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22. PETROGRAPHICAL AND CHEMICAL STUDY OF BASEMENT BASALTS FROM THEGALAPAGOS SPREADING CENTER, LEG 701

Christine Laverne, Laboratoire de Géologie, BP 5, 97490 Ste. Clotilde (Reunion), France DOMand

Gerard Vivier, Institut Dolomieu, L.A. CNRS n° 69, 38031 Grenoble Cedex, France

INTRODUCTION

During Leg 70, basalts from the region adjacent tothe Galapagos Spreading Center were recovered at foursites (Sites 506, 507, 508, and 510). Previous to this, Leg54 of the Deep Sea Drilling Project (DSDP) had recov-ered basalts from the same general area—at Sites 424and 425 (Fig. 1).

The objectives of this chapter are the following:1) To present the mineralogical and chemical fea-

tures of fresh basalts recovered during Leg 70 in theGalapagos Rift area;

2) To compare these basalts with those recovered onLeg 54 and to complete a general model for the origin ofbasalts recovered from the Galapagos Spreading Center;

3) To assess changes in the mineralogical and chemi-cal composition of these oceanic lavas brought aboutprimarily by their reaction with seawater;

4) To determine whether a relationship exists amongthe ages of the basalts, their degree of fractionation,and their alteration.

ANALYTICAL TECHNIQUESMinerals were identified by polarizing microscopy and analyzed by

electron microprobe. Computerized electron-microprobe analyses werecarried out at the Centre Océanologique de Bretagne, Brest (Auto-mated Camebax). During the analyses, the accelerating voltage waskept at 15.0 kV. The electron-beam incidence of the Camebax has atake-off angle of about 42°, and the limit of detection is about 500ppm. The precision of the method used is about 1%.

Major, minor, and trace elements were analyzed in bulk-rocksamples (after the usual grinding and homogenization in an agatemortar) by X-ray fluorescence (CGR Alpha 2020; rhodium tube), atthe Institut Dolomieu, Grenoble.

FRESH ROCK PETROGRAPHYBasalt textures and estimated amounts of magmatic

minerals are summarized in Table 1. Young crust at Sites506, 507, and 508 yielded nearly identical samples of fine-to medium-grained, aphyric to sparsely Plagioclase (or,more rarely, pyroxene) phyric basalts. Textures rangefrom glassy to variolitic to subophitic, with the grainsize of the groundmass minerals increasing with distancefrom the glassy rims, as is usually observed in oceanicbasalts. Plagioclase (An63; see analysis 1, Table 2) is thedominant phenocryst phase (< 5%), with clinopyroxenephenocrysts less abundant (<4%). Plagioclase and

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

clinopyroxene appear also as microlites and in rockswith quenched textures. Olivine was not observed inthese basalts. Titanomagnetite is the dominant opaquephase (10-15%) of the bulk rock, and primary spherulesof sulfide minerals occur. Vesicles range from 1 % to 8%by volume.

Site 510 is characterized by fine- to medium-grained,moderately (3-10%) Plagioclase phyric, sparsely (1%)olivine phyric basalt. The olivine is Mg rich (Fo86_87; seeanalyses 2, 3, 4, Table 2). Titanomagnetite is less abun-dant (5-8%) than in basalts from the other sites, and novesicles have been observed.

Comparison of the phenocryst assemblages and Fe-Ti oxide abundances seems to indicate that basalts fromSite 510 are more primitive than are those from Sites506, 507, and 508. This is confirmed by our study ofbulk-rock chemistry.

BULK COMPOSITIONThe samples which we analyzed chemically were, as

often as possible, pairs of the same hand specimenwhich exhibited (1) apparently fresh rock and (2) alteredrim. Homogeneous samples of "fresh" rock were easilyisolated.

Whole-rock analyses and normative mineralogy offour samples from Site 510 and eight samples from Sites506, 507, and 508 are presented in Table 3. For compari-son with "parental and differentiated magmas," all theanalyses have been normalized to 100% after water wasremoved and all iron reduced (see Table 4).

Site 510. Fresh basalt samples from Site 510 havehigh A12O3 (15.07 to 16.35 wt.%), MgO (7.81-8.49wt.%), CaO (12.91-13.21 wt.%) and relatively high Ni(157-170 ppm), Cr (406-446 ppm), and Cu (100-110ppm) contents.

The high MgO content and the presence of normativeolivine (Fo66) in one sample corresponded to the petro-graphic observation of modal olivine (Fo78_80). Olivinecontains 200 to 400 ppm of Ni and up to 900 ppm of Cr(see Table 3). Normative quartz ranges between 0.00%and 0.75%.

The chemical composition of Site 510 basalts is veryclose to that of the parental magma of the GalapagosSpreading Center described by Clague and Bunch (1976),with FeOVMgO = 0.99 to 1.06 and MgO content =7.81% to 8.49%.

High Cr and Ti content and low Sr (53 to 57 ppm), Ba(13 to 19 ppm), and V (264 to 275 ppm) content reflect a

375

C. LAVERNE, G. VIVIER

10°N

i Siqueros Fracture Zone

05°W

Cocos Nazca Ridge 69

-é-

100° 95° 80" 77°W

Figure 1. Location of Sites 506, 507, 508, and 510 in the region adjacent to the Galapagos Spreading Center. Also shown areSites 424 and 425, Leg 54.

Table 1. Textures, magmatic minerals (estimated percentages), and secondary-mineral occurrencein basalts from Sites 506, 507, 508, and 510.

Groundmass

Sample(interval in cm) Texture

Phenocrysts

ol cpx pi ol cpx pi tm ve/vo

Secondary Minerals

Clay Minerals8

2 3 4 5 6 7

506G-2-1, 10-14506G-2-1, 49-52

506G-2-1, 126-123

507B-1-1, 29-32

507C-3-1, 2-55O8B-3-1, 12-15508B-3-1, 32-345O8B-3-1, 43-46510-9-1,4-7

510-9-1, 44-46

510-9-3, 31-36

510-11-1,79-82

HyalopiliticSubvariolitic

GlomerophyricHyalopilitic

GlomerophyricSubophitic to inter-

granularHyalopiliticHyalopiliticHyalopiliticHyalopiliticHyalopilitic

SubglomerophyricHyalopilitic



Subglomerophyric

— — — — 45 40 15

— 2 2 —

— — 1 —

— — 7

— 10

— — 6

40

43

4140374040

35

31

40

43

40

43

4140374040

45

41

42

44

12

12

1513101414

7

8

5

5

2(ve)

l(ve)

3(ve)7(ve)8(ve)6(ve)6(ve)

l(vo)

2(vo)

2(vo)

l(vo)

Note: tm = titanoma Clay minerals are

lagnetite; ve/vo = vesicles and voids; pi = Plagioclase; cpx = clinopyroxene; ol = olivine; and c = carbonate,numbered according to Table 3.

very low degree of (or even no) fractionation from theparental magma.

