppm), and Ni (65 ppm). It resem-bles closely the basalts from Holes 420 and 421.

Hole 423

Five analyzed samples of basalts from this hole aresimilar to basalts from Holes 420 and 421; that is, theyare characterized by high Ti (TiO2 = 2.02 to 2.22%)high FeO/MgO (1.63 to 1.86), and low Al (A12O3 =13.34 to 13.77%). Zr and Y in these basalts are alsohigh. On the basis of their FeOVMgO and SI (Solid-ification Index) values, they can be divided into twosub-groups. The analyses of these sub-groups given inTable 5 a (Cores 423-1 and 2) reveal that there are nomajor differences between the two types, which is inkeeping with the limited variation in the petrography ofthese basalts. The Core 423-1 type, found in the upperpart of the hole, most likely represents a slightly lessfractionated magma than that of Core 423-2.

Holes 428 and 428A

Chemical analyses of basalts from these holes are giv-en in Tables 2, 3, and 4. There are no large-scale vari-ations in the chemistry of these basalts. The aphyricolivirie basalts from Hole 428 and the upper part ofHole 428A are similar to those of the doleritic unit in thelower part of Hole 428A, except that the MgO contentof the doleritic unit and its SI values are slightly higherthan those of the olivine basalts. Thus, the olivine

676


TABLE 2 - Continued

lie

50.001.53

16.002.227.70

0.207.00

11.852.630.24

0.190.78

100.34

1.4322.4031.3321.73

13.973.921.852.930.44

211

49.881.51

14.912.437.05

0.197.44

11.442.560.24

0.190.76

99.60

1.4421.9828.9022.43

16.903.031.962.900.45

278

50.831.62

14.842.477.80

0.197.35

11.042.560.25

0.190.80

99.94

0.601.50

21.8928.5620.88

21.10_

1.923.110.44

279

51.231.54

15.054.236.00

0.177.00

11.052.630.24

0.190.84

100.18

1.161.43

22.4828.8720.67

20.09_

1.892.950.44

280

51.531.56

14.843.885.60

0.177.29

11.252.630.25

0.210.64

99.70

1.451.50

22.5128.2621.83

19.21_

1.752.990.50

428A

281

50.581.56

14.533.446.80

0.167.59

11.042.500.17

0.200.80

99.37

0.781.02

21.5328.4121.23

21.65_

1.912.990.47

282

51.081.53

14.243.077.30

0.178.09

11.042.500.14

0.180.88

100.22

0.640.84

21.3527.4621.72

22.70_

1.932.940.42

283

50.381.48

15.062.957.20

0.177.97

11.442.520.18

0.200.66

100.18

1.0721.4729.4621.51

18.892.421.882.830.46

284

50.581.50

14.452.707.20

0.157.91

11.442.520.16

0.190.80

99.60

0.9621.6527.9723.05

21.140.021.862.890-45

285

50.381.55

14.312.947.20

0.177.91

11.042.500.16

0.190.94

99.29

0.280.97

21.5727.8821.75

22.21_

1.902.990.45

286

49.881.15

16.L92.975.20

0.147.29

12.252.500.08

0.131.32

99.10

0.4821.7033.5522.66

16.261.281.522.240.30

429A

287

49.981.19

15.823.805.20

0.137.29

12.052.430.14

0.131.12

99.28

0.8521.0132.5522.56

18.020.711.672.300.30

288

50.631.62

14.314.357.45

0.197.22

10.652.500.32

0.200.52

99.96

0.461.91

21.3527.1320.45

22.94-

2.183.110.46

289

50.481.62

14.383.287.80

0.187.15

11.042.500.26

0.190.60

99.45

0.581.56

21.4627.6521.86

21.28-

2.063.120.45

TABLE 3Trace Element Contents (ppm) and FeO*/MgO Ratio, Basaltic Rocks, Leg 54

Sample

RK201202203204205

206207208209210

211212213214215

216217218219220

221222223224225

226227228229230

231232233234235

236237238239240

241242243244245

Ni

8269937492

6349626975

9856666741

50114122107103

6563564747

5244384466

4449445040

4952564952

5449565946

Cu

5160595257

5450646586

5058555848

4659505552

5359456055

5953615559

4859555955

5461885561

6561665761

Zn

9996

10594

103

10496999996

10985748570

6872587261

65104909898

9887

103103

98

8298949894

117115107103110

10911311510798

Y

4544394545

5348514947

4943293435

2428242520

3137404643

4639383837

3639373835

3837403840

4043403540

Sr

111108121120113

126114128126132

130104140148136

142182167176172

109122122125132

13062686665

6665646064

6868686866

7165716168

Zi

160154161164164

212197179171168

184185122119116

10010810298

105

144160155163164

160132121129116

109114116110110

125123130129130

128131125115123

Rb

9126

1210

tl486

12

341I1

11111

111I1

11111

21112

11113

32121

Li

56574

55766

56576

55775

67664

16686

58988

56655

56__

-

FeO*/MgO

1.651.671.441.531.69

2.052.131.581.601.67

1.621.731.231.241.24

1.281.071.040.991.02

1.461.631.661.771.80

1.861.961.951.961.91

1.942.021.961.891.89

1.962.071.992.002.08

2.142.102.112.071.92

SI

31.1931.2733.5532.4031.55

27.1626.3431.7931.5730.53

31.4530.3237.9937.7337.58

37.0340.2941.2341.6541.55

34.3131.5231.2229.3129.48

28.5629.2929.2029.2629.61

29.6928.6229.4530.2429.99

29.3328.4429.0828.9728.16

27.7327.9927.8528.2430.01

Sample

RK246247248249250

251252253254255

256257258259260

261262263264265

266267268269270

271272273274275

276277278279280

281282283284285

286287288289

Ni

5656492936

5647505970

6262707070

5656996546

5062526269

7069665965

7456626780

6268756959

140138

8986

Cu

5770525349

6461636874

6872687279

7174717250

4650466977

6460726360

6762525959

5360796667

76744853

Zn

1109283

117115

9694817475

7173688095

7585697982

9486827878

7474827066

7068646672

7072716263

73628282

Y

3845454035

3029302427

2424161925

2527272446

4546422624

2927272928

2127282130

2926242526

20223229

Si

6568666,759

5755555752

5454624952

57605954

130

124120110128124

128130128130136

128139128131145

145136125131139

139136115114

Zr

106113115121126

7782747161

6250414845

49504945

190

18619319499

103

10710210710194

949897

120100

9699

1019795

6466

100106

Rb

21122

11111

11111

11111

11134

22112

21121

12221

1134

Li

_

-__-

32221

11111

2222-

__-_-

____-

____-

____-

____

FeO*/MgO

1.981.911.911.921.90

1.531.541.411.331.37

1.291.191.131.201.33

1.271.321.261.251.92

1.871.911.801.311.41

1.341.281.341.321.35

1.371.351.351.391.24

1.301.231.231.241.24

1.071.171.571.49

SI

29.1429.9629.8629.9630.30

34.6934.6936.9538.1137.44

38.7640.7942.0840.4837.97

39.5837.8838.8439.0828.71

29.3329.1130.1736.7234.79

36.3637.0536.1136.6736.00

35.3735.9135.9834.8337.10

37.0238.3438.2838.6038.19

40.4138.6533.0634.06

Notes: FeO = total iron as FeO. SI = 100 MgO/MgO + Fe2θ3 + FeO + Na2θ + K2O.

677


TABLE 4

Rare-earth and Additional Trace Element Contents of Selected Samples, Leg 54

Sample

R K 2 6 1

262

263

264

265

266

267

268

269

270

271

272

273

274

275

276

277

278

279

280

281

282

283

284

285

286

287

288

289

Na2θ

n.d.

2.08

2.12

2.09

3.21

3.002.92

3.09

2.77

2.66

2.70

2.83

2.61

2.77

2.70

2.75

2.73

2.61

2.85

2.88

2.89

2.77

2.81

2.76

2.90

2.74

2.83

2.81

2.77

F e 2 θ 3

10.97

11.55

10.89

11.32

13.40

13.9614.01

14.29

10.32

9.94

10.26

10.03

9.76

10.36

10.01

9.63

11.169.25

10.41

9.57

10.34

10.53

10.079.41

10.33

7.73

9.38

13.02

11.10

Sc

47

49

43

47

43

42

41

44

4039

41

41

39

41

41

41

45

38

43

44

42

41

42

41

42

39

42

45

44

Cr

267

274

254

268

97

106

90

110304

282

275

305

268

287

301

314

332

267

307

332

284

281

314

357

314

355

400

126

129

Co

47

49

46

49

44

43

42

46

40

39

41

41

4041

41

41

43

37

43

45

42

41

41

39

41

56

61

46

43

Hf

< l

< l

< l

< l

4.5

4.4

5.4

4.7

2.5

1.3

3.3

2.7

2.6

1.6

1.4

2.5

3.1< l

1.4

2.8

< l

< l

2.6

2.7

1.3

3.5

< l

3.4

2.6

La

1.0

1.2

1.3

0.99

7.1

7.0

6.6

7.03.8

4.9

4.0

4.3

4.0

4.0

4.3

3.9

4.4

4.0

4.0

4.2

4.2

4.1

4.3

3.94.4

2.4

2.6

4.2

4.3

Ce

3.9

3.8

4.1

3.6

18

16

16

17

12

11

10

10

10

10

10

11

10

9.5

10

13

10

9.5

11

9.6

10

6.5

8.1

12

13

Nd

5.1

5.5

5.4

5.1

15

18

16

15

9.09.5

11

11

9.69.5

9.8

9.5

10

9.7

11

11

10

11

11

8.5

9.0

8.69.4

10

11

Sm

2.1

2.2

1.9

2.1

5.8

5.8

5.65.8

3.43.4

3.5

3.5

3.4

3.63.5

3.4

3.63.3

3.6

3.7

3.5

3.4

3.33.2

4.0

2.62.8

3.8

3.7

Eu

0.86

0.91

0.84

0.88

2.2

2.1

1.9

2.1

1.261.35

1.33

1.401.34

1.32

1.36

1.34

1.39

1.31

1.45

1.47

1.39

1.33

1.30

1.18

1.35

1.061.21

1.47

1.44

Gd

3.4

3.2

3.4

3.2

6.6

6.5

6.8

6.9

4.34.3

5.5

4.5

4.1

4.0

4.2

4.7

5.1

3.8

5.3

5.0

5.1

4.7

5.1

3.9

4.5

3.3

3.7

6.2

4.6

Tb

0.65

0.65

0.56

0.61

1.3

1.4

1.4

1.4

0.85

0.77

0.70

0.87

0.75

0.84

0.95

0.91

0.900.75

0.810.84

0.87

0.78

0.81

0.70

0.88

0.72

0.78

1.1

0.88

Ho

1.0

0.97

0.95

0.96

1.8

2.01.8

2.31.1

1.2

1.01.1

1.3

1.31.4

1.1

1.2

1.1

1.2

1.3

1.0

1.3

1.2

1.1

1.3

1.1

1.1

1.2

1.4

Tm

0.43

0.47

0.40

0.43

0.93

0.87

0.78

0.77

0.500.52

0.58

0.49

0.59

0.530.54

0.53

0.46

0.36

0.45

0.51

0.52

0.46

0.51

0.50

0.46

0.45

0.43

0.51

0.58

Yb

2.7

2.7

2.6

2.8

5.1

4.9

4.6

5.0

2.8

2.8

2.8

3.2

2.8

3.0

3.0

3.0

2.8

2.6

3.0

3.2

3.0

3.0

3.2

2.7

3.3

2.62.5

3.4

3.2

Lu

0.49

0.43

0.39

0.44

0.74

0.690.72

0.75

0.53

0.42

0.46

0.49

0.48

0.45

0.48

0.41

0.49

0.43

0.53

0.57

0L41

0.46

0.42

0.39

0.54

0.40

0.45

0.57

0.55

TABLE 5a

Average Compositions of the Magmas Found at the EPR Sites, Leg 54

Hole

Age (m.y.)

