41. COMPARISON OF CHEMICAL AND MAGNETIC STRATIGRAPHY OF BASEMENT ROCKSAT DSDP SITES 332 AND 395

James H. Natland, Deep Sea Drilling Project, Scripps Institution of Oceanography, La Jolla, California

ABSTRACTDeep drilling into oceanic basement at Sites 332 (36° 53 'N, 33°

39 "W; 583 m) and 395 (22 ° 45 'N, 46 ° 05 'W; 572 m) provides an op-portunity to compare the origin of a major portion of Layer 2 at twoplaces on the Mid-Atlantic Ridge. The sites are similar in that base-ment at each consists dominantly of relatively few chemically distinctbasaltic units (18 in Holes 332A and 332B; 9 in Holes 395 and 395A).These units occur as composites of basaltic pillow lavas, thin flows,and glassy breccia zones as thin as a few meters and as thick as 209meters. Only two closely spaced dolerite units, totaling 15 metersthick (in Hole 395A), have been confirmed as intrusive at either site.Holes 395 and 395A also contain gabbroic and ultramafic cobblesand boulders, interpreted as talus, both above and below the top-most aphyric basal unit. No plutonic rocks were cored at Site 332.

The chemical groups at both sites can be divided into stronglyphyric types and moderately, sparsely, or aphyric types (less phyrictypes). The first group contains 15 to 25 per cent or more ofphenocrysts of olivine, Plagioclase, clinopyroxene, or combinationsof these minerals. Many of the phenocrysts appear to have ac-cumulated by mechanical processes (gravity settling or floating, andflow differentiation). The proportion of strongly phyric basalts inboth Hole 395A and Hole 332B, curiously, is 38 per cent. Among thephyric basalt types in Hole 332B are olivine-rich basalts (oceanites),representing about 5 per cent of the interval cored. All other phyricbasalts at both sites have Plagioclase as the predominant phenocryst,with lesser olivine and/or clinopyroxene.

For the most part, the less phyric basalt types cannot be related toeach other, or to phyric basalts, by crystal fractionation involvingphases present in the rocks. Instead, chemical differences appear tohave resulted largely from variations in the extent of melting in themantle. All basalts are low-K and -Ti mid-ocean ridge basalts, butthose of Site 332 are typically lower in Ti, Zr, and other low-partition-coefficient elements than those of Site 395. They thus appear torepresent greater partial melts of the mantle than those of Site 395.Basalts of Site 395, however, appear to derive from a distinctly moredepleted mantle than those of Site 332, and have La/Sm of about0.8. At Site 332, less phyric basalts and most strongly plagioclase-phyric basalts have La/Sm of about 2.2. The olivine-rich basalts andoceanites have La/Sm of about 1.5.

At Site 332, there is evidence for minor crystal fractionation andaccumulation between some less phyric types that erupted more orless simultaneously. Less phyric and strongly phyric types are in-terbedded repeatedly at intervals of tens of centimeters to meters inthe deeper parts of Hole 332B. Apparently, either these came fromdifferent sources and magma chambers at essentially the same time,or the interval has been repeated numerous times by faulting. A thirdpossibility is that the phyric basalts are closely spaced narrow in-trusives into the less phyric basalts. The rock types at both Site 332and Site 395 suggest that to explain the lava sequences, there musthave been at least two magma reservoirs, repeatedly tapped.

There is little evidence for magma mixing among less phyric typesat either Site 395 or Site 332, and none between strongly phyric andless phyric types. These is considerable mineralogic evidence for mix-ing within the phyric basalts of Site 395, but mixing has not beenevaluated for phyric basalts of Site 332.

657

J. H. N ATL AND

Site 332 contrasts with Site 395, in that chemical types requiringdifferent sources recur repeatedly in the holes, but can have differentmagnetic inclinations or polarities. Magnetic inclinations in someunits are not the same in Hole 332B as in chemically and spatiallycorrelative units in Hole 332A. Fault replication of units may explainmany of these relationships. Assuming that all repetition of units wascaused by faulting and/or intrusives, only 11 distinct chemical typesof basalts need be defined at Site 332. The alternative is that magmatypes chemically almost identical either persisted unchanged in thecrust or mantle for periods long with respect to secular changes andeven reversals in the earth's magnetic field, or were generated underidentical conditions. At Site 395, although reversals occur in the sec-tion, units are not repeated by faulting, and all have inclinationsclose to the theoretically predicted ±40° for the latitude. Inclina-tions at Site 332 are generally much shallower than the predicted± 56°. The reversals at Site 395 imply that lavas took up to I05 yearsto accumulate, with thousands or tens of thousands of years betweeneruptive events (Johnson, Paleomagnetism of Igneous Rocks, thisvolume).

At both Site 332 and Site 395, the aphyric and less phyric basalttypes represent the most fractionated basalt compositions, with astrong mode of Mg/(Mg + Fe) at about 0.60 to 0.65. This contrastswith a much wider range of compositions toward Fe- and Ti-enrichment exhibited by dredged and drilled rocks from such typicalnon-rifted fast-spreading centers as the East Pacific Rise, and it con-trasts also with Iceland.

INTRODUCTION

Two holes where penetration into basement exceeded500 meters have been drilled into Miocene or youngeroceanic crust on the Mid-Atlantic Ridge. The first ofthese was Hole 332B, drilled on DSDP Leg 37. Alongwith the other holes drilled on that cruise, it was de-signed in part to complement the extensive geophysical,submersible, and deep-tow operations and observationsof Project FAMOUS (French-American Mid-OceanUndersea Study), conducted primarily on the axial por-tion of the Mid-Atlantic Ridge 35 kilometers to the east(Figure 1). Hole 332B penetrated 586 meters of base-ment, and was accompanied by a pilot hole, 332A, ex-tending more than 300 meters into basement (Aumen-to, Melson, et al., 1977).

DSDP Legs 45 and 46 were designed to drill one ormore deep holes into Mid-Atlantic Ridge oceanic crustas standards of comparison for two transects to be con-ducted during IPOD ocean-crust drilling in the NorthAtlantic. The two transects were (1) a series of holes onyoung oceanic crust between Iceland and the proposedtargets for Legs 45 and 46 near 22 °N, to test the possibleinfluence of the Iceland and Azores geochemical anom-alies on the Mid-Atlantic Ridge, and (2) a series of holesaway from the Mid-Atlantic Ridge, into progressivelyolder crust, to explore "crustal aging" phenomena, aswell as temporal variations in magmatic processes andmantle source compositions along a "flow line" awayfrom the Mid-Atlantic Ridge.

Two holes involving repeated re-entries were drilledon Legs 45 and 46. The first, 395A, penetrated 572meters of basement and was accompanied by a pilothole with 93 meters basement penetration. These holesand Hole 396 were drilled on Leg 45. Hole 396 served asa pilot hole for Hole 396B, drilled on Leg 46, which

penetrated 263 meters into basement. The locations ofSite 332, 395, and 396 are shown on Figure 1, and listedin Table 1.

Deep drilling into oceanic basement at Sites 332 and395 provides an opportunity to compare major portionsof Layer 2 at two places on the Mid-Atlantic Ridge. Inaddition, some of the objectives of the transect betweenIceland and Site 395 and 396 can now be addressed, in-asmuch as Site 332 is part of that transect. This papercompares the basic chemical and magnetic stratigraphyof Sites 332 and 395. Other comparisons will be drawn,especially with the FAMOUS area axial rift, Iceland,and eastern Pacific fast-spreading ridges. The purposesof this paper are to evaluate magmatic processes on theMid-Atlantic Ridge, to consider lateral variations in thecrust by attempting correlations between closely spacedholes at the two sites, and to estimate the extent ofcrustal modification by faulting. To the extent thatmagnetic stratigraphy constrains what can be said aboutmagmatic and tectonic processes, the magnetic structureof the crust will also be considered. This is a subjectwhich deep crustal drilling has complicated enormously.This paper will not consider Holes 396 and 396B, sincethey concern mainly the results of Leg 46 and will bedealt with elsewhere (Heirtzler, Dmitriev, et al., 1978;Natland, 1978).

