INTRODUCTION SECONDARY MINERALS IN …30. SECONDARY MINERALS IN BASALT FROM THE COSTA RICA RIFT, HOLES 501 AND 504B, DEEP SEA DRILLING PROJECT LEGS 68, 69, AND 701 Victor B. Kurnosov,

30. SECONDARY MINERALS IN BASALT FROM THE COSTA RICA RIFT, HOLES 501 AND 504B,DEEP SEA DRILLING PROJECT LEGS 68, 69, AND 701

Victor B. Kurnosov, Igor V. Kholodkevich, and Valerii M. Chubarov, Far-East Geological Institute,Vladivostok, U.S.S.R.

andAlla Ya. Shevchenko, Institute of Oceanology, U.S.S.R. Academy of Sciences, Moscow, U.S.S.R.

ABSTRACT

Basalt samples recovered during DSDP Legs 68, 69, and 70 from a 550-meter-thick section in two holes near theCosta Rica Rift (Holes 501 and 504B) were found to contain the following secondary minerals: trioctahedral and dioc-tahedral smectite, chlorite, mixed-layer clays, talc, hematite, pyrite, foujasite, phillipsite, analcime, natrolite, thom-sonite, gyrolite, aragonite, calcite, anhydrite, chalcocite, Fe-hydrosilicate, okenite, apophyllite, actinolite, cristobalite,quartz, and magnesite. A less positive identification of bismutite was made. A mineral rich in Mn and minerals withstrong reflections at 12.9 Å and 3.20 Å remain unidentified.

Trioctahedral smectite replaces glass and olivine in the basalt groundmass. The other secondary minerals occur inveins. The distribution of the secondary minerals in the basalt section shows both hydrothermal and oxidizing-nonoxi-dizing zonation.

Most of the secondary minerals formed under alkaline, nonoxidizing conditions at temperatures up to 120° C. Anacidic regime probably existed in the lowest portion of basalt. Oxidative diagenesis followed nonoxidative diagenesis inthe upper part of the section. Oxidative diagenesis is characterized by the absence of celadonite, rare occurrences of di-octahedral smectite, and widespread hematite and phillipsite.

INTRODUCTION

Secondary minerals in basalts drilled from Holes 501and 504B near the Costa Rica Rift were identified andanalyzed, and an attempt was made to infer the physicaland chemical conditions under which they formed.

METHODSSecondary minerals were identified chiefly by X-ray diffraction

analysis. Infrared spectral analysis, electron microscopy, scanning elec-tron microscopy (SEM), and electron diffraction were also used. Chem-ical compositions were determined by wet chemical analysis and elec-tron microprobe techniques. Secondary minerals in basalt groundmasswere studied in the < 1 µxn and 1 to 10 µm fractions.

SECONDARY MINERALS IN BASALTGROUNDMASS

Trioctahedral smectites are the principal secondaryminerals in the basalt groundmass. Talc, which was foundin Sample 504B-5-2, 142-146 cm, seems to have comefrom a thin veinlet. Minor chlorite was identified in thegroundmass of basalt from Hole 501 only (Table 1).

Smectites, which formed by the alteration of inter-stitial glass and olivine, were identified by X-ray diffrac-tion analysis in all samples studied. The basal reflection(001) varied from 13.6 to 15.2 Å, and the ò-parametervaried from 9.18 to 9.21 Å (Sample 504B-7-2, 64-68cm, 9.19 Å; 504B-24-1, 125-129 cm, 9.18 and 9.21 Å;504B-70-1, 17-20 cm, 9.21 Å). As shown in Figure 1,the ib-parameter suggests that the smectites are triocta-

Table 1. Occurrence of secondary minerals in basalt groundmass fromCosta Rica Rift, Holes 501 and 504B, Legs 68, 69, and 70.

Sample(interval in cm)

501-10-2, 10-1212-1, 71-7314-4, 69-7115-1, 109-11115-3, 134-13617-3, 18-2020-1, 39-41

504B-3-1, 96-994-1, 62-644-3, 108-1114-4, 42-454-5, 87-905-2, 142-14686-1, 65-696-3, 3-77-2, 64-688-3, 61-669-1, 142-14710-3, 17-2012-1, 39-4313-4, 40-4415-1, 24-2816-1, 45-4916-4, 22-2617-1, 30-3418-1, 45-4919-1, 84-8720-1, 110-11321-2, 126-13021-5, 63-6622-1, 106-10924-1, 125-12826-1, 15-1827-2, 55-5828-3, 69-7332-2, 137-14033-1, 86-8834-1, 140-14235-1, 70-7237-1, 15-2039-2, 97-10247-2, 130-14049-1, 122-12661-2, 145-14967-1, 45-4970-1, 17-20

Sub-bottomDepth (m)

265.70273.81288.29293.19296.44305.28328.49278.96280.64284.08284.92286.87290.92299.15301.53309.64320.11326.92337.67361.89366.40375.24384.45385.72393.30398.45403.84413.10423.76427.63431.06449.25463.15468.05478.68510.37517.36526.90535.30552.65572.97640.80657.22753.40800.45827.17

< l µm

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Saponite

1 to 10 µm

TiS

TV

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Cann, J. R., Langseth, M. G., Honnorez, J., Von Herzen, R. P., White, S. M., et al.,Init. Repts. DSDP, 69: Washington (U.S. Govt. Printing Office).

