12. CHEMICAL COMPOSITION OF SEDIMENTS FROM SITES 506, 507, 508, AND 509,LEG 70, DEEP SEA DRILLING PROJECT1

S. A. Moorby and D. S. Cronan, Applied Geochemistry Research Group, Geology Department, Imperial College,London, S.W.7, United Kingdom

INTRODUCTION

Since its discovery in 1974 (Klitgord and Mudie,1974), the Galapagos mounds hydrothermal field has re-ceived much attention. Sediment samples were takenduring Leg 54 of the Deep Sea Drilling Project (DSDP)and by other expeditions to the area (e.g., Corliss et al.,1978). While a hydrothermal origin for the mounds sed-iments has been generally accepted, several differenttheories of origin for the mounds themselves have beenproposed (e.g., Corliss et al., 1978; Natland et al., 1979;Williams et al., 1979).

One of the aims of DSDP Leg 70 was to return to themounds field and, using the new hydraulic piston cor erdescribed elsewhere in this volume, to obtain more com-plete recovery of mounds sediments than had previouslybeen possible. It was our hope that this would help inour understanding of the nature and origin of these de-posits. In this chapter, we describe the results of chemi-cal analysis of over 250 sediment samples taken duringthe course of Leg 70.

SAMPLESSamples were taken from every hole that was piston

cored at Sites 506, 507, 508, and 509. A total of morethan 250 samples were analyzed—these having been se-lected at varying intervals in the cores, depending on thelithological changes seen visually and by smear slide ex-amination. The major sediment types recognized, de-scribed in detail in the site chapters, are in summary asfollows:

Pelagic OozeA siliceous foraminifer-nannofossil ooze occurs in

every hole and is the typical pelagic sediment in thisarea. A decrease in biogenic silica content to almost zerowith increasing depth is typically seen in all holes on andnear the mounds themselves. At all holes the surface pe-lagic sediment differs significantly from that at depthand is therefore discussed separately.

Hydrothermal SedimentTwo major types of hydrothermal deposits were rec-

ognized: Mn-oxide-rich sediment and nontronite.

Honnorez, J., Von Herzen, R. P., et al., Init. Repts. DSDP, 70: Washington (U.S.Govt. Printing Office).

The first type, Mn-oxide-rich sediment, is always theuppermost hydrothermal sediment and consists of twosubtypes:

1) Hard fragments of Mn-oxide crust. This was en-countered in all the mounds holes (i.e., Holes 506, 506C,507D, and 509B) although it was only comparativelyabundant in Hole 509B and, because of its nature, washighly disturbed by the piston-coring operation.

2) Mn-oxide rich mud. This sediment type consistsessentially of "normal" pelagic sediment containingabundant, finely disseminated Mn oxides. It was en-countered only at Hole 509B, where it occurred abovethe Mn-oxide crusts.

The predominant component of the second majortype of hydrothermal deposit, nontronite, which was re-covered from all the mounds holes, is a green mineral ofthe smectite group. Two subtypes of this sediment werealso recognized:

1) Dark green coarsely granular material.2) Paler green, nongranular, or "compact," transi-

tional sediment.The latter material typically occurs between normal

pelagic sediments and granular nontronite, both aboveand below the nontronite, and is regarded as beingtransitional between the two sediment types.

ANALYTICAL METHODS

After drying and crushing, the samples were digested in a mixtureof hydrofluoric, nitric, and perchloric acids. Final solutions weremade up in IM HC1 and sprayed on an ARL 34000 inductively coupledargon plasma emission spectrograph. Simultaneous determination ofK, Mg, Ca, Al, Mn, Fe, Li, Be, Cd, Sr, Ti, V, Cr, Co, Ni, Cu, Zn, Pb,Mo, and P was made on each sample using a program developedwithin the Applied Geochemistry Research Group, incorporating driftand blank correction. Silica was determined by atomic absorptionspectrophotometry after a separate digestion in hydrofluoric andhydrochloric acids in sealed containers. Accuracy and precision werechecked using in-house and international reference materials andduplicates. Details are summarized in Table 1.