LITHOLOGIC UNITSThe scientific staffs of Legs 37 and 45 identified three

types of units in igneous basement at Sites 332 and 395:(1) lithologic units, (2) chemical units, and (3) magneticunits. These are summarized on Figures 2 and 3. Thesefigures, and the discussion that follows, are basedprimarily on the relevant site chapters in this volumeand in Volume 37 (Aumento, Melson, et al., 1977).

658

CHEMICAL AND MAGNETIC STRATIGRAPHY

<3337£»411

417A-1417D

" é 4 1 8 B

\V\c3_B53N Vema\ v F.'z. V"N

15'

30° 15°

Figure 1. Locations of DSDP North Atlantic Ocean crustal sites drilled on Legs 37, 45, 46, 49, and 51 through 53. Sites 412through 414 are in the Mid-Atlantic Ridge median rift in the FAMOUS area. Inferred crustal ages out to 180 m.y. are shown.

Lithologic units are defined on the basis ofmacroscopic features of the rocks (presence of pillowfragments or breccias, occurrence of thicker flows or in-trusives, etc.), the abundance and proportions of phe-nocrysts, and the occurrence of other distinctive rocktypes (gabbros, serpentinites, etc.).

At Site 395, 27 lithologic units were thus defined, in-cluding four in Hole 395 (Figure 2). Four of these (two

each in Holes 395 asnd 395A) include plutonic rocks.Four others were defined because they are largely or en-tirely breccias. Two others are dolerite intrusions, close-ly spaced near the bottom of Hole 395A. Most litho-logic units are pillow basalts or fragments thereof, andeither lack phenocrysts or contain abundant largePlagioclase and lesser olivine phenocrysts, or Plagio-clase, olivine, and clinopyroxene phenocrysts. Altera-

659

J. H. NATLAND

TABLE 1Location of Sites

Site

332

395

Location

36°57.72'N33°38.46'W(332B~100mNWof 332A)22°45.35'N46°04.90'W(395 <50 m from395A*)

WaterDepth

(m)

1806-1818

4484

Depth toBasement

-1958 (332A)-1989(332B)

4577 (395)-4538 (395A)

*Not resolvable using satellite fixes.

tion increases generally downhole. In the upper part ofHole 395A, brownish gray altered zones are abundantnext to joint or fracture surfaces, and many such zonescontain cracks lined with secondary carbonates or clays.Alteration and fractures are more pervasive deeper inthe hole (Cores 52 through 67).

Eighteen lithologic units at Site 332 (7 in Hole 332A,11 in 332B) were also defined on the basis of macro-scopic variability and the presence and abundance ofphenocrysts. The major lithologic units are shown onFigure 3. The Leg 37 shipboard party divided these in-to 89 sub-units (Aumento, Melson, et al., 1977, Chapter2). Twelve of these sub-units contained breccia zones,but no intrusives were positively identified. Most sub-units contain at least some phenocrysts, but the major-ity of basalts are only sparsely or moderately phyric,with Plagioclase and rare olivine phenocrysts. Someunits are distinctly porphyritic, including one stronglyplagioclase-phyric unit at the top of Hole 332B and anumber of thin strongly olivine-phyric units deeper in332B. I follow Macdonald (1968) in terming tholeiiticbasalts with abundant olivine "oceanites," to contrastthem with alkalic picrites. A distinctive plagioclase-olivine-clinopyroxene phyric and glomerophyric basaltoccurs in the deepest core of Hole 332A.

From paleontological data, the basalts of both Site332 and Site 395 appear to have accumulated in an axialrift setting. The oldest sediments at the two sites areabout 3.7 and 6.1 million years old, respectively.

CHEMICAL UNITS AND THEIR SIGNIFICANCEDuring Legs 37 and 45, one or more samples per core

were analyzed for major elements on board ship byX-ray fluorescence spectrometry (Bougault, 1977;Bougault et al., this volume). On Leg 45, Ni, Cr, Sr, andZr were also routinely determined. According to thesedata, all the basalts of Sites 332 and 395 are low-K2Omid-ocean ridge basalts.

In addition to the shipboard work on Leg 37, severalother studies included numerous analyses of Site 332basalts (Flower et al., 1977; Blanchard et al., 1976;Dmitriev, 1977; Gunn and Roobol, 1977). Each defineda downhole chemical stratigraphy differing somewhatfrom the others but without a uniform nomenclature.

Apart from the Leg 45 shipboard analyses, severalother papers in this volume include numerous chemical

Figure 2. Basement stratigraphy, Site 332. Unit boundariesassigned on basis of percentage of recovery of units with-in cores taken as percentage of interval cored.

660

CHEMICAL AND MAGNETIC STRATIGRAPHY

Figure 3. Basement stratigraphy, Site 395. Unit boundaries assigned on basisof percentage of recovery of units within cores taken as percentage of in-terval cored.

661

J. H. NATLAND

analyses of Site 395 basalts (Graham et al.; Zolotarevand Choporov; Propach et al.; Rhodes et al.). Thesestudies either adhere to the chemical unit identificationand nomenclature based on the shipboard analyses, orelse make no attempt at redefinition.

Byerly and Wright (1978) defined a coherentchemical stratigraphy for all Leg 37 basalts by makingquantitative corrections for interlaboratory chemicaldifferences and for the effects of alteration. Severalother studies demonstrated important trace-element dif-ferences among Site 332 basalts (Blanchard et al., 1976;Schilling et al.. 1977; Bence and Taylor, 1977; O'Nionsand Pankhurst, 1977; Puchelt et al., 1977), but nonehas attempted to integrate these data systematically intoa presentation of chemical stratigraphy.

In order to provide the most comprehensive possiblecomparison with Site 395,1 defined units for Site 332 byre-examining all data, correlating chemical and litho-logic changes, incorporating trace-element data, andassessing possible chemical units for coherence ofmagnetic inclinations. These were the procedures usedfor Site 395. Site 332 proved considerably more complexthan Site 395, and this is itself significant. The re-mainder of this section will deal largely with Site 332;this is primarily because the amount of data obtained onSite 332 is vast, chemical variations are subtle, and thesite is complex—many units repeat several times in oneor both of Holes 332A and 332B. To address the prob-lems of correlating units between the two holes, and toconsider possible structural modifications of the crustthat might be evident in the chemical and magneticstratigraphy, it was necessary to develop as clear a pic-ture of Site 332 as possible.

For this paper, I have considered two primarysources of major-element data: the shipboard data(Bougault, 1977; Bougault et al., this volume) and thedata of the NASA Johnson Space Center (Blanchard etal., 1976; Rhodes et al., this volume). Comparisons ofinterlaboratory standards (Wright, 1977) and of averageanalyses of homogeneous basaltic units (Byerly andWright, 1978; Appendix, this volume) indicate that cau-tion is necessary when comparing data of two or morelaboratories. The procedure I followed was to evaluatechemical stratigraphy separately for the data from eachlaboratory, then combine the conclusions for each. Thiswas a simple matter for Site 395, since unit boundariesfor the two laboratories were identical. For Hole 332Bthe data coverage for the two laboratories was almostentirely adequate, but only the shipboard data com-prehensively covered Hole 332A. One unit in Hole 332Awas defined only by analyses of another laboratory. Tocheck for completion, I scanned all data tabulated inorder of depth in Chapter 2 of Aumento, Melson, et al.(1977), and the summary of units in Byerly and Wright(1978). The units in this paper differ only in detail fromthose of Byerly and Wright, chiefly in combining someof their "chemical types" which differ only by smallaccumulations of phenocrysts from a common parent.

Some elements proved more useful than others fordiscriminating basalt types. TiO2, for example, can bemeasured with very high precision, and many of thebasalt types at Sites 332 and 395 have the interesting anduseful property that TiO2 varies extremely little (usuallyless than 0.05%) in any given aphyric or sparsely phyricbasalt chemical unit. TiO2 also shows little or no varia-tion among laboratories (Wright, 1977; Appendix, thisvolume). The differences in TiO2 between aphyric andsparsely phyric chemical units are often larger than thisby factors of 5 to 10. By and large, the group ofchemical analyses by a given laboratory within eachchemical unit also shows little variation in one or moreother oxides or trace elements that either support thedistinction based on TiO2, or define distinctionssometimes not seen with TiO2. The data can then beused to decide how, if at all, distinct chemical types arerelated.

