17. iowaite in serpentinite muds at sites 778, 779, 780

Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 125

17. IOWAITE IN SERPENTINITE MUDS AT SITES 778, 779, 780, AND 784: A POSSIBLE CAUSE FORTHE LOW CHLORINITY OF PORE WATERS1

Dietrich Heling2 and Alexander Schwarz2

ABSTRACT

The <63-µm fractions of serpentinite muds from two seamounts on the Mariana and Izu-Bonin forearcs were analyzed for mineralcomposition by X-ray diffraction and for chemical composition by X-ray fluorescence. The silt fraction of the muds consistspredominantly of chrysotile, brucite, and ample amorphous constituents. Chlorite and smectite are less abundant components.

Of special interest is the occurrence of iowaite, a brucite-like, Cl-bearing mineral with a layered structure. Iowaite was notfound in the samples from the summit site of one of the seamounts drilled; however, it is scattered throughout the strata, composingthe flanks of both seamounts investigated. No systematic change of the iowaite abundance with depth was observed.

The distribution of iowaite is confined to the surface of the flanks of the seamount. Based on the distribution on the mineraland its chemical composition, we suggest that the iowaite formed by oxidation of some of the ferrous iron in brucite containedin the serpentine mud as it contacted abyssal seawater during protrusion onto the seafloor. The resulting positive charge impartedto the brucite was compensated by the uptake of seawater chloride. Consequently, the formation of iowaite is restricted to theseafloor where oxygen and chloride are available for these reactions. The availability of oxygen is considered the limiting factor.

We conclude that iowaite formation cannot be a major cause for the low chlorinity of pore fluids inside the seamounts.

INTRODUCTION

One of the drilling objectives of Leg 125 was to obtain additionalinformation on the composition of the pore fluids circulating throughserpentinite seamounts on the Mariana and Izu-Bonin forearcs (Fryeret al., 1990). The pore fluids recovered from cores of Sites 778-780and 784 (Fig 1.) are characterized by a strong decrease in chlorinityat shallow depths below seafloor (Fryer, Pearce, Stokking, et al.,1990; Mottl, this volume). Aboard the ship the interpretation of thisdramatic decrease in chlorinity was that the fluids most likely werederived from dehydration of hydrous minerals at depth (Fryer, Pearce,Stokking, et al., 1990). The dehydration reactions generate relativelyfresh water, which flows upward and decreases the chlorinity of thepore waters.

The chloride-bearing mineral iowaite was found by X-ray diffrac-tion (XRD) analysis of the mineral content of serpentinite mudsamples. The detection of iowaite prompted our interest in addressingthe question of whether the formation of iowaite might be a possiblealternative cause for the low chlorinity of the interstitial watersrecovered from the serpentinite seamounts.

METHODS

Sample Preparation

The silt fraction (<63 µm) was separated from the bulk sample bysieve and sedimentation techniques, eliminating the sand- and gravel-sized fraction of unaltered parent rock and coarser serpentinite. Thus,only the "matrix" fraction of the serpentinite muds was analyzed.

Because the serpentinite muds strongly tended to flocculate,sedimentation methods to separate a finer fraction often failed. Thesilt fractions of such samples were obtained by using a <63-µm sieve.Careful desalting by dialysis for 24 hr did not stop flocculationcompletely; however, unlimited dialysis could not be run withoutremoving water-soluble components from the samples; in particular,

'Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1992. Proc. ODP, Sci. Results, 125:College Station, TX (Ocean Drilling Program).

Institut für Sedimentforschung, Universita^ Heidelberg, Federal Republic ofGermany.

chloride. Thus, allowance was made for minor salt contents ratherthan risk the alteration of the chemical composition of the sample.

Most of the minerals were identified by XRD of orientedspecimens. Nonoriented specimens were prepared by filling a rectan-gular sample holder of 1-cm3 volume with the powdered samples.