Note: T denotes trace.a Trace of talc(?).

573

V. B. KURNOSOV, I. V. KHOLODKEVICH, V. M. CHUBAROV, A. YA. SHEVCHENKO

9.23

9.20

9.17

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I9.10

9.05

9.01

Saponites

0 o o

0 α π +

IMontronites and Fe—smectites

""ffioπtmorillonites

Color

O Gray• Black (crust)© Black (film of clay)

Dark green with silk lusterα Light greenQ Red brown• Yellow brown -+ Clay minerals from

basalt groundmass

300 400 500 600Sub-bottom Depth (m)

700 800

Figure 1. ^-parameter of clays from veins and basalt groundmass, Hole 504B, Costa Rica Rift, Legs 69and 70, plotted versus depth below the seafloor.

hedral. The smectites consist of isometric fragments ofdifferent size and thickness (Plate 1, Fig. 1) or (rarely)elongated lamellae and needles.

Chemical analysis of the < 1 µm fraction derived frombasalt shows that the smectite is a low-K Fe-Mg-saponite(Table 2). The composition of the groundmass saponitechanges slightly through the basalt section. From about470 meters sub-bottom (Sample 504B-27-2, 55-58 cm)downward, the smectite decreases in MgO and increasesin CaO and Na2O content. The very lowest sample (Sam-ple 504B-70-1) has high levels of Fe2O3 and Fe2O3 + FeOand the lowest K2O content.

Chemical analysis shows that the < 1 µm fractiondoes not consist entirely of saponite; this fraction alsocontains Plagioclase and volcanic glass.

MINERALS IN VEINS AND VESICLES

The following minerals were identified in basalt veinsand vesicles: saponite, Fe-smectite, chlorite, talc, mixed-layer minerals, hematite, pyrite, phillipsite, foujasite,

analcime, natrolite, thomsonite, gyrolite, aragonite, cal-cite, anhydrite, Fe-hydrosilicate, chalcocite, okenite,apophyllite, actinolite, cristobalite, quartz, bismutite(?),and magnesite (Table 3). A mineral rich in Mn and min-erals with strong reflections at 12.9 Å and 3.20 Å couldnot be identified.

Clay MineralsClays from the veins and vesicles are trioctahedral

smectites (saponites) of diverse color—black and darkgreen, gray, dark green with a silk luster, light green,yellow brown, and red brown (Fig. 1). The ö-parameterof these clays ranges from 9.18 to 9.23 Å. Minor diocta-hedral smectite (b = 9.02, 9.04, and 9.06 Å) was foundin the light green, yellow brown, and red brown clays.

The black and dark green clays in the veins are low-KFe-Mg-saponites (Table 4). The MgO content of thesevarieties ranges from 12 to 20%, the Fe2O3 + FeO con-tent from 10 to 17%. Most particles are isometric inform, but some lath-shaped fragments were identified in

Table 2. Wet chemical composition (wt.%) of saponites from the < 1 µm fraction of basalt fromCosta Rica Rift, Legs 69 and 70.

Component

SiO2TiO2A12O3Fe2O3

FeOMnOMgOCaONa2OK2OL.O.I.a

Total

504B-4-1(62-64)

43.120.83

13.998.951.960.15

11.861.760.270.17

504B-7-2(64-68)

47.500.087.30

10.001.310.08

19.500.920.260.09

13.29

100.33

504B-10-3(17-20)

47.460.78

10.548.823.110.12

15.453.080.680.129.68

99.84

504B-15-1(24-28)

47.720.369.06

10.561.310.08

16.832.020.560.07

11.68

100.25

Sample (interval in cm)

504B-24-1(125-128)

45.860.69

11.009.281.960.18

16.364.710.650.19

504B-27-2(55-58)

48.140.35

12.376.672.220.13

13.925.071.130.229.38

99.80

5O4B-35-1(110-112)

44.530.77

15.059.841.440.128.919.031.140.198.83

99.85

504B-49-1(122-126)

45.050.88

13.5310.931.180.10

11.144.800.590.12

504B-61-2(145-149)

46.260.71

11.908.221.960.11

12.316.640.960.07

10.4699.60

504B-70-1(17-20)

45.581.00

10.0413.151.050.09

12.103.540.870.04

12.91

100.37

a Loss on ignition.

574

SECONDARY MINERALS IN BASALTS

Table 3. Summary of secondary minerals identified in veins and vesicles of basalts from the Costa RicaRift, Legs 68, 69, and 70.

Sample(interval in cm)

501-10-2, 10-1215-1, 109-11115-3, 134-13617-3, 18-2019-1, 26-28

5O4B-3-1, 96-994-1, 62-644-4, 42-454-5, 87-905-2, 142-1465-3, 40-556-1,65-696-3, 3-77-2, 64-687-3, 61-658-3, 61-669-1, 142-14710-3, 17-2011-1, 61-6412-1, 39-4313-4, 40-4415-1,24-2816-1, 45-4916-4, 22-2617-1, 30-3418-1,45-4919-1, 84-8720-1, 110-11321-2, 126-13021-4, 116-12021-5, 63-6622-1, 106-10923-1,32-3524-1, 125-12824-3, 125-12925-2, 62-6626-1, 15-1827-2, 55-5828-3, 69-7329-1, 31-3432-2, 137-14033-1, 86-8833-2, 14-1634-1, 140-14234-2, 18-2135-1, 70-7235-1, 110-11235-2, 14-1636-2, 25-2737-1, 15-2037-1, 55-6037-2, 75-7837-3, 14-1738-1,7-1539-2, 97-10244-1, 125-13047-2, 130-14047-3, 36-4048-1, 113-11549-1, 122-12654-1,35-3856-2, 92-9561-1, 55-6061-2, 145-14764-1,65-6866-2, 0-568-1, 83-8769-1, 77-8170-1, 17-20

