53. CHROMIAN SPINELS IN COSTA RICA BASALTS, DEEP SEA ... · ing olivine phenocrysts, obtained by electron micro-probe, is also shown. RESULTS More than 50 chromian spinels were analyzed

53. CHROMIAN SPINELS IN COSTA RICA BASALTS, DEEP SEA DRILLING PROJECT SITE 505—A PRELIMINARY INTERPRETATION OF ELECTRON MICROPROBE ANALYSES1

Toshio Furuta and Hidekazu Tokuyama, Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164 Japan

ABSTRACT

The compositions of chrome spinels of Costa Rica Rift basalts from Deep Sea Drilling Project Site 505 vary depend-ing on their occurrences as (1) inclusions in olivine crystals, (2) inclusions in Plagioclase crystals, and (3) isolated crystalsin variolitic or glassy samples. The variations are a consequence of (1) changes of melt compositions as crystallizationproceeds, and (2) contrasting behavior of olivine and Plagioclase in competition with spinels for Al and Mg. Some spi-nels have skeletal rims compositionally less magnesian than mineral cores; however, the cores do not appear to be xeno-crysts, unlike some texturally similar spinels in Mid-Atlantic Ridge basalts.

INTRODUCTION

Chromian spinels are characteristic accessory miner-als in the Costa Rica Rift basalts, occurring as tiny eu-hedral and subhedral crystals included in or attached toolivine phenocrysts or Plagioclase phenocrysts. They alsooccur in variolitic glass at pillow margins. Their para-genesis indicates that they probably are products of theearliest phase of crystallization of basaltic magma. It hasbeen suggested that some of these spinels coexisted andwere equilibrated with primitive basaltic liquid (e.g., Ir-vine, 1965, 1967; Sigurdsson and Schilling, 1976). There-fore, it is interesting to clarify the genesis of the chro-mian spinel and the early stage of crystallization of thebasaltic magma. We will report here on chemical com-positions of chromian spinels in submarine basalts fromDSDP Site 505 (01°54.8'N; 83°47.4'W), on the CostaRica Rift.

SAMPLES

Samples for this study were drilled at DSDP Site 505from the Costa Rica Rift during Leg 69. The crustal ageof Site 505 is estimated to be about 3.9 m.y. on the basisof magnetic anomalies (Langseth et al., this volume).The total recovery of basalt was 0.5 meters from Hole5O5A and 6.85 meters from Hole 505B. All recoveredrocks are sparsely to moderately plagioclase-olivine-phy-ric basalt with a few chromian-spinel crystals; the rocksare petrographically and chemically almost identical(Etoubleau et al., this volume). The average composi-tion of 10 whole-rock samples from 5O5A and 5O5B isgiven in Table 1. Chromian spinel occurs in olivine andPlagioclase phenocrysts and variolitic glasses (Fig. 1).Spinels included in olivine or Plagioclase are subhedralor anhedral, but most of them occur as isolated grainsand are euhedral. Some of these grains show a distinctouter rim which has a skeletal texture. Table 2 shows themodes of occurrence of spinels as inclusions in pheno-crysts and isolated grains. The Fo component of coexist-

Table 1. Average chemical composi-tion of 10 fresh basalts from Holes5O5A and 5O5B (based on ship-board XRF analyses obtained byJ. Etoubleau).

Component

SiO2

Tiθ2A12O3

FeO*MnOMgOCaONa2OK 2 O

P 2 θ 5

Weight Percent

49.540.96

16.089.530.158.70

12.682.010.050.08

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).

Total iron as FeO.

ing olivine phenocrysts, obtained by electron micro-probe, is also shown.