X-ray Diffraction Analysis

X-ray diffraction was performed by a Siemens 500 automaticdiffractometer using Ni-filtered CuKα-radiation. Scanning speed withroutine runs was 17min. Air-dried, oriented samples were scannedfrom 1° to 40° 2a Samples suspected of containing expandable clayswere run again after glycolation and heat treatment. Nonorientedspecimens were scanned from 2° to 70° 2θ. Slower scanning speedsover particular ranges were used when the identification of a par-ticular mineral phase demanded higher resolution and precision.

Semiquantitative estimates of the abundances of the mineral specieswere based on the peak areas. The calculated abundances were revisedso that the sum of all identified components totaled 100%. When applyingthis standard procedure to a quantitative estimate of minerals, we mustmake allowances for large margins of error. Because of the lack of suitablereference samples of known abundances and, above all, because of largedifferences in the degree of crystallographic order of the differentminerals, an exact estimate with a precision of 1% cannot be obtained.This is true in particular for the minerals iowaite and chrysotile. Abun-dances of iowaite will probably be overestimated. On the other hand,abundances of serpentine minerals might be underestimated. Neverthe-less, the calculated abundances are reported even in numbers below 10.This is done to show the differences in composition between the samplesrather than to put major significance on the absolute values. If thesample preparation and machine settings are kept constant, differen-ces in abundances among the analyzed samples can be evaluated withsufficient reliability.

Mineral Identification

Clay minerals were recognized in general by their basal reflectionsin oriented samples. The various nonclay minerals were detected bytheir prominent reflections unless the reflections interfered withthose of other minerals present. Brucite was recognized by its peaks at

313

D. HELING, A. SCHWARZ

45#Ni i i i r T i i i i

CHINA

I25'E 130* 135* 140* 145*

Figure 1. Location of Site 786 in the Izu-Bonin forearc. Adapted from Fryer, Pearce, Stokking, et al. (1990).

150'

314

IOWAITE IN SERPENTINITE MUDS

4.77Å and 1.79Å. Chrysotile was identified by its strong basal reflec-tions at 7.31Å and 3.65Å and by its poly type-specific reflections at2.20Å and 2.09Å. It was also recognized by its fibrous habit inscanning electron micrographs (PL 1, Fig. 1). Antigorite was notfound and lizardite occurred only in traces.

Iowaite

In certain oriented samples, distinct reflections were observed at8.045Å and 3.985Å (Figs. 2 and 3). These peaks were assigned to a"magnesium-iron-hydroxychloride" which was discovered by Kohlsand Rodda (1967) in a Precambrian serpentinite from Sioux County,Iowa. At the location of discovery it was associated with chrysotile,dolomite, brucite, calcite, magnesite, and pyrite, all of whichpostdated the serpentinization of the rock.

Kohls and Rodda (1967) assigned the peak at 8.109Å (I = 100%)to the indication 003, and the peak at 4.042Å (I = 40%) to theindication 006. The next peak in line of the integral series was 009 at2.697Å with the very weak intensity of <l. The first and second peakof the mineral under investigation in this study agrees well with thedata of Kohls and Rodda (1967). The third peak at 2.697rÅ could notbe detected with certainty, probably because of its weak intensity. Thebasal spacing of iowaite is 24.33Å. The complete integral series ofbasal reflections was readily recognized and is proof of the layeredstructure of the mineral. The mineral was described as displaying agreasy luster, bluish in color, and having a soapy feel. All of theseproperties agree well with the mineral found in some serpentinesamples of Leg 125. Scanning electron micrographs show the platy,hexagonal structure of iowaite (PL 1, Fig. 2). Chlorite was recognized

by its weak 001 reflection at 14.30Å, which did not shift uponglycolation, and its much stronger 002 peak at 7.15Å. Smectite wasrecognized by its basal reflection at 12.62Å, which shifted to 17Åafter glycolation. Chlorite-smectite (corrensite) was identified byits reflections at 28.90Å and 14.20Å and by its swelling propertiesupon glycolation.

The chemical composition of iowaite derived from the analysis byKohls and Rodda (1967):

4Mg(OH)2FeOCl«H2O (n = 1-4).