Site 507 and 508. Basalts from Sites 507 and 508 arericher in Na, K, Ti, P, Mn, V, and Sr than are thosefrom Site 510. Their higher SiO2 content and normativequartz amount (1.92-4.17%) identify them as quartztholeiites.

They are very rich in FeO* and TiO2 (11.81-13.11wt.% and 1.82-2.09 wt.%, respectively) and correla-tively rich in V (379-409 ppm) and poor in A12O3(12.69-13.40%). They can, therefore, be classified asferrobasalts (Melson et al., 1976) and are similar toother ferrobasalts already dredged from the area of theGalapagos Spreading Center (Anderson et al., 1975;Schilling et al., 1976; Clague and Bunch, 1976; Fodor etal., 1980).

The medium content of Ni (118-148 ppm) and Co (61-103 ppm) and the high FeOVMgO (1.77 to 1.97) ratioindicate a substantial fractionation of Fo-rich olivinefrom the parental magma. The simultaneous destabil-ization of Cr spinels with this olivine would explain therelatively low Cr (110-164 ppm) content observed.

Compared with Site 510 basalts, the fractionationcorresponds also to higher contents of Sr (75-87 ppm),Ba (23-48 ppm), and Na2O (2.29-244 wt.% instead of1.84-1.88 wt.%) and lower CaO (10.53-10.79 wt.% in-stead of 12.91-13.21 wt.%).

Site 506. Basalts from Site 506 are also ferrobasaltsand are very similar to those from Sites 507 and 508.However they are slightly poorer in TiO2 (1.56-1.78wt.%), MgO (7.17-7.28 wt.%), V (347-375 ppm), andCr (176-190 ppm) than the latter. Normative quartz is

376

PETROLOGY AND CHEMISTRY OF BASALTS

Table 2. Microprobe analyses of magmatic minerals.

Component

5O7C-3-1,2-5

(Plagioclase)

Major oxides (wt.%)

SiO2

A12O3

FeO*MnOMgOCaONa2OK2OTiO2

NiOCr 2 O 3

P2O5

Total

53.7429.15

0.78—0.29

12.424.00—0.02—0.030.23

100.65

Cation proportions

SiAlFeMnMgCaNaTiNiCrP

Total

AnAbOrFoFa

2.4181.5460.029—0.0190.5980.349——0.0010.009

4.969

63.1936.81———

Sample(interval in cm)

510-9-1,94-96

(Olivine)a

40.580.02

12.630.15

47.140.34———0.130.030.03

101.06

0.9970.0010.2600.0031.7270.009—

0.002—0.001

3.001

86.6813.32

510-9-1,94-96

(Olivine)a

40.680.04

12.800.11

47.030.290.01——0.130.05—

101.14

0.9950.0010.2630.0021.7220.008—

0.0030.001—

2.999

86.5413.46

510-9-3,31-36

(Olivine)

40.910.03

12.980.18

46.640.310.040.04————

101.12

1.0050.0010.2670.0041.7080.0080.002

——

2.996

86.3313.67

also definitely less abundant (1.54-1.91%). These fea-tures allow us to classify Site 506 basalts as intermediatebetween those from Site 510 and those from Sites 507and 508. This intermediate character appears in all thediagrams illustrating the relationship among the freshbasalts or their alteration states (Fig. 2, 6, 7).

Comparison with Site 424 and 425

Fodor et al. (1980) described basalts from Sites 424and 425, which were drilled during Leg 54 in the area ofthe Galapagos Spreading Center. Site 424 is located nearSites 506, 507, and 508, whereas Site 425 is located nearSite 510 (Fig. 1).

A comparison with average normalized analyses ofparental and differentiated basalts from the GalapagosSpreading Center proposed by Clague and Bunch (1976)and with basalts from Sites 424 and 425 is presented inTable 4.

Site 510 basalts are very close to the parental magma.Site 425 basalts represent moderately fractionated oce-anic tholeiites by mainly Plagioclase plus olivine frac-tionation in the proportion 3:1 (crystallized: 28%, Fodoret al., 1980).

Basalts from Sites 507, 508, and 424 are close to dif-ferentiate 3 (Clague and Bunch, 1976). Up to 60% ofthe parental magma has fractionally crystallized to formthese ferrobasalts. For Clague and Bunch (1976) theseferrobasalts are formed by fractionation of Plagioclase,clinopyroxene, and olivine in the proportions 9.3:7.7:1(differentiate 3). The calculated proportions given byFodor et al. (1980) are respectively 11.3:12.8:1 and 50%of crystallized magma for the same ferrobasalts.

From Figure 2 and Table 4, one may infer that theSite 425 parental magma corresponds to a tholeiitewhose composition is very close to those of basalts de-scribed above. Fractionation would result in ferrobasalt(Sites 424, 507, 508), and Site 506 basalts would be anintermediate, close to those from Sites 507 and 508.Thus, fractionation increases from Site 510 to Site 424basalts as follows: 510, 425, 506, 507-508, 424.

A linear correlation exists between the paleomagneticage and the degree of fractionation of basalts from theGalapagos Spreading Center except for those from Site506. The exceptional feature of Site 506 basalts will befurther underlined in the alteration study. As shown inFigure 2, the more evolved basalts are the youngest.

Site

Age (m•y )

510

2.73

425

1.80

506

0.54

507

0.69

508

0.85

424

0.62

COLORED ALTERATION HALOS

In basalts from the Galapagos Spreading Center, al-teration appears as colored halos which are subparallelto exposed surfaces and fissures. The halos are darkerthan the fresh basalt and thus were called "black halos"on the ship.

Most basalt samples from Site 506 are fresh, but athin (<4 mm) alteration halo occurs in a few samples.At Site 507, the texture of the basalt clearly controls thealteration. Subophitic basalts are completely fresh where-as those that are hyalopilitic show dark alteration halos(maximum thickness: 15 mm). At Sites 508 and 510, allof the samples exhibit thick altered halos ranging, re-spectively, up to 50 mm and from 5 to 40 mm.