Magma Group

Core Interval

Siθ2TiO2

A12O3

Fe 2 O 3FeO

MnOMgOCaONa2θK2O

P 2 θ 5 +H 2 O

Total

Trace elements

Ni( i i

ZnYSr

ZrRbLiFeO*/MgOa

SI

423

1

423-1

5, CC tc7-1

50.792.14

13.654.696.600.216.68

10.362.700.36

0.221.05

99.72

in ppm

60529738

122

158161.66

32.10

.6

423-2

>8-1

50.382.09

13.605.626.53

0.196.28

10.382.770.37

0.200.85

99.26

49589845

129

162141.85

29.89

421-1

2-1 to 3-1(25 cm)

49.972.63

13.516.626.60

0.225.989.912.610.55

0.201.24

100.04

5652

10051

120

205452.13

27.56

421

3.4

421-2

3-1 (110 cm)to 4-1 & bit

51.092.13

13.176.085.43

0.216.61

10.912.620.50

0.171.20

100.12

72659848

124

178661.65

32.03

420

3.4

420-1

13, CC to17-1

50.641.93

14.184.926.25

0.196.64

11.062.530.43

0.141.12

100.03

8256

10044

115

16110

51.61

32.74

429A-1

1-1 to 2-1(14 cm)

49.931.17

16.003.395.20

0.147.29

12.152.460.11

0.131.22

99.19

139756721

138

651_

1.1340.25

429A

4.6

429 A-2

2-1 (100 cm)to 3-1

50.551.62

14.353.827.63

0.197.18

10.852.500.29

0.200.56

99.74

58508231

115

1033_

1.5434.13

427

ca. 3.4

427-1

9-1 to10-3

50.052.58

13.793.988.88

0.206.609.772.970.09

0.301.03

100.24

53488645

121

1911_

1.8929.94

422-1

7-1 to9-1

50.261.58

14.093.696.59

0.197.89

11.572.710.12

0.130.83

99.65

56527431

142

114161.26

38.24

422

1.7

422-2

9-2 to9-5

49.261.41

15.383.545.44

0.218.35

11.722.760.20

0.140.92

99.33

112546624

175

104161.03

41.94

422-3

10-1

50.272.03

13.652.948.00

0.207.25

11.212.720.31

0.170.96

99.69

65536531

109

144161.51

34.64

428

2.0

428-1

5-4 to6-2

49.901.56

14.704.026.38

0.197.48

11.682.600.14

0.190.60

99.44

67707727

127

1033_1.34

36.99

428A-1

1-1 to 1-2(57 cm)

50.031.59

14.853.666.70

0.227.57

11.642.650.10

0.190.69

99.89

65657528

129

1032_

1.3237.20

428A

2.0

428A-2

1-2 to 3-1(36 cm)

50.571.56

15.163.196.91

0.197.22

11.412.590.25

0.190.78

100.02

67606826

135

1001_1.35

36.38

429A-3

3-1 to7-2

50.601.52

14.523.027.14

0.177.89

11.202.500.16

0.190.82

99.73

67656826

135

981_1.25

38.66

aTotal iron as FeO.

678


TABLE 5bAverage Compositions of the Magmas Found at the Galapagos Rift Sites, Leg 54

Hole

Age (m.y.)

Magma Group

Core Interval

SiO 2

TiO 2

A1 2O 3

F e 2 O 3

FeO

MnOMgOCaONa2OK2O

P 2 θ 5 +H2O

Total

424

424-1

4-6 to7-1

50.871.80

12.913.129.98

0.216.44

10.362.400.09

0.181.00

99.36

Trace elements in ppm

NiCuZnYSr

ZrRbLiFeO*/MgOa

SI

4859

1013866

121161.99

29.65

424A

424A-1

4-1(11-44 cm)

51.851.77

12.832.32

10.70

0.225.98

10.522.340.11

0.191.49

100.32

5263

1114168

130262.14

28.18

424B

0.6 to 0.62

424B-1

5-1(58-91 cm)

50.721.78

13.023.449.67

0.206.09

10.042.440.08

0.181.91

99.57

5862

1113866

1201_2.10

28.49

424B-2

5-2(11 cm)6-1 (115 cm)

51.251.75

13.472.77

10.30

0.216.579.982.330.13

0.190.92

99.84

5260964267

1141_1.95

30.10

424C

424C-1

2-1 (19 cm)3-1 (56 cm)

50.921.88

12.832.13

10.83

0.236.63

10.702.290.13

0.160.70

99.43

3351

1163863

1242_1.92

30.41

425-1

7-1(36-101 cm)

51.041.35

13.783.019.05

0.197.60

11.432.170.07

0.120.75

100.53

5262953056

801_1.55

35.18

425-2

7-1 (133 cm)7-2 (77 cm)

51.091.20

14.003.148.30

0.188.08

11.742.000.03

0.100.74

100.60

5565782756

721_1.38

38.04

425

ca. 1.8

425-3

7-2 (102 cm)8-1 (50 cm)

50.161.19

14.143.667.55

0.178.08

11.961.870.05

0.120.65

99.60

6671732553

621_1.34

38.76

425-4

8-1 (97 cm)9-1 (76 cm)

50.430.96

14.323.067.18

0.168.41

12.111.740.03

0.110.85

99.36

6771742055

461_

1.1841.81

425-5

9-1 (100 cm)9-2 (30 cm)

50.570.97

14.533.937.23

0.178.19

11.771.870.07

0.090.75

100.14

6175852656

481_1.31

39.19

425-6

9-2 (118 cm)9-3 (77 cm)

50.181.01

14.143.657.30

0.178.39

11.952.180.01

0.100.73

99.81

8272742556

471_1.26

39.64

aTotal iron as FeO.

basalts from these holes probably represent a quicklycooled portion of the same magma that gave rise to thedoleritic unit.

The average compositions of magmas found in theseholes are given in Table 5a, while Cores 428-1 and 2 arethe averages of olivine basalts; Core 428A-3 is theaverage of doleritic rocks. The magma group of Cores428-1 and 428A-1 are similar and differ from that ofCore 428A-2 only with regard to K2O content. Thehigher K2O content of Core 428A-2 may not be a geneticfeature but be related to later alteration, since in allother respects it is similar to the magma group of Cores428-1 and 428A-1. As stated above, the olivine basaltsub-groups have slightly higher FeOVMgO and lowerSI value as compared with the sub-group of Core 428A-3,representing doleritic rocks.

Besides the major and trace element data, REE andsome additional trace elements for these basalts are alsogiven in Table 4; the rare-earth pattern for these basaltsis shown in Figure 2. No significant differences occur inthe total REE or their distribution with respect to depthof sample or type of basalt. All samples are depleted inlighter rare earths as compared with heavier ones. TheLa/Sm (e.f.) is always less than one, similar to ocean-ridge tholeiites described by Kay et al. (1970), Schilling(1971), and others.

Like REE, Sc, Cr, Co, Hf, Ni, Cu, Zn, Y, Sr, Zr, andRb do not show any significant variation. In general,basalts from these holes are very similar to the upperdoleritic unit from Hole 422. For a quick comparison,some of the data are tabulated below:

SiO2

TiO 2

A12O3

MgOCaONa 2 OZrSrYFeOVMgOSI

Hole 422,Magma Group,

Core 422-1

50.261.58

14.097.89

11.572.7111414231

1.2638.24

Holes 428 and428A, Average of

17 Analyses

50.361.55

14.817.53

11.462.5810113226

1.3037.30

Hole 429A

Basalts from this hole belong to two chemicallydistinct types. The average chemical composition ofthese types is given in Table 5a. The younger olivinebasalts from the upper part of the hole (Magma Group,Core 429A-1) are low-Ti, low-K basalts. The Al and Cacontents of these basalts are also high (A12O3 = 16%,CaO = 12.15%). The FeOVMgO ratio is 1.13, and theSI value is 40.23. Compared with these, aphyric basaltsfrom the lower part of the hole (magma Group, Core429A-2) have low to moderate Ti (1.62%), relativelyhigh K, and lower Al and Ca. The FeOVMgO ratio ofthese basalts is 1.54, and SI value is 34.13.

679

aooo

La Ce

Hole 428

Nd

5-4 (6-9)

6-1 (24-28)

6-2 (59-63)

Sm Eu Gd Tb Ho Tm Yb Lu

Hole 428A

i i I

2-1 (86-90)

3-1 (32-36)

3-1 (81-84)

6-1 (79-82)

7-2 (20-23)

J ILa Ce Nd Sm Eu Gd Tb Ho Tm Yb Lu

20

10

4-3 (44-47)

5-3(108-112)

20

10

20

I I

La Ce

Hole 428A

I I I I

1-1 (5-8)

1-2 (55-57)

1-2 (87-90)

1-2 (101-104)

1-3 (105-109)

1-4 (48-53)

I I INd Sm Eu Gd Tb Ho Tm Yb Lu

Figure 2. REE chondrite-normalized distribution patterns of basalts from Holes 428 and 428A (Sample intervals in cm.)


There are also significant differences in the trace ele-ment contents of these groups. In group 1, Ni, Cr, andCu are significantly higher and Zr, Y, and Rb lowerthan in group 2.

The REE patterns of these basalts are given in Figure3. The total REE for group 1 is half the value for group2; however, all patterns are depleted in light REE. TheLa/Sm (e.f.) for group 1 is 0.58, and for group 2, 0.71.The La/Yb (e.f.) is 0.58 and 0.76, respectively.

Petrographically and chemically, group 2 basaltsfrom this hole appear to be closely related to other nor-mal fabric basalts from Holes 420, 421, and 423; how-ever, they are more primitive than the basalts fromHoles 420, 421, and 423. On the other hand, group-1magma from Hole 429A differs significantly from EPRnormal fabric basalts — with respect to both their majoroxide and trace element chemistry, and it may not there-fore have been formed at the EPR crest. The magneticdata of these basalts also suggest that they are youngerthan indicated by the marine magnetic anomaly forHole 429A. To a certain extent, these basalts resemblethe group-2 basalts from Hole 422. In the latter hole,also the doleritic unit postdates the rocks that pro-duced Anomaly 2 (Olduvai normal event, 1.79 to 1.64m.y.), although the hole is located well within theAnomaly 2 (See Site Report for Holes 422 and 429A).