This is illustrated in Figure 4 for Site 395, and inFigures 5 through 7 for Site 332. For Site 395, the Leg 45shipboard party adopted the mnemonic symbol A for allaphyric basalt units, distinguishing them by numericalsubscripts, and P for all phyric basalts. For Site 332, Iadopted the symbol S for sparsely or moderately phyricbasalts; aphyric basalts are rare, and are part of largersparsely phyric or moderately phyric units. I retained Pfor all porphyritic units. Units in Hole 332A which haveno counterparts in Hole 332B are designated byalphabetic subscripts. These units are shown on Figures2 and 3, and the data clusters for each unit are identifiedon Figures 4 through 7.

The diagrams illustrate essentially flat TiO2 trendsfor all aphyric or sparsely phyric basalt units. Up to 1per cent or more variation can occur in MgO, total ironas FeO*, and CaO, in any given unit; TiO2 varies byonly 0.05 per cent or so. Because the Site 395 basaltswith flat TiO2 trends are aphyric; the distinctions amongunits are very sharp. None of the papers dealing withSite 332 basalts had identified similar trends, and itwas important, once having seen them at Site 395, todetermine if similar trends occur at Site 332. Theydo, although there is more overlap and the varia-tion in TiO2 is greater, probably because of the effect ofa minor percentage of phenocrysts in the typically small(10 cm3) samples. Similar flat TiO2 trends occur at Site396 (Hole 396B), and have been explained by Kirkpat-rick (1978) as a consequence of simultaneous olivinefractionation and Plagioclase resorption. Figures 4through 7 show that many of the flat TiO2 trends do notfollow single-mineral control lines on all diagrams; norcan they be explained by combinations of olivine andPlagioclase fractionation. Kirkpatrick's explanation isconsistent with observations of rare, rounded (resorbed)Plagioclase phenocrysts in aphyric or sparsely phyricbasalts from Sites 395 and 396 (Figure 8; Dungan et al.,1978), and could have arisen simply by magma ascent.Fujii et al. (this volume) have established that Site 395chemical type A2 has both Plagioclase and olivine near

662

CHEMICAL AND MAGNETIC STRATIGRAPHY

MgO (%)

FeO* (%)

2.0

o

1.0 -

1c

- A3

i

A4

1

1

-

A2•—7—•-

112 11 10

CaO (%)

Figure 4. Variation diagrams for Site 39 5. Aphyric units only are shown. Data from Bougault et al. (this volume) recalculateddry, reduced (total iron as FeO*), and normalized to 100 per cent. Representative mineral control lines are based on Nat-land (unpublished data) and analyses listed in Byerly and Wright (1978) from various sources. Where a range of composi-tions for a given mineral might be involved, the full range is shown, but the more likely potential control is closer to thethicker control lines. (A) TiO2-MgO. (B) TiO2-FeO*. (C) TiO2-CaO.

only olivine. During magma ascent at constant tempera-ture, this basalt would have moved off a two-phase(olivine and Plagioclase) cotectic, to a lower pressureregime where olivine alone crystallized. Previously

formed Plagioclase would then have been partiallyresorbed.

To show how narrow are the compositional rangesfor the aphyric, sparsely phyric, and moderately phyric

663

J. H. NATLAND

1.2

1.0

^ 0 . 8

0.6

0.4

SD

Sparsely PhyricBasalts, Hole 332A,Shipboard Data(except SA ; Gunn andRoobol, 1977)

inopyroxene compositions

22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6

15

Figure 5. Variation diagrams for Hole 332A sparsely phyric basalts. Data fromBougault (1977) recalculated dry, reduced (total iron as FeO*) and norma-lized to 100 per cent. Control lines are based on analyses listed in Byerlyand Wright (1978) and plotted with the same conventions as in Figure 4.(A)TiO2-MgO. (B) TiO2-FeO*. {C)TiO2-CaO.

664

CHEMICAL AND MAGNETIC STRATIGRAPHY

~i 1 1 1 1 1

0 clinopyroxene compositions

Sparsely Phyric Basalts Hole 332Band all Phyric Basalts Holes 332A and332B Shipboard Data (Bougault, 1977).

25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5

CaO (%)

Figure 6. Variation diagrams for Hole 332B sparsely phyric basalts and Holes 332A and 332Bphyric basalts. Data from Bougault (1977) recalculated dry, reduced (total iron as FeO*),

_ and normalized to 100 per cent. Data corrected for CaO in CaC03 Control lines plotted~~ from same analysed as in Figure 5. (A) Ti02-Mg0. (B) TiO2-FeO*. (C) TiO2-CaO.

665

J. H. NATLAND

clinopyroxene compositions

Sparsely Phyric BasaltsHole 332B and allPhyric Basalts Holes332A and B (Blanchard,et al., 1976)

23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

clinopyroxene compositions

13 12 11 10

linopyroxene composition

16 15CaO (%)

Figure 7. Variation diagram for Hole 332B sparsely phyric basalts and Holes 332A and 332B phy-ric basalts. Data from Blanchard et al. (1976) recalculated dry, reduced (all iron as FeO*), andnormalized to 100 percent. Data corrected for CaO in CaCO^• Control lines plotted from sameanalyses as in Figure 5. (A) TiO2-MgO. (B) TiO2-FeO*. (C) TiO2-CaO.

666

CHEMICAL AND MAGNETIC STRATIGRAPHY

0.1mmFigure 8. A small rounded Plagioclase xenocryst in a

Site 395 typeA3 aphyric basalt, Sample 45-395-35-1,109-133 cm (#3). The xenocryst has a thin rim ofPlagioclase needles formed during groundmass cry-stallization.

the liquidus at elevated pressures, whereas at 1 atmithasunits of Sites 332 and 395, Tables 2 and 3 list averageanalyses and standard deviations of these units from thetwo sites. The Site 395 data are from Bougault et al.(this volume). For Site 332, the data of Bougault (1977)and Blanchard et al. (1976) are listed. The data of Blan-chard et al. are meager for Hole 332B, so for that holeonly data of Bougault (1977), and one analysis of Gunnand Roobol (1977) are shown. Where full analyses areavailable, the data are listed dry, reduced, and nor-malized to 100 per cent. For the data of Bougault andcoworkers, where Na2O, P2OS, and MnO were generallynot determined for Site 332, iron listed as Fe2O3 wasfirst converted to FeO, the difference added to loss onignition, and the average analysis normalized to 100 percent minus the adjusted loss on ignition.

In general, especially when trace-element data havebeen considered, the various aphyric and sparsely phyricchemical types have come to be viewed as distinct mag-ma batches unrelated by crystal fractionation (Blan-chard et al., 1976; Bougault et al., this volume; Rhodeset al., this volume). Some elements of this conclusion, interms of major elements, can be appreciated by exami-nation of the flat TiO2 trends of Figures 4 through 7. Itis evident that the internal variation for each group isnot "pointing toward" that of any other group on oneor more of the three variation diagrams shown on eachfigure. Furthermore, combinations of olivine, Plagio-clase, and clinopyroxene in many cases cannot produceone group from another by crystal fractionation. Thiswas shown for Site 332 by Byerly and Wright (1978),who identified three distinct sparsely phyric parentalcompositions which they related to five other sparselyphyric types by crystal accumulation, and only one typeto another sparsely phyric type by crystal fractionation.In addition, they identified seven other distinct and in-dependent phyric magma types.

One aspect of the problem of magma batch in-dependence at Site 332 depends on rare earth elements.