Thus, the widths of the altered halos increase withage from Site 506 to 508—i.e., between 0.55 and 0.85m.y. (Fig. 3).

SECONDARY MINERALS

Alteration halos in basalts from Sites 506, 507, 508,and 510 are characterized (both in hand specimens andunder the microscope) by secondary minerals in vesiclesand interstitial voids. In addition, olivine is replaced bysecondary minerals in Site 510 basalts. Feldspar and py-roxene are always fresh.

Because of the small size and number of samples, itwas not always possible to make a thin section represen-tative of a complete alteration rim. Nevertheless, weidentified various kinds of clay minerals, whose charac-teristics are summarized in Table 5. We have chosen torepresent their chemistry on a FeO*-MgO-K2O triangle(Fig. 4), for the behavior of these elements during al-teration is particularly interesting. Because of the lack

377

Table 3. Bulk-sample chemical analyses (major elements in wt.%, monocations in %, and CIPW norms).

Component506G-2-1,

10-14a

Major oxide (wl.%)

SiO2

A12O3

Fe 2 O 3

FeOMnOMgOC a O

Na2OK2O

TiO2

P2O5H 2O

49.8313.562.069.900.227.23

11.102.260.131.780.071.66

506G-2-1,49-52b

49.6613.602.229.910.217.17

11.142.280.101.650.081.15

506G-2-1,49-52a

50.2913.55

1.8510.100.187.12

11.162.240.151.560.111.46

506G-2-1,121-123a

50.7113.80

1.869.810.197.28

11.242.270.131.580.091.10

507B-1-1,29-32a

48.9312.692.73

10.660.226.64

10.532.360.141.900.131.68

5O7C-3-1,2-5b

48.8012.985.758.350.175.689.852.450.321.850.123.29

507C-3-1,2-5a

50.6413.352.40

10.110.236.46

10.642.440.071.890.140.86

5O8B-3-1,12-15°

48.3112.585.059.000.216.39

10.092.290.351.960.152.53

5O8B-3-1,12-15a

50.4213.052.57

10.440.286.57

10.532.300.172.090.161.61

Sample(interval in cm)

5O8B-3-1,32-34b

49.0513.016.038.750.246.279.972.300.502.050.171.87

508B-3-1,32-34a

51.0713.392.729.440.266.63

10.792.400.202.080.170.98

5O8B-3-1,43-46b

49.6312.874.729.120.216.51

10.302.180.491.710.142.17

5O8B-3-1,43-46a

49.7113.402.209.830.226.66

10.792.290.241.820.131.52

510-9-1,4-7 b

49.0815.222.546.650.187.59

12.781.790.200.800.032.30

510-9-1,4-7 a

49.8915.84

1.487.040.158.01

13.091.840.040.79tr

1.17

510-9-1,44-46°

49.2315.522.726.410.157.55

12.781.800.250.770.021.31

510-9-1,44-46a

49.9216.351.706.780.187.81

13.211.860.050.83tr

1.07

510-9-3,31 -36b

49.0415.532.966.240.147.92

12.981.850.180.790.021.57

510-9-3,31-36a

49.3616.08

1.756.760.148.13

13.121.880.050.810.021.94

510-11-1,79-82°

49.1815.162.836.410.168.30

12.801.840.260.81tr

0.93

510-11-1,79-82a

49.6315.07

1.537.000.168.49

12.911.860.050.85

tr

1.24

Total 99.

Trace Elements (ppm)

Ba

CoCr

Cu

NiSrVRb

FeOFeOVMgOOx°

Cation (%)

SiAl

F e 3 +F e 2 +

Mn

MgCaNa

KTiPΣ = IOO

CIPW Norm

qor

aban

dihyil

mtapol

a fresh.° altered.