Hole 429A

20

10

5

1-1 (52-54)

Hole 427(SFZ)

Four analyzed samples of basalts from this hole areidentical in their chemistry, which is in keeping withtheir uniform petrography. They can be characterized ashigh-Ti, high-Fe basalt. The FeOVMgO ratio of thesebasalts varies from 1.80 to 1.97, and the SI value from30.17 to 28.71.

The REE patterns of these basalts are given in Figure4. All the basalts from this hole are LREE-depleted;however, depletion of LREE is less than at Site 428 and429 basalts. The total REE is also almost double that ofthe Site 428 and 429 basalts.

Amongst trace elements, Zr shows the highest con-centration (186-194 ppm). Y is also high (42-46 ppm).Cr, Ni, and Cu concentrations are low.

From Glomar Challenger underway records, weknow that some of the troughs in the SFZ are charac-terized by a relatively flat floor and reflective basement.These structures may represent intact down-faultedblocks into which flows had been ponded. Hole 427 wasdrilled to determine if the chemistry and petrography oftrough rocks were similar or comparable to those offabric basalts erupted from the EPR proper, namelybasalts from Holes 420, 421, 423, and 429A.

Hole 427 is located on the southern flank of the SFZ(Figure 1) and may originally have belonged to the blockof oceanic crust to the south. Assuming a spreading ratesimilar to that of the EPR north of the fracture zone,the calculated age of the site comes to approximately 3.4m.y. (distance of Hole 427 from the ridge crest is 196km). It should be mentioned here that the age ofsediments at this site ranges from only 0.2 to 1.2 m.y.(see Site Report).

If the basalts have an age in the order of 3.4 m.y.,and if the nature of basalts erupted at the EPR proper,

20

10

2-1 (12-14)

2-1 (101-104)

20

10

20r3-1 (93-96)

30j

201

Hole 427

9-1 (15-18)

9-3 (84-87)

9-5 (81-85)

10-3(65-68)

La Ce Nd Sm Eu Gd Tb Ho Tm Yb Lu

Figure 3. REE chondrite-normalized distribution pat-terns of basalts from Hole 429A (Sample intervals incm).

La Ce Nd Sm Eu Gd Tb Ho Tm Yb Lu

Figure 4. REE chondrite-normalized distribution pat-terns from Hole 427 (Sample intervals in cm).

681


north and south of the fracture zone, are similar, thenthe chemistry of Hole 427 basalts might well be similarto that of Hole 420 and 421 basalts.

Table 6 presents a comparison of analytical datafrom Hole 427 with Holes 420 and 421 and the EPRcrest north and south of the SFZ. This shows that: 1)Broadly speaking all analyses are of high-Ti, high-Fe,low-Al, and low-Ca basalts. 2) Except for K, Hole 427basalts are similar to glassy basalts from Hole 421; thetotal alkalies in both, however, are of similar order. 3)There are significant differences between the two dredgedEPR crest basalts (SD5-1 and V2023) and present SFZbasalts in respect to enrichment level of Ti and K.

Galapagos Rift

Holes 424, 424A, 424B, and C

These four holes form a north-south transect acrossa geothermal mounds field approximately 22 km southof the GR. The magnetic crustal age varies from 0.6m.y. at Hole 424C to 0.62 m.y. at Hole 424A. Holes 424and 424A are located on mound-like features, and theother two holes are located off mounds. Analyses of 24samples from these sites are given in Tables 2 and 3, andthe averages are given in Table 5b. An analysis of datain these tables reveals that there are no significant dif-ferences in the composition of basalts from these holes;only Hole 424B basalts show minor differences in chem-istry downhole (they form two groups). The uniformchemistry of these basalts is in keeping with theiruniform petrography.

All basalts from these holes have high Fe, high Ti,and low Al. Another interesting feature of their chem-istry is their exremely low K, despite very high Fe, Ti,and a high FeOVMgO ratio. The total alkali content of

TABLE 6Comparison of Analytical Data from Hole 427 with Holes 420,

421, and EPR Crest North and South of Fracture Zone

SiO2

TiO2

A12O3

FeO (total Fe)MnO

MgOCaONa2OK 2 O

P 2 θ 5

NiSrZr

1

50.712.13

13.6511.08

0.20

6.5210.80

2.600.480.16

73120175

2

49.972.63

13.5112.56

0.22

5.989.912.610.550.20

56120205

3

50.052.58

13.7912.46

0.20

6.609.772.970.090.30

53121191

SD5=1

50.702.19

13.6012.71

0.22

6.9411.48

3.570.270.25

51185

-

V2023

48.302.26

14.3011.70

-

6.7010.102.750.18-

58107

-

Notes:1 = Average of Holes 420 and 421.2 = Average of glassy basalts from Hole 421.3 = Average of Hole 427 basalts.SD 5-1 EPR crest north of SFZ (Batiza et al., 1977).V 2023 EPR crest south of SFZ (Kay et al., 1970).

these basalts is generally below 2.60 per cent. Thesechemical features are also common to some of thedredged basalts reported from this area by Anderson etal. (1975) and Schilling et al. (1976). Two of the dredgehauls — Southtow 7, D-17, and Cocotow 4, 6D-1 and6D-2 — together with Holes 424, 424A, B, and C, forma north-south transect across the GR at 86° W latitude.Fe and Ti contents of Cocotow 4, 6D-1 and 6D-2 aresimilar to Site 424 basalts, but the Southtow 7, D-17basalts have higher Ti and slightly lower Fe than Site424 basalts. The low Al in high-Fe basalts is a featurecommon to both drilled and dredged samples.

Like major oxides, trace elements in these basaltsshow only limited variation. Their Zr content variesfrom 114 to 130 ppm. Strontium in these rocks is ex-tremely low (63 to 68 ppm); Y is about 40 ppm. Con-centrations of Ni and Cu are also low.

In comparison with EPR normal fabric basalts from9° N, these basalts are more fractionated as is evidentfrom their higher total iron, and higher FeOVMgOratio; despite this, their total alkalies, Zr, Rb, and Srcontents are much lower than those of the EPR normalfabric basalts.

Hole 425

Basalts from this hole (located 84 km north of Site424 on the northern flank of the GR), show somewhatdiverse chemistry. The hole lies just beyond the Olduvaimagnetic boundary (slightly older than 1.8 m.y.) in asediment-filled topographic depression characterized byhigh heat flow (~5 HFU; Williams et al., 1974); fur-thermore, it is within, or very close to, the zone of large-amplitude magnetic anomalies associated with the GR.Although the hole lies on Matuyama-age reversed, mag-netized crust (very close to the boundary of the normalOlduvai event), rocks in the upper part of the hole shownormal polarity and in the lower part reverse polarity. Itis not unlikely that the upper part belongs to theOlduvai normal event and the lower part belongs toMatuyama. In addition, the deviations from zero in-clinations are considerable in this hole. Peterson andRoggenthen (this volume) believe these magnetic fea-tures to be a consequence of tilting.

Chemical analyses of 14 samples of basalts from thishole are given in Tables 2, 3, and 4. On the basis ofpetrographic types, the rocks can be grouped into sixchemical sub-groups. The average composition of thesesub-groups is given in Table 5b. The data reveal that thebasalts from this hole generally have low Ti, moderateFe, and relatively high Mg. The A12O3 content in thesebasalts is about 14 per cent, and the CaO content about12 per cent. The total alkalies content is the lowest forall sites drilled during the Leg. In comparison with thebasalts from Site 424, these rocks are much poorer in Tiand Fe and richer in Mg. Their alkali content also ismuch lower than that of the Site 424 basalts.

Chemical types of Cores 425-1, 2, 3, and 4 (Table 5b)belong to petrographic types 1, 2, 3, and 4 (Appendix),

682


respectively; the chemical type of Core 425-5 is againaphyric basalt (petrographic type 1), and the chemicaltype of Core 425-6 represents the petrographic type 5.The rocks belonging to the chemical type of Cores425-1, 2, and 3 have normal polarity, and those belong-ing to Cores 425-4, 5, and 6 have reversed polarity.Aphyric basalts constituting type 1 differ from types 2and 3 (pyroxene-plagioclase sparsely phyric basalts andphyric-glomeroporphyritic basalts) in having higher ti-tanium, higher total iron, alkalies, and lower mag-nesium. Slight differences or none at all between types 2and 3 are in keeping with their similar mineralogy. Thechemical type of Core 425-4 differs significantly fromthe overlying types in having lower Fe, lower Ti, andhigher Mg. The FeOVMgO ratio of this group is alsomuch lower than that of the others. These changes inchemistry can be traced to the presence of olivine in theglomerocrysts of these basalts. Aphyric basalts belowthis unit (chemical group of Core 425-5) differ fromaphyric basalts from type 1 in having higher MgO andlower TiO2 and total iron. This chemical type is similarto that of Core 425-6 found at the bottom of the holeand probably represents chilled margin of the petro-graphic type 5. The REE data of the last two sub-groupsgiven in Table 4 support this contention. An interestingfeature of the REE data is the extreme depletion ofLREE as compared with HREE. The normalized pat-terns of these basalts given in Figure 5 are stronglyLREE-depleted; however, HREE show an enrichmentof about 20 times of chondritic values.

Cores 425-1, 2, and 3 show systematic variation indownhole chemistry at this hole. TiO2 decreases from1.35 to 1.19 per cent, total iron (as FeO) from 11.76 to10.84 per cent, and alkalies from 2.24 to 1.92 per cent.A12O3, MgO, and CaO show an increase from 13.78 to14.14 per cent, 7.60 to 8.08 per cent, and 11.43 to 11.96per cent, respectively. FeO*/MgO decreases from 1.55to 1.34, and SI increases from 36.18 to 38.76. Amongstthe trace elements, Ni shows an increase from 52 to 66ppm, and Zr decreases from 80 to 62 ppm. The chem-istry of Core 425-4 immediately below Core 3 may rep-resent a chemical break, because its MgO is significantlyhigher and total iron and TiO2 are lower than in unit 3.The FeOVMgO ratio of this unit is 1.18 and the SIvalue is 41.81. Below this unit, however, the ratioFeOVMgO again shows an increase, and the SI value adecrease. If the chemical unit of Core 425-4 is altogetherneglected, then downhole variation in chemistry be-comes more systematic; that is, from the top to the bot-tom TiO2 decreases from 1.35 to 1.01 per cent, totaliron (as FeO) from 11.76 to 10.58 per cent, and MgO in-creases from 7.60 to 8.39 per cent. The ratio FeOVMgOdecreases from 1.55 to 1.26, and the SI value increasesfrom 35.18 to 39.64. Amongst trace elements, Ni in-creases from 52 to 82 ppm and Zr decreases from 80 to47 ppm.

In view of these facts, it is not unlikely that the entiresequence of rocks found at this site was erupted at thetransition of the Matuyama and the Olduvai normalevent, and that the older Matuyama rocks have a nega-

Hole 425

9-1(134-137)

20

10

5

3

20

10

5

3

20

10

5

3

20

10

5

9-2 (27-30)

9-2(118-121)

9-3 (73-77)

j lLa Ce Nd Sm Eu Gd Tb Ho Tm Yb Lu

Figure 5. REE chondrite-normalized distribution pat-terns of basalts from Hole 425 (Sample intervals incm).

tive polarity and the younger ones belonging to Olduvaihave normal polarity. In general, it can be said that therocks with negative polarity are less evolved than thosehaving positive polarity.

