34. BASALT WEATHERING ON THE EAST PACIFIC RISE AND THE GALAPAGOSSPREADING CENTER, DEEP SEA DRILLING PROJECT LEG 54

Susan E. Humphris,1 Department of Geology, Imperial College of Science and Technology, Prince Consort Road,London SW7 2BP, England

William G. Melson, Department of Mineral Sciences, Smithsonian Institution, Washington, D.C.and

Robert N. Thompson, Department of Geology, Imperial College of Science and Technology,Prince Consort Road, London SW7 2BP, England

ABSTRACT

The mineralogical and geochemical changes during the earlystages of weathering of two suites of basalts from the East PacificRise and from the hydrothermal mound area near the GalapagosSpreading Center have been investigated. Three compositionalgroups of smectites were observed: an FeO * and K2O-rich variety, anMgO-rich type most closely resembling saponite, and a group withcompositions intermediate between them. Other secondary mineralsincluded carbonates, particularly an Mn-rich variety, hydrated ironoxides, and minor pyrite.

Geochemical comparison of the weathered rims and fresh inte-riors of individual samples indicated an increase in the K2O and Rbcontents and Fe2O3/FeO ratio, and a slight decrease in the CaO con-tent in the weathered rims, even though their H2O

+ contents wereoften less than 1 per cent.

The similar weathering processes that appear to have affectedboth suites of rocks, and the absence of any secondary minerals thatare formed exclusively under hydrothermal conditions in the samplesfrom the hydrothermal mounds area, would indicate that either cir-culation may be confined to a few narrow fractures — therebyrestricting high-temperature alteration to the margins of basalts ad-jacent to the fractures — or that most of the hydrothermal mineralsare formed at greater depths in the basaltic column.

INTRODUCTION

Interactions between sea water and rock at varioustemperatures in the oceanic crust have recently beenstudied in attempts to assess their importance for ele-mental fluxes into and out of the oceans (e.g., Thomp-son, 1973; Humphris and Thompson, 1978; Donnelly etal., in press; and Humphris et al., in press). Most ofthese studies have been confined to rocks dredged ordrilled in the Atlantic Ocean. Although our knowledgeof mineralogical and chemical variations during reac-tions with sea water is improving, our understanding ofalteration processes as a function of age, basalt chem-istry, and tectonic setting is very limited.

The purpose of this paper is to investigate the earlystages of weathering (i.e., less than 3 m.y.B.P.) in twosuites of samples recovered during Leg 54 which origi-nated at different spreading centers in the Pacific Ocean

1 Present address: Sea Education Association, Woods Hole, Mas-sachusetts.

— the East Pacific Rise and the Galapagos SpreadingCenter. The basalts erupted at these two spreading cen-ters are chemically distinct from each other (Fodor etal., Srivistava et al., and Humphris et al.; all this vol-ume) and from Mid-Atlantic Ridge basalts.

In addition, the samples studied also represent sever-al different tectonic settings. Three major structural fea-tures were drilled in the region of the East Pacific Risenear 9°N (Figure 1): the East Pacific Rise flank fabric(Sites 420, 421, 423, and 429), the east-west trendingOCP Ridge (Sites 422 and 428), and the Siqueiros frac-ture zone (Site 427). The Galapagos Spreading Centerdrilling included a north-south transect across a hydro-thermal field 22 km south of the spreading center, whichhad previously been surveyed by Klitgord and Mudie(1974) and Lonsdale (1977) — Site 424. The basalts wereoverlain by hydrothermal sediments and so could havebeen subjected to a thermal regime different from thatof the other sites. Site 425 was located 62 km north ofthe Galapagos Spreading Center in an area character-ized by high heat flow.

773

m10°N

9"N

SIQUEIROS FRACTURE ZONE

PT-4 SITES(419-423,426 429)

105°W 104"W

Figure 1. The location of sites drilled on Deep Sea Drilling Project Leg 64 near the East Pacific Rise and Galapagos Spreading Center (inset).

BASALT WEATHERING

ALTERATION MINERALOGY

Weathering processes affecting all the rocks re-covered on Leg 54 have generally been confined to theinfilling of veins and vesicles by clay minerals and car-bonates, and to the formation of surface coatings onsamples from both the East Pacific Rise and the Gala-pagos Spreading Center. In addition, many of thesamples exhibit a well-defined rim, which is usually 1 to3 cm wide and pale grey in color. Minor alteration ofphenocrysts and groundmass phases and infilling ofvesicles appear to have been responsible for this zona-tion. These weathered rims indicate that many of therock surfaces have been exposed to sea water, suggest-ing that the oceanic crust in this area consists of highlyjointed flows or small pillows.