28114

19073

151

83375

< I O11.751.630.160

47.7115.30

1.487.930.18

10.3211.394.200.161.280.06

1.540.76

19.1226.4723.0820.61

3.382.980.16

26193193

77

148

72

355< I O

11.911.660.168

47.6115.37

1.607.950.17

10.2511.444.240.121.190.06

1.370.59

19.2926.5723.1120.52

3.133.210.18

30

105184

8014173

347

< I O11.771.650.142

48.0715.26

1.338.070.15

10.1411.434.150.181.120.09

1.820.88

19.9526.4723.1621.10

2.962.680.26

27

97

17682

140

75349

< I O

11.481.580.146

48.0815.42

1.337.780.15

10.2911.424.170.161.130.07

1.910.76

19.2027.0823.0521.02

3.002.690.21

2369

11754

117

73

381< I O

13.121.980.187

47.7314.592.008.700.189.65

11.014.460.171.390.11

1.920.82

19.9623.6122.8819.843.603.950.30

28141110

54

17876

37111

13.532.380.383

48.2115.114.276.900.148.36

10.434.690.401.370.10

5.831.89

20.7323.4719.9812.283.518.330.28

3161

110

5612275

381

< I O12.27

1.900.176

48.5915.10

1.738.110.199.24

10.944.540.091.360.11

3.560.41

20.6425.2621.9019.183.583.470.33

24

120155

53116

68

376< I O

13.552.120.336

47.6114.613.757.420.189.39

10.654.380.441.450.13

4.222.06

19.3723.0121.2115.093.727.320.35

30

103

15652

148

76403

< I O

12.751.940.181

48.4114.771.868.380.239.40

10.834.280.211.510.13

3.761.00

19.4624.7821.7419.763.963.720.37

24

93146

53128

73394

13

14.182.260.383

47.5014.854.397.090.209.05

10.354.320.621.490.14

5.142.95

19.4623.6919.9814.073.898.740.40

4198

16457

14887

409

< I O

11.881.790.206

48.6015.02

1.957.510.219.41

11.004.430.241.490.14

4.171.18

20.3025.1722.3117.702.953.940.40

29126

129

5913071

363

20

13.372.050.318

48.1114.713.447.390.179.41

10.704.100.611.250.11

4.632.89

18.4423.8821.4516.153.246.840.33

2890

12666

13380

379

< I O

11.811.770.168

48.1315.29

1.607.960.189.61

11.194.300.301.330.11

2.431.41

19.3725.5722.2819.263.453.180.30

16100

418101

15558

267

< I O

8.941.180.256

47.1917.25

1.845.350.15

10.8813.173.340.250.580.02

1.671.18

15.1432.9024.5116.171.513.680.07

1868

41611015757

266

< I O

8.371.040.159

47.1317.63

1.055.560.12

11.2813.253.370.050.56tr

0.680.23

15.5634.8424.4118.78

1.502.14

-

15125

39994

15054

264

< I O

8.861.170.276

47.1517.52

1.965.130.12

10.7813.113.340.310.550.02

1.751.47

15.2333.5224.0115.72

1.463.940.04

1581

422100169

55

275

< I O

8.311.060.184

46.9318.11

1.205.330.14

10.9413.303.390.060.59tr

0.750.29

15.7336.1123.8317.90

1.572.46

I

1479

411

94

15153

273

< I O

8.901.120.299

46.6817.422.124.970.11

11.2413.243.410.220.570.02

1.031.06

15.6533.5324.7315.78

1.504.290.04

1364

406104

157

53

273< I O

8.341.030.189

46.6117.891.245.340.11

11.4413.273.440.060.580.02

_

0.2915.9025.2824.0018.37

1.532.530.040.10

1476

41082

15054

269

< I O

8.961.080.284

46.7216.972.025.090.13

11.7513.033.390.320.58tr

0.591.53

15.5632.3325.0916.98

1.534.10

-

1984

446107

17053

268

< I O

8.380.990.164

47.1216.86

1.095.560.13

12.0113.133.420.060.61tr

0.310.29

15.7332.6225.4319.31

1.612.21

-


Table 4. Average normalized analyses of basalts from the Galapagos Spreading Center.

Oxide(wt.%)

SiO2A12O3

FeOMnOMgOCaONa2OK2OTiO2

P2O5FeOVMgO.

Parenta

50.3715.998.970.158.00

12.842.450.071.090.071.12

a Clague and Bunch,b Fodor et al ., 1980.

Site 510

50.7316.168.520.168.28

13.351.900.050.84tr.1.03

1976.

Differentiate l a

50.2715.5210.100.166.65

12.283.130.231.580.111.52

Site 425b

52.0814.4210.45v0.17

7.6111.942.130.041.050.081.37

Differentiate 2 a

50.1914.4312.010.187.11

11.192.700.191.860.151.69

Site 506

51.1813.8811.870.207.34

11.362.300.141.670.091.62

Site 507-508

51.2513.4612.720.256.74

10.882.430.161.990.141.94

Site 424b

51.4513.4613.530.216.39

10.412.480.081.880.152.12

Differentiate 3a

51.3013.1913.790.205.80

10.602.600.142.170.202.38

of X-ray diffraction data we cannot precisely identifythese clay minerals. But by comparing their chemistryand optical characteristics to other data (Bolhke et al.,1981; Honnorez et al., 1978; Mevel, 1980) we are able toassume that most of these clay minerals belong to thesmectite group.

Site 506. In Sample 506G-2-1,49-52 cm (analyzed forbulk rock chemistry), the only secondary mineral is aheterogeneous orange-to-brown material occurring ininterstitial voids and in a very tiny vein. No vesicles wereobserved in the alteration halo. A vesicle in-filling witha center of orange material and a rim of greenish clayhad been observed on board ship in Sample 506G-2-1,90-93 cm.

Site 507. In Sample 5O7C-3-1, 2-5 cm, the vesiclesand other voids are filled by yellowish green clay, byorange-to-brown material, or both; if by both, theorange brown mineral occurs in the vesicle rim, whereasthe yellowish green one occurs in the center of the vesi-cle. The analyses of both materials occurring in thesame vesicle are given in Table 6 (analyses 1 and 2). Theyellowish green mineral is an K, Fe-rich clay, chemicallyclose to a nontronite.

The shipboard scientific party had first identified theorange to brown material as iron oxide and/or hydrox-ide. But it is too rich in silica (14-20%) and poor in iron(FeO* = 50-63%) to be a pure hydroxide. On the otherhand, it is too poor in silica and rich in iron to be a puresmectite. Thus, this material is probably a mixture ofiron oxyhydroxides and clay mineral. The K, Fe-richclay seems to have been formed before the orange brownmaterial.

Site 508. The altered rim of Sample 5O8B-3-1, 12-15cm clearly shows a zonational pattern. From the ex-terior to the center of the sample, one observes in theinterstitial voids and vesicles: (1) brown to brownishgreen clay (analysis 5, Table 6), associated with an or-ange brown material. The clay is very FeO* rich (39%)and slightly K2O rich (3%); (2) green to yellowish greenclay (analysis 4, Table 6) in the vesicle rims, and orangematerial (analysis 3, Table 6) in the vesicle cores (Plate1); (3) green to yellowish green clay (Plate 1); (4) nothing.

Based on both chemical analyses and the respectivelocations of these various minerals, we believe that thebrownish green clay is a former yellowish green claywhich has undergone an initial oxidation accompanied

by loss of SiO2 and gain of FeO*. The arrow in Figure 4indicates this trend.

The following succession of vesicle in-fillings was ob-served, from the exposed surfaces to the core of Sample5O8B-3-1, 43-46 cm: (1) brownish black material; (2)brown material (Plate 2); (3) brownish orange materialin the vesicle rims and brown material in center; (4)brownish green clay; (5) nothing.

Thus, if we consider the brownish green clay as a ref-erence for the start of clay oxidation, one could say thatsecondary minerals of Sample 5O8B-3-1, 43-46 cm haveall undergone oxidation, whereas in Sample 5O8B-3-1,12-15 cm, oxidation did not reach the innermost sec-ondary products. Further, the brownish black materialis perhaps chemically very close to an iron oxyhydrox-ide, suggesting that oxidation is more important in themore external part of the altered halo. Oxidation ap-pears to have progressed toward the center of the basalt.

Site 510. Basalts from Site 510 contain olivine micro-phenocrysts and/or phenocrysts. Alteration appears notonly as interstitial void in-fillings but also as a replace-ment of olivine.

In Sample 510-9-1, 94-96 cm, two zones are recog-nized in the alteration halo:

1) The outer zone contains olivine crystals whosecores are replaced by a K, Fe-poor, Mg-rich pale greenclay (analysis 10, Table 6), whereas their external part iscomposed of a K, Fe-rich, Mg-poor clay mineral (analy-sis 9, Table 6), chemically close to the "protoceladonite"found in Holes 417A, 417D, and 418A (Mevel, 1979).This replacement is sometimes very incomplete, withalteration products occurring only along cracks andaround the olivine crystals. Olivine is sometimes com-pletely replaced by clay minerals. Miarolitic voids arefilled with the Fe, K-rich dark green clay mineral (analy-sis 11, Table 6). These observations suggest that the Mg-rich pale green clay mineral at Site 510 was able to formonly because it could take Mg from the Mg-rich olivineand also explain why the pale green clay mineral doesnot occur in basalts from Sites 506, 507, and 508.