Chemical Variation Across the Galapagos Rift

Hole 425 on the northern flank of the Rift and Site424 on the southern flank, along with the previously re-ported dredge haul Southtow 7, D-17 and Cocotow 4,6D, form a north-south transect at 86° W. De Steiguer41, D1A, DIB, D3A, D3B, and D4 form another tran-sect at 85° 30' W. Data from these sites offer, there-fore, an opportunity to study chemical variations acrossthe GR. Furthermore, both the drilled sites and all thedredges are located within, or very close to, the zone ofhigh-amplitude magnetic anomaly associated with theGR between 85°and 88° W. According to Anderson et

683


al. (1975), this zone is the result of intrusion and extru-sion of large amounts of Fe-rich tholeiite at the ridgecrest from the time of the formation of Anomaly 2 up tothe present.

Figure 6 shows variation in certain elements andratios with distance from the ridge crest around 86° W.Generally speaking, samples close to the ridge crest arerich in Fe and Ti; away from the crest their concentra-

4

3

2

1

16

14

12

0

8

6

4

16

14

12

10

8

BRUHNESNORMALEPOCH0-0-69 m.y.

0 36'N0 43'0 49'0

MATUYAMA

102' 109' 1 24'Ni i

FeO/MgO

Al 2 θ3

MgO

FeO

Tiθ2

424 D1A D17 D6 D3A D4 425

Figure 6. Variations in the concentrations of selectedmajor oxides and in FeO/MgO ratio across the GR at86° W latitude. The shaded area indicates the ridgecrest region. Site 424 and 425 data are from the pres-ent work, D-17 data are from Anderson et al. (1975),andDIA, D6, D3A, D4 data are from Schilling et al.(1976).

tion decreases. Likewise, the ratio FeOVMgO shows aprogressive decrease away from the crest. On the otherhand, Mg and, to certain extent, Al show a progressiveincrease away from the crest. This is contrary to theview of Schilling et al. (1976) who report "There seemsto be no systematic variation in Fe and Ti enrichmentwith distance or age across the ridge in the 85°-87° Warea."

NOMENCLATURE AND CLASSIFICATION OFBASALTS

Generally, petrographic (phenocryst contents) andchemical criteria are used for classifying basaltic rocksfrom an oceanic environment. In this paper, we do notuse petrographic criteria because the large number ofrocks studied by us usually lack phenocrysts and areeither glassy or very fine grained. The chemical schemesfor classifying or distinguishing basaltic rocks are eitherbased on empirical boundaries such as the tholeiite/alk-ali basalt boundary proposed by Chayes (1966) and thealkali-silica diagram (Kuno, 1968; MacDonald and Kat-sura, 1964), or are based on the normative compositionof basalt or its parental liquid. Normative composition,particularly the degree of silica saturation, as indicatedby the presence or absence of quartz, nepheline, orolivine in the norm, can in principle apply to anybasaltic liquid, provided the oxidation state of iron istaken into account. Furthermore, normative classifica-tions are useful in relating natural (especially nearlyglassy) basalts to the results of experimental petrology.

Following the procedure of Coombs (1963), basaltsin this study are classified according to their norm. Forthe purpose of the norm calculation, instead of choos-ing an arbitrary value of 1.5 per cent Fe2θ3, as was doneby Kay et al. (1970) and some others, we have decided tofix the ratio Fe2O3/FeO at 0.15, since the lowest ob-served Fe2O3 content of unaltered submarine basaltranges from 1.0 to 1.5 per cent and the total Fe contentis in the vicinity of 10 per cent. We chose a lowFe2θ3/FeO ratio for two reasons: (1) we find largevariation in Fe2θ3/FeO with almost constant total Fe,and (2) the H2O content of these basalts is more than 0.5per cent, a value normally reported for the ocean-ridgebasalts. Furthermore, since it is unlikely that shallowmagmatic processes result in the reduction of an oxi-dized liquid, we have assumed that the Fe2O3 contentsof ocean-ridge basalt parental liquids are similar to theobserved low values.

The present basalts outlined in Figure 7 are highlyvariable in normative chemistry. There are no rockswith alkalic affinities, but most of them are quartz-normative. This finding is contrary to that of Chayes(1965) and Kay et al. (1970), who report that most of theocean-ridge tholeiites are neither quartz- nor nepheline-normative. To a certain extent our finding is similar tothat of Kempe (1975), who reports that more than 25per cent of the drilled oceanic basalts are quartz-normative; in our study, however, over 70 per cent ofsamples are.

Except for a few samples, all EPR normal fabricbasalts are quartz-normative with a diopside (di) max-

684


DSDP Leg 54 Holes

D 420421

• 422

O423

*424,A,B,C• 425O427x 428.A* 429.A

Figure 7. ol-di-hy-q diagram for Leg 54 basalts.

imum of about 53 per cent. SFZ basalts (Hole 427) arealso quartz-normative, but their di maximum is about44 per cent. On the other hand, basalts from OCP moatsites (Holes 422, 428 and 428A) by and large are olivine-normative with a di maximum of about 57 per cent, afigure which is typical of most drilled basalts (seeKempe, 1975). The GR sites are also quartz-normative;however, their di maximum is much lower — about 44per cent.

On the alkali-silica diagram (Figure 8), all samplesfall in the field of tholeiites as given by Macdonald and

THOLEIITES

Figure 8. Alkali-silica variation diagram for Leg 54 ba-salts. The tholeiites/alkali basalt boundary is afterMacdonald and Katsura (1964). Hole symbols aresame as in Figure 7.

Katsura (1964). It is interesting that the samples fromthe GR plot in a field less alkalic than that of EPR sam-ples. The variation in total alkalies is large, in spite oflimited variation in silica.

In recent years a number of schemes using trace ele-ments have been propounded to relate basic volcanicrocks to magma type and tectonic setting. Cann (1970)and, later, Pearce and Cann (1971 and 1973) have pro-pounded schemes based on Ti, Zr, Y, and, for less al-tered rocks, Sr. A scheme using TiO2, K2O, and P2O5

was put forward by Pearce (1975), and one using Ti, Zr,Y, Nb, and P by Floyd and Winchester (1975). Smithand Smith (1976) have discussed the usefulness of thesevarious schemes in the classification of basalts.

There is a positive correlation between the pairsTi-Zr and Ti-Y in ocean-floor basaltic rocks; tholeiitesmay show a proportional trend, whereas alkali basaltshave a horizontal trend. Figures 9 and 10 are TiO2-Zrand TiO2-Y plots of present basalts along with some ofthe data for DSDP Legs 16, 34, 37, and 52. The dia-grams show a remarkable positive correlation betweenthese element pairs, a finding which is similar to that ofCann (1970) who reports positive correlation betweenthose element pairs in ocean-floor basaltic rocks fromdifferent ocean areas. Furthermore, amongst the Leg 54

i DSDP Leg 54 Holes

π 420 425A 421 o 427• 422 × 428.AO 423 * 429.A* 424,A,B,C + DSDP Ieg37

1.5 2.0Tiθ2 (%)

2.6

Figure 9. Covariation of TiO2 and Zrfor Leg 54 basalts.DSDP Leg 3 7 data are our unpublished data.

DSDP Leg 54 Holes

a 420 O 427421 × 428.A

a 422 * 429 AO 423 v DSDP leg 52* 424,A,B,C, + DSDP leg 37• 425

1.5 2.0Tiθ2 (%)

Figure 10. Covariation of TiO2 and Y for Leg 54 ba-salts. DSDP Leg 37 and 52 data are our unpublisheddata.

685


basalts, EPR normal fabric basalts and SFZ basaltshave the highest concentrations of Ti, Zr, and Y.

The pair TiO2-P2θ5 also shows two trends: one forthe tholeiitic suite, where an increase in TiO2 is matchedby a proportional increase in P2O5, and the other for thealkalic suite where there is a threefold increase in P2O5with each doubling of TiO2. Both trends converge near1 per cent TiO2 and 0.1 per cent P2O5, and it is thereforenot possible to distinguish rocks from the two suiteswith extremely low Ti and P by means of this plot.Figure 11 is a TiO2-P2O5 diagram of the present rocks.Like the Ti-Zr and Ti-Y diagrams, this one also shows aproportional trend. The dashed line is the ridge cresttrend.

The K2O-TiO2 diagram (Figure 12) has been used todistinguish, and speculate on, the relationships betweendifferent types of basaltic rocks. It is interesting that thedata from GR sites fall on a narrow K2O and wide TiO2

range, whereas those from EPR sites show a near-pro-portional trend; the variation seems to be a continuousone. Furthermore, the SFZ basalts plot in a field of theirown.

4*&+ O•

•

Tiθ2 (%>

Figure 11. Covariation of TiO2 and P2O5for Leg 54 ba-salts. Dashed line indicates the ridge crest tholeiitestrend. Hole symbols are same as Figure 7.

Tiθ2 (%)

Figure 12. K2O - TiO2 variation diagram for Leg 54 ba-salts. Hole symbols are same as in Figure 7.

Concentrations of individual elements have also beenused to characterize rocks from mid-oceanic ridges.Low K2O and SiO2 and high A12O3 contents have allbeen put forward as characteristics of these rocks (Engelet al., 1965; Nicholls, 1965; Green and Ringwood, 1967;Kay et al., 1970; and Cann, 1971). None of the samplesstudied can be called high-alumina basalt, since theA12O3 content of the present samples is commonlyabout 14 per cent. The range of K2O is also fairly large,0.01 to 0.6 per cent. The Si is generally above 50 percent. Thus, the characterization by Engel et al. (1965) ofocean ridge basalts as being high in Al, low in K and Si,is not supported by the present data. Table 7 comparesthe averages of the present work with the averages givenby Engel et al. (1965) as well as other averages from theAtlantic, Pacific, and Indian oceans. The table clearlyreveals that present basalts are lower in Al and muchhigher in Ti and Fe as compared with the averageoceanic tholeiite of Engel et al. (1965). In comparisonwith the Atlantic, the present basalts have higher Fe andTi and lower Al and Mg. The present EPR normalfabric basalt average matches closely the EPR averageof Melson et al. (1976), except for the Ti and K contentswhich are higher. On the other hand, when all EPR sitesof Leg 54 — that is, normal fabric basalt, OCP moatbasalt, and SFZ basalts — are taken together, the aver-age comes very close to that of Melson et al. (1976), ex-cept for Fe which is lower. As regards the GR basalts,they appear to be a type in themselves characterized byextremely low Al, low Mg, and low total alkalies as wellas high Fe. In the light of this, we suggest that use of"oceanic tholeiite" as a blanket term for ocean-floorbasalts be dropped and that the latter be referred to asolivine tholeiite, quartz tholeiite, or high-aluminatholeiite, as the case may be. The present basalts areolivine and quartz tholeiites.