ANALYTICAL RESULTSThe analytical results are summarized in Table 2.

Overall, the data compare well with other publisheddata for Galapagos mounds sediments (e.g., Dymond etal., 1980; Hoffert et al., 1980; Schrader et al., 1980a).Direct comparison is difficult in many cases, however,because material collected on Leg 54 (as a result of ex-tensive drilling disturbance) was often a mixture of thesediment types described above. For example, on Leg 70no evidence was found of any of the mixed Fe- and Mn-

269

S. A. MOORBY, D. S. CRONAN

Table 1. Precision and accuracy of chemical analysis.

Element

KMgCaAlMnFe

LiBeCdSrPTiVCrCoNiCuZnPbMo

ReplicateSample

Precision

± 1 0 %± 3 %± 5 %± 8 %± 5 %± 5 %

± 1 0 %± 6 %± 8 %± 7 %± 6 %± 6 %± 5 %± 2 0 %± 5 %± 8 %± 6 %± 3 %± 8 %± 1 2 %

StandardSO-3

Accepted

1.25.1

14.83.00.0521.5

__

217480

190044261216175214—

Found

(<K

1.64.6

14.22.80.0491.34

__

205530

18005124

913164720—

Red ClayStandard

Accepted

)

1.91.479.50.455.6

66__—

4900

10065

120100130

——

Found

1.81.49.10.425.8

64————

4400—

10462

110103147

——

Mn-noduleStandard

Mangenknollen-probe-4

Accepted Found

— —1.7 1.72.8 • 2.0

31.8 31.44.6 4.1

_ _— —— —— —— —

3000 2800— —20 30

1700 1560— —

15000 150001580 1540330 350— —

rich sediments reported extensively on Leg 54 (see, forexample, Hekinian et al., 1978; Schrader et al., 1980b).

The average compositions of each major sedimenttype (pelagic ooze, Mn-oxide-rich sediment, nontronite,transitional sediment) show no major differences be-tween different holes at the same site or even betweensites. The nontronite sediments in particular show veryconstant composition throughout the mounds holes. Thechemistry of each type will be described separately.

Pelagic Sediments

Surface Pelagic Sediment

At all holes, a surface pelagic-sediment layer occursup to 30 cm thick, which is brown in color in contrast to

the grey-green of the underlying pelagic sediment (seesite summaries). This layer represents the uppermost ox-idized layer of the sediments in which the Mn is presentpredominantly as the dioxide. It is a typical feature ofthe sediments in the region (Bonatti et al., 1971) and wasencountered in Leg 54 sites in the mounds area (Dy-mond et al., 1980). These surface sediments are enrichedin Mn, Ni, Cu, Zn, and P compared to the underlyingpelagic sediment (see Table 2). The Mn enrichment re-sults from its diagenetic remobilization and reprecipita-tion near the surface (Lynn and Bonatti, 1965), and se-lective chemical leaching of the samples (Varnavas etal., this volume) indicates that the other enriched ele-ments are associated with the Mn phase. The surface pe-lagic sediment is similar in composition in most moundsholes, off-mounds holes, and nonmounds holes, butsmall variations do occur.

Basal Pelagic Sediment

Samples immediately overlying basement were ana-lyzed from nine holes. Their average composition is givenin Table 3 and compared with that of basal sedimentsfrom several areas on the East Pacific Rise (EPR) (Dy-mond et al., 1973; Boström et al., 1976; Cronan, 1976;Heath and Dymond, 1977). While the nine samples doshow some variation, there is no significant differencein the composition of basal sediments from the differ-ent hole types (i.e., mounds, off-mounds, nonmounds).Three holes (506, 507F, and 509B) show some basal en-richment of Fe, but even in these samples Fe, Mn, Cu,Ni, and Zn are much lower, and Al and Ti much higher,than typical values for EPR basal metalliferous sedi-ments (Table 3). The metal ratios in these sediments(Fig. 1) also show that they are more closely associatedwith pelagic sediments and biogenic material than EPRbasal-metalliferous sediments. The basal sediments inthe mounds hydrothermal field, therefore, show little or

Table 2. Average bulk-chemical composition of the sediment types recognized onLeg 70.