e g gSub-bottom £ J IDepth (m) U X

265.70 y y293.19296.44 y305.28 y319.36 y278.% y280.64 y284.92 y286.87290.92 y292.90299.15301.53309.64311.11320.11326.92337.67 y344.11361.89 »366.40 y375.24 y384.45 y385.72393.30 *398.45403.84 .413.10423.76426.66427.63431.06 *448.32 .449.25 .452.52459.12 v463.15468.05 <•478.68484.31 *510.37 *517.36518.14 .526.90527.18 *535.30535.60 *536.14 y545.25552.65 .553.05 .554.75 >555.64 >561.57572.97 t612.25 »640.80641.36648.13657.22 *692.35707.92751.05753.40773.65792.50809.83 ,818.77 .827.17

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a Smectite and mixed-layer chlorite-montmorillonite.

Sample 504B-29-1, 31-34 cm. The structure of smectitein black clay is shown in Tables 2 and 4.

Analyses were made of green saponite clays that oc-cur as globules in veins in Samples 501-15-1,109-111 cmand 504B-16-1, 45-49 cm. The clays are saponites with acontent of MgO up to 22% and a maximum K2O contentof 0.33% (Table 5). Black clay located along the walls ofa vein in Sample 501-15-1, 109-111 cm has higher ironand alumina contents, lower magnesium content, andsignificantly higher K2O content (2.4%) (Table 5).

Green clays in veins from the zeolite zone (Samples504B-34-2, 18-21 cm; 504B-35-2, 14-16 cm; and 504B-38-1, 7-15 cm) were also studied. In Sample 504B-34-2,

18-21 cm a brecciated glassy crust is composed of fineblocks of glass altered to smectite. These blocks consistof dark green saponite with 18% MgO and 0.63% K2O(Table 5). The centers of the blocks consist of lightergreen Fe-Mg-saponite with 4.17% K2O. In the samesample there is a white smectite vein at the glassy crust/basalt contact. Its chemical composition is as follows:SiO2, 50.13; TiO2, 0.01; A12O3, 8.93; FeO, 6.26 (total Feas FeO); MnO, 0.17; MgO, 19.83; CaO, 1.25; Na2O,0.94; K2O, 1.12%; total, 88.64%.

In Sample 504B-35-2, 14-16 cm, first green and thendark green (Fe-MgO-saponite) clays occur as one pro-ceeds from the central part of a vein (which consists of

575


Table 4. Chemical composition (wt.%) of saponites from clay veins of basalts fromthe Costa Rica Rift, Legs 69 and 70.

Component

SiO2TiO2A12O3Fe2O3

FeOMnOMgOCaONa2OK2OL.O.I.a

Total

504B-5-2(142-146)

Light Green

45.520.226.204.957.200.13

19.281.79

5O4B-1O-3(17-20)

Gray

47.600.136.204.557.030.09

21.230.751.710.20

10.18

99.67

5O4B-16-1(45-49)

Light Green

1.490.33


504B-32-2(137-140)

504B-32-2(137-140)

Color of Clay

Black

6.8014.172.94

1.250.62

Black

39.470.419.93

11.605.890.13

12.562.251.320.40

15.69

99.65

5O4B-49-1(122-126)

Black

14.255.934.91

1.120.19

504B-61-1(55-60)

Dark Green

44.970.085.803.206.050.07

20.352.130.950.02

11.1199.74

504B-70-1(17-20)

Black

13.109.732.78

1.710.03

a Loss on ignition.

Table 5. Chemical composition of clay minerals from veins of basalt, Costa Rica Rift, Legs68, 69, and 70.

Component

SiO2

TiO2

A12O3FeOh

MnOMgOCaONa2OK2O

Total

501-15-1(109-111)

(a)

44.660.025.349.130.06

22.030.641.200.33

83.41

501-15-1(109-111)

(b)

43.430.78

13.5615.770.18

10.190.670.222.46

87.24

5O4B-16-1(45-49)

(c)

52.720.014.369.190.02

19.030.341.950.11

87.72


5O4B-16-1(45-49)

504B-34-2(18-21)

Color of Clay

(d)

49.040.034.85

10.470.03

18.640.522.360.03

86.02

(e)

53.350.49

11.0314.090.19

10.551.730.184.17

95.77

504B-34-2(18-21)

(0

46.440.73

13.2011.490.41

18.481.970.160.63

93.52

5O4B-35-2(14-16)

(0

42.980.027.02

12.520.15

15.100.551.464.48

84.41

504B-35-2(14-16)

(g)

46.001.54

11.7616.650.16

13.120.790.421.91

92.35

504B-38-1(7-15)

(g)

43.120.16

11.9011.510.25

13.512.480.400.60

83.95

a Green (globules from center of vein).b Black (basaltward from center of vein).<• Light green (rim of green globule).d Green (center of globule).e Green into dark green.* Dark green.8 Green.h Total Fe as FeO.

natrolite and calcite) toward basalt. Of the two clays,the dark green clay is richer in K2O (4.48%) and Na2O(1.46%) (Table 5). The green clay has higher aluminaand iron contents, slightly lower MgO content, and low-er K2O (1.91%) and Na2O (0.42%) content.