Surface CoatingsClay minerals constitute the most abundant surface

coating and are found lining depressions in rock sur-faces and infilling vugs in samples from the East PacificRise and the Galapagos Spreading Center. They areusually pale green or blue in color and form coatings ofup to 1 mm thick. Scanning electron microphotographsindicate that these surface coatings are composed of acomplex network of ramose (branching, twig-like) struc-tures which project from the rock surface (Plate 1, Fig-ure 1). Details of these ramose structures are shown inPlate 1 (Figure 2), which illustrates the radiating and in-terwoven nature of the clay mineral plates. Several sam-ples of this surface coating were removed for X-ray dif-fraction study. Glycerol saturation of oriented samplesresults in expansion of the basal spacing from 14Å to17.5Å, which subsequently collapsed on heating. A 060peak of 1.533Å was observed, indicating that this smec-tite is trioctahedral.

The smectites are often associated with later-stagecarbonates. Large vugs up to 1.2 cm in diameter (in Sec-tion 427-9-2, Piece 4) show two generations of calcite,and calcite veins are common in many samples. X-raydiffraction and qualitative analysis using an energy dis-persive X-ray spectrometer (EDAX) were used to iden-tify a manganese-rich calcite. The latter occurred as acream, globular surface deposit found on samples fromHole 427 in the Siqueiros fracture zone (Plate 2, Figure1). Scanning electron microphotographs (Plate 2, Fig-ures 2 and 3) indicate that these globules are composedof parallel columns of stacked platelets. X-ray diffrac-tion data are given in Table 1 and, although the majorcalcite peaks are clearly distinguishable, the whole pat-tern is slightly displaced owing to the manganese substi-tution. A manganese-rich carbonate has been observedreplacing plagioclase phenocrysts in highly weatheredbasalts from the Bermuda Rise (Humphris et al., inpress), but its existence as a surface deposit has thus farnot been reported from ocean floor basalts.

Hydrated iron oxides cover the surface of manysamples, and these are often associated with glassy mar-gins of pillows or flows that are partially weathered.Pyrite occurs in small surface depressions (e.g., Section

TABLE 1XRD Data for Manganese-Rich Calcitea

hkl

104018116113202110

d Spacing (Å)

2.9581.8681.8122.2242.0352.462

Intensity

Very strongStrong

Medium (broad)Medium

WeakVery weak

Section 427-9-2, Piece 5; RadiationFeKα.

427-11-1, Piece 3) or in vesicles (e.g., Section 425-9-3,Piece 5), but is extremely rare.

Basalt Alteration

It should be emphasized that all of the samples showonly the early stages of weathering — i.e., infilling ofsome vesicles and veins, very occasional phenocryst al-teration, and minor patchy replacement of the ground-mass. Generally, less than 2 per cent of the rock hasbeen affected, and this is confined to the outer wea-thered rim. Basalts from Site 425 near the GalapagosSpreading Center appear to be the most altered, with upto 4 volume per cent being affected. Samples with doler-itic textures (i.e., from the OCP Ridge Sites 422 and428, and from some sections of the Siqueiros fracturezone, Site 427) appear to have been less affected by wea-thering (less than 1 vol. %) than the quenched basalts.Alteration of phenocrysts is observed in only a fewsamples, and olivine appears to be the most affectedphase. It is pseudomorphed by fine-grained aggregatesof green smectite, which are often associated with hy-drated iron oxides (eg., Section 427-11-1, Piece 4), oroccasionally with cubes of pyrite (e.g., Section 428A-1-1, Piece 13). This smectite varies in color from brightgreen to yellow-green, and exhibits blue-green inter-ference colors. In Hole 424C, drilled on the GalapagosSpreading Center, the olivine is replaced by a low bire-fringent, orange-brown smectite. In Section 429A-1-1,Piece 5, some plagioclase phenocrysts are replaced bywhite to pale green clay minerals with an infilling ofcalcite, but plagioclase alteration did not occur at anyother site.

The groundmass of many basalts contains smallpatches of clay minerals. Bright green smectites are themost abundant, but light brown and orange-brown clayminerals do occur. The orange-brown smectites alsoform narrow rims outlining crystals. A relatively com-mon feature is color zonation of the larger patches fromgreen rims to light brown to red in the centers. The inte-riors are opaque and dark red, and resemble hydratediron oxides. However, attempts to analyze these bymicroprobe indicate that they probably consist of inti-mate mixtures of iron oxides and clay minerals. Calciteoccurs as a minor secondary mineral replacing thegroundmass, particularly in samples from Sites 428 and427, where it may or may not be associated with smec-tites.

775

S. E. HUMPHRIS, W. G. MELSON, R. N. THOMPSON

Vesicles commonly contain secondary minerals in theweathered rims of the basalts, but are usually empty inthe interiors. They are filled with: (1) bright green smec-tites; (2) light brown smectites; (3) a fine-scale zonationof green rims and brown interiors; (4) bright orange-brown smectites, similar in appearance to those rim-ming crystals; (5) dark red mixtures of hydrated iron ox-ides and smectites; (6) green or brown smectite liningwith a core of chalcedony (Site 429A); and (7) in Section425-7-1, Piece 6, a green rim infilled with white radiat-ing needles of a zeolite. Different vesicle fillings occurwithin a single sample. However, the proportion oforange-brown and light brown smectites to green smec-tites appears to increase toward the margin of thesamples. This may be related to the oxidation state ofthe iron in the smectite and not to a compositionalchange.