Element

KMgCaAlSiO->MnFe

LiBeCdSrTiVCrCoNiCuZnPbMoP

No. ofSamples

PelagicSediments

0.26 (0.79)0.70 (2.1)

26.81.2 (3.7)

14.8 (44.8)0.26 (0.79)1.25 (3.8)

8.3 (25)0.3 (0.9)1.7 (5.2)

960 (2900)570 (1730)67 (200)30 (91)

9 (27)85 (260)84 (255)

140 (420)17 (52)

1.5 (4.5)440 (1330)

90

SurfaceSediment

0.250.65

23.81.2

15.22.11.26

5.10.31.6

1050520

503012

161102177

161.3

675

11

BasalSediment

0.220.77

30.71.16

10.00.191.48

Nontronites

1.52.20.400.28

43.50.11

20.2

(ppm)

7.20.31.6

116071558261263587618_

430

9

8.40.274.2

47117

1731

3172156164.9

250

88

TransitionalSediment

1.12.15.42.2

35.90.199.96

210.52.7

31099095471077

11817121

3.9460

58

Mn-oxideRich

Sediment

0.541.54.00.858.6

33.31.1

650.251.7

600325963714

17394

14265

3901130

13

Mottles

0.471.0

24.71.10.00.233.8

8.70.34—

780495

42326

4858

102101.5

360

11

Note: Figures in parentheses are data recalculated on a carbonate-free basis.

270

CHEMICAL COMPOSITION OF SEDIMENTS

Table 3. Comparison of composition of basal sediments recovered onLeg 70 with basal metalliferous sediments from the East PacificRise.

Basal Sediments

Element Uncorrected

K 0.22Mg 0.77Ca 30.7Al 1.2SiO2 10.0Mn 0.19Fe 1.5

Ti 715V 58Cr 26Co 12Ni 63Cu 58Zn 76P 430No. Analyzed 9

Corrected for NoCaCO3

Content

1000

300

µ 100

a

30

10

• Leg 70) nontronites

-I

Carbonate-Free Basis

0.913.2

4.841.3

0.86.2

295024011050

260240310

18009

Yes

G EPR basal sediments

A

Boströmet al.(1976)

m

_1.6

_7.6

23.0

(ppm)

1500700

37130640

1200750

100035

Yes

\ OBauer deep sediments

• Leg 70- Mn crusts

0.0 0.1

•BM(

B

0.2 0.3Al

Heath and DymondDymond et al.

(1977) (1973)

— 1.07

0.26 3.28.8 17.3

12.4 4.941.0 27.0

— —— —

640 4701900 1200410 —

_ _Not 6

KnownYes No?

Cronan(1976)

1.22.11.52.7

23.26.2

20.7

——

4708000

_—25

No(non-

carbonate)

BM = Biogenic MateriaTM = Terrigenous MaterialBAS = Basaltic Material

Leg 70pelagic 5 0 6 C

sediments i 509 506D

^^--«lėfC05O8

509B 5 0 7 H \ ^ ^ ^ β

0.4 0.5 0.6

Al + Fe+Mn

1

0.7

Figure 1. Metal ratios in Leg 70 basal and other sediments (after Bos-tröm et al., 1976; other data from Bischoff and Rosenbauer, 1977).

