27. CLAY-SIZED MINERALS FROM CORES OF THE SOUTHEAST PACIFIC OCEAN,DEEP SEA DRILLING PROJECT, LEG 351

Z.N. Gorbunova, P. P. Shirshov Institute of Oceanology,U.S.S.R. Academy of Sciences, Moscow, U.S.S.R.

ABSTRACT

Terrigenous illite, chlorite, kaolinite, and small amounts of ver-miculite occur in sediments at the four sites drilled during DSDP Leg35. Montmorillonite (mix-layered clay), which can result from altera-tion of volcanogenic material on land and under water, increasedand the amount of chlorite plus kaolinite decreased in the oldersediments from Sites 322, 323, and 325. The trend in illite content isnot well defined.

In the Pliocene-lower Miocene age sediments, the crystallization ofmontmorillonite was more or less constant and increased noticeablyonly in horizons above the basalts (Sites 322 and 323) as a result ofhydrothermal activity. The montmorillonite content increases withdepth without a change of its crystallization, perhaps due to thegreater influx of montmorillonite into the older sediments, or as theresult of postdepositional changes of the clay minerals. This sametendency exists in most oceanic sediments. Other diagenetic but non-clay minerals, clinoptilolite, cristobalite, and goethite, occur in thecores.

INTRODUCTION

The suites of clay minerals, the degree of mineralcrystallization, and the content of amorphous materialare related to their provenance, depositional mechanismand diagenetic alterations. These minerals are stable andcan be transported great distances from their sourcewithout changes to their basic crystallochemical proper-ties (Rateev et al., 1968; Griffin et al., 1968; Biscaye,1965; Hayes, 1973). Illites, kaolinites, chlorites, andmontmorillonites, the four principal groups of clayminerals, can be transported to the ocean from land, butonly montmorillonite usually occurs in mix-layered se-quences and can be formed in substantial amounts inmarine sediments from the alteration of the volcanicrocks in contact with seawater (Skornyakova et al.,1971; Rex, 1967).

The composition of clay minerals in hydrothermallyaltered sediments is rather distinctive. For example,well-crystallized montmorillonites as well as triocta-hedral vermiculites and illite occur in the sediments ofthe South Pacific Basin (Gorbunova, 1974). The metal-liferous sediments, which were located near the EastPacific Rise, are characterized by montmorillonitic min-erals in the very initial stages of crystallization.

The deep cores of Leg 35 provide samples to evaluateclay mineral transformations during low-temperaturediagenesis. We previously studied the coring resultsfrom the northeastern Pacific Ocean (Hayes, 1973),where oceanic crust subduction provides deep burial ofthe sediment and an increase of illitic layers in the

'Translated and edited from Russian.

mixed-layered montmorillonite-illite was expected as aresult of diagenesis, but a reverse trend was noted atSites 174, 175, and 176. Hayes (1973) explained this inthat burial of the sediments occurred shallower than3000 meters, which is not sufficient for low-temperaturediagenesis. Perry and Hower (1970) noted that in theGulf of Mexico alteration of montmorillonite into micaoccurred only below 2000 meters depth of burial. In theeastern equatorial zone of the Pacific Ocean, Zemmels(1973) noted a high degree of diagenesis and related thisto the presence of clinoptilolite, pyrite, cristobalite, andbarite in the sediments. In his opinion, montmorillonitealters to mica in the deeper layers of the sediment. In thewestern part of the Pacific Ocean, Okada and Tamuda(1973) noted changes in the interlayered water content inmontmorillonites: one layer occurred in shallow sedi-ments and two layers occurred in the deeper ones.

METHODSThe < 1 µm minerals were studied after dispersal with

tripolyphosphate and isolation by decantation. At Site323 we also studied bulk samples. For mineral deter-minations we used the X-ray diffractometer Dron-I witha scintillation counter with Cu k; radiation and a nickelfilter. Samples were transferred by pipette to 2 glassslides and dried at room temperature. Since the sampleswere mostly of light color, the contaminating Fe2U3were thought to be insignificant and not removed.

All samples were analyzed following magnesiumsaturation, glycerin saturation, heating to 520°C for onehour, and after \N HC1 treatment to determine thestability of the minerals and differentiate betweenchlorite and kaolinite. Some samples were treated withIN KOH to determine the nature of mix-layered mont-

479

Z. N. GORBUNOVA

morillonite and vermiculite and help to distinguish ver-miculite from chlorite (Hayes, 1973).