Representative microprobe analyses of the clay min-erals from both the East Pacific Rise and the GalapagosSpreading Center are reported in Table 2; each result isthe average of at least five analyses. For all sites, themajor type of smectite is enriched in FeO* (expressingtotal Fe) and K2O. This occurs as pseudomorphs afterolivine, replacing the groundmass and infilling vesicles.Its color varies from bright green to light brown and itscomposition is generally characterized by low A12O3, 26to 30 per cent FeO*, 1 to 5 per cent MgO, and up to 6per cent K2O content. The bright orange-brown smectitethat rims crystals and replaces the groundmass containsa higher concentration of iron — up to 36 per cent inSection 429A-1-1, Piece 5. Two other distinctly differ-ent compositions can be observed. The white to palegreen smectite replacing the plagioclase phenocrysts ap-pears to resemble saponite most closely in composition(Table 2, analyses 20 and 21), and contains 22 to 24 percent MgO and 5 to 6 per cent FeO*. This mineral wasidentified only in Hole 429A, and only in this mineral-ogical setting.

The other compositional variation observed is inter-mediate between the Fe-rich and Mg-rich types, and thissmectite occurs as a bright green vesicle filling in SampleSection 428A-3-1, Piece 11 (Table 2, Analysis 8). It con-tains 22 per cent FeO* and 14 per cent MgO, and mayrepresent either a fine-grained mixture of two smectitesor a mixed-layer clay mineral.

All the microprobe data from this study are plottedon a triangular SiO2-FeO*-MgO diagram, with theweight percentages of K2O plotted on a vertical axis(Figure 2). These are compared with clay mineral com-positions from 110-m.y.-old basalts from the BermudaRise (Humphris et al., in press) and from 40-m.y.-oldbasalts from the Peru Trench (Scheidegger and Stakes,1977). This comparison shows that Fe- and K-enrichedsmectites are the most abundant type of clay mineralfound in the basalts studied from both the Atlantic andPacific Oceans. However, the clay minerals found in theEast Pacific Rise and Galapagos Spreading Center ba-salts are generally more Fe-rich than those from the Ber-muda Rise. This could be related either to the higherFeO* content of the more evolved Pacific basalts, whichmay influence the composition of the fluid from which

the clay minerals formed, or to later cation exchange ofthe clay minerals from the Atlantic.

The "saponite-type" smectites replacing phenocrystsin Hole 429A are some of the most Mg-rich clay miner-als analyzed from ocean floor basalts, and their forma-tion is probably the result of rigidly localized physicaland chemical conditions. A separate compositional groupintermediate between the Mg-rich and Fe-rich end mem-bers is comprised of smectites from the Peru Trench andfrom the Bermuda Rise basalts. The absence of these in-termediate smectites in the young East Pacific Rise andthe Galapagos Spreading Center basalts suggests they areformed at a later stage of alteration, either by precipita-tion from a fluid of different composition or by cationexchange in pre-existing clay minerals.

GEOCHEMISTRY

Although all of the basalts recovered during Leg 54are less than 5 m.y. old and appear relatively fresh, theearly stages of alteration and formation of clay mineralsshould be reflected in changes in the bulk chemistry.Thirteen samples, for which the weathered rims previ-ously discussed could be separated from the fresh cores,were selected for X-ray fluorescence (XRF) analyses.This approach permits direct comparison of the com-position of the weathered rims with that of their freshprecursors; hence, it eliminates errors involved in as-suming the composition of the fresh rock. Precision andaccuracy of the analytical methods used are discussed inHumphris et al. (this volume). Details of the lithology,texture, and petrology of the selected samples are pre-sented in Table 3.

Major- and selected trace-element analyses are pre-sented in Table 4. Although most of the H2O+ contentsare less than 1 per cent, with only three rims showinghigher concentrations, it is clear that some of the oxidesand elements, particularly Fe2O3, K2O, and Rb, showsignificant alteration effects.

To make comparisons of the fresh interiors to thealtered rims more meaningful, it is necessary to normal-ize the data. This can be accomplished by taking theratio of each oxide to one that is relatively unaffected byweathering, such as alumina or titania. If the concen-trations of both these oxides are unchanged by weather-ing, then their ratio in the fresh sample interior shouldbe equal to that in the altered rim. This is illustrated inFigure 3, which demonstrates that the ratio of A12O3/TiO2 remains virtually unchanged during alteration ofthese samples. Thus, either A12O3 or TiO2 could be usedfor normalization; we will, however, use A12O3 in thisdiscussion since it is present in higher concentrations.