no affinity with basal sediments recovered elsewhere onthe East Pacific Rise. Dymond and his colleagues (1980)analyzed basal sediments from holes on Leg 54 and foundsome similarities between this material and basal sedi-ment from 6°S on the East Pacific Rise. They concludedthat there was evidence for the formation of metal-en-riched basal sediments in the mounds area. Their con-clusion, however, was based primarily on the similarityof the Fe/Mn ratios of the two sample groups. In viewof the contamination problems of Leg 54 samples, andthe markedly different Al, Ti, Mn, Fe, Ni, Cu, and Zncontents of mounds sediments compared with typicalmetalliferous sediment, we do not believe that markedbasal metal enrichment of the sediments occurs at anyof the sites drilled in the mounds area. On the evidence

of Figure 1, the composition of these sediments can inmost cases be accounted for simply in terms of a mix-ture of biogenic, basaltic, and terrigenous material.

Another feature of the basal sediments is their lowAl/Ti ratio compared to the other sample groups ana-lyzed (Table 6). This probably results from the commonoccurrence of basalt fragments in the sediment im-mediately above basement, these basalts being particu-larly Ti-rich (Hekinian et al., 1978; Mattey and Muir,1980).

Other Pelagic Sediments

The pelagic sediments from all holes cored weremainly siliceous carbonate oozes. In surface and near-surface sediments, the ratio of SiO2 to CaCO3 is about1:5, but with increasing depth the biogenic-silica contentfalls to almost zero (see site chapters). The decrease isnot gradual but seems to occur within a fairly narrowdepth interval, which is at shallower depth in moundsholes than off-mounds and nonmounds holes (Honno-rez, Von Herzen, et al., 1981). While dissolution of bio-genic silica within the sediment column is a widespreadphenomenon (Calvert, 1974; Chester and Aston, 1976),it is highly temperature dependent (Dapples, 1967). Thevariations seen may, therefore, result from differentheat-flow characteristics at the different holes (see Hon-norez, Von Herzen, et al., 1981). In order to facilitatecomparison with data for Leg 54 sediments, the pelagicsediment data in Table 2 and elsewhere in this chapterhave not been corrected for carbonate or biogenic silicacontent except where stated. On this basis, the data com-pare well with those for pelagic sediments recovered onLeg 54 (Dymond et al., 1980; Shrader et al., 1980a) andwith other analyses of sediments from this area (Bonattiet al., 1971). While Mn shows some variation betweenholes there is generally very little variation in the aver-age composition of this sediment type between holes,and there is no significant difference in the compositionof pelagic sediments in mounds, off-mounds, or non-mounds holes.

Hydrothermal Sediment

Mn-Oxide Crust

Because of the fragmented nature of the Mn-oxidecrusts and the nature of the piston-coring operation,doubt must be cast on the stratigraphic positions ofsome of the crusts analyzed. This applies to the twosamples from Hole 507D (in Core 2, Section 1) and toSample 509B-2-1, 98-100 cm. The average chemicalcomposition of all crusts analyzed is given in Table 4,together with, for comparison, the composition of hy-drothermal crusts from various localities analyzed byother workers. Some of the minor and trace metalsshow significant variations between holes; in particularLi is enriched and Sr depleted in Hole 507D samples,while Mo is enriched in Holes 506 and 507D. Some dif-ferences occur in the composition of crusts analyzed inthis work compared with mounds crusts analyzed byHekinian et al. (1978) and Schrader et al. (1980a). Inparticular Mn is slightly lower while SiO2, Al, Fe, and V

271

S. A. MOORBY, D. S. CRONAN

Table 4. Comparison of bulk composition of Mn-oxide crusts and sediments from Leg 70 with other Mn-rich deposits.