An improved structural formula was given by Allmann and Donnay(1969) on the basis of their chemical analysis:

= 4,

with the actual composition expressed as:

[Mg0.775Feπ

0.o22Feiπ

0.2o3(OH)2]0 203+[Cl0.2o3(H20)o.6]

0 2 0 ^

According to this structural formula the mineral is composed oftwo subunits. The first consists of four brucite layers associated withone layer in which a ferric iron (Fe3+) ion is coordinated by twohydroxyls, yielding a positive charge. The second subunit consists ofone Cl-ion coordinated by three H2O molecules, resulting in onenegative charge by which the positive charge of the first subunit isneutralized. Kohls and Rodda (1967) found that the water is given offgradually as the temperature rises to 280° C, and the structure beginsto collapse at 315° C.

8-

I (003) 8.045 A

C (OO2)7.318A

(006) 3.985 A

C(004)3.653A

8.0 12.0 16.0 2θ70 2470 2IU) 3270 36.0

TWO - THETA (DEGREES)

Figure 2. XRD pattern of chrysotile and iowaite (oriented, air-dried), Sample 125-784A-36R-1, 43-45 cm.

) 0 4 0 . 0

315


"1 0 8.0 22.0 29.0 36.0 43.0 50.0

TWO - THETA (DEGREES)

57.0 64.0 71.0

Figure 3. XRD pattern of chrysotile and iowaite (nonoriented, air-dried), Sample 125-784A-36R-1, 43^-5 cm.

Carbonates

Most of the aragonite exceeded 63 µm in size and was separatedfrom the silt fraction used for mineral analyses. Traces that werepresent in the analyzed fraction were identified by reflections at 3.40Åand 1.97Å, but were not considered in the quantitative evaluation.Aragonite is visible in the sand fraction as needles up to 5 mm long.

Magnetite

Magnetite was identified by its distinguishing reflections at 2.53Åand 1.48Å, as well as by its magnetic properties.

X-ray Amorphous Constituents

Amorphous matter, present in most samples, was detected as ahump in the base line of the XRD patterns between 20° and 30° (2θ).We attempted a rough estimate of the abundances of the amorphousfractions by measuring the height of the hump. Because of the highNi-content determined by trace element analyses, the amorphousfractions probably contain predominantly garnierite, commonly as-sociated with chrysotile. Silica and alkali-earth-hydroxides arepresumed to be additional amorphous constituents of the "garnierite"fraction. Clay minerals in abundances below the limit of detectionmight also be included (Figs. 2 and 3).

X-ray Fluorescence Analyses

The compositions of both the major elements and the trace ele-ments of selected samples were analyzed by X-ray fluorescence(XRF) using a Siemens wavelength dispersive, sequential X-rayfluorescence spectrometer, Model SRS 300. Major element composi-tions were determined with melt tablets, produced by mixing thesample with 9 parts of lithium tetraborate (Spectromelt A1000). Traceelement concentrations were determined using pressed powdertablets. Loss on ignition was determined in the temperature rangebetween 105° C and 1050° C.

Thermogravimetric Analyses

Thermogravimetric (TGA) analyses were conducted using aDuPont Model 1090 thermoanalytic system, equipped withchromel/alumel thermoelements and a thermoanalytic balance, inorder to study the decomposition of iowaite by raising the tempera-ture. Only samples containing iowaite and chrysotile exclusively wereanalyzed by TGA. These minerals are easily distinguished by theirdifferent decomposition and conversion temperatures.

The first peak observed starts at 270° C and reaches its maximumat 293° C. At the latter temperature the chloride of the iowaite is givenoff (Fig. 4). Zeolitic water has been gradually released. The secondpeak appears at 315° C and displays its maximum at 340° C, probably

316

112

lββ••

104--

xn

100--

96 -

9 2 -

88 •

84--

605°C


-• 1 . 2

•• 1 . 0

- - 0 . 8

H 1 1 *

- - 0 . 6

-– 0. 4

100 200 300 400 500 600 700Temperature ( C)

—t ( 1 i T' 1--0. 2800 900 1000 1100

DuPont 1090

-Pσ>

- - 0 . 2 J.