The most significant variations in chemical composi-tion are observed in the iron concentrations. All thesamples indicate that there is an increase in the total ironconcentration during weathering (Figure 4), althoughthis increase is not as pronounced in the more crystal-line samples from the OCP Ridge and the GalapagosSpreading Center. Mineralogically, this is exhibited bythe high iron content of the smectites replacing thegroundmass and forming pseudomorphs after olivine.Oxidation has also been an important weathering pro-

776

cess — the Fe2O3 content of the rims is greater than thatof the cores (Figure 5) — and the ferrous componentshows only minor variations in concentrations (Figure6). This would indicate that important changes in theconcentrations of oxides occur during the earlier stagesof weathering, which would not be evident from a bulkchemical analysis of a rock and its H 2O+ content. Suchvariations, particularly in the iron concentration, mayaffect conclusions drawn from norm calculations andfrom magma fractionation modeling.

The slightly lower CaO contents of most of the al-tered rims in comparison with the fresh cores (Figure 7)

BASALT WEATHERING

are in agreement with the decrease in calcium generallyobserved in more highly weathered basalts (e.g., Hart,1970; and Thompson, 1973). Strontium also shows a de-crease in concentrations in the weathered rims. Theslightly higher CaO contents of the rim shown by two ofthe samples are probably a consequence of either minorreplacement of the groundmass by calcite or of the in-corporation of calcite veinlets into the sample.

The concentrations of MgO show some scatter aboutthe line and, in general, appear to show only minor vari-ations (Figure 8). However, higher MgO contents in therims are indicated for a few samples. Since there is no

TABLE 2Representative Microprobe Analyses of Clay Minerals from Basalts of the East Pacific Rise and the Galapagos Spreading Center

Wt. %Oxide

SiO2

A12O3

FeO* a

MgOCaOK2ONa2O

TiO 2

MnO

Total

Wt. %Oxide

SiO2

Al 2 θ3FeO* a

MgOCaOK2ONa 2OTiO 2

MnO

Total

l b

49.706.17

26.142.910.245.360.140.100.05

90.83

14°

49.970.97

30.243.720.375.990.030.030.05

91.37

2C

47.463.44

27.753.040.095.880.170.060.09

87.97

15P

49.890.65

28.743.990.286.330.010.030.04

89.97

3 d

47.204.61

27.702.970.185.840.060.050.12

88.73

161

49.220.59

30.033.570.406.120.070.040.05

90.10

4 e

47.914.14

26.933.080.225.900.090.100.10

88.46

17r

42.581.04

34.297.890.532.580.100.030.10

89.15

5 f

48.453.30

28.133.150.166.010.040.070.07

89.39

18 s

43.290.45

35.585.380.293.770.060.030.07

88.91

68

48.370.07

31.913.360.844.660.050.030.04

89.32

19*

40.100.77

35.519.420.461.230.050.040.08

87.66

7 h

50.840.22

28.204.740.644.730.030.030.03

89.46

20 u

50.443.626.19

22.830.250.270.050.060.05

83.77

81

47.253.04

22.4714.20

1.510.520.070.070.45

89.58

21 V

50.953.265.70

23.600.200.300.090.040.06

84.22

9J

49.890.32

28.374.530.724.380.040.030.04

88.31

22 W

52.603.77

26.942.390.523.100.070.050.15

89.60

10 k

47.463.12

31.641.830.303.410.160.020.09

88.04

23 X

53.632.99

27.492.360.402.770.070.050.15

89.91

I I 1

51.044.72

28.351.830.792.870.470.150.15

90.36

24y

51.203.82

28.501.480.502.520.080.060.05

88.22

1 2 m

48.574.32

29.152.080.263.930.210.050.08

88.65

25Z

52.654.70

28.931.560.832.130.110.070.09

91.05

1 3 n

53.620.43

22.616.540.314.960.010.020.13

88.63

2 6 a a

51.540.42

24.285.450.376.070.100.030.06

88.32

2 7 b b

52.030.36

27.874.200.406.270.090.030.08

91.33

a FeO* = total iron expressed as FeO.bGreen clay mineral associated with hydrated iron oxide pseudomorphing olivine phenocryst: Section 427-11-1, Piece 4;CGreen clay mineral replacing groundmass: Section 427-11-1, Piece 4;^Green clay mineral pseudomorphing olivine: Section 427-11-1, Piece 4;eGreen clay mineral after olivine: Section 427-11-1, Piece 4;fYellow-green vein filling: Section 427-11-1, Piece 4;IGreen clay mineral replacing groundmass: Section 428A-3-1, Piece 11;^Green clay mineral replacing groundmass: Section 428A-3-1, Piece 11;*Green smectite infilling vesicle: Section 428A-3-1, Piece 11;JGreen smectite replacing groundmass: Section 428A-3-1, Piece 11;^Orange-brown smectite replacing groundmass: Section 424C-2-1, Piece 4;^Orange-brown smectite pseudomorphing phenocryst: Section 424C-2-1, Piece 4;mOrange-brown clay mineral replacing groundmass: Section 424C-2-1, Piece 4;"Interior of vesicle: Section 429A-1-1, Piece 5;°Outer rim of same vesicle: Section 429A-1-1, Piece 5;PGreen smectite infilling small vesicle: Section 429A-1-1, Piece 5;QGreen vesicle filling: Section 429A-1-1, Piece 5;rOrange-brown clay mineral replacing groundmass: Section 429A-1-1, Piece 5;sBrown clay mineral replacing groundmass: Section 429A-1-1, Piece 5;tßrown clay mineral replacing groundmass: Section 429A-1-1, Piece 5;uWhite-pale green smectite replacing plagioclase phenocryst and associated with calcite: Section 429A-1-1, Piece 5;vPale green smectite replacing plagioclase phenocryst: Section 429A-1-1, Piece 5;wGreen smectite infilling vesicle: Section 421-3-1, Piece 20;xGreen vesicle filling: Section 421-3-1, Piece 20;vGreen smectite replacing groundmass: Section 421-3-1, Piece 20;zGreen smectite replacing groundmass: 54-421-3-1-20;aaGreen rim of vesicle infilled with zeolite: Section 425-7-1, Piece 6; andbbGreen vesicle filling: Section 425-7-1, Piece 6.