CQ

o•B

>>

a.

s•ö

o

• d

ceg

ft

<n<N

ro

α>

fN <*>

.3iQ

T 3

IT3

1(/)

s an

d

8>>

73

•Sj13

o

εΛ

u

1

T 3

C

Bl

au

lt

SJ.30

CQ

-α

Jc

Bl

ult

3O

αa

*Ha

Xy

la

CQ

"3

3o

αa

•α

Ü

cCQ

;au

lt

3O

CQ

"2a

Bla

nch

3cdMj3O

CQ

Bo

ug

a

a3Occ

3

1o

PQ

llll

t

s>3o

CQ

ja

ult

o

αa

c

c

3

ao

'S

sti

u

In

v

CQ

ro

CQ

ro

αa<N

ro

CQ< N

ro

αa<Nro

CQ

roro

CQ< N

eα

ro

CQ

33

2

CQrN

ro

<

33

2

<<N

ro

<

33

2

<

ro

<<NCOro

<t

33

2H

o

< N

-J?

(6

)

J?

i t

-—'

ro

<N^

ro< Λ

--^

< N

< N

VO

uö~

fo

vθ

CQ

óo

<

Zr

Q

•—*U

CΛ

oo

CQ

<

'c

o — < o o o o o o o o

t— o •* VD r o— σ\ <N oo o

O v o σ N r ~ • < N < N d ö d d

io oo r~ <N

r̂ rf r̂ o o o o o

o o o o o o o o o o

• o o

.58

9

00

.44

6

.27

12,

.10

ON<N

.12

0

oo°. 1 1

d d d d o o o o o

<O ^^ vo 00 ON <O <O ro »—i »-Ho o r o < N • ^ r N - H O O O Od ö d d d > d d ö d > d

00 O\ to 00 O O O

O v~> io o ~-* ro <Λ

od •* σ\ vd <N <N d

^ vo

d d

VO <O <N •Tf o o o o

o o o o o o o o o o

O O O O O O O O+1 +1 +1 +1 +1 I +1 +1 +1

O < N V D T J ^ H σ \ r - - c - -< O < O ^ H < O O O < N O ^ - HO < O C > t ~ ^ < N O ^ H O

d d d d d

o o o o o

<N ro rt σ\ r—σ\ oo <Ni vo vpd r j σs r•- rN<O —I ^H

o

σ\

oiθ

<o

o

o

ONON

o

o

o

<N

o

r^

o

<oro

O

<NVD

*""

O

o

σ>ro

<N

<N

O

o

o

•<t

roO

<oo

o

o

o

^J•<N

o

o

o

o

<7\

o

o

o

r o - H —π

6 *

fi"3 '8

ε r r

P JT4

667

J. H. N ATL AND

TABLE 3Site 395, Major-Element Average Values and Standard Deviations

for Aphyric Basalt Units (volatile-free concentrations, wt. %)(from Bougault et al., this volume)

Unit

Hole

No. Samples

A2*

395

6

A2

395A

6

A3

395A

16

A4

395Aabove

Core 61

4

A4

395AbelowCore 61

4

SiO2

A12O3

Fe2O3 (t)

MnO

MgO

CaO

Na2O

K2O

TiO2

Total

Lol

49.370.32

14.930.12

12.060.150.180.018.580.20

10.530.032.660.020.110.021.620.010.150.01

100.01

-0.93

49.450.23

15.010.14

12.310.230.180.018.530.15

10.600.092.640.040.140.031.640.010.170.01

49.730.14

15.140.22

11.200.190.180.017.610.25

11.290.112.460.060.210.051.720.020.170.01

49.370.77

15.000.36

11.170.230.180.018.200.14

10.970.322.400.040.200.081.590.010.140.01

100.67 99.71 99.22

-0.98 -1.34 -1.9

48.900.29

15.170.10

11.460.230.200.018.400.18

11.090.162.5

0.230.021.620.020.150.01

99.72

-2.87

In general, two groups can be distinguished: those withLa/Sm between 1.8 and 2.4, and those with La/Sm be-tween 1.2 and 1.6 (Figure 9). This distinction is alsouseful in defining chemical stratigraphy (Figures 10 and11). In constructing these last two figures, I used thedata of Blanchard et al. (1976), Bence and Taylor(1977), Schilling et al. (1977), and O'Nions andPankhurst (1977). The data of Puchelt et al. (1977)show similar variations, but not the consistency of theother laboratories, so are not plotted on Figures 10 or11. Among the four remaining laboratories, as can beseen, few rare-earth data are available for Hole 332A(Figure 10).

All these laboratories reported relatively light-rare-earth-depleted and light-rare-earth-enriched patterns forSite 332 basalts, but only Blanchard et al. (1976) clearlyidentified which types of basalts showed the relativelydepleted patterns. By plotting La/Sm for all theselaboratories versus depth, it became clear that all therelatively depleted basalts at Site 332 are porphyritic,and all but one of these (Pi) are olivine-rich basalts oroceanites (olivine-controlled basalts of Byerly andWright, 1978). Figure 9, on which only the data of Blan-chard et al. (1976) and Rhodes et al. (this volume) areplotted, illustrates the impossibility of deriving Site 332sparsely phyric basalts from the phyric basalts withlower La/Sm. Where relative shifts in La/Sm can beproduced by clinopyroxene fractionation (or by leavingit as a residuum during partial melting), shifts of themagnitude shown on Figure 9B would cause drasticchanges in scandium, not seen on Figure 9A. The figure

also illustrates the fundamental dichotomy between Site395 and Site 332. This dichotomy was also demon-strated by Bougault et al. (this volume) for other low-partition-coefficient trace elements (Ta, Th, Tb, etc.).On Figure 9, not only do the general fields for the twosites fall on completely different parts of the diagrams,but the variation of La/Sm within one particular sparse-ly phyric unit of Site 332 (Unit S4) exceeds that of the en-tire group of Site 395 basalts. This implies that theevolution of even a single magma type at Site 332 wasmore complex than at Site 395, and that crystal frac-tionation and accumulation patterns in the lavas wereoverprinted on fundamentally heterogeneous magmatypes, even in a single eruptive sequence.

Another important contrast is that Site 332 basaltshave overall lower TiO2 and Zr than Site 395 basalts,even though the mantle sources for the two suites ap-pear to be similar in these elements (the basalt abun-dances fall on a single line on the TiO2-Zr diagram ofBougault et al., this volume). They also have lower Na20(Tables 2 and 3). These relations imply that Site 332basalts represent greater partial melts of the mantle thanSite 395 basalts, even though that mantle appears to beenriched, by comparison, in light rare-earth and certainother trace elements.

How did the Site 332 basalts develp so heterogeneousan enrichment in light rare-earth elements? Mysen andHolloway (1977) argued from experimental andtheoretical evidence that rare-earth heterogeneitiescould be produced in an initially homogeneous mantlesource by partial extraction of magmas, then by contin-uing the melting of the remaining refractory mantleresiduum. Langmuir et al. (1977) extended this conceptto FAMOUS area basalts by proposing that "dynamicmelting," involving different ranges in the extent ofmelting in the mantle, variably efficient filter pressing,re-melting of refractory residues, and zone refining,could produce differences in rare-earth abundancessuch as those at Site 332. Crystal fractionation and theeffects of pressure on shifting phase boundaries duringmelting could produce other variations. The rare-earthdata of Langmuir et al. (1977) for FAMOUS areabasalts and one basalt from Unit P2 of Site 332 are alsoplotted on Figure 9. They illustrate that essentially thesame complex processes are occurring in the FAMOUSrift today as those which occurred at Site 332 when itwas in an axial rift setting along the same segment of theMid-Atlantic Ridge.

Rhodes et al. (this volume) and Dungan et al. (thisvolume, and 1978) argued that an important process inSite 395 and Site 396 phyric basalts is that of magmamixing. Their evidence is mineralogical and experimen-tal. Calcic cores of strongly zoned plagioclases andmagnesian cores of olivines could not be produced in1-atm crystallization experiments on whole-rock com-positions of Site 395 and Site 396 phyric basalts. Theequilibrium plagioclases were more sodic and the oli-vines more iron rich than the cores of the phenocrysts.In addition, liquid inclusions in the phenocrysts weremore "primitive" than natural glass compositions fromthe quench margins of phyric basalt pillows. These rela-

668

CHEMICAL AND MAGNETIC STRATIGRAPHY

2.5

2.0 -

1.5 -

1.0 -

0.5

A

.9 clinopyroxene c

• . # *

C

•PA

•

fi

u-–—>•—

y

—.