-•0.0

Figure 4. TGA diagram showing the decomposition of iowaite and chrysotile, Sample 125-784A-36R-1, 43-45 cm.

caused by the emission of water from the brucite layers, leading tothe complete decomposition of iowaite. The third peak appears at500°C and reaches its maximum at 605° C, when the water of thechrysotile is released. The peaks assigned to the iowaite agree wellwith the differential thermoanalytic (DTA) data given by Kohlsand Rodda.

RESULTS

Mineralogical Composition

The results of the mineralogical analyses of the Sites 778, 779,780, and 784 are shown in Figures 5 through 8.

Chrysotile is the prevalent mineral in nearly all investigatedsamples. Lizardite occurs only in minor amounts and was not givenfurther consideration. Iowaite occurs in many samples from Sites 778,779, and 784, but was not found in samples from Site 780. Brucite ispresent in considerable amounts at Site 780. At all other sites it is aminor component and often absent. Apparently brucite is more abun-dant where iowaite is absent. We suspect garnierite is present in allsamples in moderate abundances. Aragonite occurs only in samplesfrom Conical Seamount. Magnetite occurs in the coarse fractionsof a few samples. Chlorite was found in some samples, but in smallamounts. Smectite was found in Sample 125-778A-11R-1, 56-58cm, and corrensite in Samples 125-778A-13R-1, 83-85 cm, and125-779A-36R-1, 80-82 cm.

Chemical Composition

The results of the major and trace element composition of theanalyzed samples are listed in Tables 1 and 2. The chemical composi-tion of the serpentine matrix is uniform. The major element composi-tion is characterized by high concentrations of SiO2, MgO, Fe2O3, and

a low concentration of A12O3. The SiO2/MgO ratio is near one, as ischaracteristic for serpentines. In samples containing only serpentineminerals the ratio is >l; in samples containing iowaite the ratio is <l.Because of the high water content of both serpentine and iowaite, theloss on ignition is high. The maximum values of 17.26% are found insamples containing iowaite. The trace element composition is markedby exceptionally high concentrations of Ni and Cr. Co, V, Zr, and Znare also relatively abundant. The concentrations of Rb, Th, and Y arebelow the detection limit of 5 ppm. The concentrations of Sr resultfrom the presence of aragonite, where Ca is partly substituted by Sr.

The locations of iowaite occurrences in the analyzed samples aregiven in Table 3.

DISCUSSION

The most interesting finding of our investigation is the occurrenceof the rare mineral iowaite. According to all the evidence found,iowaite is thought to have formed from brucite by uptake of chloride.This conclusion is consistent with the structural similarities of iowaiteand brucite. Moreover, the abundances of iowaite in the serpentinitemuds of the seamounts in the Mariana and Izu-Bonin forearcs corre-late approximately negatively with the abundances of brucite. Brucite[Mg(OH)2] generally contains some ferrous iron as a substitute forMg2+ in the layer lattice. As cited by Deer et al. (1962), the FeOcontent of brucite can reach a maximum of 10%. The authors suggestthat the greater the contribution of fayalite, the greater the Fe content.

When some ferrous iron in brucite is oxidized to the ferric stateby the oxygen in seawater, one positive charge per oxidized Fe2+ inthe brucite elementary cell is left unsatisfied. This positive chargewould readily be compensated by a Cl-ion from seawater if the C r isintroduced as an interlayer anion between the brucite layers. Iowaiteis formed by this uptake of chloride. Accordingly, the formation of

317


Table 1. Major element concentrations of selected samples from Sites 778,779,780, and 784.

Sample(interval in cm)

125-778A-1R-1, 57-60125-779A-2R-3, 88-90125-780C-1R-1,65-69125-784A-

34R-2, 106-10835R-1,98-10035R-2, 3 9 ^ 136R-1,43-4536R-1,64-6639R-1,60-6240R-1,48-5041R-1,48-5042R-1,71-7344R-1,76-7845 R-1,20-22

Depth(mbsf)