In order to establish the nature of the octahedrallayers, we surveyed randomly oriented samples in aspecial holder to obtain the 060 reflection. The dioc-tahedral montmorillonites were treated with lithiumchloride in order to distinguish the nontronite-beidellitesubgroup from the montmorillonite (Green-Kelly,1953).

The illite polymorphs could not be distinguishedbecause of the excessive feldspar content which pro-duces similar reflections. Many samples were studiedwith an electron microscope in order to identify crystalswith a specific morphology.

The quantitative ratios of the groups of clay mineralswere evaluated according to Biscaye (1965) and, ac-cordingly, chlorite and kaolinite were calculated simul-taneously.

In the <l µm fraction we also discovered nonclayminerals: quartz, feldspar (common and abundant),amphiboles (determined by the 8.5A peak), clinop-tilolite (determined by a peak at approximately 9.0A),cristobalite (by the 4.04Å peak), and goethite (by the4.18Å peak).

RESULTS OF INVESTIGATIONSCores from four sites were studied. Sites 322 and 323

were located on the Bellingshausen Abyssal Plain andSites 324 and 325 were on the Antarctic continental rise.

Site 323 was most thoroughly studied. The total sub-bottom penetration was 731 meters, and the cored inter-val was divided into six lithologic units, ending in basalt(see site summary report, this volume). The basalt isoverlain with brown claystones, enriched with hydrousferric oxide and manganese, as well as zeolites. This typeof sediment was probably formed from subaqueous vol-canic exhalations near spreading centers. Terrigenousmaterial, for example, quartz and feldspar, occurs above638 meters. Apparently, turbidity currents were sig-nificant, especially in the upper units to a subbottomdepth of about 500 meters. The material, however, iswell sorted, which is indicative of reworking by bottomcurrents.

In sediments from this site we encountered clayminerals of the montmorillonite, illite, chlorite, andkaolinite groups. The montmorillonite occurs in mix-layer forms with illite. The magnesium-saturated com-plex is characterized by a broad and asymmetrical max-imum diffraction peak at approximately 15.2Å, with aglycerine reflection at 18Å (001); the consecutive ordersof reflection are usually expressed by broad asym-metrical peaks. Reflections 002 and 003 are usuallydiminished or completely lacking. The degree of thecrystallization of montmorillonite is average or high assuggested by the v/p ratio (Biscaye, 1965) which rangesfrom 0.6 to 0.9.

The stability of montmorillonite treated with IN HC1changed at various horizons. From the sediment surfaceto a depth of nearly 600 meters, the montmorillonitesdisintegrated rather easily, and only in the lower parts ofthe section are they partially preserved. Consequently,we believe that the composition of the octahedral layerschanges in the deeper layers. The chemical data revealeda high content of barium here.

The 060 reflection near 1.501Å is characteristic ofdioctahedral clay minerals. A reflection at 1.530Å,possibly that of quartz, was encountered in samplesthroughout the cored section. Small amounts of trioc-tahedral minerals (if they do exist) could have beenoverlooked.

The dioctahedral illite is characterized by reflectionsat 10Å, 5Å, 3.3Å, which do not change upon heating orsaturation with glycerin. Reflections 001 and 002 areasymmetrical, indicating the mix-layer character of themineral. The illites are stable in IN HC1 throughout theentire sediment sequence.

The chlorite is characterized by reflections at 14.2Å(001), 7.1Å (002), 4.7Å (003), and 3.54Å (004), which donot change when saturated with glycerin or variouscations. On heating to 520°C, reflection 001 changes to13.8Å, whereas the others disappear or weaken. Chloritereflections overlap those of kaolinite and cannot alwaysbe resolved. However, treatment with hydrochloric acidallowed the recognition of a small amount of kaoliniteon a background of chlorite. The reflection near 7Å,which is characteristic of both kaolinite and chlorite,frequently is broadened towards greater 2θ angles (ap-proximately 7.3Å) and is related to the presence ofserpentine or halloysite. In samples which consist almostentirely of montmorillonite, there is a wide asym-metrical peak at approximately 7Å, which probably re-flects a complicated mixture of serpentines, zeolites, andother minerals.