777

S. E. HUMPHRIS, W. G. MELSON, R. N. THOMPSON

r1ù Wt. % K2O

70/-

t . t

t

50 40 30

Fed*

1050

Figure 2. SiO2-FeO*-MgO triangular diagram, with wt. % K2Oplotted on a verticalaxis, presenting the composition of secondary clay minerals found in some weath-ered oceanic basalts (FeO expresses total Fe). = clay minerals from the EastPacific Rise and Galapagos Spreading Center (this study); = clay mineralsfrom the Bermuda Rise, Atlantic Ocean (Humphris et al., in press); = clayminerals from the Peru Trench (Scheidegger and Stakes, 1977).

evidence for uptake of significant quantities of MgO inthe compositions of the smectites from these samples,these variations in the MgO content may be a reflectionof inhomogeneous distribution of primary phases with-in the thin flows or pillows.

Previous studies have shown that K2O is generallytaken up during basalt weathering, and it has been sug-gested that this process may be an important sink forpotassium in the oceans (e.g., Melson and Thompson,1973; Donnelly et al., in press; and Humphris et al., inpress). The weathered rims of the young basalts fromthe East Pacific Rise and the Galapagos SpreadingCenter also show overall increases in the K2O content ofthe rocks (Figure 9) with concomitant increases in theRb concentrations. This is the result of formation of theK- and Fe-enriched smectites in these samples. The K2Oconcentrations observed in the fresh interiors of themore crystalline samples are lower than those found inthe finer grained basalt. This may indicate that weather-ing has affected the interiors of the fine-grained basaltsas well as the rims, whereas the crystalline cores have re-tained a K2O content closer to their original levels.Ocean floor basalts clearly provide a sink for K andbegin to remove this element from sea water during thevery early stages of weathering. These results suggestthat the bulk rock K2O content is affected by the initialweathering processes which consist mostly of clay min-eral formation in vesicles, and which may affect only 1per cent of the rock. Many of the basalts dredged ordrilled from the ocean floor contain smectite-filled vesi-cles, suggesting that the early stages of weathering are acommon feature. These observations would thereforesupport the contention that weathering can be an impor-tant sink for potassium in the oceans.

Variations in Na2O concentrations are small andshow no consistent trends, suggesting that the earlystages of weathering do not alter the Na2O contents ofthe basalts (Figure 10). This is in contrast with studies ofmore highly weathered basalts, in which uptake ofNa2O has been observed (Hart, 1970).

DISCUSSION AND CONCLUSIONSThe major type of alteration in the basalts from Leg

54 is the formation of smectites. There is no evidencefor their absolute temperature of formation, but thelack of zeolites and chlorite suggests that alteration oc-curred predominantly at low temperatures. Smectiteformation, the presence of calcite, and the increase inthe Fe2O3/FeO ratio in the weathered rims indicate thatalkaline, oxidizing conditions (which could be providedby sea water) prevailed during alteration of these rocks.The occurrence of chemically similar smectites from theEast Pacific Rise, the Galapagos Spreading Center, andthe Bermuda Rise suggests that K- and Fe-enrichedsmectites can be formed regardless of variations in ba-salt chemistry. If these smectites are typical of the earlystages of low-temperature weathering of basalts, thiswould indicate that the chemical variations in the com-position of the basalts are initially unimportant. Thismay be explained by the observation that olivine and theinterstitial material are usually the first to be altered. Itis not until other phases, such as plagioclase and pyrox-ene, are altered, that the effects of variations in basaltchemistry are detected. For instance, the smectites fromthese young basalts contain very little alumina.However, it is likely that as weathering advances, andplagioclase is altered, the smectites formed will be morealuminous.