2) In the inner zone, olivine is replaced by a very palebrown clay mineral (analysis 12, Table 6), which is MgOrich (17%), K2O poor (0.1%), and contains significantA12O3 (2.6%). It also fills interstitial voids. Unfortu-nately, this mineral is unstable under the microprobebeam. Hence/its analysis must be regarded as semi-

379


60

55

50

SiO2

-

A.TH.

/

mi

CA/"/TH

-

*• 1

g

~ 8

(wt.

7

6

MgO

-

-

-

\ -

400

_ 300

α

200

100

Cu

-

s

1

rCr

-

r

400 -

1 0 0 -

13

12

11

10

CaO

-

-

i

tV

1

-

-

V\

f -

FeO*/MgO

Legend •= Site 506 • = Site 507 4 = Site 508 ^ = Site 510 Δ = Site 424 v= Site 425

Figure 2. Bulk chemical variations in fresh rocks from the Galapagos Spreading Center: weight percent oxides vs. FeOVMgO.

quantitative. A similar mineral, called "light tan smec-tite," is described by Bohkle et al. (1981) and Honnorezet al. (1978), in basalts from Leg 46, Hole 496B.

In Sample 510-9-3, 31-36 cm, we observed the mostcomplete zonal alteration pattern. From the exposed

surface to the core of the sample, the following se-quence is found:

1) Orange to brown material;2) Dark green clay mineral (analysis 13, Table 6) +

orange brown material (Plate 2) ± dark brown clay

380


Age(m.y.)

0.54

0.69

0.85

2.73

Site

506

507

508

510

0i i i

5I

Distance (mm

1l |

31

15I l I I I I

mmw////// |

2

1

3

7

^ – Exposed surface or crack

"Iddingsite"? (=Brown material) |

Yellowish green smectite

Dark green smectite

m.Pale brown smectite

ite "jI Alfm

tite J

AlteredRim

Fresh Rock

Figure 3. Variation in average thickness and nature of fillings in altered rim with age of rock.

Number J

Table 5. Features of clay minerals occurring in Leg 70 Galapagos basalts.

Type

1

2

3

4

5

6

7

Color

Orange to brown

Yellowish green

Brownish green

Pale green

Olive green

Pale brown

Dark brown

Color withCrossed Polars

Abnormaldark orange ordark brown

Abnormalgreenish gray

Abnormal darkbrownish gray

Light gray toyellow(1st order)

Abnormal darkolive to black

Abnormalbrownish gray

Dark grayto black

Habitus

Granulous

Radial or notradial needles

Small slabs

Fibers

Very tiny crystals(granulous aspect)

Tiny slabs (slightlygranulous aspect)

Slight granulous

Mode ofOccurrence

ves-vo-ol

ves-vo

ves-vo

ol

ves-vo-ol

vo-ol

ol

PrincipalChemical Features

Very high FeONo K2OLow Siθ2Medium FeO*Medium K2OHigh SiO2

High FeO*Medium K2OMedium Siθ2Low FeOVery low K2OHigh SiO2

Medium FeOHigh K2OLow FeO*NOK2OMedium Siθ2Low FeO*Medium K2OHigh Siθ2

50-63%

14-20%27-30%

2-3%50%39%

3%45%8-10%

1%48%

26-28%5-8%

6%

47%24%

2-3%52%

Note: ves = vesicle filling, vo = interstitial void filling, ol = olivine replacement.

mineral (analysis 16, Table 6) ± carbonates (analysis 17,Table 6). The calcium carbonate is rich in both MgO(2.10-2.50%) and MnO (1.22-1.74%);

3) Dark green clay minerals;4) A very pale brown clay mineral (analysis 14, Table

6) ± calcium carbonate ± pale greenish brown claymineral (analysis 15, Table 6; see also Plate 3).

The pale brown clay mineral was not observed inother samples from Hole 510. Thus, freshly cored rockis directly in contact with the dark green clay mineralzone, which is without any doubt the more abundant al-teration product in basalts from Hole 510.

Figure 5 represents schematically the FeO*, MgO,and K2O variations in the main clay minerals from Site510 as a function of their distance to an exposed surfaceor crack.

The sequence from dark green clay mineral to brownmaterial can be explained as following. During a firststage, a K, Fe-rich/Mg, Al-poor clay mineral would pre-cipitate from percolating fluids in interstitial voids. Theearly formed clay minerals would be transformed laterinto the brown Fe-rich material as a result of oxidation.

It is not possible to say whether the pale brown clayformed prior to or contemporaneously with the darkgreen clay.

Comparison of Alteration at Site 506-508 and Site 510

Three main features allow us to distinguish young ba-salts (Sites 506, 507, and 508) from older ones (Site 510)on the basis of their alteration (see Table 1):

1) Potassium-rich dark olive clay mineral is very rarein young basalts; however such basalts do contain amoderately rich-K, yellowish green clay. The first char-acteristic could be explained if the time of seawater-ba-salt interaction were too short to allow for the forma-tion of a K-rich clay mineral.

2) The K-poor pale brown clay mineral was not ob-served at Sites 506, 507, and 508. The same explanationas above is proposed. On the other hand, the presenceof this mineral could be related to bulk-rock chem-istry—i.e., basalts from Site 510 are richer in Mg thanare those from Sites 506, 507, and 508.

3) Carbonates are frequent at Site 510, very rare atSite 508, and were not observed at Sites 506 and 507.

381


Table 6. Selected microprobe analyses of secondary minerals.