In Sample 504B-38-1, 7-15 cm, green clay occurs inassociation with analcime, calcite, and gyrolite. It con-sists of Fe-Mg-saponite with a K2O content of 0.60%(Table 5).

The gray clay is another Fe-Mg-saponite with lowK2O content (0.20%). MgO content is 21.23%, andFe2O3content is 4.55% (Table 4). Particles are lathshaped or ribbonlike (Plate 1, Fig. 2). Under the scan-ning electron microscope, the gray clay shows a rosettetexture (Plate 2, Figs. 1 and 2). Traces of chlorite werealso identified in this clay (Samples 504B-15-1, 19-1 cmand 21-2 cm).

The dark green clay with a silk luster is also saponitewith lath-shaped particles. Its natural texture is shownin Plate 2, Figure 3.

The light green clay consists of saponite, and its chem-ical composition is close to that of the gray clay (Ta-ble 4). Infrared patterns of light green clay show onlysaponite reflections. Particles are lath shaped (Plate 1,Fig. 3).

The yellow brown clay is similar to the light greenclay.

The red brown clay consists of saponite and finelydispersed hematite, with traces of dioctahedral Fe-smec-tite (ö-parameter, up to 9.06 Å). The clay particles areisometric in shape (Plate 1, Fig. 4).

A comparison of the chemical composition of theclay minerals in the veins and basalt groundmass showsthat the clay minerals in the veins are usually higher inMgO, lower in Fe2O3 and A12O3, and sometimes higherin K2O (Samples 504B-5-2, 504B-10-3, 504B-29-1, and504B-61-1). Light green clay in a vein in the oxidizedzone (Sample 504B-5-2) is not distinguished by its chem-ical composition.

Other Minerals

All the secondary nonclay minerals that were foundare enumerated in the preceding material. They wereidentified by X-ray diffraction analysis (Joint Commit-tee on Powder Diffraction Standards, 1974).

The unknown mineral rich in Mn in Sample 501-10-2,10-12 cm was also analyzed by microprobe techniques(Table 6). The central part of the mineral contains19.84% MnO (P-trace) and 2.53% MgO. Narrow zonesanalogous in composition to the central part of the min-

576


Table 6. Chemical composition(wt.%) of a Mn-rich mineralfrom basalt of Costa Rica Rift,Sample 501-10-2, 10-12 cm,Leg 68.

Table 8. Chemical composition (wt.%) of vein mineralsfrom basalts of Costa Rica Rift, Leg 70.

Component

SiO2

TiO2

A12O3

FeOa

MnOMgOCaONa2OK2O

Total

a Total Fe as

Centerof

Mineral

33.290.036.960.09

19.842.53

19.060.020.01

81.84

FeO.

Rimof

Mineral

41.020.01

12.162.854.19

23.152.730.830.11

87.05

eral alternate with zones containing about 4% MnO andup to about 23% MgO, with higher A12O3, SiO2, FeO,Na2O, and K2O contents and much lower CaO contents(to 2.73%). We failed to identify the Mn-rich mineral.

Chalcocite was identified in Sample 504B-35-2, 14-16 cm from microprobe data. Weak 3.76,2.94, and 2.139Å X-ray reflections in clay from Sample 504B-3-1, 94-96 cm were attributed to bismutite.

The results of microprobe analyses of analcime andnatrolite are given in Table 7. Natrolite has a high K2Ocontent (up to 5%). Part of a vein in another sample ismilk white, and the chemical composition of this part ofthe vein, which according to X-ray diffraction analysisconsists of analcime (5.57, 3.42, and 2.92 Å), calcite(3.03 Å), and an unidentified mineral with a strong re-flection at 12.9 Å, is shown in Table 8. The CaO con-tent of this part of the vein is high (up to 24.49%). It ispossible that the 3.03 Å reflection attributed to calciteactually belongs to the mineral with the strong reflectionat 12.9 Å. This mineral appears to be in an isomorphous

Table 7. Chemical composition (wt.%) of vein minerals frombasalts of Costa Rica Rift, Legs 68, 69, and 70.

Component

SiO2

TiO2

A12O3

FeOaMnOMgOCaONa2OK2O

Total

504B-38-1(7-15)

Analcime

57.390.02

21.95—0.01—0.02

15.230.01

94.65


5O5B-35-2(14-16)

501-15-1(109-111)

Mineral

Natrolite

53.390.02

16.360.23——0.347.615.08

83.02

Foujasite

51.630.24

15.970.100.020.183.076.840.07

78.12

501-15-1(109-111)

Phillipsite

52.100.26

18.950.06——0.988.704.70

85.76

Component

SiO2

TiO2

A12O3

FeOd

MnOMgOCaONa2OK2O

Total

5O4B-38-1(7-15)

(a)

49.570.017.580.05——

24.491.940.07

83.72


504B-38-1(7-15)

504B-5-3(Piece 391, 40-55)

Color and/or Mineral

(b)

53.150.011.770.010.02—

32.940.100.03

88.02

(c)

36.55

32.890.04——

13.693.64—

86.81

Note: Dash denotes that element was not found.a Milk white (analcime, 12.9 Å-mineral, and calcite).b Gray.c Thomsonite (analyzed by N. N. Pertsev).d Total Fe as FeO.

mixture with analcime. The chemical composition of thegray part of the same vein is also shown in Table 8.