Samples

Mn-Crusts(Av. of 8)

Mn-Rich Mud(Av. of 4)

Mounds Mn-Crust(Schrader et al., 1980a)

KΛΛIIΠHÇ \ΛΠ fVnct1V1UU1IU5 lVIll~V^>I Udi

(Hekinian et al., 1978)Mounds Mn-Crust

(Corliss et al., 1978)Gulf of Aden

(Cann et al., 1977)Various Localities

(Toth, 1980)Hydrogenous Mn

Nodules, Pacific Ocean(Cronan, 1975)

Continental BorderlandNodules (Cronan, 1972)

K(%)

0.71

0.39

0.41

π ~>i

0.6

1.4

0.75

Mg(%)

1.6

1.3

2.5

1 QI . "

1.4

1.8

1.7

Ca(<%)

0.98

8.7

4.1

n QSU."o

1.5

1.5

2.0

Al(%)

0.24

1.2

0.12

n A\J.*rl

0.19

0.69

0.13

3.1

SiO2(%)

3.1(n = 2)

14.0(n = 2)

0.81

A 71U. /1

1.7

7.8

0.92

17.8

Mn(%)

47.0

21.8

45.3

CA Q34.y

50.0

37.9

45.6

19.8

34.0

Fe Li(%) (ppm)

0.66 100

1.3 6.4

0.22 —

0.26 —

2.7 —

0.18 —

12.0 —

1.6 —

Sr(ppm)

660

675

390

_

320

850

P(ppm)

1200

1020

_

—

_

2350

Ti(ppm)

28

475

60

—

1070

6700

600

V(ppm)

107

85

18

—

—

530

310

Co(ppm)

13

12

11

4.7

30

17

3400

75

Ni(ppm)

124

200

83

470

395

640

6300

970

Cu(PPm)

80

115

48

100

82

320

3900

650

Zn(ppm)

93

215

160

378

310

1030

680

—

Pb(ppm)

72

58

—

—

—

10

850

60

Mo(ppm)

540

160

—

—

—

—

440

720

are all higher in our samples. This may be explainedpartly in terms of a greater admixture of diluting pelagicsediment in our samples, although the Ca content indi-cates that little, if any, carbonate material is incor-porated. Levels of Ni, Cu, and Zn in the samples weregenerally less than 200 ppm, Pb less than 100 ppm, andCo less than 20 ppm. None of the crusts analyzedshowed comparatively high Ni, Cu, and Zn values as didseveral analyzed by Corliss et al. (1978).

The low trace-element content of hydrothermally de-rived Mn-rich crusts has been regarded (Bonatti et al.,1972; Toth, 1979; and others) as evidence of their rapidaccumulation and of a higher ratio of Mn to trace met-als in hydrothermal solutions than in normal seawater.The comparatively high concentrations of some metalsin hydrothermal crusts, however, have led to the sugges-tion that they either may be supplied to some extent byhydrothermal solutions (Zn, As, Hg: see Toth, 1980), orthey may be incorporated by a mechanism other than sur-face adsorption (U: see Scott et al., 1974). Zinc, after Feand Mn, has been found to be the metal taken into solu-tion in highest concentration by hydrothermal leaching(Seyfried and Bischoff, 1977); Toth (1980), therefore,used the Co/Zn ratio as an indicator of the degree of hy-drothermal, as opposed to hydrogenous, supply of tracemetals to ferromanganese-oxide deposits. Using the samecriterion the crusts analyzed in this study show evidenceof a predominantly hydrothermal supply of metals. Datafor Li in hydrogenous Mn-oxides are lacking, but thecomparatively high levels of this element in Leg 70crusts may also indicate hydrothermal supply. The en-richment of Mo in the crusts is particularly striking: av-erage values are as high as in the Mn-, Ni-, and Cu-richhydrogenous nodules of the northeast Pacific. The high-est Mo values are similar to those in diagenetically Mn-enriched nodules from the Mexican continental margin(Cronan, 1972), which otherwise show a very differentchemical composition.

At none of the sites drilled was Mn-oxide crust pres-ent at the sediment-water interface, but there was al-ways a certain thickness of typical pelagic ooze abovethe crusts, indicating that Mn-oxide deposition may notbe occurring at the present time. However, we do not

know where the various holes were located with respectto the active chimneys. At Hole 509B much of the sedi-ment capping the Mn-oxide crust was markedly enrichedin Mn.