0.584.000.67

312.26320.28321.20329.72329.93358.50367.98377.78387.61407.06415.26

SiO2

38.5335.0738.92

39.9740.0041.6536.4835.0139.9140.2138.6637.6039.1539.27

MgO

37.4233.3635.96

36.4836.5537.3238.8438.2238.3937.8038.1938.4037.5538.03

Na2O

0.000.720.35

0.120.020.070.050.000.000.000.000.020.070.07

Fe2O3

7.276.457.91

8.966.927.707.628.217.146.337.137.356.806.68

MnO

0.100.080.12

0.310.210.120.100.100.070.090.110.100.090.09

Major elements (%)

TiO2

0.060.020.10

0.030.020.020.010.030.020.020.020.020.030.03

P 2 O 5

0.030.020.03

0.010.010.010.020.000.000.000.010.010.010.01

CaO

1.068.781.18

0.311.320.560.060.270.240.280.410.480.870.84

K2O

0.130.160.15

0.030.020.150.130.010.010.010.010.010.010.01

A12O3

0.480.000.86

1.001.070.000.001.210.001.041.231.161.331.26

LOI

14.7014.0013.50

13.2514.1012.3016.5017.2613.4613.8514.6715.1514.6214.11

Total

99.7899.1399.08

100.47100.2499.7299.81

100.32100.2499.63

100.44100.30100.53100.40

Note: LOI = loss on ignition.

Table 2. Trace element concentrations of selected samples from Sites 778,779,780,and 784.

Sample(interval in cm)

125-778A-1R-1, 57-60125-779A-2R-3, 88-90125-780C-1R-1,65-69125-784A-

34R-2, 106-10835R-1,98-10035R-2, 39-4136R-1, 43—4536R-1,64-6639R-1, 60-6240R-1,48-5041R-1,48-5042R-1, 71-7344R-1, 76-7845R-1, 20-22

Depth(mbsf)

0.584.000.67

312.26320.28321.20329.72329.93358.50367.98377.78387.61407.06415.26

Ni

151415871455

27682650173816212235270222322349210819321903

Cr

1086876921

650664

149815711309580886

2002160011331960

Trace elements (ppm;

Co

897090

11293

10298

12718713812210610495

V

524655

2929464630302931313129

Cu

986

188

<5<5105

<51212198

i

Zr

208615

8431161426202316141514

Sr

1311196

22

137140

9<510812911512110610499

Zn

442224

5645262877414137373434

Table 3. The occurrence of iowaite in the analyzed samples from seamountson the Mariana and Izu-Bonin forearcs.

Seamount Site Position Iowaite occurrence

Conical 778

779

780

Halfway up thesouthern flank

Halfway up thesouth-eastern flank

Near summit

TorishimaForearc

784 Western flank

In most samples between 0 and100 mbsf

In two of the samples analyzedfrom 0 to 300 mbsf

In none of the 13 samples takenfrom the depth interval from0 to 10 mbsf, and in none ofthe 11 samples from thedepth interval from 10 to160 mbsf

In four of 17 samples analyzedfrom 0 to 420 mbsf

Note: Fryer and Mottl (this volume) found iowaite in a subsample from Site 780 (summitof Conical Seamount) at 1.0 mbsf.

318


5 ~

SITE 778

Samplescore section interval

1 1 57-601 1 122-124

1 2 140-1431 3 67-701 4 23-261 4 104-1071 5 46-492 1 36-392 3 25-28

Chrysotile [%]20 40 60 80

i i i i

Garnierite20 40

i i

lowaite (%]20 40 60 80

i i i i

Brucite20 40

i i

Chlorite [%]20 40 60 80

i i i i

§10 -i"SiS 2 0

o<D

nCL

a

40

34

5

6

7

9

1212

13

CC

1

1

1

1

1

11

1

13-1513 -15

11 -13

65-67

56-58

86-88

56-58

28-3060-62

83-85

-

-

-

60 -

80

100 -

Figure 5. Mineral composition of serpentinite muds, Site 778.

iowaite depends on two processes: (1) some Fe2+ of the brucite has tobe oxidized to Fe3+, and (2) Cl" has to be supplied to balance thelayer charge.