Okenite was identified from its 19.6 Å reflection inmaterial from an analcime-filled vein in Sample 504B-35-1, 70-72 cm.

Ano unidentified mineral with a strong reflection at3.20 Å (possibly fairchildite) was found in Samples 501-10-2, 501-15-3, 504B-5-2, 5O4B-1O-3, and 504B-34-2.

Vein foujasite (identified by a strong reflection at14.25 Å) was found in Sample 501-15-1,109-111 cm to-gether with phillipsite. The chemical composition offoujasite is shown together with that of phillipsite in Ta-ble 7. Fe-hydrosilicate (Table 9) was detected in thesame sample by microprobe analysis.

A red ferruginous film found in Sample 504B-18-1,45-49 cm was studied by X ray. Hematite was identifiedby its 3.65, 2.70, 2.50, 2.20, 1.83, and 1.68 Å reflec-

Table 9. Chemical composi-tion of vein iron mineralfrom basalt of Costa RicaRift, Sample 501-15-1,109-111 cm, Leg 68.

Component

SiO2

TiO2

A12O3

FeOa

MnOMgOCaONa2OK2O

Total

10.93—3.06

63.970.392.810.430.140.18

81.92

7.360.052.49

62.430.452.230.300.140.03

75.48

Note: Dash denotes that element was not found.a Total Fe as FeO.

Note: Dash denotes that ele-ment was not found.

a Total Fe as FeO.

577


tions. Reflections at 2.77, 2.50, 2.14, and 1.70 Å wereattributed to magnesite.

Thomsonite, which was identified by N. N. Pertsevon the basis of its chemical composition (Table 8), wasnot identified by X-ray diffraction analysis, probablybecause it is present in very small amounts.

An aragonite vein and pyrite are shown in Plate 3,Figures 1 to 4. A sphere of unknown composition oc-curs on the pyrite in Sample 504B-61-1, 55-60 cm (Plate3, Figs. 5 and 6). We have found analogous shapes be-fore (during a study of gold morphology). Excellent tri-octahedral Fe-Mg-chlorite was revealed together withvein smectite and analcime in Sample 504B-34-2, 18-21cm.

MINERAL ASSOCIATIONS

Vein minerals identified in the Costa Rica Rift basaltsoccur in the following principal associations: black clay+ hematite ± light green clay and phillipsite; black clay+ pyrite; black clay + gray clay ± phillipsite; blackclay + aragonite; black clay + analcime + other Ca-,Na-, and K-minerals; black clay + anhydrite + quartz ±apophyllite; and green clay in pores.

Black clay is widespread, present in thin veins throughthe whole basalt section. Thick layers of black clay werefound mainly in brecciated glassy crusts. Hematite andpyrite divide the basalt into two significant parts: an in-terval with hematite from 278 to 570 meters sub-bottomand an interval with pyrite from 570 to 827 meters sub-bottom. Light green clay, which occurs as thin, 1-mm-thick layers that cover the black clay, is present mainlyto the depth of 570 meters sub-bottom.

The vein mineral assemblages can be used to dividethe interval from 278 to 570 meters sub-bottom intothree parts in turn: from 278 to 427 meters sub-bottom,427 to 527 meters sub-bottom, and 527 to 570 meterssub-bottom.

Gray clay occurs at depths up to 427 meters sub-bot-tom. It exists as shapeless lumps and clearly outlinedfragments and occurs mainly in brecciated glassy crusts.It also occurs in veins up to 1 cm wide with black clay,dark green clay with a silk luster, and phillipsite (Plate2, Figs. 4 and 5). The contact between gray and blackclays is distinct and metasomatic. Bismutite(?), actino-lite, and chlorite were identified in the gray clay. In thesame part of the basalt section (to a depth of 427 meterssub-bottom) black clay lines veins that are filled with ar-agonite. In this part of the section talc, thomsonite, cris-tobalite, a mineral rich in Mn, foujasite, Fe-hydrosili-cate, magnesite, and a mineral with a reflection at 3.20Å were also detected.

In the interval from 427 to 527 meters sub-bottomhematite and phillipsite were identified in veins linedwith black clay.

In the interval from 527 to 570 meters sub-bottom theblack clay lines veins that are filled mainly with anal-cime. Natrolite, okenite, gyrolite, calcite, phillipsite,chalcocite, chlorite, and an unidentified 12.9 Å mineralwere also found in association with the analcime. Theseveins occur mainly in the glassy crusts and brecciatedglass.

Pyrite is a common mineral in the interval from 570to 827 meters sub-bottom, where it is associated withblack clay. A zone within this interval (751 to 809 meterssub-bottom) has veins that consist of anhydrite andquartz and have black smectite linings. Apophyllite wasfound in association with anhydrite in Sample 504B-61-1,55-60 cm.

Pores filled with green clay are present all throughthe basalt section. To a depth of 527 meters sub-bottom,the clay is light green in color; below that depth it isdarker. In Sample 504B-32-2, phillipsite was found inpores with the clay. The texture of smectite in a pore isshown in Plate 2, Figure 6.