The clay minerals at Site 323 were determined in boththe bulk and <l µm fractions. The relative abundancesof the major groups of clay minerals are in both casesrather similar (Figure 1). Kaolinite abundance in thishole is minor, and because it is poorly resolved fromchlorite, the two were calculated together. Mont-morillonite content shows a downhole increase (Table1). In the upper sediments of the hole (Pliocene), themontmorillonite content ranges from 30% to 40%,whereas below, in Units 2 and 3, horizons containing asmuch as 70% montmorillonite are common. In litho-logic Units 4 and 5, almost pure montmorillonite withtraces of other clay minerals was found in the bulk and<l µm fractions. Electron microscope examination(Figure 2a, b) revealed particles with cloudy contoursand elongated rods with distinct edges (zeolites), whichare characteristically associated with montmorillonite.Goethite was also found in these horizons.

The degree of crystallization of the montmorilloniteincreases in the lower horizons; this is expressed by adistinct increase in the intensity of the 001 reflection,which cannot be attributed solely to the increase inabundance (Figure 3). The monomineralic compositionof these sediments can probably be attributed to hydro-thermal exhalations at spreading ridge crests.Analogous sediments were found on the East PacificRise (Böstrom and Peterson, 1969); however, the natureof the clay minerals differs substantially in that on therise, the montmorillonite is in its initial stage of crystal-lization. Montmorillonites of a similarly high degree ofcrystallization were also found in the hydrothermally al-tered surface sediment layers in the southern PacificOcean Basin (Gorbunova, 1974).

Units 1 to 3 contain, in addition to the minerals notedabove, illite (up to 40%), chlorite, and, to a lesser degree,

480

MINERALS FROM CORES, SOUTHEAST PACIFIC OCEAN

GO1—

ZD

CM

U J

LU

z:LU

o

Q .

U J

o

ID

LU

C_>O

2 :LLJ

< :

O

1

CD

OLI

DA

M?)

]in1—

UJQ

100

200

300

400

500

600

700-1

MONT.

100%

——HBB

1 ...—

M i l

mmmsec=.

^ ^ ^ ^

ILLITE

100%

~——!

= -

^ _

111

C H L O . + K

100%

1 ^

— -

r

= .

NONCLAY

CRIS

CLIN.

CLIN.

CLIN.

GOET.GOET.

CLIN.

MONT.

100%

ILLITE

100%

CHLO.+K

100%

1—

OO

LLJ

<C

IOCE

NE

1Q-

1I0C

ENE

REN

ca:CQ

o~

CD

m1—

UJQ

100

300

400

bUU

MONT.

100%

ILLITE

100%

CHLO.+K

100%NONCLAY

CLIN.

CLIN.

<l µm FRACTIONS

SITE 322

<l µm FRACTIONS BULK SAMPLES

SITE 323

Figure 1. Distribution of clay minerals at Sites 322 and 323.

kaolinites (i.e., typical minerals of terrigenous prov-enance) along with quartz and feldspar. In electronmicroscope micrographs, these minerals occur as par-ticles with distinct outlines (Figure 2c, d). The followingdiagenetic minerals were also encountered at variouslevels of Site 323 in the < 1 µm fraction: clinoptilolite inthe third and fourth lithological units, cristobalite in thesecond unit, and goethite in the fourth unit. The rela-tive abundances of these minerals vary. The content ofchlorite and kaolinite has a tendency to decrease downthe hole. The trend in illite content is unclear; in thelithologic Units 4 and 5 it disappears almost completely.In general, there does not seem to be any correlation

between the distribution of clay minerals and lithologicunits at this site. The only major trend seems to be theone noted above of an increase in montmorillonite and adecrease in the chlorite plus kaolinite content with depthand thus with age. Superimposed on this trend are a fewsharp breaks. For example, a sharp decrease in mont-morillonite abundance and the appearance of cristo-balite occurs at a depth of approximately 400 meters.However, only lithologic Units 4 and 5 are char-acterized by a distinct mineral assemblage.