Mn-Oxide-Rich Mud

Both the physical appearance and smear-slide exami-nation of the Mn-oxide-rich mud, which was found onlyat Hole 509B, showed it to consist essentially of typicalpelagic ooze admixed with varying amounts of finely di-vided Mn oxides. The average composition of four sam-ples of this mud is given in Table 4. Comparison of thedata with those for the crusts and pelagic ooze (Table 2)shows that the mud is intermediate in composition be-tween that of the crusts and the pelagic ooze, therebyconfirming the visual observation that the material is amixture of these two sediment types.

Nontronite: GranularThe granular dark green clay recovered from the

mounds has been identified by previous workers as pre-dominantly a nontronite (Corliss et al., 1978; Hekinianet al., 1978; Hoffert et al., 1980), and X-ray analysis ofsamples collected on Leg 70 confirmed this (Honnorezet al., this volume; Kurnosov et al., this volume). Com-pared with the carbonate-free fraction of the pelagicoozes, the nontronites are significantly depleted in Ca,Al, Mn, Li, Be, Sr, Ti, V, Cr, Co, Ni, Cu, Zn, Pb, and Pand enriched in K and Fe; Mg, SiO2, and Mo are neitherenriched nor depleted. The generally low trace-metalcontent and lack of significant correlations of minormetals with Fe (Table 5) suggest that the nontronites arevery pure precipitates and that trace elements are incor-porated within impurities rather than within the non-tronite itself. The elements Ca and Al also show little orno correlation with Fe. Donnelly (1980) has proposedthat K and Mg are essential components of the non-tronite structure and that their weak negative correla-tion with Fe and positive correlation with each othersuggest that in pure nontronite these elements may in-deed occupy definite crystallographic sites, which, how-ever, may also be occupied by Fe. The positive correla-tion of Fe with P confirms the close association of P

272

CHEMICAL COMPOSITION OF SEDIMENTS

Table 5. Correlation coefficients for elements in Leg 70 nontronites.

Mg Ca Al Mn Fe Li Sr Co Ni Cu Zn Pb

MgCaAlMnFeLiSrTiVCoNiCuZnPbPMo

0.53

-0.26

0.28

-0.250.58 0.46

0.580.930.700.390.510.63

0.720.39 0.43

0.32 0.33 0.51 0.79 0.580.65 0.75 0.49 0.83

0.31

0.66 0.420.51

Note: Sample population = 88; minimum significant correlation at 99% confidence level = 0.25.

with hydrothermal Fe phases (Froelich et al., 1977) al-though its mechanism of incorporation into nontroniteis not known. Of the other elements, Ti, V, Ni, and Cushow significant correlations with each other and withAl, suggesting their presence in included impurities rath-er than in the nontronite itself. The correlation betweenAl and Ti is particularly good (Fig. 2), and the Al/Tiratio is very similar to that in the pelagic oozes and othersample groups (Table 6). This suggests that little or noAl is actually incorporated within the nontronite struc-ture. A major feature of the nontronites is their veryconstant composition both with depth within single holesand between holes.

Determination of the Fe3 + /Fe2+ ratio was carried outon 15 nontronite samples. The results (Table 7) show thatsmall variations in this ratio do occur. In particular it isinteresting to note that the single brown sample (507D-2-1, 66-68 cm) had the highest Fe3 + /Fe2+ ratio of anyanalyzed, while the most intensely green sample (506-3-2, 6-8 cm) had the lowest.