PHYSICAL AND CHEMICAL CONDITIONS FORTHE FORMATION OF SECONDARY MINERALS

The composition of the secondary minerals recoveredfrom the Costa Rica Rift basalts indicates that they orig-inated mainly under alkaline conditions. Acidic condi-tions probably prevailed only in the lowest part of thebasalt section, where pyrite and quartz are present.

The secondary minerals that form under nonoxidiz-ing conditions are the most common in the Costa RicaRift basalts. Trioctahedral smectite and (in the lowerpart of the section) pyrite are the most reliable indica-tors of these conditions.

Saponite formed in the basalt groundmass under theinfluence of the "autoclave" effect. Bass (1976) showedthat smectite can form in the groundmass of basalts in aclosed system with low oxygen activity and high pHvalues. His results were confirmed when saponite wasproduced during the experimental alteration of glassybasalt by hot seawater under closed and semiclosed con-ditions (Bischoff and Dickson, 1975; Hajash, 1975; Ko-tov et al., 1978; Mottl and Holland, 1978; Kholodkevichet al., 1981).

The conditions for the formation of smectite in thebasalt pores are like the conditions in the basalt ground-mass. The main mineral in the pores is saponite.

Although most of the secondary minerals formedunder non-oxidizing conditions, the mineral assemblag-es in the upper part of the section of the Costa Rica Riftbasalt (in the interval from 278 to 570 meters) indicatethat there oxidizing conditions prevailed. Of prime im-portance in drawing this conclusion is the presence ofhematite, together with traces of dioctahedral smectite;the latter is most likely Fe-smectite. The phillipsite asso-ciated with these minerals can form under oxidizingconditions, as it has in the recent pelagic sediment of thePacific. This mineral association differs from that de-termined for the oxidizing diagenesis of either the re-cent basalt of the East Pacific and Atlantic mid-oceanicridges (Bass, 1976; Robinson et al., 1977; Seyfried et al.,1976, 1978) or the ancient basalt from the Nauru Basin(Kurnosov et al., 1981), where the chief minerals of ox-idative diagenesis are hydrous ferric oxides and celadon-ite. It should also be noted that the oxidative diagenesisin the Costa Rica Rift basalts is not as great in impor-tance as in some other mid-oceanic ridges (e.g., thosedrilled during Legs 34 and 37).

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Temperature measurements near the base of Hole504B (Becker et al., this volume) indicate that the high-est temperature of formation of the secondary mineralsin the Costa Rica Rift basalts did not exceed 120°C. Thetemperature measured at the sediment/basalt contact isabout 60°C. Where smectites are present their tempera-ture of formation can be inferred from the temperatureof formation of the rest of the mineral assemblage. Thistemperature ranges from a high of about 100°C (wherethe smectite is associated with anhydrite and quartz) to alow of about 25 °C (where the smectite is associated withhematite, cristobalite, pyrite, and phillipsite). The lowtemperature smectites are black clay, which occurs inthe basalt groundmass and veins and formed during theinitial stages of basalt heating, and light green and redbrown clays, which were produced by oxidative diagen-esis. The idea that the light green and red brown clays(as well as the black clays) formed at about 25 °C is ad-vanced by Seyfried et al. (1976 and 1978).

SOURCES OF MATERIAL

Experiments on basalt/seawater interaction (Bischoffand Dickson, 1975; Kotov et al., 1978; Mottl and Hol-land, 1978; Mottl et al., 1979; Kholodkevich et al.,1981) show that glassy tholeiitic basalt has all the com-ponents necessary for the formation of the secondaryminerals identified in the Costa Rica Rift basalts. How-ever, some endogenous components are also present inthe Costa Rica Rift basalts (fluorine and potassium inapophyllite; sulfur in anhydrite, pyrite, and chalcocite;and CO2 in aragonite and calcite). Some of the second-ary mineral cations also appear to be endogenous, espe-cially in the lower part of the basalt section.

HISTORY OF SECONDARY MINERALFORMATION

Pillow lava and submarine flows typically go througha deuteric stage of alteration and halmyrolysis and thenreheat as basalts pile up. In the pillow lavas of the CostaRica Rift, the deuteric stage and halmyrolysis did notproduce secondary minerals (or at least none that havesurvived subsequent alteration). Studies of certain oce-anic basalts (Kurnosov et al., 1978) show that secondaryminerals do not necessarily form in glassy crusts in thedeuteric stage or during long contact with cool sea water(halmyrolysis). Furthermore, no minerals formed in thepillow lava glassy crusts during the deuteric stage of hal-myrolysis under oxidizing conditions. This observationdisagrees with Bass's (1976) view that oxidative diagene-sis occurs during the initial stages of basalt cooling.

The deuteric stage may however be of great impor-tance for the formation of secondary minerals in thesills and thick flows (tens of meters) when seawater pene-trates the solidifying lava slowly. Such deuteric mineralswere reported in the Nauru Basin basalts recovered dur-ing Leg 61 (Kurnosov et al., 1981).

The secondary minerals in the Costa Rica Rift basaltsformed later, while the basalts were being reheated. Thereheating was rapid because this zone is relatively hot(the temperature measured at the sediment/basalt con-tact in the Hole 504B was about 60°C [Becker et al., thisvolume]). Early during this reheating, while tempera-

tures were still relatively low, saponite grew in fracturesand pores, replaced basalt groundmass material, andfragmented glassy crusts. Material for the formation ofsaponite was produced by basalt/seawater interaction.