According to Bogdanov (this volume) the mineralogyof silt-sized fractions and the chemical data indicate up-ward migration of hydrothermal solutions to a depth of

481

Z. N. GORBUNOVA

TABLE 1Composition <l µm Fractions of Leg 35 Siltstones

Sample(Interval

in cm)

Site 322

1-2, 26-301-3, 70-762-2,113-1163-1, 18-204-1, 65-674-2, 57-605-1, 81-856-1, 30-339-2, 143-146

10-1, 101-105

11-1,42-46

11-2,45-49

11-3, 24-32

11-4,17-24

11-5,36-40

Site 323

1-1, 50-601-1, 140-1501-2, 24-431-3, 78-881-4, 118-1272-1, 128-1303-1, 88-943-2, 14-203-2,61-706-1, 100-1106-1, 140-1427-2,105-1107-3, 18-248-1,133-14310-1,133-12210-2, 75-8510-3, 99-102

10-3, 106-114

11-1,22-32

11-2,137-148

12-1, 110-114

12-2,9-18

12-2, 73-77

13-5,63-70

13-5, 106-115

13-6, 145-150

14-2, 1-814-2, 65-72

ApproximateDepth (m)

18.220.2

193.1195.2352.6354.0391.8438.8469.9

487.0

505.4

506.9

508.2

509.7

511.4

76.076.977.379.381.2

162.3256.9257.6258.1342.5343.9363.0368.7409.3504.1505.2507.0

508.0

550.7

553.4

559.1

599.6

600.2

623.6

624.1

625.9

637.5638.1

Age

PliocenePliocenePlioceneLate MioceneLate MioceneLate MioceneBarrenBarrenEarly tomid. MioceneEarly tomid. MioceneEarly tomid. MioceneEarly tomid. MioceneEarly tomid. MioceneOligocene toearly MioceneOligocene toearly Miocene

PliocenePliocenePliocenePliocenePliocenePlioceneLate MioceneLate MioceneLate MioceneEarly MioceneEarly MioceneEarly MioceneEarly MioceneBarrenBarrenBarrenOligocene toearly MioceneOligocene toearly MioceneOligocene toearly MioceneOligocene toearly MioceneOligocene toearly MioceneOligocene toearly MioceneOligocene toearly MioceneOligocene toearly MioceneOligocene toearly MioceneOligocene toearly Miocene

9

Montmor-illonite

(%)

375345684664636582

64

100

100

100

100

73

272738334364tr356465547770157451

100

66

38

41

25

65

72

40

64

49

6957

Illite(%)

393126182620243511

25

tr

tr

tr

tr

27

42474337383040283685462330581842tr

34

47

39

58

27

28

43

23

37

2538

Chloriteand

Kaolinite(%)

24162914271613tr

7

11

tr

tr

tr

tr

tr

31261930196

6037trtrtrtrtr27

87

tr

tr

15

20

17

8

tr

17

13

14

65

NonclayMinerals

Clin.Clin.

Crist.

Clin.Cün.

Clin.

Clin.

482

MINERALS FROM CORES, SOUTHEAST PACIFIC OCEAN

TABLE 1 - Continued

Sample(Interval

in cm)

14-2, 128-13515-1,52-6015-2, 91-10015-3, 29-3515-4, 54-6315-5, 89-9815-6, 19-2616-1,57-6216-3, 32-3716-4, 83-9218-2, 80-8618-3, 130-13918-4, 115-12418-5, 14-2018-5, 65-73

ApproximateDepth (m)

638.8655.5657.4658.3660.0661.9662.7665.0667.8669.8695.3697.3698.6699.1699.6

Age

77777

Late DanianLate DanianLate DanianMid. DanianMid. DanianMaestrichtianMaestrichtianMaestrichtianMaestrichtianMaestrichtian

Montmor-illinite

(%)

100100100100100100100100100100100100100100100

Illite(%)

trtrtrtrtrtrtrtrtrtrtrtrtrtrtr

Chloriteand

Kaolinite(%)

trtrtrtrtrtrtrtrtrtrtrtrtrtrtr

NonclayMinerals

GoethiteGoethite

Goethite

GoethiteGoethite

Clin.

Clin.Clin.Clin.Clin.

Note: Feldspars and quartz were found throughout.

<l µm B

f

<l µm

<l µm

Figure 2. Electron microscope photos of the sediments of Site 323 (A, B) authigenic montmorillonite and clinoptilolite;(C, D) terrigenous illite and chlorite, x 12,000.

553 meters in the sedimentary column (Unit 3).However, the composition of clay minerals does notchange appreciably between Units 2 and 3.

Site 322 in the Bellingshausen Abyssal Plain wasdrilled to a depth of 544 meters. The Hthologic sequenceis divided into five Hthologic units terminating in basalt.