?.8

.9

f

Si \

.7

^•Sz

^. olivine

.o

332

~~ 395- ^ Averaç

I

es of

i

all Site 395 groups

30 35 40 45Se (ppm)

50 55 60

2.5

2.0

1.5

1.0

0.5

F Langmuir, et al. (1977)FAMOUS rift samples

<B Langmuir, et al. (1977)P2 sample

2.5 3.0 3.5 4.0

Yb (ppm)

Figure 9. (A) La/Sm versus Se for Sites 332 and 395 and FAMOUS basalts (after Blan-chard et al, 19 76). (B) La/Sm versus Yb for Sites 332 and 395 and FAMOUS basalts(after Blanchard et al, 1976). Data from Blanchard et al. (1976), Rhodes et al. (thisvolume), and Langmuir et al. (1977). Liquid lines of descent are shown for single-mineral fractional crystallization. Index numbers refer to the fraction of the systemthat is liquid.

tions suggested mixing of "primitive" lavas chargedwith phenocrysts into magma chambers containingmore fractionated magmas, thus producing the strongzoning on the phenocrysts.

Detailed examination of crystal morphologies in thephyric basalts of Site 395 (Natland, this volume) con-firms that this process more than likely occurred, butthat the patterns of phenocryst crystallization and zon-ing are only partially explained by mixing. More thanone "primitive" composition and more than one "frac-tionated" composition were probably involved, andcrystallization occurred at a wide range of undercool-ings, even among the phenocrysts.

Magma mixing has not been evaluated for Site 332,but photomicrographs in Blanchbard et al. (1976) show

strongly zoned plagioclases with glass inclusions, andlarge megacrysts and glomerocrysts similar to those inthe phyric basalts of Site 395. In addition, Dungan et al.(1978) evaluated the mineralogical evidence for mixingin some Site 396 basalts with relatively low phenocrystabundances, sufficiently low to show flat TiO2 trends.In essence, although aphyric, sparsely phyric, ormoderately phyric basalts appear to be compositions in-depenedent of phyric basalt compositions, they too canshow mineralogical evidence for a mixing history. Thiscould be the explanation for the fairly great range inLa/Sm shown by some Site 332 sparsely phyric basalts(e.g., Site 332, Type S4 on Figure 8).

This may help explain the lack of variability inLa/Sm at Site 395. There are no olivine-rich basalts or

669

J. H. N ATL AND

Core TiO 2

2000-

2100 ^

2 2 0 0 -

2285.5-J

AVERAGECHEMICAL MAGNETIC

TYPE INCLINATION

2 N R

10

13

14

15

16

18

19

20

22

23

24

25

26

28

29

30

31

32

33

34

35

36

37

38

39

40

i

\

~>

SB

SD

So

SD

SD

La/Sm

2

INTERVALS BETWEEN FIRST AND LASTOCCURRENCE OF CHEMICAL TYPE

+33°

+53°

-38°

16.5°

ΘΘ©

Figure 10. 7702 versus depth, average magnetic inclinations of Hall and Ryall (1977) supemnits versus depth,La/Sm versus depth, and stratigraphic range of chemical units for Hole 332A. Magnetic inclinations in paren-theses are representative values in intervals where inclinations are scattered. Sample depths assigned assumethat recovery within a core is porportional to the interval cored. Ti02 data from Bougault (1977) and Blan-chard et al. (1976), normalized as in Tables 1 and 2 (see text). Rare-earth element data from Blanchard et al.(1976) and Schilling et al. (1977).

670

CHEMICAL AND MAGNETIC STRATIGRAPHY

oceanites at Site 395, and as argued earlier, Site 395basalts appear to represent smaller degrees of melting inthe mantle than basalts of Site 332. Of all the mineralsin the probable mantle source assemblage of these ba-salts (Plagioclase peridotite; Fujii et al., this volume),clinopyroxene is the one most likely to affect La/Smduring melting (Frey et al., 1974; see Figure 8). Byerlyand Wright (1978) inferred that clinopyroxene left in themantle residuum by partial melting was a principalcause of compositional differences in Site 332 magmatypes. Fujii et al. (this volume) showed that, for a Site395 aphyric basalt composition, near-eutectic melting ofa Plagioclase lherzolite at 8 kilobars pressure could haveproduced the basalt. At a depth of about 25 milometers,then, partial melting would contribute components ofolivine, Plagioclase, and clinopyroxene to the melt innear-constant proportions through a fairly wide rangein the extent of melting, and no shift in La/Sm wouldoccur, even though absolute rare-earth element abun-dances could vary. With depletion of the earliest meltingphase of this assemblage (Plagioclase), melt composi-tions would begin to change, perhaps abruptly (Pres-nall, 1969), and with them, the proportions of clino-pyroxene entering the melt. An increased proportion ofclinopyroxene would reduce La/Sm.

I propose that at Site 395 none of the basalts repre-sent partial melts which involved depletion ofPlagioclase in the mantle — that they were essentially"eutectic" melts of Plagioclase peridotite. At Site 332,the sparsely phyric basalts also represent this type ofmelting, but of a distinctly less depleted mantle sourcewith perhaps a larger degree of partial melting (e.g.,lower TiO2, Na2O, etc.). But several phyric basalts,most of them olivine controlled, represent greater par-tial melts, or separate partial melts of a refractory(plagioclase-free?) residuum, so they have much moreabundant olivine and reduced La/Sm. Dispersion ofLa/Sm within any sparsely phyric unit could haveresulted from minor hybridization between phyric andsparsely phyric basalt types.

Apart from the olivine-rich basalts and oceanites ofSite 332, the ranges of compositions at Sites 332 and 395are very similar. Figure 12 contains a histogram ofMg/(Mg + Fe) for all basalts of Legs 37, 45, and 46 (in-cluding Holes 396 and 396B). In constructing thishistogram, unit averages were first compiled using ship-board data (Bougault, 1977; Bougault et al., thisvolume; Bougault et al., 1978), then the resultingMg/(Mg + Fe) values were weighted according to thethicknesses of the units. As can be seen, there is a strongmode between Mg/(Mg + Fe) values of 0.60 to 0.65. TheLeg 37 data are skewed toward higher Mg/(Mg + Fe),largely because of the olivine-rich basalts and oceanites.The restricted compositions and similarity of basaltsrecovered on Legs 37, 45, and 46 can only be appreci-ated by comparison with similar histograms constructedfor Iceland (Figure 12B, based on number of analysesrather than unit thickness) and for the East Pacific Rise(Figure 12C, based on considerably less cumulativethickness of drill holes than the Mid-Atlantic Ridge).

Rocks from the Mid-Atlantic Ridge drilled through Leg46 essentially do not include basalts withMg/(Mg + Fe)<0.55, whereas such basalts comprisenearly 50 per cent of the basalts of Iceland and thoserecovered by drilling the East Pacific Rise. The fun-damental contrast between the slow-spreading Mid-Atlantic Ridge and these other portions of the earth'socean ridge system, then, is that basalts of a uniformand comparatively restricted range of compositions areproduced.

In detail, what occur are individual magma batchesnot related to each other by crystal fractionation, andcarrying a small internal variation probably producedon ascent from the mantle or the deep crust. Two con-trasting magma types occur, one aphyric or sparselyphyric, the other strongly phyric. Elsewhere in thisvolume (Chapter 18), I have suggested that at Site 395this requires at least two magma chambers and twoplumbing systems, repeatedly replenished: one whicheliminates phenocrysts or prevents them from growing,and one which concentrates them (by flotation, crystalsettling, and/or flow differentiation). At Site 332, twoor more magma chambers are also required, one for thegroup of sparsely phyric high-La/Sm basalts, one forthe phyric lower-La/Sm basalts. The rare-earth data ofLangmuir et al. (1977) on basalts from the Mid-Atlantic Ridge median rift in the FAMOUS area showsimilar variations, and so more than one magmachamber seems required there as well. The rare-earthdata thus do not support the single-magma-chamberhypothesis for the FAMOUS area, proposed by Bryanet al. (1977) and based on major-element analyses of asuite of basalts including those analyzed by Langmuir etal. (1977). Site 332 includes most of the same types ofbasalts, and there the rare-earth data are supported by adetailed chemical stratigraphy based on many analysesof individual magma batches and chemical types. Amineralogically zoned median rift magma cupola at Site332, similar to that of Bryan et al. (1977), cannot haveproduced interbedded olivine-rich and sparsely phyricbasalts of diverse light-rare-earth-element enrichment.At least two magma chambers isolated from each otherare required. One of these chambers could have pro-duced some of the Plagioclase accumulative basaltsfrom the sparsely phyric basalts, however (except forUnit P, of Hole 332B, which has too low La/Sm).