Because of the abundance of methane in the serpentine muds,reducing conditions should prevail inside the seamounts. No oxygenis contained in the pore fluids. Thus, the oxygen for the oxidation ofthe ferrous iron of brucite is supplied by seawater. According toMantyla and Reid (1983) the O2 concentration of the abyssal seawateron the Mariana forearc is near 100 mg/L. The chloride that must beincorporated with brucite to form iowaite could be supplied byseawater when muds protrude onto the seafloor. If this deduction istrue, iowaite is formed on the seafloor and therefore could be foundon the flanks of the seamounts where the brucite-bearing serpentinemud protruded and mixed with seawater as it flowed down the flanks.Our samples of the muds on top of the seamount lack iowaite. Fryerand Mottl (this volume) found iowaite in one subsample from Site 780(near the summit of Conical Seamount) at 1.0 mbsf. The abundanceof the iowaite in this sample was not specified. We suspect that thisiowaite formed by reacting with seawater chloride that migrated downto 1.0 mbsf by diffusion, possibly at a time when the rate of protrusionof serpentinite muds was low. Generally, however, the adventive,pressure-induced flow of seawater down into the diapir must beconsidered unlikely. The fluid vents observed at the seafloor (Fryeret al. 1987) prove that the pressure gradient is directed upward similarto the thermal gradient or the flow of compaction fluids.

The conclusion that seawater does not seem to penetrate deeplyinto diapiric seamounts is in accordance with Haggerty's (1987)interpretations, deduced from the chemical and isotopic compositionof aragonite found in serpentinites from Mariana forearc seamounts.Haggerty concluded that fluids other than normal seawater migrate

through the sediments. According to Haggerty's hypothesis, iowaitecould form only in oxidizing environments. Therefore, iowaite cannotform unless the muds have contact with seawater. The iowaite foundtoday in strata below the seafloor is supposed to have been formedearlier (i.e., before the iowaite-bearing strata were covered byyounger flows). The oxidizing potential on the seafloor is con-sidered the limiting factor in iowaite formation. Other factorsmight be the duration of exposure on the seafloor, grain size, ormixture with seawater.

Iowaite is sometimes associated with brucite, but brucite is usuallyabsent where iowaite is the prevailing mineral phase. Where theaccompanying brucite is missing, it might be totally transformed toiowaite. Where brucite is present, the oxygen supply might not havebeen sufficient to completely transform to iowaite.

It is possible that some chloride is included in the serpentinitemuds in the course of the serpentinization process (Rucklidge, 1972).However, as long as only chloride, but no oxygen, is added to thebrucite-bearing serpentine muds, iowaite cannot be formed.

CONCLUSIONS

Iowaite, a rare, brucite-like, chloride-bearing mineral was foundirregularly distributed in the serpentinite mud flows of two seamountsof the Mariana and Izu-Bonin forearcs. The mineral probably formedfrom brucite produced along with serpentinites at greater depths.

The oxidation of ferrous iron to ferric iron in brucite, accompaniedby the uptake of Cl~ to balance the charge, forms iowaite. The essentialoxygen is thought to be supplied by seawater. Accordingly, iowaite isformed after the brucite-bearing, serpentine muds have protrudedonto the seafloor. The oxidation of the ferrous iron generates a layer

319


0 -i

5 -

core

12

2

222

Samples

section interval1 71-741 61-63

2 99-1013 88-903 138-1404 38-40

Chrysotile [%]

20 40 60 80i i i i

SITE

Garnierite

20 40i iI

I

779

Iowaite [%]

20 40 60 80I I ! I

Brucite

20 40i i

Chlorite [%]

20 40 60 80! I I I

§10"δ<Dco 50

100-

α>Q

4

13

1819

23

3233

3637

1

1

11

1

11

11

50-52

66-68

51 -5339-41

60-62

72-7434-36

80-8223-25 -

150-i

2 0 0 -

2 5 0 -

300


charge, which is compensated by chloride, taken up from the seawater.

The oxidation of ferrous iron and the abundance of brucite are

considered to be the limiting factors in iowaite formation.

The distribution of the analyzed iowaite seems to be independent

of depth. Therefore, it is unlikely that iowaite is formed diagenetically

(i.e., under considerable overburden). The independence of depth

further shows that the temperatures in the sampled depth intervals do

not affect the iowaite stability. In accordance with the data obtained

in this study, we conclude that the formation of iowaite cannot be

interpreted to be a major cause for the low chlorinity of the fluids from

the investigated seamounts.