483

Z. N. GORBUNOVA

I 003-J+Q

3.23.3 3.5 4.7 5.0 10 14 18

Figure 3. Diffraction pictures of clay minerals, showing thedifferences in depth of the sediments at Site 323.

The clay mineral composition is similar to that at Site323 (Figure 1, Table 1). The montmorillonite is basicallya mix-layered structure with illite, in which the 002reflection is diffused and the 001 reflection is broad. Thehigh reflection intensity and the v/p ratio (Biscaye,1965) are indicative of average to good crystallization.Montmorillonite is abundant (over 50%) throughout thehole and increases in abundance with depth in theNeogene sequence. At a depth of 500 meters, mont-morillonite is the only significant mineral at severalhorizons, with only trace amounts of other mineralspresent. Treatment with hydrochloric acid showed thatthis mineral is unstable except in lower horizons. In con-trast to Site 323, goethite was not detected. Chlorite pluskaolinite decrease in abundance with depth. The 7Åpeak is complicated and indicates the presence of severalminerals: chlorite, kaolinite, serpentinite, and others.The occurrence of kaolinite is indicated by the presenceof the 7Å peak after treatment with IN HC1.

Illite content averages 30% and ranges from 11% to62% except for several horizons near 500 meters where itoccurs in trace amounts. Small amounts of feldspar andquartz were found in the same horizons. This suggeststhat the terrigenous influence is minimal at these levels.By analogy with cores from Site 323, hydrothermal for-mation for the montmorillonite at a depth of approxi-mately 500 meters can be assumed.

The diagenetic mineral clinoptilolite was observed atmany levels, and its maximum content was at 381 and438 meters. There is at this site, as at Site 322, no distinctcorrelation between clay mineral composition andlithologic units. However, the same tendency of an in-crease of montmorillonite and a decrease of chlorite andkaolinite with depth was found. The variations noted byBogdanov (this volume) for the mineralogy of the coarsefraction that reflect various sources areas do not occurin the clay mineral composition at Sites 322 and 323.

Site 324 was drilled to a depth of 218 meters on thelower part of the Antarctic continental rise. Thesediments consist almost completely of terrigenousclays. The clay content ranges from 30% to 40% formontmorillonite, 40% to 50% for illite, and about 25%for chlorite plus kaolinite. Treatment with IN HC1 in-dicates that the montmorillonite is not too stable asshown by the decrease in intensity or disappearance ofthe 001 reflection. As at Sites 322 and 323, the degree ofits crystallization is high. Illite and chlorite pluskaolinite also have sharp, intense reflections indicativeof good crystallization. However, because of the highfeldspar content, the polymorphs of illite could not bedetermined. The illite content at Site 324 is considerablyhigher than at all the other Leg 35 sites (the sedimentsare of Pliocene age). Kaolinite plus chlorite are aboutequally abundant. This is clearly expressed by treatmentwith IN HC1 as the 7Å reflection remains intense.However, the complexity of this peak suggests thathalloysite and serpentinite may also be"present. Quartzoccurs throughout the section, as does clinoptilolite, butthe latter occurs only in trace amounts.

Site 325 was drilled to a depth of over 700 meters onthe continental rise of the northwestern part of the Ant-arctic Peninsula. The deepest layers are represented bydetrital sediments which are overlain by terrigenoussediment. The material here is more poorly sorted thanat Site 324.

The clay minerals at this site show characteristicallyweaker X-ray reflections than those at Site 324. As atprevious sites, montmorillonite is represented by an un-stable mix-layered clay with illite. There is a generaldownhole increase in montmorillonite content with aminimum at a depth of 140 meters (Table 2, Figures 4,5). In the lowermost layer, at a depth of 710 meters, thedegree of crystallization is very high as shown by diffrac-tograms with a narrow intense peak at 17.6Å afterglycerin treatment. In the upper horizons the 001 peakof montmorillonite is wide and the v/p ratio (Biscaye,1965) fluctuates from 0.5 to 0.6, whereas in the lowerhorizons it is approximately 0.9. The lithologic data in-dicate that the lower horizons were redeposited(Bogdanov et al., this volume). The illite content is lowerthan at Site 324 and ranges from 25% to 30%. Maximumchlorite plus kaolinite (about 50%) were found at adepth of approximately 170 meters. Feldspar and quartz