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BASALT WEATHERING

TABLE 3Samples Subjected to Bulk Chemical Analyses of their Fresh Interiors and Weathered Rims

Specimen Location Lithology Texture

Petrographic Notes

Fresh Interior Weathered Rim

Section 420-14-1, EPR flank fabricPiece 3

Section 421-3-1, EPR flank fabricPiece 14

Section 421-3-1, EPR flank fabricPiece 18

Section 421-4-1, EPR flank fabricPiece 5

Sample 423-5- EPR flank fabricCC-1

Section 425-7-1, 62 km north ofPiece 6 Galapagos Spreading

Center

Section 425-9-3, 62 km north ofPiece 5 Galapagos Rift

Section 428A-2-1, South side of OCPPiece 2 Ridge

Section 428A-6-1, South side of OCPPiece 6 Ridge

Section 428A-6-1, South side of OCPPiece 14 Ridge

Section 429A-2-1, EPR flank fabricPiece 8

Section 429A-2-1, EPR flank fabricPiece 10

Section 429A-2-1, EPR flank fabricPiece 16

Interior of highlyjointed flow

Fragment of highlyjointed thin flow

Fragment of highlyjointed thin flow

Fragments of jointedflow or pillow

Edge of thin,basaltic flow

Aphyric; fine-grained Plagioclase, clinopyrox-ene, minor olivine, glass

Fine-grained; iso-lated pcs with glo-merophyricclusters

Fine-grained;glomerophyric

Glomerophyric

Fine-grained;variolitic

Interior of cooling Intersertalunit

Interior of cooling Holocrystalline,unit equigranular

Interior of thin flow Cryptocrystalline,massive

Interior of slowlycooled unit

Interior of slowlycooled unit

Pillow basalt orhighly fractured thinflow

Pillow basalt orhighly fractured thinflow

Pillow basalt orhighly fractured thinflow

Equigranular

Equigranular

Fine-grained; vario-litic to intersertal

Fine-grained; vario-litic to intersertal

Fine-grained; vario-litic to intersertal

Plagioclase and olivinephyric

Plagioclase and olivineglomerocrysts.Empty vesicles

Plagioclase, olivine andclinopyroxene. Somevesicles filled

Plagioclase, clinopyrox-ene; dominantly glassymatrix

Plagioclase, clinopyrox-ene, with minor glass.Vesicles empty

Plagioclase, clinopyrox-ene and opaques

Plagioclase, clinopyrox-ene and minor olivine

Plagioclase, clinopyrox-ene and opaques

Plagioclase, clinopyrox-ene and opaques

Plagioclase phyric withplagioclase, clinopyrox-ene and opaques

Plagioclase phyric

Plagioclase phyric withplagioclase, clinopyrox-ene and glass

Green smectite patchilyreplacing groundmass.Zoned vesicles.Weathering - 2%

Green and brown smec-tites replacing ground-mass and filling vesicles.Weathering < 1%

Patchy replacement ofgroundmass. Filledvesicles.Weathering < 1%

Patchy groundmassalteration. Vesiclesfilled with green smec-tites or zoned.Weathering - 1%

Green and orange-brown smectites fillvesicles and replacegroundmass. Sulfides.Weathering ~ 2%

Green and brown smec-tites. Sulfide minerali-zation.Weathering - 1%

Green clay mineralsreplacing groundmass.Weathering - 1 %

Groundmass patchilyreplaced by smectites,sometimes with sulfide.Weathering < 1%

Patchy groundmassreplacement by brownsmectites.Weathering - 1 %

Green-brown smectitesreplacing groundmass.Weathering - 1 %

Green smectites replac-ing groundmass andinfilling vesicles.Weathering < 1%

Green smectites replac-ing groundmass andinfilling vesicles.Weathering < 1%

Green smectites replac-ing groundmass andinfilling vesicles.Weathering - 1 %

The lack of any secondary minerals formed exclusive-ly under hydrothermal conditions at Site 424 in thehydrothermal mounds area near the Galapagos Spread-ing Center has important implications for circulation offluids through the oceanic crust. Although overlain byhydrothermal sediment deposits, the samples recovered

show only traces of weathering, resembling those at allother sites. There are two possible explanations for thisobservation. The first is that even in highly jointedflows such as those that constitute the crust in this re-gion, circulation may be confined to a few narrow frac-tures, thereby restricting reactions to a thin margin of

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S. E. HUMPHRIS, W. G. MELSON, R. N. THOMPSON

TABLE 4Core and Rim Chemical Analyses for Individual Samples Recovered During Leg 54

Specimen: 423-5-CC, Piece 1 428A-2-1, Piece 2 428A-6-1, Piece 6 428A-6-1, Piece 14 421-3-1, Piece 14