A further increase in temperature toward the lowerpart of the section resulted in vein mineralization, withthree assemblages appearing from the base upward: an-hydrite-quartz with apophyllite; zeolite; and aragonite.Gray clay veins formed within the aragonite zone. Theblack clay that comprises the vein linings formed at high-er temperatures than the black clay that formed duringthe first stages of the reheating of the basalt. Low tem-perature saponite clay remained stable during the tem-perature increase.

The mineral zones identified reflect the temperaturefacies of hydrothermal alteration. The source of F, CO2,and part of the S, as well as a number of cations, such asK in apophyllite, was endogenous.

The appearance of such high temperature minerals astalc, actinolite, and chlorite in the upper part of the sec-tion breaks the temperature zonation. Bismutite, whichexperiments have shown to form at 400-500 °C (Mottland Holland, 1978), was identified in the same part ofthe section.

The occurrence of these high temperature minerals inan otherwise low temperature zone can be explained bythe different degree of saturation of the solutions withcations in the upper and lower parts of the basalt. Witha sufficient degree of saturation temperature need notbe so high.

The second type of zonation identified in the CostaRica Rift basalts, which is based on the distribution ofpyrite and hematite in the section, corresponds to thetransition from nonoxidizing to oxidizing conditions.This boundary lies approximately 570 meters below theseafloor and 300 meters below the sediment/basalt in-terface. Oxidative diagenesis occurred during the finalstage of secondary mineral formation, when tectonicmovements caused old fractures to reopen and new onesto originate. New seawater, rich in oxygen, penetratedthese fractures. Oxidative diagenesis manifested itselfby the appearance of hematite, which was followed bythe formation of phillipsite and of red brown, light green,and yellow brown clays. These rare minerals, whichform under oxidizing conditions, are superposed on allother mineral assemblages to a sub-bottom depth of 570meters.

The participants in Legs 69 and 70 noted both normaland reverse zonation in the distribution of clays in basaltveins. This can be accounted for not only by pH behav-ior but by the reopening of previously healed fractures.The rarity of hematite, phillipsite, and light green andyellow brown clays in the basalt section results from thefact that old fractures did not reopen everywhere.

The minerals that result from oxidative diagenesis inthe Costa Rica Rift differ from those identified in someother mid-ocean ridges (Bass, 1976; Robinson et al.,1977) in that potassium is present in phillipsite insteadof celadonite and nearly all Fe+3 is fixed in hematite.

A comparison of secondary minerals and the condi-tions of their formation in the Cretaceous off-ridge mag-matic complex of the Nauru Basin (Kurnosov et al., 1981)

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with the Cenozoic pillow lavas of the Costa Rica Riftshows that the lava eruption mechanism determines thetiming of the initial stage of secondary mineral forma-tion in basalts and the composition of the resulting min-erals. The eruption of thick flows and the intrusion ofsills during Nauru Basin magmatism were immediatelyfollowed by the formation of secondary minerals—thatis, the minerals formed during the deuteric stage. Wherethin flows and pillow lavas have erupted, secondaryminerals do not form in the deuteric stage but begin tooriginate during a later stage of heating. Therefore, dur-ing the formation of off-ridge basement in CretaceousNauru Basin (Shcheka, 1981; Shcheka and Kurenzova,in press), the conditions of secondary mineral formationand those of cation flux into the ocean and sedimentswere probably different from those at the Costa RicaRift.

CONCLUSIONS

Secondary minerals in tholeiitic basalts of the CostaRica Rift were formed by seawater/basalt interactionand endogenous element addition during the heating ofthe pillow lavas. The secondary minerals formed mainlyunder reducing alkaline conditions over a wide temper-ature interval. An acidic regime for secondary mineralformation was identified in the lower parts of the sec-tion.

Two types of zonation in vein mineralization wereidentified in the distribution of the secondary mineralsin the basalt section: hydrothermal zonation and oxida-tive-nonoxidative zonation.

An interesting phenomenon is the occurrence of hightemperature minerals in the upper part of the sectionand their absence in the lower part. We explain this bythe different degree of saturation of the solutions withdissolved cations in the upper and lower parts of the ba-salt section.

Oxidative diagenesis manifested itself in the final stag-es of secondary mineral formation to a sub-bottom depthof 300 meters. The oxidative mineral assemblages differfrom those known in some other portions of the mid-ocean ridges in the absence of celadonite and the pres-ence of phillipsite. The minerals formed during this stageof alteration are less abundant than those formed undernonoxidizing conditions in the Costa Rica Rift.

ACKNOWLEDGMENTSWe thank Dr. P. Robinson and Dr. E. Lelikov for reviewing the

manuscript, and T. Bortina, G. Yudina, N. Ryapolova, V. Piskunova,

and L. Kovbas for analytical work and helping to put the manuscriptinto shape.

REFERENCESBass, M. N., 1976. Secondary minerals in oceanic basalt, with special

reference to Leg 34, Deep Sea Drilling Project. In Yeats, R. S.,Hart, S. R., et al., Init. Repts. DSDP, 34: Washington (U.S. Govt.Printing Office), 393-432.

Bischoff, J. L., and Dickson, F. W., 1975. Seawater-basalt interactionat 200°C and 500 bars: implications for origin of sea-floor heavy-metal deposits and regulation of seawater chemistry. Earth Planet.Sci. Lett., 25:385-397.