484

MINERALS FROM CORES, SOUTHEAST PACIFIC OCEAN

TABLE 2Composition < l µm Fractions of Leg 35 Mudstones

Sample(Intervalin cm)

Site 324

1-2, 126-1311-3, 124-1321-4,125-1311-6,31-382-1, 83-922-2,61-702-3, 55-572-4, 27-362-5, 35-452-6, 95-1043-1, 137-1463-2, 136-1453-3,67-773-4, 47-573-5, 99-1003-6,138-1484-2, 113-1234-3, 30-404-4, 56-645-2, 66-835-3, 25-356-2, 19-307-1,69-787-3, 110-1207-6,119-1298-3, 61-71

Site 325

1-1, 133-1441-2, 26-361-4, 81-922-2, 39-493-2, 11-243-3, 76-873-4, 26-365-1, 55-575,CC6-1, 145-1507-1, 131-1357-2, 138-141

8-1,45-49

8-2, 119-128

8-3, 23-25

9-3, 90-98

10-1, 35-39

10-2, 26-30

ApproximateDepth (m)

11.813.214.716.847.849.150.551.853.355.476.978.479.280.582.584.4

106.6107.3109.0134.7135.7143.7149.2155.6160.2174.1

35.335.839.3

168.9177.1180.3181.3405.0414.0481.9519.8520.4

612.9

615.2

615.8

644.9

708.8

710.3

Montmor-i

Age

PleistocenePleistocenePleistocenePleistocenePleistocenePleistocenePleistocenePleistocenePleistocenePleistocenePleistocenePleistocenePleistocenePleistocenePleistocenePleistocenePleistocenePliocenePliocenePliocenePliocenePliocenePliocenePliocenePliocenePliocene

PliocenePliocenePliocenePliocenePliocenePliocenePlioceneMioceneMioceneMioceneBarrenEarly Oligocene toearly MioceneEarly Oligocene toearly MioceneEarly Oligocene toearly MioceneEarly Oligocene toearly MioceneEarly Oligocene toearly MioceneEarly Oligocene toearly MioceneEarly Oligocene toearly Miocene

illonite(%)

3625

trace4616173736204035333531254237273533263729252227

473542202529263849456050

40

65

30

50

53

71

Illite(%)

4052654647454137503739434041463638454242474046464749

323631262018263633302534

42

20

46

24

27

17

Chloriteand

Kaolinite

242335

837372237302326242528292225282325272325293124

212927545553482618251516

18

15

24

26

20

12

NonclayMinerals

Clin.Clin.Cün.Clin.Clin.Clin.Cün.Clin.Cün.Cün.Clin.Clin.Clin.Clin.Cün.CUn.Clin.Clin.Clin.

Am.Am.Am. Clin.Am. Cün.Am.Am.Am.Am. Cün.Am. Clin.Crist. Cün.Am.Clin.

Cün.

Cün.

Crist. Cün.

Clin. Am.

Cün.

Cün.

Note: Feldspars and quartz were found throughout.

occur throughout the section, and actinolite was noteddown to 400 meters. At this depth a change inprovenance of the sedimentary material occurs (seeBogdanov et al., this volume). Below 400 meters, thediagenetic minerals cristobalite and clinoptilolite occur.

As at Sites 322, 323, there seems to be no relation ofclay mineral composition to lithologic changes.

DISCUSSION OF RESULTS

Sites 322 and 323, are mineralogically similar. Withthe exception of the upper horizons, montmorillonite in-creases with depth and is the predominant mineralthroughout. It is moderately to well crystallized and is

485

Z. N. GORBUNOVA

UNIT

S

AGE

PLIO

CENE

DEPT

H

100

200

MONT.

100%

ILLITE

100%

CHLO.+K

100%

™?

-*-

NONCLAY

CLIN.

CLIN.

CLIN.

< l µm FRACTIONS

SITE 324

Figure 4a. Distribution of clay minerals at Site 324.

often mix-layered with illite. At both sites the basal sedi-ments above basalt are nearly pure montmorillonite.

At Site 323 montmorillonite occurs together withgoethite and clinoptilolite, whereas at Site 322 onlyclinoptilolite co-occurs. The nearly monomineralic com-position and high degree of crystallization of thesesediments (even in bulk samples from Site 323) are in-dicative of a hydrothermal origin and submarine dia-genesis. Such mineral assemblages have been previouslyrecognized in the surface layers of hydrothermallyaltered sediments of the southern basin of the PacificOcean (Gorbunova, 1974). The dark interlayers, en-riched with ferromanganese hydroxides, have also beennoted in many areas of the Pacific Ocean (Cook et al.,1970; Cook and Zemmels, 1971).