Outer Outer Outer OuterCore Rim Core Rim Core Rim Core

OuterRim Core

OuterRim

421-3-1, Piece 18

CoreOuterRim

SiO2a

A12O3

Fe2O3

FeOMgOCaONa2OK 2 0TiO2

P2O5MnOH 2 0 +

CO 2

Total

RbbSrYZrNb

50.8113.994.827.186.72

10.522.940.482.050.230.200.820.07

100.83

————

51.1814.432.078.016.84

11.053.090.312.140.220.210.590.01

100.15

-———

50.8014.655.635.416.99

11.512.860.501.510.160.160.650.01

100.88

9122

2596

4

51.5615.71

3.155.276.85

12.383.300.221.580.160.160.300.09

100.73

——_-

49.7415.074.175.667.73

11.702.880.271.380.160.160.610.09

99.62

5127

259ü

4

50.6315.472.616.537.67

12.222.870.151.450.140.160.670.15

100.74

—-_—

50.7014.994.995.577.24

11.962.950.431.470.150.170.870.12

101.61

10121

2795

4

50.5215.23

3.326.067.38

12.512.940.161.480.150.160.550.27

100.73

1127

2993

4

50.4114.005.386.676.46

10.732.820.522.060.210.200.910.07

100.44

811242

1326

51.5514.63

3.426.386.64

11.583.010.352.240.220.210.670.09

100.99

113548

1417

49.8813.835.366.786.38

10.732.890.572.120.190.201.020.21

100.16

_--——

51.0214.54

3.836.426.54

11.403.000.342.250.230.200.410.49

100.67

_---—

10 12

(AI2θ3/TiO2) interior

14 16

Figure 3. Al2O3/TiO2 ratios for the weathered rims andfresh interiors of basalts from the East Pacific Riseand the Galapagos Spreading Center. The solid linerepresents equal ratios in the rim and interior.

basalt adjacent to those fractures. This would indicatethat hydrothermal alteration affects only a very smallpercentage of the oceanic crust, while low-temperaturereactions affect a much larger volume of basalt.

The second explanation is that most of the hydrother-mal minerals are formed at greater depths in the basaltcolumn, and so were not recovered during drilling.

0.6 0.7 0.8

(FeO*/AI O ) interior

0.9

O = East Pacific Rise basaltsΔ OCP Ridge basaltsO = Galapagos Spreading Center basalts

Figure 4. FeO*/Al2O3 ratios in altered rims andfresh interiors, where FeO* expresses total Fe.

780

BASALT WEATHERING

TABLE 4 - Continued

421-4-1,

OuterRim

50.5413.685.377.056.22

10.642.870.482.180.200.210.670.11

100,22

410846

1426

Piece 5

Core

50.9314.422.367.856.12

10.833.050.362.270.200.160.510.09

99.15

013649

1464

420-14-1,

OuterRim

50.8414.207.185.385.83

10.782.850.671.960.190.171.450.06

101.56

_

—

—-

Piece 3

Core

52.1415.07

2.296.206.40

11.563.000.312.040.170.200.470.08

99.93

013542

1345

429A-2-1,

OuterRim

50.3014.215.016.286.85

11.362.800.501.650.150.180.690.07

100.05

9111

30103

6

Piece 8

Core

52.1615.03

1.656.487.00

11.643.090.231.710.190.190.770.05

100.79

___-

429A-2-1,

OuterRim

50.5914.195.146.286.87

11.202.870.511.640.160.190.890.11

100.64

_

_

-

Piece 10

Core

52.3215.052.176.137.00

11.953.160.311.700.170.190.690.04

100.88

—__-

429A-2-1,

OuterRim

50.5114.195.726.106.86

11.082.920.501.660.210.181.270.05

101.25

_

__

-

Piece 16

Core

51.6515.02

2.536.107.05

11.863.010.271.700.180.190.600.04

100.20

_

—__

—

425-7-1,

OuterRim

51.5713.634.897.726.95

10.812.450.431.390.180.190.600.01

100.82

8543274

4

Piece 6

Core

51.8214.18

2.698.737.39

11.502.390.051.430.130.200.340.13

100.98

1573577

1

425-9-3,

OuterRim

50.8514.61

3.776.867.85

12.012.180.340.980.110.170.800.06

100.59

---—

Piece 5

Core

51.5314.73

1.677.818.16

12.512.260.071.020.100.190.530.12

100.70

2552443

2

aOxide concentrations in wt. %.^Concentrations in ppm.

0.6

Figure 5. Fe2O3/Al2O3 ratios for weathered basalts ofthe East Pacific Rise and the Galapagos SpreadingCenter.

ε 0.5-

o_CN

<

OCM

0.4 -

o.;

1—

oo

/

Δ /

oo /

Δ

—1

\

/

/

D

1

V

o

/f

α

i

0.3 0.4 0.5

interior

0.6

Figure 6. FeO/Al2O3 ratios in altered rims and freshbasalt interiors.

Although hot hydrothermal fluids could percolate up-ward through the basalt, the highly fractured surfacelayers permit considerable mixing and dilution of thesefluids by sea water, although some of the fluids mustreach the sea floor to deposit the hydrothermal sedi-ments. Hence, most of the evidence for hydrothermalcirculation will be masked in the surface crustal layersby low-temperature rock-water interactions.

781

S. E. HUMPHRIS, W. G. MELSON, R. N. THOMPSON

0.85

0.80 -

CO

O

<oo

0.70 0.75 0.80

interior

0.85

Figure 7. CaO/Al2O3 ratios in basalts displaying wea-thered rims.