Hajash, A., 1975. Hydrothermal processes along mid-ocean ridges: anexperimental investigation. Contrib. Mineral. Petrol., 53:205-226.

Joint Committee on Powder Diffraction Standards, 1974. SelectedPowder Diffraction Data for Minerals (1st ed.): Swarthmore, Pa.

Kholodkevich, I. V., Kotov, N. V., and Kurnosov, V. B., 1981. Exper-imental study of secondary changes of volcanic glassy rocks inpure and model sea water at higher P-T parameters. In MineralTransformations of Rocks of the Oceanic Substratum (Epigenesisand Initial Metamorphism): Moscow (Nauka), pp. 67-72.

Kotov, N. V., Kurnosov, V. B., and Kholodkevich, I. V., 1978. Mod-eling of natural volcanic rock alterations in pure and standard sea-water at higher P-T values. Lithol. Mineral Res., 13:450-459.(Translated from Litologiya i Poleznye Iskopaemye, 1978, No. 4,pp. 78-89.)

Kurnosov, V. B., Kholodkevich, I. V., and Shevchenko, A. Ya., 1981.Secondary minerals of basalts from the Nauru Basin, Deep SeaDrilling Project Leg 61. In Larson, R. L., Schlanger, S. O., et al.,Init. Repts. DSDP, 61: Washington (U.S. Govt. Printing Office),653-671.

Kurnosov, V. B., Skornyakova, N. S., Murdmaa, I. O., Kashintsev,G. A., Narnov, G. A., and Shevchenko, A. Ya., 1978. Underwaterweathering of glassy basalts on the ocean floor. Lithol. MineralRes., 13:113-121. (Translated from Litologiya i Poleznye Iskopae-mye, 1978, No. 1, pp. 134-143.)

Mottl, M. J., and Holland, H. D., 1978. Chemical exchange duringhydrothermal alteration of basalt by seawater—I. Experimentalresults for major and minor components of seawater. Geochim.Cosmochim. Acta, 42:1103-1115.

Mottl, M., Holland, H. D., and Corr, R. F., 1979. Chemical exchangeduring hydrothermal alteration of basalt by seawater—II. Experi-mental results for Fe, Mn, and sulfur species. Geochim. Cos-mochim. Acta, 43:869-884.

Robinson, P. T., Flower, M. F. J., Schminke, H.-U., and Ohnmacht,W., 1977. Low temperature alteration of oceanic basalts, DSDPLeg 37. In Aumento, F., Melson, W. G., et si., Init. Repts. DSDP,37: Washington (U.S. Govt. Printing Office), 775-793.

Seyfried, W. E., Shanks, W. C , and Bischoff, J. L., 1976. Alterationand vein formation in Site 321 basalts. In Yeats, R. S., Hart, S. R.,et al., Init. Repts. DSDP, 34: Washington (U.S. Govt. PrintingOffice), 385-392.

Seyfried, W. E., Jr., Shanks, W. C , III, and Dibble, W. E., Jr., 1978.Clay mineral formation in DSDP Leg 34 basalt. Earth. Planet. Sci.Lett., 41:265-276.

Shcheka, S., 1981. Igneous rocks of Deep Sea Drilling Project Leg 61,Nauru Basin. In Larson, R. L., Schlanger, S. O., et al., Init.Repts. DSDP, 61: Washington (U.S. Govt. Printing Office),633-646.

Shcheka, S. A., and Kurenzova, N. A., in press. Magmatic complexesof the ocean. Soviet Geology.

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Plate 1. Electron micrographs of saponites from Leg 69 and 70 basalts. 1. Saponite from basalt groundmass, × 8000; Sample 504B-70-1,17-20cm (size is <2 µm). 2 to 4. Saponite from veins. (2) Gray clay. Sample 504B-10-3, 17-20 cm; × 7000. (3) Light green clay. Sample 504B-5-2,142-146 cm; × 7000. (4) Red brown clay. Sample 504B-2-1, 108-110 cm; × 12,000.

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Plate 2. Electron micrographs of minerals from Leg 69 and 70 basalts. 1. Gray clay, × 300; Sample 504B-7-2, 64-68 cm. 2. Gray clay in ba-salt, × 500; Sample 504B-9-1, 142-147 cm. 3. Dark green clay with a silk luster, × 300; Sample 5O4B-1O-3, 17-20 cm. 4. Phillipsite on thegray clay, × 300; Sample 504B-9-1, 142-147 cm. 5. Same sample, × 3000. 6. Smectite(?) in basalt, × 1500; Sample 504B-37-1, 55-60 cm.

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Plate 3. Electron micrographs of minerals from Leg 69 and 70 basalts. 1. Vein aragonite in basalt, × 50; Sample 501-15-3, 134-136 cm. 2.Same sample, × 1500. 3. Pyrite in black clay, × 4500; Sample 504B-61-2, 145-149 cm. 4. Pyrite in black clay, × 5000; Sample 504B-70-1,17-20 cm. 5. Nondetermined sphere on the pyrite, × 300; Sample 504B-61-1, 55-60 cm. 6. Same sample, × 1000.

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Documents

INTRODUCTION SECONDARY MINERALS IN …30. SECONDARY MINERALS IN BASALT FROM THE COSTA RICA RIFT, HOLES 501 AND 504B, DEEP SEA DRILLING PROJECT LEGS 68, 69, AND 701 Victor B. Kurnosov,