Nontronite: Transitional SedimentThe nongranular pale green sediment which typically

occurs between pelagic ooze and granular nontronite

180 270Ti (ppm)

360 450

Figure 2. Scatter plot of Al vs. Ti in nontronites from all Leg 70mounds holes.

horizons in the mounds holes was not reported fromLeg 54, almost certainly because of the extensive drillingdisturbance resulting from rotary drilling. Because ofits position between "pure" granular nontronite and"pure" pelagic ooze, the transitional material is of par-ticular importance in interpreting the mechanism of for-mation of the hydrothermal sediments themselves. Theaverage composition of transitional sediments at eachmounds hole is given in Table 8. More variation occursfrom hole to hole in this sediment type than in pelagicoozes and pure nontronites. The average composition ofall transitional sediments recalculated on a carbonate-free basis is intermediate between that of pelagic oozes(on a carbonate-free basis) (Table 8) and pure non-tronite (Table 2), suggesting that the transitional sedi-ment is a mixture of these two sediment types. The hole-to-hole variation in transitional sediment composition isprobably largely the result of variations in the propor-tions of these two components.

Some horizons of pelagic ooze in mounds holes werecharacterized by green mottles (see site summaries). Sev-eral samples of this material were analyzed (Table 2).Their composition agrees with the notion that these mot-tles are areas of incipient transitional sediment forma-tion.

DISCUSSIONThe chemical data presented above enable us to ad-

dress some of the problems associated with the hydro-thermal mounds field. One of their most interesting fea-tures is that the surface pelagic sediment and much ofthe pelagic sediment at depth shows little or no evidenceof the hydrothermal activity which has given rise to themounds. The basal sediments in the mounds area showlittle evidence of the patterns of metal enrichment typi-cal of basal sediment on the EPR flanks. However, ifsuch sediments were at one time present in this area thenthe hydrothermal activity giving rise to the mounds hasaltered these sediments, perhaps by dissolving and re-mobilizing their initial hydrothermal component. Thefact that normal pelagic ooze capped all the moundsthat were cored suggests either that the mounds had notbeen active for some time, at least where drilled, or thatnontronite formation is largely a subsurface processwhich does not affect the surface pelagic sediment. Twofactors favor the latter theory. First, the presence of

273

S. A. MOORBY, D. S. CRONAN

Table 6. Al/Ti ratios in different sediment types at each site.

SedimentType 506 506B 506C 506D

Hole

507D 5O7F 5O7H 508 509 509BAll SitesAverage

Pelagic OozeSurface SedimentBasal SedimentTransitional

SedimentNontronitesMottlesMn-Oxides

23 21 23 21 22 21 20 20 19

22 — 23 — 22 22 — — — 21

— 28 — 21 21 — — — 30

Table 7. e2+ ratios in selected nontronite samples.

Sample(interval in cm)

506-2-2, 147-149506-3-2, 6-8506-3-3, 60-625O6C-3-1, 124-126506C-4-1, 38-40506C-4-1, 127-129506D-2-3, 72-74507D-2-1, 66-68507D-5-1, 104-1065O7D-6-1, 80-82509B-3-2, 135-137509B-4-2, 51-53509B-4-3, 40-42509B-4-3, 88-90

Fe Total(%)

20.018.616.820.019.720.219.321.522.616.620.517.420.417.0

Fe2 +(%)

0.771.680.770.840.420.700.910.211.121.140.780.850.630.60

F e3 + / F e 2 ^

26112224472921

102201526203228

Fe2 + as Woh of Total Fe

494.5423.551574533.5

Table 8. Average composition of transitional sediment from each holesampled.