Oxide(wt.%)

SiO2

A12O3

FeO*MnOMgOCaONa2θK2OTiO 2

NiOC r 2 O 3

P2O5

Total

1

14.960.63

49.250.061.790.280.310.540.22—

0.010.35

68.39

2

49.913.74

30.690.052.870.750.193.510.270.06—0.04

92.10

3

16.381.33

63.510.201.330.520.130.600.05—0.010.28

84.35

4

49.771.48

27.780.021.550.190.122.630.010.100.08—

83.74

5

45.151.34

39.080.081.810.540.182.97——0.110.06

91.32

6

51.230.58

28.230.065.460.630.154.98——0.270.06

91.63

7

51.910.408.62—

19.730.480.241.020.010.140.08—

82.64

8

53.061.51

26.710.135.590.510.086.500.08———

94.16

9

52.520.92

26.46—5.450.430.236.930.01

——

92.93

10

48.250.779.900.17

16.920.520.360.890.050.03——

77.86

11

52.030.38

29.380.064.060.400.067.22—

0.050.010.03

93.67

12

40.032.585.820.05

16.690.870.780.350.12—0.04—

67.32

13

51.740.31

25.790.124.620.270.107.81——0.01—

90.77

14

47.112.408.690.15

19.730.470.220.220.080.020.04—

79.13

15

47.682.11

12.550.04

18.390.730.350.340.13——0.02

82.33

16

52.280.39

24.240.03

10.360.730.152.520.07—0.020.05

90.84

17

0.06—0.501.332.33

53.210.03——0.050.071.01

58.60

Note: 1 = Sample 5O7C-3-1, 2-5 cm, orange material core of vesicle, 2 = Sample 507C-3-1, 2-5 cm, yellowish-green clay mineral in vesicle rim; 3 = Sample5O8B-3-1, 12-15 cm, orange material in vesicle core; 4 = Sample 508B-3-1, 12-15 cm, yellowish green clay mineral in a vesicle rim; 5 = Sample5O8B-3-1, 12-15 cm, brownish green clay mineral in vesicle; 6 = Sample 510-9-1, 44-46 cm, dark green clay mineral septum between two fresh olivineislets; 7 = Sample 510-9-1, 44-46 cm, pale green clay-mineral septum around the clay mineral analyzed in 6; 8 = Sample 510-9-1, 44-46 cm, dark greenclay mineral partly replacing olivine; 9 = Sample 510-9-1,44-46 cm, dark green clay-mineral replacing olivine rim; 10 = Sample 510-9-1,44-46 cm, palegreen clay mineral replacing olivine core and associated with clay mineral analyzed in 9; 11 = Sample 510-9-1, 44-46 cm, dark green clay mineral in amiarolitic void; 12 = Sample 510-9-1, 44-46 cm, pale brown clay mineral replacing olivine; 13 = Sample 510-9-3, 31-36 cm, dark green clay mineralpartly replacing olivine; 14 = Sample 510-9-3, 31-36 cm, pale brown clay mineral replacing olivine; 15 = Sample 510-9-3, 31-36 cm, pale greenishbrown clay mineral replacing olivine; 16 = Sample 510-9-3, 31-36 cm, dark brown clay mineral replacing olivine; 17 = Sample 510-9-3, 31-36 cm, car-bonate associated with dark green clay mineral and replacing olivine. Most analyses are representative of several made of optically similar materials,which gave practically identical results.

FeO* Legend

D Brown materialM Brownish green clay mineralV Yellowish green clay mineralΔ Dark green clay mineral• Dark brown clay mineralif Pale green clay mineral•<J>•Pale brown clay mineral

MgO K , 0

Figure 4. FeO*-MgO-K2O diagram for secondary clay minerals inbasalts. (Full = Site 510; empty = Sites 506, 507, 508.)

The formation of these carbonate minerals may be ex-plained by a longer period of interaction between therocks and seawater.

The following secondary mineral sequence was ob-served on the ship (from the vein walls to the center):pyrite, various clays, and Fe-rich brown material. Thisindicates that conditions changed from reducing to oxi-dizing during alteration. But we did not observe this se-quence in any of our samples.

BULK-ROCK CHEMISTRY OF ALTERED BASALTS

Most of the "fresh" samples we studied display al-tered halos. In this case, both fresh and altered rocks

S 60

J 400 20o

LL

0

sf 81 4O

CM n

s? 2 0

"iddingsite

pale brownsmectite

dark green smectite

10i

12 14

exposed surface fresh rock

Figure 5. Variation of the FeO*, K2O, and MgO average contents. Inclay minerals: distance to exposed surface.

have been separated and analyzed (Table 1). As the al-tered halos are zoned (see Fig. 3), the analyses representthe chemical composition of the entire altered area.

We have chosen to show chemical variations in theoxides in terms of their oxidation ratio (O°). This ratiois expressed by the atomic ratio of Fe3 + /FeT.

382


Representative points of pairs of fresh and alteredrocks are connected by tie lines in Figures 6 and 7, whichshow the following:

1) A systematic oxidation of iron, with an Ox° gapranging between 0.20 and 0.25 for "fresh" and alteredrocks, respectively. The Fe content increases similarly(Table 3).

2) Alteration is generally accompanied by hydration.However, three samples (Samples 506G-2-1, 49-52 cm;510-9-3, 31-36 cm; and 510-11-1, 79-82 cm) show theopposite trend—i.e., the fresh basalts are more hy-drated than their altered rim. In such cases the "ap-parently fresh" basalt must have been hydrated beforebeing oxidized. The explanation for this is unclear.

3) K2O content drastically increases with alteration.Fresh basalts contain from 0.04 to 0.24 wt.% K2O,whereas altered rims contain 0.18 to 0.50 wt.% K2O.

4) Most altered rims are slightly richer in Co than thefresh equivalents of the same pair. It is noticeable thatthe Co content in Sample 506G-2-1, 49-52 cm increasedgreatly during alteration (193 ppm). The opposite wasobserved in Hole 396B of Leg 46 by Honnorez et al.(1978) for the brown rims where olivine was replaced byiron oxyhydroxides.

5) Fresh samples from Site 510 are very poor inP2O5. Thus, any transformation resulting from a fluidphase induces no perceptible effect, or it induces an in-crease in phosphorus during the first stage of alteration.

6) During alteration, content of all other elementsdecreases more or less. In fact, the more abundant anelement is in the fresh basalt, the more it decreases withalteration.

7) Sample 506G-2-1, 49-52 cm behaves erratically:The fresh sample represents some kind of transition be-tween basalts from Sites 507 and 508 and those fromSite 510. Furthermore, during alteration, it often showsopposite trends with respect to other sites, with a cleargain of Co (90 ppm) Ti, Ni, and V; a slighter gain ofMg?, Na, and Al; no change in Ca; and a loss of K andH2O. This could be explained in terms of a primarycompositional difference between "fresh" and alteredsamples.

8) Finally, an important gain in Ni occurs during al-teration in Sample 507C-3-1, 2-5 cm.