DISCUSSION AND CONCLUSIONS

We have shown that Leg 54 basalts have highlyvariable petrography and chemistry. All EPR normalfabric basalts are cryptocrystalline to fine-grainedaphyric plagioclase-pyroxene basalt. The OCP moatbasalts by and large are fine- to medium-grained plag-ioclase-pyroxene ± olivine basalts. The SFZ basalts arefine- to medium-grained plagioclase-pyroxene basalts.The normal fabric basalts show textures ranging fromhyalopilitic to intersertal. The OCP moat basalts andSFZ basalts have intersertal to ophitic texture. The GRbasalts from Site 424 are similar to EPR normal fabricbasalts in their petrography. The largest petrographicvariation at any site of Leg 54 is among Hole 425basalts. On the basis of texture and mineralogy, basaltsfrom this hole can be grouped into four distinctpetrographic types.

There are also significant differences in chemistryand normative mineralogy of basalts from variousregions. EPR normal fabric basalts and SFZ basalts arehigh-Ti, moderate to high-Fe quartz tholeiites with lowNi and high Zr. The OCP moat basalts, on the otherhand, are low- to moderate-Ti, low-FeOVMgO olivinetholeiite. Their Y and Zr are significantly lower than the


TABLE 7Average Compositions of Oceanic Basalts and Basaltic Glasses Related to Spreading Ridges in the Atlantic,

Pacific, and Indian Oceans

Siθ2Tiθ2A12O3

FeO*

MgOCaONa2θK2O

1

50.671.28

15.459.67

8.0511.72

2.510.15

2

50.551.69

13.9511.04

7.1211.00

2.460.18

3

50.331.81

14.3410.36

7.1711.09

2.640.26

4

50.652.07

13.7110.89

6.5710.69

2.610.43

5

50.201.52

14.789.70

7.7211.52

2.620.17

6

50.052.58

13.7912.46

6.609.772.970.09

7

50.841.54

13.4411.94

7.0510.88

2.220.08

8

50.191.77

14.8611.33

7.1011.44

2.660.16

9

49.941.51

17.258.71

7.2811.68

2.760.16

10

49.611.43

16.0111.49

7.8411.32

2.760.22

11

50.731.19

15.1510.32

7.6911.84

2.320.14

Notes:1 = Atlantic 155 glass analyses, including FAMOUS data and DSDP Leg 37 (Bryan et. al., 1976).2 = Equatorial East Pacific, Leg 54, 89 basalt analyses (this work).3 = EPR, Leg 54, all sites around 9°N, 51 basalt analyses (this work).4 = EPR normal fabric basalt, Leg 54, 20 basalt analyses (this work).5 = OCP moat basalt, Leg 54, 25 basalt analyses (this work).6 = SFZ basalt, Leg 54, four basalt analyses (this work).7 = GR, Leg 54, 38 basalt analyses (this work).8 = EPR, 38 glass analyses (Melson et al., 1975).9 = Average oceanic tholeiite (Engel et al., 1965).

10 = Average oceanic tholeiite (Cann, 1971).11 = Indian Ocean, 10 glass analyses (Melson et al., 1975).

EPR normal fabric and SFZ basalts. The GR basalts,insofar as their major oxides are concerned, display thecomplete chemical spectrum of the EPR normal fabricbasalts, OCP moat basalts, and the SFZ basalts, exceptthat their total alkalies are lower. Site 424 basalts arehigh-Ti, high-Fe quartz tholeiite, and Hole 425 basaltsare low-Ti, low- to moderate-Fe quartz tholeiite. Thepresence of normative quartz in Hole 425 basalts is aconsequence of their low alkalies. Another character-istic feature of GR basalts is their extremely low Sr andlow to moderate Zr, despite high Ti and Fe.

The old concept that the deep ocean floor consists ofbasaltic rocks that are more or less uniform in composi-tion and represent a primary magma composition is nolonger valid. During the past few years several workershave demonstrated that ocean-floor basaltic rocks havesystematic chemical differences and often show a mod-erate iron enrichment trend of fractionation oblique tothe FeO-MgO side of an AFM diagram. The mineralsparticipating in the shallow fractionation process havegenerally been inferred to be olivine and plagioclase(Kay et al., 1970; Miyashiro et al., 1969; Shido et al.,1971; Scheidegger, 1973; Schilling, 1975). Bunch and LaBorde (1976), Clague and Bunch (1976) and Batiza et al.(1977) have noted the importance of clinopyroxene asfractionating phase and have propounded models withclinopyroxene and plagioclase as the dominatingphases, with subordinate olivine for basalts occurring ateast Pacific mid-ocean spreading centers. Furthermore,these authors have demonstrated that high-Fe and Tibasalts as well as ferrobasalts occurring at these centerscan be derived by fractionation from normal mid-oceanridge tholeiites.

According to Kempe (1975), in many of the DSDPbasalts "differentiation patterns in moderately longdrilled basalt cores show two main types of fractiona-tion with respect to Fe/Mg and (normative) Ab/An.

Enrichment in iron and albite normally occurs upward(? within each flow), while sudden reversal of this trendmay indicate new effusions of magma from its source.Repetitive step-like patterns thus result". Miyashiro(1975) reports that the FeOVMgO ratio for abyssaltholeiites varies from 0.8 to 2.1, but most of the freshsamples have a ratio less than 1.7. Analytical data inTable 3 reveal that the FeOVMgO ratio of the presentrocks is highly variable, ranging from 1.0 to 2.14. TheEPR normal fabric basalts show a variation from 1.50to 1.80, except for two samples from Hole 421 whichhave a ratio above 2.0. The OCP moat basalts havelower FeOVMgO than normal fabric basalts, the rangebeing 1.0 to 1.40. Basalts from the SFZ (Site 427) alsoshow high FeOVMgO. GR basalts show considerablevariation, while at Hole 425 the ratio varies from 1.13 to1.54; at Site 424 the ratio varies from 1.89 to 2.14. As tothe downhole variation at different sites, the reversal oftrends as pointed out by Kempe (1975) also occurs insome of our drill holes.

To further test the possibility of crystal fractionation,the data have been plotted on an AFM diagram (Figure13). This reveals that (1) there are two trends, one markedby the EPR sites and the other by the GRZ sites, (2) theGalapagos samples plot in a field which is less alkalicthan that of EPR basalts, (3) both trends are slightlyoblique to the MgO-FeO side of the AFM triangle andindicate a moderate iron enrichment, (4) EPR normalfabric basalts and SFZ basalts are more enriched in ironand alkalies than OCP moat basalts, (5) there seems tobe a gap between the normal fabric and moat fields ifthe two samples from the lower part of Hole 429A andthe sample from the bottom of Hole 422 are neglected,(6) the largest variation for any site is shown by thesamples from Hole 425, and (7) Site 424 samples aremore enriched in iron than Hole 425 samples and thereis a gap between the two. The difference in Fe enrich -

687


DSDP Leg 54 Holes

α 420Δ 421• 422o 423

• 424,A,B,C• 425o 427× 428.A• 429.A

Figure 13. AFM diagram of Leg 54 basalts.

ment at two GR sites may be related to the respectiveages of the basalts at these sites; the younger Site 424basalts are more enriched in Fe than the older Hole 425basalts.

In Figure 14 we compare the present data with NorthAtlantic data. It is interesting that (1) EPR normalfabric basalts mark a continuation of the North Atlantictrend and plot in a more iron-enriched field than the lat-ter, (2) OCP moat basalts plot in the field of NorthAtlantic basalts, (3) the Galapagos samples, despitehigher Fe enrichment, are clearly less alkalic than theNorth Atlantic basalts.

Generally speaking, basalts from modern ridges dif-fer from one another, especially in terms of FeOVMgO

Figure 14. AFM diagram comparing Leg 54 basalts withNorth Atlantic basalts. Open squares are Leg 54, andsolid squares are North Atlantic data.

and TiO2. In Figure 15 we show distribution ofFeOVMgO and TiO2 in all Leg 54 basalts, our un-published data from Leg 37, DSDP Leg 16 data fromYeats et al. (1973), and DSDP Leg 34 data from Bunchand La Borde (1976). The dashed boundary in the figureindicates the field of FAMOUS glasses as given byBryan et al. (1976). The diagram reveals several in-teresting points: (1) There is broad covariation of all thedata plotted, despite the samples being from differentoceans, (2) The field of FAMOUS glasses encloses asignificant portion of the plotted data, although the lat-ter are distributed over a wide range of ages andenvironments in different oceans, (3) A fairly largenumber of Pacific analyses significantly exceed thosefrom the Atlantic in both FeOVMgO and TiO2, (4) AllEPR normal fabric basalts, SFZ, and GR Site 424basalts plot in the direction of higher FeOVMgO andhigher TiO2 and by and large outside the field ofFAMOUS glasses, (5) The OCP moat basalts plot with-in the field of FAMOUS glasses with relatively largevariation in FeOVMgO as compared with TiO2, and (6)Hole 425 basalts also plot within the field of FAMOUSglasses and show a proportional variation in FeOVMgO and TiO2.

x //

/

/

/

: ; /

DSDP Leg 54 HolesD420

421• 422O423* 424,A,B,C* 425o 427× 428.A* 429.Av DSDP leg 16» DSDP leg 34+ DSDP leg 37

Tiθ2 (%)

Figure 15. Covariation of FeO*/MgO and Tiθ2forLeg54 basalts. DSDP Legs 16 and 34 data after Yeats etal. (1973) and Bunch and LaBorde (1976), respective-ly. Leg 37 data are our unpublished data. The dashedboundary encloses the field of FAMOUS glasses asgiven by Bryan et al. (1976).

688


In general, Leg 54 basalts are characterized by higherFeO and higher TiO2 than Atlantic basalts. The higherFeO and higher TiO2 may be related to the fasterspreading rate in the Pacific. Melson et al. (1976) alsoshow that the EPR and Juan de Fuca Ridge averages arecharacterized by distinctly higher FeO and TiO2 thanthe Atlantic and Indian ocean averages.

P, Ti, Zr, and Y are generally enriched in alkali ba-salts relative to tholeiitic basalts. Most of the ocean-floor and ridge tholeiitic rocks have less than 1.8 percent TiO2 and less than 120 ppm Zr. Oceanic islandtholeiites have higher Ti and Zr contents (Floyd andWinchester, 1975; Cann, 1970). All EPR normal fabricbasalts and the SFZ basalts have more than 1.8 per centTiO2 and more than 150 ppm Zr. The OCP moat basaltshave lower Ti and lower Zr, the range being similar tothat of ocean-floor and ridge basalts. The GR basaltsshow a very wide range; as TiO2 varies from 0.96 to 1.88per cent, Zr varies from 46 to 130 ppm. The high degreeof positive correlation between the pairs Ti-Zr and Ti-Yin the present tholeiites may either be attributed to thefact that these elements are strongly partitioned intoresidual liquid as crystallization proceeds (their parti-tion coefficients into solid phases is nearly zero) andtheir absolute abundances are related to the degree offractional crystallization which the melts have under-gone, or else the positive correlation is a result ofvarious degrees of partial melting of a homogeneousmantle. A more realistic approach is that the level ofabundances of these elements as well as some otherssuch as P and K in these basalts is related to both frac-tional crystallization and the degree of partial melting.Relatively high Zr contents of EPR normal fabric ba-salts and SFZ basalts may indicate a slight enrichmentof Zr in the source of these rocks.