ACKNOWLEDGMENTS

The financial support of Deutsche Forschungsgemeinschaft

through grant number He 532/9-1, which made this study pos-

sible, is gratefully acknowledged. The comments of P. Fryer and

two anonymous reviewers on an earlier draft helped improve the

paper considerably.

Deer, W. A., Howie, R. A., and Zussman, J., 1980. An Introduction to theRock-forming Minerals (Vol. 5): London (Longman Group Ltd.).

Fryer, P., Haggerty, J. A., Tilbrook, B., Sedwick, P., Johnson, L. E., Saboda,K. L., Newsom, S. Y., Karig, D. E., Uyeda, S., and Ishii, T., 1987. Resultsof studies of Mariana forearc serpentinite diapirism. Eos, 68:1534.

Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1990. Proc. ODP, Init. Repts.,125: College Station, TX (Ocean Drilling Program).

Haggerty, J. A., 1987. Petrology and geochemistry of Neogene sedimentaryrocks from Mariana forearc seamounts: implications for emplacement ofthe seamounts. In Keating, B., Fryer, P., and Batiza, R. (Eds.),Seamounts, Islands, and Atolls. Am. Geophys. Union., AGU Monogr.Sen, 43:175-185.

Kohls, D. W., and Rodda, J. L., 1967. Iowaite, a new hydrous magnesiumhydroxide-ferric oxychloride from the Precambrian of Iowa. Am.Mineral, 52:1261-1271.

Mantyla, A. W., and Reid, J. L., 1983. Abyssal characteristics of the WorldOcean waters. Deep-Sea Res. Part A, 30:805-833.

Rucklidge, J., 1972. Chlorine in partially serpentinized Dunite. Econ. Geol,67:38^0.

REFERENCES

Allman, R., and Donnay, J.D.H., 1969. About the structure of iowaite. Am.Mineral., 54:296-299.

Date of initial receipt: 1 October 1990Date of acceptance: 8 October 1991Ms 125B-176

320


0 -i

5 -

SITE 780

Samples

core section interval

i

1 85-69 —~?1 75-77 ~-/.i 100-102 -/V1 134-138 V2 48-SO •s /

///3 96-100 -////,I 3 125-127 -///l,

3 138-140^/74 5β-58 J4 83 - 86 J

2 CC • J

1 20-22 -/

Chrysotile [%]20 40 60 80

i i i i

Garnierite20 40

—

lowaite [%]20 40 60 80

i i i i

Brucite20 40

i i

Chlorite [%]20 40 60 80

i i i i

O10-

oα>n 50

α>Q

100

1 5 0 -

4

7

8

9

12

15

16

18

1

1

1

1

1

1

1

1

17-19

16-18

10-12

2 - 4

9-11

19-21

27-30

129-131

—


321


300-i

O350"JSα>w

α>

a.α>Q

400

SITE 784

Samples

core section interval

34 2 106 -108

35 1 98-10&.

36 1 43 - 4 5 - ^

37 1 68-70

38 1 98-100

39 1 19-21.39 1 38-40-^;39 1 60 - 6 2 ^39 1 135-13740 1 4 8 - 5 0 ^

41 1 48-50

44 1 76-78

45 1 20 - 22-^_45 2 101-104^

Chrysotile [%]

20 40 60 80I I I I

Garnierite

20 40i i

i I

I i

lowaite [%]

20 40 60 80I I I !

Brucite

20 40i i

Chlorite [%]

20 40 60 80I I I !


322


10 µm

Plate 1. Scanning electron micrographs of serpentinite muds. 1. Relict structure of an olivine crystal, overgrown by chrysotile fibers, Sample 125-780B- 1R-1,65-67 cm. 2. Plate structure (arrow) and hexagonal symmetry (small arrow) of iowaite, Sample 125-784A-36R-1, 43-45 cm.

323

17. iowaite in serpentinite muds at sites 778, 779, 780

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