Instead, both Site 332 and Site 395 suggest that close-ly spaced magma reservoirs with virtually no linkagesupplied basalts alternately or simultaneously to thesites. I believe that this is consistent with a generallysmall magma chamber size at the Mid-Atlantic Ridgemedian rift, that multiple magma chambers are dis-persed, isolated, and virtually unconnected along therift. These erupt and are replenished infrequently (atperiods long with respect to secular variations and evenreversals in the earth's magnetic field). This would seema natural consequence of a slow rate of magma genesisat a slow-spreading rifted median ridge. On the EastPacific Rise and Iceland, which differ structurally andmorphologically from the Mid-Atlantic Ridge near Sites

671

toTiO2 (%)

Core 0 1

2100^

AVERAGEMAGNETIC

INCLINATION 1La/Sm

2

(-3.6)(-39.7)

(-67.8)

- 1 T

-31°©

INTERVALS BETWEEN FIRST AND LASTOCCURRENCE OF CHEMICAL TYPE

HEAVY LINE = RELATIVELYDEPLETED COMPOSITIONS

LIGHT LINE = RELATIVELYUNDEPLETED COMPOSITIONS

2500

2562.5

Φ-®

- θ ©

©

Figure 11. TiO2 versus depth, average magnetic inclination of Hall and Ry all (1977) superunits versusdepth, La/Sm versus depth, and stratigraphic range of chemical units for Hole 332B. Magnetic in-clinations in parentheses are representative values in intervals where inclinations are scattered. Depthassignments as in Figure 10. Data sources as in Figure 10, plus rare-earth element data ofO'NionsandPankhurst (1977) and Bence and Taylor (1977).

oX

n>r

O

J. H. N ATL AND

40 -

-MID-ATLANTIC RIDGE

7 SITES

-

A

22° N| 1 LEGS 45, 46 953 m

3 7 ° N £ | ] LEG 37 1100 m

2053 m

20

ICELAND153 ANALYSES

20

EAST PACIFIC RISE15 SITES Q LEG 54

ü LEG 34

1 1 LEG 16

C

218.8 m

80.0 m

21.0 m

319.8 m

35-40 40-45 45-50 50-55 55-60 60-65Mg/(Mg+Fe)

65-70 70-75 75-80

Figure 12. Histogram ofMg/(Mg+Fe) for DS DP basalts fromthe Mid-Atlantic Ridge Sites 332 through 335 (Bougault,1977); 395 (Bougault et al, this volume); and 396 (Bou-gault et al, this volume and 1978), compared with datafrom the East Pacific Rise (Yeats et al, 19 73; Rhodes etal, 1976; and Rosendahl, Hekinian et al, in preparation).Histogram is constructed by weighting according to thethickness of clearly identified chemical units. Icelanddata are based on number of analyses (35 from Sigvalda-son, 1974; 118 from Jakobsson. 1972).

332 and 395, the wider range in magma types requiresthat something entirely different be going on, somethingthat promotes more extensive fractionation of thebasalts. Elaboration of this must await detailed con-sideration of basalt compositions from these areas; suchconsideration is beyond the scope of this paper.

MAGNETIC UNITSMagnetic units were defined in much the same way

on Legs 37 and 45 (Hall and Ryall, 1977; Johnson,Paleomagnetism of igneous rocks, this volume). Themagnetic inclinations of minicore samples taken fromone or more oriented basalt pieces per core weremeasured. An oriented piece has an external surface cutby coring and a length greater than the core diameter.On Leg 37, minicores were taken from full-diametercores. On Leg 45, they were taken from the workinghalf of split cores. Apart from the effects of instrumen-tal precision, inclinations can be in error by any amounta hole diverges from the vertical (generally less than 2°),by amounts caused by irregularities in the cored (outer)surface of the piece of basalt sampled, and by anydivergence of the minicore sampler from a vertical rela-tionship to the base plate supporting the basalts duringsampling. Johnson, in Chapter 7 of this volume, deter-mined that in adjacent samples from the same piece, thelatter two errors could result in a combined difference ininclination of 10°. In general, however, the errors areless than this (or do not combine), and the resultingaverage inclinations of magnetic units have standard

deviations of perhaps 5°. Magnetic units defined forSites 332 and 395 are shown on Figures 2 and 3 bymeans of average inclinations or, where necessary, arange of inclinations.

In Hole 395A, basalts are either normally or reverselymagnetized with an inclination of ±40°, matching theinclination expected from the latitude of the site. Allmagnetic and chemical unit boundaries coincide.Johnson (Paleomagnetism of igneous rocks, thisvolume) concluded that the basalts were emplaced overa period of time when short-period reversals within alonger polarity epoch were occurring. He proposed thatthe individual volcanic episodes were short (< I02 years)with respect to the time involved in secular variations ofthe earth's magnetic field (hence the strong coherence ofeach magnetic unit), and that the intervals between themwere long (I03 to I04 years), allowing reversals of theearth's field to occur.

Site 332 is enormously more complex magnetically.Hall and Ryall (1977) first determined mean inclinationson each lithologic sub-unit (as defined by the Leg 37shipboard party), in several cases dividing these intofurther magnetic units, then grouped as many of theseas possible into magnetic superunits. These superunitsare essentially equivalent to the magnetic units of Site395, and it is their mean magnetic inclinations which areshown on Figure 2. In Holes 332A and 332B, there arenine instances where one or more changes in inclinationoccur within lithologic sub-units. In the lower part ofHole 332B, four magnetic superunit boundaries occurwithin intervals of distinct chemical types (Figure 2). Inlithologic Units 2, 8, and 9 of Hole 332B, scattered incli-nations occur, and no magnetic superunits were defined.Where no magnetic data were determined for certaincores, question marks are shown on Figure 2.

Inclinations at Site 332 are generally much lower thanthe ±56° predicted for the latitude of the site, andaverage - 4 ° . An exception is the upper part of Hole332A, which has an average inclination of +53° andincludes four chemical types not present in Hole 332B.The corresponding interval in Hole 332B has a negativeaverage inclination of -30.6° and consists of onechemical type.

Lithologic Units 3 of Hole 332B and 6 of Hole 332Ahave very similar negative magnetic inclinations (Figure2), and are similar chemically (Table 2). The inclinationsof lithologic Units 4 of Hole 332B and 7 of Hole 332Aare also similar, and the basalts of the two units are bothphyric types. But phyric basalt PB is relatively lessdepleted in light rare-earth elements (Figure 11) thanphyric basalt P2 (Figure 10), so the two types do notmatch chemically.

Hall and Ryall (1977) speculated that low inclinationsof Site 332 and the poor correlation between the upperparts of Holes 332A and 332B result from a combina-tion of "tectonic breaks" and possible eruption duringa long-term departure from an axial dipole field. Theypointed out that to explain the low inclinations entirelyby faulting would require a dip to the south of 30° to40°, whereas, so far as is known, only small dips(3°-10°) to the west occur in the area. Macdonald

674

CHEMICAL AND MAGNETIC STRATIGRAPHY

(1977), citing inversions of near-bottom east-westmagnetic profiles near Sites 332 and 333, lent support tothe concept that the Site 332 basalts erupted during amagnetic transition. Apparently, shortly after theGauss-Gilbert reversal, a short-period reversal of theearth's field also occurred (Atwater and Mudie, 1973;Klitgord, 1974), and may have lasted 25,000 years. Thismay have extended the effective duration of the longerterm Gauss-Gilbert reversal and the consequent erraticbehavior of the earth's magnetic field. Site 332 is in acrustal zone which probably recorded this complexbehavior of the field between the Gauss and Gilbertepochs.