0.55

E 0.50

CO

O

<o

0.45

0.400.40

Figure 8. MgO/Al2O3 ratios in basalts from the EastPacific Rise and the Galapagos Spreading Center.

One of the most remarkable features of the alterationmineralogy and associated chemical variations is thesimilarity between all the sites, even though they repre-sent different thermal and tectonic settings. This sug-gests there is an initial set of reactions that ocean floorbasalts undergo after eruption, regardless of the prevail-ing environmental conditions. If this is the case, theK-and Fe-enriched smectites represent the initial re-sponse of the cooling basalt to reaction with sea water.These early alteration processes must therefore occurvery shortly after the emplacement of the basalts on thesea floor.

0.05

0.01 0.02

(K2O/AI2O3) interior

0.03

Figure 9. K2O/A12O3 ratios in basalts, showing uptakeof K2O during the early stages of weathering.

0.140.14 0.16 0.18 0.20

(Na2O/AI2θ3) interior

0.22

Figure 10. Na2O/Al2O3 ratios in weathered basalts fromthe East Pacific Rise and the Galapagos SpreadingCenter.

782

BASALT WEATHERING

ACKNOWLEDGMENTS

We wish to thank I. L. Gibson for permission to use theXRF equipment at Bedford College, and G. F. Marriner forassistance. The British Museum (Natural History) providedthe CHN analyzer for H2O

+ and CO2 determinations. Thanksare also due to R. Curtis, P. Grant, and D. Bailey forassistance in the laboratory at Imperial College, and to EugeneJarosewich and Charles Obermeyer for help in running theelectron microprobe at the Smithsohian Institution.

This research was supported by NERC Grant No. GR3/2946.

REFERENCES

Donnelly, T., Thompson, G., and Salisbury, M. H., in press.The chemistry of altered basalts at Site 417, Deep Sea Drill-ing Project Leg 51. In Donnelly, T., Francheteau, J.,Bryan, W., Robinson, P., Flower, M., Salisbury, M., etal., Initial Reports of the Deep Sea Drilling Project, v. 51,52, 53, Part 2: Washington (U.S. Government Printing Of-fice).

Hart, R., 1970. Chemical exchange between seawater and deepocean basalts. Earth Planet. Sci. Letts., v. 9, p. 269.

Humphris, S. E. and Thompson, G., 1978. Hydrothermalalteration of oceanic basalts by seawater. Geochim. Cos-mochim. Act, v. 42, p. 1017.

Humphris, S. E., Thompson, R. N., and Marriner, G. F., inpress. The mineralogy and geochemistry of basalt weather-ing, Holes 417A and 418A. In Donnelly, T., Francheteau,J., Bryan, W., Robinson, P., Flower, M., Salisbury, M., etal., Initial Reports of the Deep Sea Drilling Project, v. 51,52, 53, Part 2: Washington (U.S. Government Printing Of-fice).

Klitgord, K. D. and Mudie, J. D., 1974. The GalapagosSpreading Centre: a near bottom geophysical survey. Geo-phys. J. Roy. Astr. Soc, v. 38, p. 563.

Lonsdale, P., 1977. Deep-tow observations at the moundsabyssal hydrothermal field, Galapagos Rift. Earth Planet.Sci. Letts., v. 36, p. 92.

Melson, W. G. and Thompson, G., 1973. Glassy abyssal ba-salts, Atlantic sea floor near St. PauFs rocks: petrographyand composition of secondary clay minerals. Bull. Geol.Soc. Amer., v. 84, p. 703.

Scheidegger, K. F. and Stakes, D. S., 1977. Mineralogy, chem-istry and crystallisation sequence of clay minerals in alteredtholeiitic basalts from the Peru Trench. Earth Planet. Sci.Letts., v. 36, p. 413.

Thompson, G., 1973. A geochemical study of the low temper-ature interaction of seawater and oceanic igneous rock.Trans. Am. Geophys. Un., v. 54, p. 1015.

783

S. E. HUMPHRIS, W. G. MELSON, R. N. THOMPSON

PLATE 1Section 427-9-2, Piece 9A

Figure 1 Pale blue clay mineral composed of a complex net-work of branching structures lining a vug.Magnification: ×300

Figure 2 Detail of the branching structures illustrating theradiating and interwoven nature of the clay miner-al plates.Magnification: ×1500

784

BASALT WEATHERING

PLATE 1

' » • .

785

S. E. HUMPHRIS, W. G. MELSON, R. N. THOMPSON

PLATE 2Section 427-9-2, Piece 12

Figure 1 Cream, globular deposit of manganese-rich calciteon rock surface.

Figure 2 Detail of surface deposit exhibiting parallel col-umns of stacked platelets.Magnification: ×860

Figure 3 Finer detail of manganese-rich calcite.Magnification: ×2400

786

PLATE 2

BASALT WEATHERING

1cms

787