In summary, the first alteration stage is hydrationand oxidation. The apparent variations in oxide contentduring this alteration are: an increase in H2O, K2O,FeO*, and a slight decrease in SiO2, A12O3, MgO, CaO,and Na2O.

Various authors (Matthews, 1971; Hekinian, 1971;Thompson, 1973; Hart et al., 1974; Shido et al., 1974;Honnorez, in press) agree that the Fe3 + /Fe2+ ratio andthe content of K2O and H2O increase in oceanic basaltsaltered at low temperature. Most of the authors (Mat-thews, 1971; Thompson, 1973) also report that Ca, Mg,and Si decrease. Al content of altered rocks decreasesaccording to Hart et al. (1974) and Thompson (1973),but remains unaffected according to most of the otherauthors (Hekinian, 1971; Hart, 1970; Thompson, 1973).The behavior of Na is very controversial.

In their study of alteration of pillow basalts in Hole396B (IPOD, Leg 46), Honnorez et al. (1979) observed

chemical variations. They calculated the values of thesevariations, assuming that A12O3 is immobile duringalteration. For them, an apparent gain of FeO*, A12O3,TiO2, CaO, and Na2O merely results from actual lossesof MgO and SiO2. This normalization was possible be-cause of the large number (77) of fresh basalts analyzedin Hole 396B and because of the homogeneity of thevalues.

It is not possible here to balance variations by an av-erage Al2O3-content value for all sites because of theevolutive feature discussed in our study of fresh basalts.

Site by site, average A12O3 content is 15.84 wt.% forSite 510, 13.64 wt.% for Site 506, and 13.18 wt.<Vo forSites 507 and 508. (The last two were grouped becauseof their compositional similarities.) Normalization ofA12O3 sheds light on the behavior of some elements. In-creases of H2O, K2O, and FeO* content persist, but thevariations in content of SiO2, MgO, CaO, and Na2Obecome insignificant.

If one considers hydration to be the first alterationstage, changes in the concentrations of other elementscan be estimated from normalized concentrations ofcations on a wt.% water-free basis (Table 3). When wecancel the effects of hydration in this way, we find asignificant gain in Fe and K and a slight loss of Mg. Theother element variations (i.e., Si, Ca, and Na) show avery slight statistical decrease, which is not clearly sig-nificant. The small variations in Al content are more ir-regular, but the most important change is decreasingvariations, mostly in basalts from Sites 508 and 510.Potassium content can increase by a factor of five andshows the largest relative variations.

In conclusion, the first alteration stage of apparentlyfresh oceanic basalt is mainly characterized by oxidationand hydration, leading to a "dilution" of elements: anapparent decrease of SiO2, MgO, and CaO. The K2O-rich fluid phase causes an increase in this element in thealtered rock. The altered rims are also slightly more richinFe.

A further alteration stage, observed in a "fresh" ba-salt which is already oxidized and hydrated, is mainlycharacterized by a loss of SiO2 and MgO (Honnorez etal., 1979). In this case, the K2O behavior during altera-tion becomes erratic, but potassium content tends to in-crease.

GENERAL CONCLUSIONYoung crust from the Galapagos Spreading Center

(Sites 506, 507, and 508) is represented by a fine- tomedium-grained basalt with some Plagioclase and clino-pyroxene phenocrysts, abundant titanomagnetite, butno olivine. Site 510 basement, which is older, is repre-sented by a fine- to medium-grained, Plagioclase, Mg-rich olivine phyric basalt. Titanomagnetite is less abun-dant. Phenocryst assemblages, Fe-Ti oxide abundance,and bulk-rock chemistry indicate that Site 510 basaltsare more primitive than are those from Sites 506, 507,and 508. Comparison with a previous study (Fodor etal., 1980) of basalts from Leg 54 (Sites 424 and 425)confirms that the composition of Site 510 basalts is veryclose to that of the tholeiitic parental magma. Frac-tionation increases from Site 510, 425, 506, 507, and 508

383


52

51

3? 50

49

48

- SiO2

•

•

1

i T

17

16

ss 15

j*"* 14

13

12

" AUO-,

-

| 1

| i "

SS

14

13

12

11

10

Q

T~- CaO

-

j

i i

-

=—

i i

1.0 -

0.1

Legend O= Site 506 D= Site 507 0= Site 508 ft = Site 510 = Site 424 v= Site 425Black symbol = fresh rock; empty symbol = altered rim

Figure 6. Bulk-chemical variations with alteration: weight oxides vs. O°.

to Site 424, and results in ferrobasalts in the last threesites. The drastic decrease of A12O3, MgO, CaO, Cr,and Cu and the important increase of FeO*, TiO2, V,and Na2O, Sr, Ba indicate fractionation of olivine andspinel plus Plagioclase from the primitive tholeiite. No-tice that the more evolved basalts are youngest.

Alteration of these basalts appears as dark rims,whose thickness increases with age of basalts from Site506 to Sites 508 and 510. The most common and abun-dant minerals in the altered rims are clay minerals(probably smectites), occurring in vesicles, miaroliticvoids, and, eventually, as olivine replacement (Site 510).

384


400 -

125

100

αS

~ 75

50

25

_Sr

_

-

-

_

0.2 0.3 0.4 0.1 0.2 0.3 0.4

Legend O= Site 506 D= Site 507 0= Site 508 TV = Site 510 = Site 424 v = Site 425Black symbols = fresh rock; empty symbols =altered rim

Figure 7. Bulk-chemical variations with alteration: ppm vs. O°.

The altered rims show concentric zonation, marked bydifferent types of minerals, indicating that the alterationof Galapagos Spreading Center basalts is characterizedby (at least) three stages:

1) A reducing stage (pyrite formation), also found byHonnorez et al. (1978) in basalts from Leg 46, Hole396B.

2) A nonoxidizing or slightly oxidizing stage (withformation of various clay minerals, particularly thosethat are K-rich), together with hydration.

3) An oxidizing stage ("iddingsite" and brownishgreen clay mineral formation), during which an oxida-tion front moves toward the innermost parts of thealtered rim.