CORRELATIONS BETWEEN HOLESAND THE PROBLEM OF REPLICATION

OF UNITS AT SITE 332

Holes 395 and 395A were so close together that nodifference in their locations could be resolved bysatellite fixes (Chapter 7, this volume). But the averageinclination of chemical Unit A2 in Hole 395 is 16° lessthan in Hole 395A, and one chemical unit, P1} occursonly in Hole 395. Johnson (Paleomagnetism of igneousrocks, this volume) attributed the difference in inclina-tions to faulting.

At Site 332, where Hole 332A is 107 meters fromHole 332B, nearly 330 meters can be correlated betweenthe two holes. But as Figure 2 shows, nothing abovechemical Unit Si correlates between the holes; within Siin Hole 332A are several interlayers of type SD not pres-ent in Hole 332B, and as already discussed, phyric typePB at the bottom of Hole 332A is not the samechemically as the stratigraphically equivalent Unit P2 inHole 332B. Because SD recurs in Hole 332A but not inHole 332B, it is shown to pinch out between the holes onFigure 2. Units SA through Sc in Hole 332B, with theirpositive inclination, are shown to overlie Pi in 332Bsomewhere between the two holes on Figure 2. SAthrough Sc probably erupted after a magnetic reversalwhich followed the eruption of P,, whose negative in-clinations are consistent with all deeper units in 332B.

Below Unit P4 in Hole 332B, the stratigraphybecomes very complex. Assuming that the holerepresents strictly an eruptive sequence, the recurrent in-terbeds of types S3, P5, and S4 require availability ofseveral strongly contrasting magma types throughseveral possible shifts in the earth's magnetic field. It isin this lower part of Hole 332B that four magneticbreaks (superunit boundaries) occur within chemicalunits.

To consider the significance of the poor correlationbetween Holes 332A and 332B, and the downhole com-plexities of Hole 332B, it is necessary to establish a firmor at least consistent basis for evaluating eruptive se-quences and possible tectonic replication of units.Elsewhere I have defined "rules of stratigraphic order"(Natland, 1978) to deal with basalt drill-hole chemicaland magnetic stratigraphy, but Site 332 presents anunusual array of problems. Without biostratigraphic in-formation, we cannot be talking rigorously about time-stratigraphic units. With poor recovery of contacts and

without being able to view the lateral extent of whateverunits we define, inferences must be based entirely onchemical and magnetic stratigraphy. Magneticstratigraphy might give us some idea of the time se-quence of events, but faulting and the possibility of in-strusions can make this very difficult to apply.

Two independent sets of judgments must be made,one concerned with the significance of magnetic breaksin a basaltic sequence, the other dealing with how con-fidently magma types can be identified and consideredto represent discrete eruptive events. Where Site 395 wasstraightforward on both counts, Site 332 strains thelimits of the possible. If indeed the Site 332 basaltserupted during a period of erratic behavior of theearth's magnetic field, and if faulting has been shown tohave occurred in several places (e.g., magnetic breakswithin lithologic or chemical units), then what can con-fidently be said about the history of the earth's magneticfield at the site? And to what extent is it possible toestablish an eruptive sequence?

On the other hand, if chemical types are only verysubtly distinct, and recur in a lava sequence severaltimes through a number of possible magnetic transi-tions, to what extent can we say that a particular magmacomposition persisted in the crust essentially unchangedthrough a long period of time (tens or hundreds ofthousands of years, according to the magneticsspecialists)? On Figures 10 and 11 I have indicated theoverlap of availability of chemical types throughmagnetic transitions at Site 332. At several times at leasttwo, and in some cases three, magma types wereavailable to be tapped alternately or in sequence. Thiscontrasts with no overlap whatever at Site 395.

Estimates of chemical-type overlap at Site 332 de-pend critically on defining chemical units, and thendeciding whether they are co-magmatic (simply relatedby crystal fractionation or accumulation). This is by nomeans easy for this site. The crucial question is this: Domagma types represent single eruptive events, or a time-transgressive sequence of eruptive events? If a sequenceof events, how was the sequence spaced with respect tovariations in the earth's magnetic field? At Site 332, wehave discussed the possibilities that crystal accumula-tion, fractionation, and magma mixing have occurred.Byerly and Wright (1978) have shown that virtuallyidentical Site 332 sparsely phyric basalt types can bederived from different parents by different mechanisms(two of their chemical types which derive from differentparents I have combined into type Si). Though this maybe, it soon becomes evident that this type of coincidenceleaves little to be said with any confidence about erup-tive sequences. Subtly but distinctly different magmatypes may have been part of the same eruptive event,may even have had different parents, may have beenhybrids. Elsewhere (Natland, 1978), I have stated that"similar but not identical chemical units may have beenpart of the same eruptive sequence if, for example,magmas were being supplied simultaneously from morethan one source, or if they reached the surface throughdifferent conduit systems. Factors other than crystalfractionation (such as magma mixing, fractional resorp-

675

J. H. NATLAND

tion of minerals, especially Plagioclase, zone refining,local source heterogeneities, leaving various proportionsof residual liquids behind, etc.) may have caused thesubtle differences in composition. The interpretationthat they were part of the same eruptive event isstrengthened if they have the same or similar magneticinclinations, and the same stratigraphic level in twoholes, but is not precluded if they are different. Faultingmay have occurred. The eruptive 'event' may have beenseveral events closely spaced in time."

With this in mind, I shall state the case for maximumreplication of units at Site 332, on the basis of Table 2and Figures 5 through 7.

1) Unit P5 may represent a complex dike swarmfeeding unit P3 (or P5 may be P3 repeatedseveral times by faulting).

2 ) Units S and S3 may be equivalent.3 ) Units SA, Sc, and S4 may be equivalent.

Units S2, S5, and SD may be equivalent.5) Bnit P4 may be partially hybridized with any

one of the sparsely phyric types (see especiallyFigure 7A). The "olivine-controiled" oortionof the hybrid could be equivalent to P2.

The number of distinct chemical types would bereduced from 18 to 11. The site would have to be viewedas complexly faulted, with relatively few magma typesrepeated numerous times. The mechanism for the fault-ing could be (1) formation of inward-facing normalfaults in the median rift, with throws between 20 to 200meters, to form the inner wall of the rift (Macdonaldand Luyendyk, 1977), and in the rift mountains,outward-facing faults appear to be 100 to 1000 metersapart (Macdonald and Atwater, 1978).faults per hundred meters have been observed on the in-ner wall of the FAMOUS area median rift (Macdonaldand Luyendyk, 1977), and in the rift mountains, faultsappear to be 100 to 1000 meters apart (Macdonald andAtwater, 1978).

Since we do not know the rate of changes in theearth's magnetic field during a magnetic transition, andthe basalts of Site 332 may be complexly faulted, there isvery little we can say about the frequency of eruptiveevents at the site. In this respect, Site 395 is a betterguide to the rate at which Layer 2 can accumulate on theMid-Atlantic Ridge.

ACKNOWLEDGMENTS

This paper would not have been possible without theanalytic work and shipboard descriptions of many people,most of them cited in the text.

I would like to thank Bill Melson for constructive criticismof the manuscript.

REFERENCES

Atwater, T.M. and Mudie, J.M., 1973. Detailed near-bottomgeophysical study of the Gorda rise, /. Geophys. Res., v.78, p. 8665-8686.

Aumento, F., Melson, W.G., et al., 1977. Initial Reports ofDeep Sea Drilling Project, v. 37: Washington (U.S.Government Printing Office).

Bence, A.E. and Taylor, S.R., 1977. Petrogenesis of Mid-At-lantic Ridge basalts at DSDP Leg 37 Holes 332A and 332B

from major and trace element geochemistry. In Aumento,F., Melson, W.G., et al., Initial Reports of the Deep SeaDrilling Project, v. 37: Washington (U.S. GovernmentPrinting Office), p. 705-710.