In Table 8 we show a comparison of EPR normalfabric basalts, OCP moat basalts, and the SFZ basaltswith regard to their petrography, physical properties,and chemistry. Also listed are some literature data fromthe SFZ. The table clearly reveals that there are signifi-cant differences in petrography, physical properties,and chemistry of basalts from these sites. From thepoint of view of major oxide chemistry, the SFZ basaltsare most evolved, having highest Fe, Ti, Zr, and FeO/MgO; the OCP moat basalts are least evolved, withlowest Fe, Ti, Zr, and FeO/MgO. The EPR normal fab-ric basalts have values intermediate between those of theSFZ and OCP moat basalts.

In the light of differences in the petrography andchemistry between the EPR normal fabric basalts andthe OCP moat basalts, as discussed in the precedingparagraphs, it may well be that the latter basalts werenot generated at the EPR proper but are related to latertransform fault activity. The OCP ridge may mark thebeginning of a new transform fault zone.

Despite having high Fe, Ti, P, and Zr, the SFZbasalts have extremely low K compared with EPR nor-mal fabric basalts and EPR crest basalts from north andsouth of the fracture zone. This may indicate a differentsource composition for these basalts. Alternatively, thismight be taken to indicate a higher degree of partial

melting of a similar large-ion-lithophile (LIL) element-depleted source, followed by extensive low-pressurefractional crystallization resulting in gradual enrich-ment of Fe and Ti. The high degree of iron enrichment(12.46% FeO) in these rocks requires some 70 to 80 percent crystallization of a liquid of the composition ofOCP moat basalt (the most primitive tholeiite recoveredduring Leg 54). Despite this, these residual liquids haveretained the LREE-depleted patterns characteristic ofnormal ridge segments, although they have a muchhigher overall enrichment (almost double that of OCPmoat basalts). This kind of REE enrichment is to be ex-pected if the crystallization process is dominated by ex-traction of plagioclase and clinopyroxene with subor-dinate olivine. According to Clague and Bunch (1976),basalts, high in FeO, TiO2, P2O5, K2O, Na2O, and SiO2,and low in MgO, CaO, and A12O3 (relative to normalmid-ocean ridge basalts) from the EPR, Galapagos, andJuan de Fuca Ridge are formed by fractionation ofplagioclase, clinopyroxene, and olivine in average pro-portion of 9.3:7.1:1 from normal ridge tholeiites. Some75 per cent of the original liquid has to crystallize toform ferrobasalts. Batiza et al. (1977) report that theirmost fractionated Siqueiros tholeiite (SD6-3, very sim-ilar to present SFZ basalts) can be derived from theirprimitive tholeiite SD8-3 (somewhat similar to OCPmoat basalts) by fractionation and removal of 47 percent plagioclase, 46 per cent clinopyroxene, and 7 percent olivine. Removal of up to 80 weight per cent ofcrystals from such a liquid is therefore necessary.

The presence of highly fractionated tholeiites intransform fault troughs of the SFZ is contrary to thefinding of Batiza et al. (1977), who report that highlyfractionated tholeiites are absent in their dredge haulsfrom the northern and southern Siqueiros transformfault troughs. According to Crane (1976), the northernSiqueiros transform fault is the presently active trans-form fault, and the southern transform fault ceased tobe active as strike slip motion and was taken up alongthe northern trough. Site 427 is certainly older than 1.2m.y. (oldest sediments at the site), and so there is everylikelihood that these basalts were produced at a timewhen the southern transform fault was still active.

Holes 423, 421, 420, and 429A form a nearly east-west transect on the west flank of the EPR with Hole429A on the oldest crust. Thus, these holes offer an op-portunity to study the composition of basalts in relationto their distance from the Rise crest. In Table 9, we listthe average composition of basalts found at these sitesas well as the analyses of other Rise crest basalts fromthe region. The table reveals that (1) among the EPRnormal fabric basalts, Hole 429A tholeiites are the mostprimitive with regard to their Ti, Mg, alkalies, Zr, Ni,and FeO/MgO and S.I. values; the magma group ofCore 421-1 (see analysis in Table 5a) seems to be themost evolved, having highest Ti, alkalies, Zr, andFeO/MgO, (2) in general it can be said that Ti, alkalies,Zr, and the ratio FeO/MgO show an increase towardthe Rise crest, and Mg, Ni, and S.I. show a decrease. Toa certain extent, Al and Ca also show an increase awayfrom the Rise crest. However, the magma group of Core

689


TABLE 8Comparison of EPR Normal Fabric Basalts, OCP Moat Basalts, and SFZ Basalts

Petrographic description

Physical propertiesa

VelocityDensityCompositionSiC-2TiO2

A12O3

FeO (total Fe)MgOCaONa2O

K 2 O

P2O5NiYSr

ZrLaYbFeO/MgO

EPR NormalFabric Basalts

Aphyric to sparselyphyric, glassy to fine-grained plagioclase-pyroxene basalts

5.2 to5.7km/s2.8 to2.88g/cm3

Average of 20 analyses50.65

2.07

13.7110.89

6.5710.69

2.61

0.430.18

6843

120

163__1.66

OCP MoatBasalts

Fine- to medium-grained olivine basalts

5.8 to 6.2 km/s2.92 g/cm3

Average of 25 analyses50.20

1.52

14.789.707.72

11.522.62

0.170.15

7526

140

1034.142.951.27

SFZ Basalts

Fine- to medium-grained

plagioclase-pyroxene basalts

5.7 km/s2.92 g/cm3

Core427-lb

50.052.58

13.7912.46

6.609.772.97

0.090.30

5345

121

1916.934.901.89

SD6-3C

48.002.70

12.8013.66

5.9810.14

3.33

0.270.23

43-

140

-

-—2.29

SD8-3C

50.300.93

15.608.599.80

12.772.41

0.060.09

130-

125

—

—-0.88

aData from the preliminary shipboard report.bAverage of four analyses from Hole 427.cSiqueiros fracture zone basalts from Batiza et al. (1977).

TABLE 9Chemical Variation Away from the EPR Crest at 9° N

Sample Location

Age

SiO2

Tiθ2A12O3

FeO (Total Fe)MgO

AlkaliesNiSrZrFeO/MgO

EPR Crest

SD5 = l a

50.702.26

13.6012.70

6.94

3.8443

185_

1.83

SD5=2a

49.801.64

14.0010.59

8.16

3.81

-_

1.30

Hole423

1.6 m.y.

50.542.11

13.6211.37

6.44

3.1153

126161

1.77

Hole421

3.4 m.y.

50.772.27

13.2711.37

6.43

3.1467

123185

1.77

Hole420

3.4 m.y.

50.641.93

14.1810.68

6.64

2.9682

115161

1.61

Hole429A

4.6 m.y.

50.551.62

14.3511.07

7.18

2.7988

115103

1.54

aFrom Batiza et al. (1977).

421-1 marks a reversal of the trend. Furthermore, thereare significant differences in the analyses of two Risecrest samples from Batiza et al. (1977). Bonatti andFisher (1971) also report large differences in Rise crestbasalts from 10° S. Yeats et al. (1973) report, withrespect to DSDP Leg 16 basalts, an increase in CaO andMgO and decrease in TiO2, K2O, and Na2O with in-creasing distance from the crest of the EPR. Here also,the trends do not extrapolate to the Rise crest data.

Evidence for a crustal low-velocity zone below theEPR crest near the Siqueiros fracture zone has beenpresented by Orcutt et al. (1976) and Rosendahl et al.(1976). The latter have interpreted this zone as a shallowmagma chamber below the EPR crest. The range of

chemical variation encountered within the EPR normalfabric basalts is certainly a result of shallow-level or, inother words, low-pressure fractional crystallization ofthe phase assemblage ol -I- pi + cpx, olivine being theleast dominant of the three. This fractionation couldtake place either in the magma chamber below the EPRor in the conduit system that replenishes the chamberfrom below. The reversal of trends at Hole 421 in allprobability marks the influx of new material to themagma chamber. Thus, a batch of magma undergoesshallow-level fractional crystallization in the low-velocity zone below the EPR. After some period, freshmaterial is added, depressing the differentiation indicesof the existing liquid which, when subsequently erupted,gives rise to a more primitive basalt than that whicherupted earlier.

Like the EPR basalts, the GR basalts are the productof low-pressure fractional crystallization of LIL-depleted melts. The degree of partial melting, however,was greater than that for EPR basalts. It is for thisreason that the GR basalts are depleted in alkalies, Zr,and Sr in comparison with EPR basalts. The contentionof Campsie et al. (1973) that the basalts from within theregion of the large-amplitude magnetic anomaly are"plume-derived," indicating chemical similarity toGalapagos Island tholeiites, is not borne out by ourresults. On the other hand, our data support Andersonet al. (1975), who observed that Fe- and Ti-enrichedbasalts from the region of the large-amplitude magneticanomaly cannot be formed by mixing of ridge tholeiite

690


and Galapagos Island tholeiite but are the result of ex-tensive shallow-fractional crystallization of normalridge tholeiite, producing Fe-enriched basalts whichprobably constitute one-third to one-half of Layer 2A.

ACKNOWLEDGMENTSThis work was done at the Institut für Petrographie und

Geochemie, Universitàt Karlsruhe, Karlsruhe, West Ger-many, during RSK tenure as AvH Fellow in that Institute. Thework was supported by a grant from the Deutsche Forschungs-gemeinschaft. Neutron activation analyses were carried outwith equipment given by Bundesministerium für Forschungund Technologic Irradiation facilities were provided by theGesellschaft für Kernforschung Karlsruhe. The help of U.Kramar in the INAA investigation is gratefully acknowledged.Last but by no means least, RKS is also grateful to the scien-tific party and crew members of the Glomar Challenger fortheir excellent cooperation during the 54th cruise.

REFERENCES

Anderson, R. N., Clague, D. A., Klitgord, K. D., Marshall,M., and Nishimori, R. K., 1975. Magnetic and petrologicvariation along the Galapagos spreading center and theirrelation to the Galapagos melting anomaly. Bull. Geol.Soc. Am.,\. 86, p. 683-694.

Batiza, R., Rosendahl, B. R., and Fisher, R. L., 1977. Evolu-tion of oceanic crust 3. Petrology and chemistry of basaltsfrom East Pacific Rise and Siqueiros transform fault. J.Geophys. Res., v. 82, p. 265-276.

Bonatti, E., and Fisher, D. E., 1971. Oceanic basalts: chem-istry versus distance from oceanic ridges. Earth Planet. Sci.Lett., v. 11, p. 307-311.

Bryan, W. B., Thompson, G., Frey, F. A., and Dickey, J. S.,1976. Inferred geologic settings and differentiation in ba-salts from the Deep Sea Drilling Project. / . Geophys. Res.,v. 81, p. 4285-4304.