Element

KMgCaAlSiO2

MnFe

LiSrPTiVCrCoNiCuZnPb

506(n = 22)

1.12.07.22.1

33.70.229.4

214004709701004311

11015018021

506C(n = 8)

1.42.22.62.5

37.10.35

10.0

18360470

110075531175

12015033

Hole

507D(n = 14)

0.932.07.01.9

37.90.089.7

211604108508447

84393

16014

5O7F(«=4)

0.732.30.83.3

37.80.099.7

(ppm)

35133620

1500160561290

13027024

5O9B(n = 10)

1.12.13.41.9

35.70.20

11.8

17380430910

8645

75084

15017

All Sites(CFB)

(n = 58)

1.22.45.42.5

41.00.22

11.4

24350530

1130109541188

13519524

PelagicSediments

(CFB)

0.82.1

273.7

44.90.83.8

252900133017302009127

260254424

52

Note: CFB = carbonate-free basis; n = no. of samples analyzed.

transitional sediment between pelagic ooze and granularnontronite horizons, both above and below the nontro-nite, strongly suggests that the nontronite has formed bythe subsurface replacement of existing pelagic ooze. Sec-ond, the formation of nontronite, either by direct pre-cipitation or by combination of previously precipitatedferric hydroxides with silica, requires conditions whichare slightly reducing or at least are not oxidizing (Har-der, 1976; Pedro et al., 1978). Formation of nontronitein the mounds, therefore, is likely to occur at depths atleast lower than the uppermost oxidized layer of pelagic

sediment. This evidence, then, strongly favors a subsur-face replacement origin for the nontronite in the mounds,as suggested by Williams et al. (1979), rather than thenontronite being a flash deposit, as suggested by Na-tland et al. (1979).

Although nontronite formation may proceed at depth,the formation of Mn-oxide crusts is only likely to occurat the sediment surface because of the need for oxidizingconditions to precipitate the Mn. Since all the Mn crustsfound on Leg 70 were covered by varying thicknesses ofpelagic ooze, it may be that the mounds that were coredhad indeed not been active for some time, at least in theimmediate vicinity cored. The Mn-rich sediment foundat Hole 509B is an interesting feature because this holecontains much more Mn-oxide crust, and at a greaterdepth, than the other mounds holes. It may be that ifMn-oxide crusts, initially formed at the mounds sur-face, subsequently become buried to sufficient depth,then they may become reduced and remobilized back tothe sediment-water interface where the process may berepeated. The Mn-rich mud above the Mn crusts at Hole509B may therefore represent a stage in this process.Such a process could be important since Fe2+, in ascend-ing hydrothermal solution, is capable of reducing Mn4+;this is therefore a possible mechanism both for reducingand remobilizing the Mn crusts and at the same time foroxidizing Fe2+ in ascending hydrothermal solutions toFe3 + , in which form it can precipitate out in nontronite.This process has been suggested by Honnorez, Von Her-zen et al. (1981) as a possible mechanism of mounds for-mation. While Mn oxides have been reported as beingextensive in the mounds field (Corliss et al., 1978; Wil-liams et al., 1979), only small amounts were recoveredon Leg 70 except at Hole 509B. This may be becauseduring the recycling of Mn discussed above, not all theMn is reprecipitated, some being lost to overlying bot-tom waters. If nontronite precipitation is to continue inthe absence of buried Mn-crust another oxidant for theFe2+ will be needed. Since the nontronite is highly gran-ular, it is possible that mounds showing appreciable to-pography could entrain some oxygenated bottom water,and this would enable nontronite precipitation to con-tinue even in the absence of buried Mn crust.

The buried nature of the Mn-oxide crusts and theinterfingering of the nontronite and pelagic ooze seen inthe mounds holes suggests that hydrothermal circula-tion in the mounds may be episodic in nature and thatperiods of normal pelagic sedimentation may be inter-spersed with pulses of hydrothermal activity which re-mobilize buried Mn crust and precipitate nontronite in

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the mounds interior. This process is similar to that pro-posed by Williams et al. (1979), although these authorssuggest that the process may be continuous.

Whatever the mechanism of precipitation of the de-posits, the nontronite is characterized by a very uniformchemical composition, both with depth within singlemounds and between widely spaced mounds. This indi-cates that the fluids from which they are formed origi-nate from a single large source which feeds the entiremounds field. The compositional variations which oc-cur in the Mn crusts may be a result of mobilization andredeposition after their initial precipitation.

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