One can distinguish young basalts (Sites 506, 507,and 508) from older ones (Site 510) in terms of altera-tion: the oldest basalts contain very K-rich dark olive

clay minerals, very K-poor pale brown clay minerals,and carbonates, whereas young basalts contain moder-ately rich-K, yellowish green clay mineral, and little orno carbonates (Site 508). This can be explained by too-short interaction between basalt and seawater. Variationin the bulk-rock chemical composition indicates that thefirst alteration stage of apparently fresh oceanic basaltsis mainly characterized by oxidation and hydration,which leads to an apparent "dilution" of elements.After normalization on a water-free basis, we find a sig-nificant gain in Fe and K and a slight loss of Mg in thealtered rocks.

ACKNOWLEDGMENT

We want to thank M. Bohn for his help during the electron micro-probe analyses. We also thank D. Shaw and T. Juteau for reviewingthis paper, and J. Honnorez for his critical comments.

385


REFERENCES

Anderson, R. N., Clague, D. A., Klitgord, K. D., Marshall, M.,and Nishimori, R. K., 1975. Magnetic and petrologic variationsalong the Galapagos Spreading Center and their relation to theGalapagos melting anomaly. Geol. Soc. Am. Bull., 86:683-694.

Bohlke, J. F. P., 1978. Alteration of deep-sea basalts from Site 396B[M.S. thesis]. University of Miami, Miami.

Clague, D. A., and Bunch, T. E., 1976. Formation of ferrobasaltat East Pacific Midocean Spreading Centers. /. Geophys. Res.,81(23):4247-4256.

Deer, W. A., Howie, R. A., and Zussman, J., 1963. Rock FormingMinerals: London (Longmans).

Fodor, R. V., Berkley, J. L., Keil, K., Hustler, J. W., Ma, M.-S., andSchmitt, R. A., 1980. Petrology of basalt drilled from the Gala-pagos Spreading Center, DSDP Leg 54. In Rosendahl, B. R.,Hekinian, R., et al., Init. Repts. DSDP, 54: Washington (U.S.Govt. Printing Office), 737-750.

Frey, F. A., Bryan, W. B., and Thompson, G., 1974. Atlantic Oceanfloor: Geochemistry and petrology of basalts from Legs 2 and 3 ofthe Deep Sea Drilling Project. /. Geophys., 79:5507-5527.

Hart, R. A., 1970. Chemical exchange between seawater and deepocean basalts. Earth Planet. Sci. Lett., 9:269-279.

, 1973. A model for chemical exchange in the basalt-sea-water systems of oceanic layer II. Can. J. Earth Sci., 10:799-816.

Hart, S. R., Erlank, A. J., and Kable, E. J. D., 1974. Seafloor basaltalteration: Some chemical and Sr isotopic effects. Contrib. Miner-al. Petrol., 44:219-230.

Hekinian, R., 1971. Chemical and mineralogical differences betweenabyssal hill basalts and ridge tholeiites in the Eastern PacificOcean. Mar. Geol., 11:77-91.

Honnorez, J., in press. The oceanic lithosphere. In Emiliani, C. (Ed.),The Sea (Vol. 7): New York (Wiley).

Honnorez, J., Bohlke, J. K., and Honnorez-Guerstein, B. M., 1978.Petrographical and geochemical study of the low-temperature sub-

marine alteration of basalt from Hole 396B, Leg 46. In Dmitriev,L., Heirtzler, J., et al., Init. Repts. DSDP, 46: Washington (U.S.Govt. Printing Office), 299-329.

Matthews, D. H., 1971. Altered basalts from Swallow Bank, an abys-sal hill in the N.E. Atlantic and from a nearby seamount. Philos.Trans. R. Soc. London, Ser. A., 268:551-571.

Melson, W. G., Valuer, T. L., Wright, T. L., Byerly, G., and Nelen,J., 1976. Chemical diversity of abyssal volcanic glass eruptedalong Pacific, Atlantic and Indian Ocean seafloor spreading cen-ters. Am. Geophys. Union Monograph, 19:351-367.

Mevel, C , 1980. Mineralogy and chemistry of secondary phases inlow temperature altered basalts from DSDP Legs 51, 52, 53. InDonnelly, T., Francheteau, J., Bryan, W., Robinson, P., Flower,M., Salisbury, M., et al., Init. Repts. DSDP, 51, 52, 53, Pt. 2:Washington (U.S. Govt. Printing Office), 1299-1318.

Miyashiro, A., 1974. Volcanic rock series in island arcs and activecontinental margins. Am. J. Sci., 274:321-355.

Schilling, J. G., Anderson, R. N., and Vogt, P., 1976. Rare earth,Fe and Ti variations along the Galapagos Spreading Center, andtheir relationship to the Galapagos mantle plume. Nature, 261:108-113.

Shido, F., Miyashiro, A., and Ewing, M., 1974. Compositional vari-ation in pillow lavas from the Mid-Atlantic Ridge. Mar. Geol.,16:177-190.

Thompson, G., 1973. A geochemical study of the low-temperatureinteraction of seawater and oceanic igneous rocks. Eos, 54:1015-1019.

Thompson, G., Bryan, W. B., Frey, F. A., Dickey, T. S., and Suen,C. J., 1976. Petrology and geochemistry of basalt from DSDP Leg34, Nazca Plate. In Yeats, R. S., Hart, S. R., et al., Init. Repts.DSDP, 34: Washington (U.S. Govt. Printing Office), 215-226.

Wilshire, H. G., 1958. Alteration of olivine and orthopyroxenein basic lavas and shallow intrusions. Am. Mineral., 43:120-145.

386


0.035 mm

0.035 mm

Plate 1. Portions of zonational pattern in altered rim (plane polarized light); Sample 508B-3-1, 12-15 cm. 1. Vesicle with yellowish green claymineral. 2. Vesicle filled with brown material, yellowish green clay mineral. (Scale bar = 0.035 mm.)

387


0.035 mm

0.035 mm

Plate 2. Portions of zonational pattern in altered rim (plane polarized light); Sample 510-9-3, 31-36 cm. 1. Olivine microphenocryst replaced bydark green clay mineral. 2. Olivine microphenocryst completely replaced by dark green clay mineral "iddingsite" (brown material) close to athin "iddingsite" vein. (The differences in color of the materials cannot be distinguished here.) (Scale bar = 0.035 mm.)

388


0.035 mm

0.035 mm

Plate 3. Portions of zonational patterns in two altered rims (plane polarized light). 1. Sample 508B-3-1, 12-15 cm; vesicle filled with brown ma-terial. 2. Sample 510-9-3, 31-36 cm; olivine phenocryst completely replaced by pale brown clay mineral carbonate. (Scale bar = 0.035 mm.)

389

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