Bunch, T. E., and LaBorde, R., 1976. Mineralogy and compo-sition of selected basalts from DSDP Leg 34. In Yeats, R.S., Hart, S. R., et al., Initial Reports of the Deep Sea Drill-ing Project, v. 36: Washington (U. S. Government PrintingOffice), p. 263-275.

Campsie, J., Bailey, J. C , and Rasmussen, M., 1973. Chem-istry of tholeiites from the Galapagos Island and adjacentridge. Nature Phys. Sci., v. 245, p. 122-124.

Cann, J. R., 1970. Rb, Sr, Y, Zr and Nb in some ocean floorbasaltic rocks. Earth Planet. Sci. Lett., v. 10, p. 7-11.

, 1971. Major element variation in ocean floor ba-salts. Phil. Trans. Roy. Soc. London, v. 268, p. 494-505.

Chayes, F., 1965. The silica-alkali balance of basalts of sub-marine ridges. Carnegie Inst. Wash. Yr. book, v. 64, p.155-163.

, 1966. Alkaline and subalkaline basalts. Amer. J.Sci., v. 264, p. 128.

Clague, D. A., and Bunch, T. E., 1976. Formation of ferro-basalts at East Pacific midocean spreading centers. /.Geophys. Res., v. 81, p. 4247-4256.

Coombs, D. S., 1963. Trends and affinities of basaltic mag-mas and pyroxenes as illustrated on diopside-olivine-silicadiagram. Mineral. Soc. Am. Special Paper 1, p. 227-250.

Crane, K., 1976. The intersection of Siqueiros transform faultand the East Pacific Rise. Mar. Geol., v. 21, p. 25-46.

Engel, A. E., Engel, C. G., and Havens, R. G., 1965. Chemi-cal characteristics of oceanic basalts and upper mantle.Bull. Geol. Soc. Am., v. 76, p. 719-734.

Flanagan, F. J., 1973. 1972 values for international geochemi-cal standards. Geochim. Cosmochim. Acta., v. 37, p.1189-1200.

Floyd, P. A. and Winchester, J. A., 1975. Magma type andtectonic setting discrimination using immobile elements.Earth Planet. Sci. Lett., v. 27, p. 211-218.

Green, D. H. and Ringwood, A. E., 1967. The genesis of ba-salt magmas. Contrib. Mineral. Petrol., v. 15, p. 103.

Kay, R., Hubbard, N. J., and Gast, P. W., 1970. Chemicalcharacteristics and origin of ocean ridge volcanic rocks. J.Geophys. Res., v. 75, p. 1585-1613.

Kempe, D. R. C , 1975. Normative mineralogy and differen-tiation patterns of some drilled and dredged oceanic ba-salts. Contrib. Mineral. Petrol., v. 50, p. 305-320.

Kuno, H., 1968. Differentiation of basalt magmas. In Hess,H. H., and Poldervaart, A., (Eds.). Basalt: The Polder-vaart Treatise on Rocks of Basaltic Composition, v. 2: NewYork (Inter Science).

Macdonald, G. A. and Katsura, T., 1964. Chemical composi-tion of Hawaiian lavas. /. Petrol., v. 5, p. 82-133.

Melson, W. G., Valuer, T. L., Wright, T. L., Byerly, G., andNelen, J., 1976. Chemical diversity of abyssal volcanic glasserupted along Pacific, Atlantic and Indian ocean sea floorspreading centres. The Geophysics of the Pacific OceanBasin, Geophysical Monograph Series, v. 19, Washington(American Geophysical Union).

Miyashiro, A., 1975. Classification, characteristics and originof ophiolites. /. Geol., v. 83, p. 249.

Miyashiro, A., Shido, F., Ewing, M., 1969. Diversity andorigin of abyssal tholeiite from Mid-Atlantic Ridge near24° and 30° N latitude. Contrib. Mineral. Petrol., v. 23, p.38-52.

Nicholls, G. D., 1965. Basalts from the deep-ocean floor. Min-eral. Mag., v. 34, p. 373.

Orcutt, J. A., Kennett, B. L. N., and Dorman, L. M., 1976.Structure of East Pacific Rise from an ocean bottom seis-mometer survey. Geophys. J. Roy. Astron. Soc, v. 45, p.305-320.

Pearce, J. A., 1975. Basalt geochemistry used to investigatepast environment on Cyprus. Tectonophysics, v. 25, p.41-67.

Pearce, J. A., and Cann, J. R., 1971. Ophiolite origin investi-gated by discriminant analysis using, Ti, Zr, and Y. EarthPlanet. Sci. Lett., v. 12, p. 339-349.

, 1973. Tectonic setting of basic volcanic rocks deter-mined using trace element analyses. Ibid., v. 19, p. 290-300.

Puchelt, H., and Kramar, U., 1974. Standardization in neu-tron activation analysis of geochemical material. Workingpaper presented at IAEA Panel Meeting on Nuclear Geo-physics and Geochemistry, 25-29 November 1974, Vienna.

Rosendahl, B. R., 1976. Evolution of oceanic crust 2. Con-straints, implications and inferences. /. Geophys. Res., v.81, p. 5305-5314.

Rosendahl, B. R., Raitt, R. W., Dorman, L. M., Bibee, L. D.,Hussong, D. M., and Sutton, G. H., 1976. Evolution ofoceanic crust 1. A physical model of the East Pacific RiseCrest derived from seismic refraction data. Ibid., v. 94, p.5294-5304.

Scheidegger, K. F., 1973. Temperature and composition ofmagmas ascending along midocean ridges. Ibid., v. 78, p.3340-3355.

Schilling, J. G., 1971. Sea-floor evolution: rare earth evidence:Phil. Trans. Roy. Soc. London A, v. 268, p. 663-706.

Schilling, J. G., 1975. Rare earth variations across normal seg-ments of Reykjanes ridge, 60°-53° N, mid-Atlantic ridge,29° S and East Pacific rise 2°-19° S, and evidence on com-position of the underlying low-velocity layer. J. Geophys.Res., v. 80, p. 1459-1473.

Schilling, J. G., Anderson, R. N., and Vogt, P., 1976. Rareearth, Fe and Ti variations along the Galapagos spreading

691


centre, and their relationship to the Galapagos mantle plume.Nature, v. 261, p. 108-113.

Shido, F., Miyashiro, A., and Ewing, M., 1971. Crystalliza-tion of abyssal tholeiites. Contrib. Mineral. Petrol., v. 31,p. 251-266.

Smith, R. E., and Smith, S. E., 1976. Comments on the use ofTi, Zr, Y, Sr, K, P, and Nb in classification of basalticmagmas. Earth Planet. Sci. Lett., v. 32, p. 114-120.

Srivastava, R. K., 1977. A comprehensive atomic absorptionand spectrophotometric scheme for the determination ofmajor and trace elements in rocks and minerals. N. Jb.Miner. Mh. Year 1977, p. 425-432.

Williams, D. L., von Herzen, R. P., Sclater, J. G., and Ander-son, R. N., 1974. The Galapagos Spreading Centre: Litho-spheric cooling and hydrothermal circulation. Geophys. J.Roy. Astron. Soc, v. 38, p. 587-ff.

Yeats, R. S., Forbes, W. C , Heath, G. R., and Schiedegger,K., 1973. Petrology and geochemistry of Leg 16 basalts,eastern equatorial Pacific. In van Andel, Tj. H., Heath,G. R., et al. Initial Reports of the Deep Sea Drilling Pro-ject, v. 16: Washington (U. S. Government Printing Office),p. 617-640.

APPENDIX

Hole 420

Five samples of basalts from Cores 13, 14, 15, and 17, are includedin this study. They are hyalocrystalline and dominated by glass com-ponents. All the samples are finely vesicular; some of the vesicles arefilled with smectites.

In thin sections, the basalts show various degrees of crystallinity.Their texture ranges from glassy-spherulitic to intersertal. They aremade up of plagioclase, clinopyroxene, and an occasional grain or twoof olivine besides glass. Among the opaque minerals, titanomagnetiteis by far the most abundant and may constitute 2 to 3 per cent of therocks. On the basis of petrography, these basalts may be considered tobe aphyric plagioclase-pyroxene basalts or simply aphyric basalts.

Hole 421

Seven samples of basalts from this hole fall broadly into two petro-graphic types, namely glassy basalts and aphyric to sparsely micro-phyric basalts. While the first type is found in Core 2 and in the upperpart of Core 3, the second type constitutes the greater part of Cores 3and 4 and the sample recovered in the bit. The main petrographicfeatures of the two types are as follows:

1) Glassy basalts:These are glassy rocks — glass constitutes 85 per cent of these

rocks — with about 10 per cent variolitic patches and 2 to 3 percent plagioclase and pyroxene microphenocrysts besides minoriron oxide minerals. The variolites are arborescent intergrowths ofplagioclase and clinopyroxene. In places, there are sheaf-likevarioles of pyroxenes. The glass present is light to dark brown incolor and shows evidence of devitrification.2) Aphyric to sparsely microphyric basalts:

These are fine-grained basalts with 1 to 2 per cent micropheno-crysts of plagioclase and/or clinopyroxene. The unit differs fromtype 1 by lower glass content (less than 20 vol. %), larger micro-phenocrysts, and smaller amount of iron oxide minerals. The tex-ture of these rocks is variolitic and intersertal.

Hole 422

Nine samples of basalts from this hole located in the northernmoat of the OCP Ridge (Figure 1) are included in this study. Mega-scopically, these basalts can be grouped into two major groups: (1)light colored, fine- to medium-grained basalts, and (2) dark colored,almost glassy basalts, similar to the basalts from Holes 420 and 421.The shipboard scientific party termed the former "doleritic rock,"and the latter "glassy basalt." For the sake of convenience, we haveadopted the same terminology in describing these rocks.

1) Doleritic rocks:There are two doleritic units at Hole 422 which are separated by

about 4.0 meters of sediments (the term doleritic has been used for

a relatively coarse grained rock that may or may not be an in-trusive). Texturally, these rocks are broadly similar; that is, mostof them show intersertal to ophitic texture. The fine-grained rocksin general show intersertal texture, whereas the medium-grainedones are strongly ophitic. Mineralogically, they are essentiallymade up of plagioclase, clinopyroxene, and patches of glass (nor-mally not exceeding 10 vol. % of the rocks). Besides these com-ponents, they contain a few grains of olivine. The amount ofolivine in the samples from the lower part of Core 9 is relativelyhigher than that in the samples from Cores 7, 8, and the upper partof Core 9. Among the opaques, titanomagnetite is most common.Occasional sulfide blebs are also seen in these rocks.2) Glassy basalt:

This type underlies the lower doleritic unit and consists of anaphyric glassy basalt which is texturally and mineralogicallydistinct from the overlying doleritic unit. The glassy groundmass,which is practically opaque, contains scattered thin, lath-likemicrophenocrysts of plagioclase and a few hourglass-zoned grainsof clinopyroxene.