Illite content does not seem to show any changes withdepth, whereas kaolinite plus chlorite decrease marked-ly. Montmorillonite is less common in the continentalrise sites (324 and 325) than at the BellingshausenAbyssal Plain sites. Well-crystallized illite and kaoliniteplus chlorite, as well as montmorillonite, occur in nor-mal abundance at Site 324. Illite is less abundant at Site325 than at Site 324, suggesting different origins for thesediments at these sites. At Site 325, there is a tendencyfor montmorillonite content to increase with depth, butunlike Sites 322 and 323, monomineralic horizons werenot encountered, although the lower horizons are char-acterized by well-crystallized montmorillonite. Ap-parently, these horizons represent redeposited material.

The distributions of chlorite plus kaolinite, as well asof illite (although less clearly) decrease. Similar trends ofincreasing montmorillonite and decreasing chlorite andillite contents with depth have been noted previously inDSDP sites in various areas of the Pacific Ocean—in thenortheastern marginal zone (Hayes, 1973), in theTasman Sea (Matti et al., 1973), and in the northwesternpart of the Pacific Ocean (Okada and Tamuda, 1973).Hayes suggested that these trends are the result of the in-

UNIT

SCM

AGE

IOCE

NE

i

α_

LU

ocεr

ZD

. M

IOCE

NE

i

OLIG

.

DEPTH

100

200

300

400

500

600

700

MONT.

100%

—

ILLITE

100%

CHLO.+K

100%NONCLAY

CLIN.

CLIN.

CLIN.

CRIS.

CRIS.

CLIN.

CRIS.

CLIN.

CLIN.

CLIN.

<l µm FRACTIONS

SITE 325

Figure 4b. Distribution of clay minerals at Site 325.

486

MINERALS FROM CORES, SOUTHEAST PACIFIC OCEAN

003-J+Q

3 . 2 3 . 3 3 . 5 4 . 7 5 . 0 10 14 18

Figure 5. Diffraction pictures of clay minerals, showing thedifferences of depth in the sediments at Site 325.

fluence of many factors on the formation of clay such as,currents, turbidites, fluctuation of volcanic activity,glaciation, lowering of sea level, degree of erosion andweathering, mechanism of transport, etc. He proposedtectonic explanations for the differences between Plio-cene and Pleistocene clays.

The increasing intensity of the 001 peak of the mix-layered montmoriUonite with site depth does notnecessarily have the same origin. This may be attributedto the increase of unstable interlayers in the mont-moriUonite (Reynolds and Hower, 1970). This couldalso be the result of the increasing degree of crystalliza-tion of the montmoriUonite in the basal sediments, or ofthe increasing abundance with depth. Between Pliocene

and lower Miocene sediments, the first two factors arenot significant. The change in the degree of crystalliza-tion (if it occurs at all) is rather uncertain. The width ofthe 001 peak seldom varies and the 002 peak is more orless clearly expressed and shows no trend with depth.Only in hydrothermal sediments above the basalt isthere any increase in the concentration and crystalliza-tion of the mineral. Thus, there is most probably an in-crease of the amount of the mineral with age. This couldbe attributed to a greater amount of volcanic activity inthe Miocene in comparison to Pliocene, although themineralogical data of the coarse fractions do not con-firm it. A direct connection between the abundance ofthe volcanogenic material and montmoriUonite can behardly expected. The montmorillonite can betransported to the sediment from distant sources andnot be formed locally. The basic source of theterrigenous materials in the cores was the weathering ofthe ancient crusts of Antarctica, which are ratherhomogeneous in composition. It is possible, thatpostdepositional alteration of the montmorillonite oc-curred because of other minerals or on account of an in-crease of its crystallization. This is hard to establish.Presently diagenesis and epigenesis are thought to be ac-companied by an increase in illite abundance, however,this process has not been observed in the DSDP cores.In any case, an alternative explanation of this questionhas not been found as yet.

ACKNOWLEDGMENTSI would like to thank Yu. A. Bogdanov for supplying me

with the material and for the useful discussions on behalf ofthe current problems of clay formation.

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