Blanchard, D.P., Rhodes, J.M., Dungan, M.A., Rodgers,K.V., Donaldson, CH., Brannon, J.C., Jacobs, J.W., andGibson, E.K., 1976. The chemistry and petrology of basaltsfrom Leg 37 of the Deep Sea Drilling Project, J. Geophys.Res., v. 81, p. 4321-4246.

Bougault, H., 1977. Major elements: analytical chemistry onboard and preliminary results. In Aumento, F., Melson,W.G., et al., Initial Reports of the Deep Sea Drilling Pro-ject, v. 37: Washington (U.S. Government PrintingOffice), p. 643-657.

Bougault, H., Cambon, P., Joron, L., and Treuil, M., 1978.Trace elements: fractional crystallization and partialmelting process, heterogeneity of upper mantle material. InHeirtzler, J., Dmitriev, L., et al., Initial Reports of theDeep Sea Drilling Project, v. 46: Washington (U.S.Government Printing Office), p. 247-252.

Bryan, W.B. and Moore, J.G., 1977. Compositional varia-tions of young basalts in the Mid-Atlantic Ridge rift valleynear lat 36°49'N, Geol. Soc. Am. Bull., v. 88, p. 556-570.

Byerly, G. and Wright, T.L., 1978. Origin of major elementtrends in DSDP Leg 37 basalts, Mid-Atlantic Ridge, J. Vol.Geotherm. Res., v. 3, p. 229-279.

Dmitriev, L., 1977. Petrochemistry of basalts and plutonicrocks, Leg 37, Deep Sea Drilling Project. In Aumento, F.,Melson, W.G., et al., Initial Reports of the Deep Sea Drill-ing Project, v. 37: Washington (U.S. Government PrintingOffice), p. 681-694.

Dungan, M.A., Rhodes, J.M., Long, P.E., Blanchard, D.P.,Brannon, J.C., and Rodgers, K.V., 1978. The petrologyand geochemistry of basalts from Site 396, Legs 45 and 46of the Deep Sea Drilling Project. In Heirtzler, J., Dmitriev,L., et al., Initial Reports of the Deep Sea Project, v. 46:Washington (U.S. Government Printing Office), p. 89-114.

Flower, M.F.J., Robinson, P.T., Schmincke, H.-U., andOhnmacht, W., 1977. Petrology and Geochemistry of ig-neous rocks, DSDP Leg 37. In Aumento, F., Melson,W.G., et al., Initial Reports of the Deep Sea Drilling Proj-ect, v. 37: Washington (U.S. Government Printing Office),p. 658-680.

Frey, F.A., Bryan, W.B., And Thompson, G., 1974. AtlanticOcean floor: geochemistry and petrology of basalts fromlegs 2 and 3 of the Deep Sea Drilling Project, /. Geophys.Res., v. 79, p. 5507-5527.

Gunn, B. and Roobol, MJ., 1977. Geochemistry of the ig-neous rocks. In Aumento, F., Melson, W.G., et al., InitialReports of the Deep Sea Drilling Project, v. 37:Washington (U.S. Government Printing Office), p.735-756.

Hall, J.M. and Ryall, P.J.C., 1977. Paleomagnetism of base-ment rocks, Leg 37. In Aumento, F., Melson, W.G., et al.,Initial Reports of the Deep Sea Drilling Project, v. 37:Washington (U.S. Government Printing Office), p.425-445.

Heirtzler, J.R., Dmitriev, L., et al., 1978. Initial Reports ofthe Deep Sea Drilling Project, v. 46: Washington (U.S.Government Printing Office).

Jakobssen, S.P., 1972. Chemistry and distribution pattern ofRecent basaltic rocks in Iceland, Lithos, v. 5, p. 365-386.

Kirkpatrick, R.J., 1978. Petrology of basalts: Hole 396BDSDP Leg 46. In Heirtzler, J., Dmitriev, L., et al., InitialReports of the Deep Sea Drilling Project, v. 46:Washington (U.S. Government Printing Office), p.165-178.

676

CHEMICAL AND MAGNETIC STRATIGRAPHY

Klitgord, K.D., 1974. Near-bottom geophysical surveys andtheir implications on the crustal generation process, sea-floor spreading history of the Pacific, and the geomagnetictime scale 0 to 6 m.y.B.P., Ph.D. thesis, Univ. of Calif.San Diego.

Langmuir, CH., Bender, J.F., Bence, A.E., and Hanson,G.N., 1977. Petrogenesis of basalts from the FAMOUSarea: Mid-Atlantic Ridge, Earth Planet Sci. Lett., v. 37, p.133-156.

Macdonald, G.A., 1968. Composition and origin of Hawaiianlavas, Geol. Soc. Am. Mem. 116, p. 477-522.

Macdonald, K.C., 1976. Geomagnetic reversals and the deepdrill hole at DSDP Site 332, /. Geophys. Res., v. 81, p.4163-4165.

Macdonald, K.C. and Atwater, T.M., 1978. Evolution ofocean ridges, Earth Planet. Sci. Lett., v. 39, p. 319-327.

Macdonald, K.C. and Luyendyk, B., 1977. Deep-tow studiesof the structure of the Mid-Atlantic Ridge crest near lat.37°N, Geol. Soc. Am. Bull., v. 88, p. 621-636.

Mysen, B.O. and Holloway, J.R., 1977. Experimental deter-mination of rare earth fractionation patterns in partialmelts from peridotite in the upper mantle, Earth Planet.Sci. Lett., v. 34, p. 231-237.

Natland, J.H., 1978. Comparison of chemical and magneticstratigraphy of Holes 396 and 396B. In Heirtzler, J.,Dmitriev, L., et al., Initial Reports of the Deep Sea DrillingProject, v. 46: Washington (U.S. Government Printing Of-fice), p. 425-430.

O'Nions, R.K. and Pankhurst. R.J., 1977. Sr-isotope andrare-earth element geochemistry of DSDP Leg 37 basalt. InAumento, F., Melson, W.G., et al., Initial Reports of theDeep Sea Drilling Project, v. 37: Washington (U.S.Government Printing Office), p. 599-602.

Presnall, D.C., 1969. The geometrical analysis of partial fu-sion, Am. J. Sci., v. 267, p. 1178-1194.

Puchelt, H., Emmermann, R., and Srivastava, R.K., 1977.Rare earth and other trace elements in basalts from theMid-Atlantic Ridge 36°N, DSDP Leg 37. In Aumento, F.,Melson, W.G., et al., Initial Reports of the Deep Sea Drill-ing Project, v. 37: Washington (U.S. Government PrintingOffice), p. 581-590.

Rhodes, J.M., Blanchard, D.P., Rodgers, K.V., Jacobs,J.W., and Brannon, J.C., 1976. Petrology and chemistryof basalts from the Nazca plate: Part 2 — major and traceelement chemistry. In Yeats, R.S., Hart, S.R., et al., InitialReports of the Deep Sea Drilling Project, v. 34:Washington (U.S. Government Printing Office), p.239-244.

Rosendahl, B.R., Hekinian, R., et al., in preparation. InitialReports of the Deep Sea Drilling Project, v. 54:Washington (U.S. Government Printing Office).

Schilling, J.-G., Kingsley, R., and Bergeron, M., 1977. Rareearth abundances in DSDP Sites 332, 334, and 335, and in-ferences on the Azores mantle blob activity with time. InAumento, F., Melson, W.G., et al., Initial Reports of theDeep Sea Drilling Project, v. 37: Washington (U.S.Government Printing Office), p. 591-598.

Sigvaldason, G.E., 1974. Basalts from the center of the as-sumed Icelandic mantle plume, J. Petrol, v. 15, p.497-524.

Wright, T.L., 1977. Chemical analyses of interlaboratory stan-dards. In Aumento, F., Melson, W.G., et al., InitialReports of the Deep Sea Drilling Project, v. 37:Washington (U.S. Government Printing Office), p.367-370.

Yeats, R.S., Forbes, W.C., Heath, G.R., and Scheidegger,K.F., 1973. Petrology and geochemistry of DSDP Leg 16basalts, eastern equatorial Pacific. In van Andel, T.H.,Heath, G.R., et al., Initial Reports of the Deep Sea DrillingProject, v. 16: Washington (U.S. Government Printing Of-fice), p. 617-640.

677