Hole 423

Five samples of basalts from Cores 5, 6, 7, and 8 are included inthis study; these are essentially fine-grained to glassy, aphyric plagio-clase-pyroxene basalts. They exhibit various textures like variolitic,hyalopilitic, and pilotaxic. There are occasional microphenocrysts ofplagioclase and clinopyroxene in these rocks. A great degree of tex-tural and mineralogical similarity exists between these rocks and thosefrom Holes 420, 421, and glassy basalts from Hole 422. The occur-rence of chilled margins in many of the samples indicates the extrusivenature of these basalts.

Holes 428 and 428A

Drilled 500 feet apart south of the OCP Ridge, in an area markingthe transition from the normal fabric to transverse ridge structure, theholes contain two types of basalts: (1) cryptocrystalline to fine-grained, aphyric olivine-bearing basalts, (2) fine- to medium-grainedmesocratic doleritic rocks similar to the doleritic rock from Hole 422(shipboard scientific party). While the former is found in Hole 428,and in the upper part (up to 15.5 m depth) of Hole 428A, the latter isconfined to the lower part of Hole 428A. Seventeen samples fromboth these holes are included in the present study. Chief petrographicfeatures of the two types of basalts are as follows:

1) Aphyric olivine basalts:As the name suggests, these are aphyric, cryptocrystalline to

fine-grained basalts occurring in Hole 428 and the upper part ofHole 428A. Microscopically, they are vesicular basalts with hyalo-pilitic to variolitic to intersertal and subophitic textures. The glassyportion in these rocks often shows sheaf-like varioles of incipientcrystals of pyroxenes and plagioclase. In some sections, speckledbrown glass is seen. Some of these rocks contain a few micro-phenocrysts of plagioclase and/or glomerocrysts of plagioclaseand olivine. The amount of olivine is variable in these rocks, butnever constitutes more than 1 to 2 per cent. Titanomagnetite is thecommon iron oxide mineral, accounting for 5 to 8 per cent.2) Doleritic rocks:

As mentioned earlier, these rocks are found below 19.5 metersdepth in Hole 428A. The top part of the unit immediately belowthe olivine basalts in Core 3 is a fine-grained aphyric type showingvariolitic to intersertal texture. The mineralogical composition isplagioclase, clinopyroxene, and iron oxide minerals, which occuras subhedral to euhedral crystals. The rock is free from olivine.The grain size increases downhole and the texture changes fromsubophitic to ophitic.Plagioclase in these rocks is lath-shaped, and generally well-

twinned. Their composition is close to that of labradorite. Clino-pyroxene is augite; it occurs as plates or large crystals (1.0 to 1.4 mm)enclosing plagioclase laths. The mineral sometimes shows twinning. Inthe lower part of the section, especially in Cores 6 and 7, one or twograins of olivine are seen. Titanomagnetite occurs as moderate- tolarge-sized, subhedral to euhedral crystals indicating a slow rate ofcooling for these rocks. Their degree of alteration is limited.

Hole 429A

This is the oldest EPR site drilled during Leg 54, and its inferredmagnetic anomaly age is 4.6 m.y. In general, the basalts from this hole

692


are fine-grained aphyric. Many of the samples, particularly from theupper part of Core 2, display chilled margins and have glassy edges.On the basis of thin section studies, the basalts from this hole can begrouped into two petrographic types: (1) aphyric olivine basalt, and(2) aphyric basalt. The former is confined to Core 1 and the upper partof Core 2; the latter is found in the lower part of the hole. Here it maybe worthwhile to recall that rocks in the lower part of the hole havenegative polarity, which agrees with the associated magnetic anomaly;in the upper part, however, the rocks show a positive polarity and aretherefore almost certainly of younger age (see shipboard report).

1) Aphyric olivine basalts:These are very fine grained basalts, showing hyalopilitic to

variolitic texture. They contain some microphenocrysts of olivineand plagioclase. In the hyalopilitic varieties, the groundmass con-sists of skeletal plagioclase laths and extremely fine sheaves ofclinopyroxenes in a matrix of devitrified glass. In the morecrystalline varieties it consists of almost equigranular plagioclase,clinopyroxene, and irregular patches of glass. The titanomagnetitegrains are skeletal and their size varies from 10 to 15 µm.2) Aphyric basalts:

These rocks differ from the earlier types in their grain size, be-ing relatively coarser grained, and in their being devoid of olivinegrains. The amount of glass in these rocks is also much less than intype-1 basalts. Petrographically, they look similar to the basaltsfrom Holes 420 and 421.

Hole 427 (SFZ)

Four samples of basalts from this hole are included in this study.Megascopically, these are fine- to medium-grained, mesocratic basaltswith a relatively large number of vesicles commonly filled in withsmectites. Microscopically, these are almost holocrystalline basaltsshowing subophitic to intergranular texture. The chief mineral constit-uents are plagioclase, clinopyroxenes, and minor amounts of titano-magnetite. Occasional phenocrysts of plagioclase and clinopyroxeneare also seen in these rocks.

Holes 424A, B, and C (GR)

Twenty-four samples of basalts from four holes drilled in the"Mounds Hydrothermal Field" close to the ridge crest of the GR areincluded in this study. The magnetic anomaly age of this site is 0.69m.y., and as such it is the youngest site drilled. Megascopically, thebasalts recovered from these holes are very similar and are fine- tomedium-grained, vesicular aphyric basalts with occasional micro-phenocrysts of plagioclase. Microscopically, these basalts show dif-ferent textures; the samples that are quenched show intersertal andvariolitic textures, whereas relatively coarser samples (fine- tomedium-grained) show intergranular texture. Mineralogically, theseare made up of plagioclase, clinopyroxene, and variable amounts ofglass and titanomagnetite. The content of titanomagnetite in theserocks is approximately 10 per cent, and the glass content varies from10 to 35 per cent. Even the relatively coarser grained samples are notdevoid of glass but contain glassy segregations which containneedlelike opaques, dendritic clinopyroxene, and sometimes micro-phenocrysts of plagioclase. In general, the petrographic studies revealthat there are no different petrographic types in these holes, and thetextural variations represent different cooling units.

Hole 425

Fourteen samples of basalts from this hole are included in thisstudy. Megascopically, these basalts show large variations in their col-or, grain size, nature of vesiculation, and in the phenocryst assem-blages both as to their size and nature (that is, they occur as eithersingle crystals or glomerocrysts, or both). In general they can betermed fine- to medium-grained, aphyric to phyric basalts without anysignificant alteration.

On the basis of the thin section studies, basalts from this site canbe grouped into five petrographic types, two of which may not bedistinct types. Furthermore, these types repeat themselves throughouteach hole. The description of the various types is as follows:

1) Fine-grained aphyric basalts:These occur in the upper part of Core 7 and in the middle of

Core 9; they are fine-grained, vesicular basalts with intersertal tovariolitic texture. Some of the thin sections, especially those fromCore 9, show pilotaxic texture. As regards the mineralogical com-

position, this is essentially made up of plagioclase, clinopyroxene,iron oxide minerals, and glass. In some thin sections, there aresuspect grains of olivine. Plagioclase is commonly lath-shapedlabradorite. Clinopyroxenes in these rocks are brownish to pinkishin color; they occur as subequant grains or needlelike crystalsshowing a fan-shaped arrangement, indicating sudden chilling.The iron oxide minerals are mostly titanomagnetite and occur asdissemination or skeletal crystals, which again suggests rapid cool-ing of these rocks. The glassy material within the rocks occurs asdark colored disseminated patches. The vesicles present are mostlyfilled with smectite.2) Pyroxene-plagioclase sparsely phyric basalts:

Like the petrographic type 1, these basalts also repeat in thehole; that is, they are found below the type-1 basalts in Core 7 andin the middle of Core 8. As compared with the type-1 basalts, theseare much coarser in grain size and have occasional phenocrysts ormicrophenocrysts of both plagioclase and clinopyroxene. Theamount of iron oxide minerals and glass present in these rocks isalso much lower than in the type-1 basalts. It is not unlikely thatthe type-1 basalts represent the chilled portions of these basalts.3) Phyric-glomeroporphyritic basalts:

These are the most abundant type of basalt found at this holeand constitute almost one-third of the total section. They make upa thick unit in Cores 7 and 8, and a thin unit at the top of Core 9.These are essentially fine- to medium-grained, almost holocrystal-line porphyritic to glomeroporphyritic rocks with minor quantitiesof glass being confined to interstices in the groundmass.The phenocrysts in these rocks comprise 5 to 8 volume per cent,

and they occur both as individual crystals and glomerocrysts. The lat-ter are made up either of plagioclase or of both plagioclase and pyrox-ene. The individual plagioclase phenocrysts range from 0.5 to 1.5 mmin size. The clinopyroxenes are between 0.5 and 1 mm in size. Theplagioclase phenocrysts may show zoning and sodic overgrowths.Some of the clinopyroxene phenocrysts show wavy extinction, suchthat an extinction band sweeps across the grain.

The groundmass of these rocks is made up of plagioclase andclinopyroxene, with minor amounts of iron oxide minerals; and itstexture varies from intergranular, intersertal, to variolitic. Occasion-ally, the groundmass pyroxenes are needle-shaped and show fan-shaped texture. The iron oxide minerals occur as skeletal crystals (onlyoccasionally do they show subequant form), indicating a moderate torapid cooling rate for these rocks. The presence of zoning, glassy in-clusions, sodic overgrowths in plagioclase crystals, and wavy extinc-tion of some clinopyroxene phenocrysts also point to rather rapidcooling. The metastable textural features indicate that the rocks arefrom flows rather than sills in which such metastable textures wouldhave been annealed.

These rocks also show some evidence of post-crystallization de-formation as shattering of grains and bending of plagioclase pheno-crysts.

4) Glomeroporphyritic ± olivine basalts:This petrographic type is found in the lower part of Core 8, and

it occurs between the petrographic type 2 above and petrographictype 3 below. There are some essential differences between thistype and type 3, which are both in the nature and the sizes ofphenocrysts as well as in the groundmass texture. The glomero-crysts of plagioclase, clinopyroxene, and olivine (rare) account forabout 15 volume per cent of the rocks. The individual phenocrystsof plagioclase are sometimes as large as 3 to 4 mm in size; they aretabular in habit and appear to be more calcic than the phenocrystsin type 3. The olivine phenocrysts, when present, are up to 1.5 mmin size, and show alteration to smectites along cracks.The groundmass of these rocks is much finer than that of the

type-3 rocks and is made up of needlelike grains of pyroxene, thinlaths of plagioclase, and glass. The iron oxide minerals are skeletal tit-anomagnetites. The nature of the groundmass and titanomagnetitesuggests a rapid cooling rate for the unit after eruption. The glomero-crysts are most likely of pre-eruption crystallization origin, when themagma composition was still on the olivine-plagioclase-pyroxenecotectic.

5) Pyroxene-plagioclase ± olivine sparsely phyric basalts:This type is found at the bottom of the hole in Core 9; it is

essentially similar to type 2, except that there are occasional grainsof olivine in the groundmass as well as microphenocrysts. In handspecimens, the rock has a bleached appearance. The amount ofiron oxide minerals is lower than that in type 2.

693

Deep Sea Drilling Project Initial Reports Volume 54 · This paper presents petrographie and...

Documents

Transcript of Deep Sea Drilling Project Initial Reports Volume 54 · This paper presents petrographie and...