RESULTS

More than 50 chromian spinels were analyzed for thisstudy, using a JXA-5 (JEOL) electron microprobe. Mostsamples were analyzed for nine elements (Si, Al, Ti, Fe,Mn, Mg, Cr, Ni, V). Twenty-four representative analy-ses of spinels and structural formulas are shown inTable 3, where the data are recalculated as structuralformulas on the basis of 4-oxygens stoichiometry. Chem-ical compositions of the spinels have relatively small va-riation (Table 3; Fig. 2), like the coexisting olivine phe-nocrysts (Fo86_89). Chromian spinels in these samples arerelatively high in the ratio Cr/(Cr + Al)—0.38 to 0.55—and moderate in the ratio Mg/(Mg + Fe2+)—0.55 to0.70. In this respect, they closely resemble spinels termed"magnesiochromite" in Mid-Atlantic Ridge basalts (Si-gurdsson and Schilling, 1976; Fig. 2A). Distinctionsamong the present spinels with different modes of occur-rence are revealed when their chemical compositions areplotted on the plane of the spinel compositional prism,Cr/(Cr + Al) versus Mg/(Mg + Fe 2 +). Spinels includedin olivine phenocrysts have relatively low values of theratio Cr/(Cr + Al) (dashed line in Fig. 2B). Spinels in-

805

T. FURUTA, H. TOKUYAMA

Figure 1. A. Reflected-light photomicrograph of a spinel with skeletal texture in Sample 505B-5-1, 31-33 cm. Spinel grain is approximately 160 µmacross and sits in variolitic glass. Chemical analyses of this grain are shown in Table 3; analysis No. 21 is on the core and No. 22 on rim of thisgrain. Small bright minerals are titanomagnetites. B. Reflected-light photomicrograph of a spinel in Sample 5O5B-3-2, 86-88 cm. Spinel grain isapproximately 25 µm across and is included in Plagioclase. C. Transmitted-light photomicrograph of a spinel in Sample 505B-4-1, 10-12 cm.Spinel grain is about 35 µm across and is included in olivine. D. Reflected-light photomicrograph of a spinel in Sample 505B-6-1, 109-111 cm.Spinel grain is about 60 µm across and is attached to olivine (lower left) and Plagioclase (right and above) in variolitic glass. Microprobe analysisacross this showing compositional zoning spinel is shown in Figure 3. E. Reflected-light photomicrograph of a spinel in Sample 505B-4-1, 10-12cm. Spinel grain is about 25 µm in diameter and is included in Plagioclase. The upper part of this grain (brighter part) is titanomagnetite. F.Reflected-light photomicrograph of a spinel in Sample 5O5B-5-1, 31-33 cm. Spinel grain is about 30 µm in diameter and is included in alteredolivine.

806

CHROMIAN SPINELS IN COSTA RICA RIFT BASALTS

Table 2. Modes of occurrence of chromian spinels as inclu-sions in phenocrysts and as isolated grains in basalts fromHoles 505A and Hole 5O5B.

Sample(interval in cm)

505A-2-1, 45-47505B-2-1, 115-117

2-2, 81-833-1, 114-1163-2, 86-884-1, 10-125-1, 31-336-1, 109-1116-2, 6-8

In InOlivine Plagioclase

- (87.0-87.3) ++ (87.5-87.8) +- (86.9-87.3) ++ (86.4-86.6)+ (?) ++ (87.0-88.5) ++ (87.3-87.5)+ (87.9-88.4) + ++ (87.4)

IsolatedGrain

+ +

++

+ ++++

+

Notes: - = not detected; + = one to five grains in each thin-section; + + = more than five grains in each thin-section;Fo component of coexisting olivine shown in parentheses.

eluded in Plagioclase phenocrysts have relatively high Cr/(Cr + Al) ratios (broken line in Fig. 2B). The Cr/(Cr +Al) ratios of spinels in variolitic basaltic glass are scat-tered; nevertheless, the range of Cr/(Cr + Al) of thesethree types of spinels in the present samples is very lim-ited compared with the range in spinels from Mid-Atlantic Ridge basalts indicated on Figure 2A (Sigurds-son and Schilling, 1976). As shown in Table 2, mostsamples have the three types of spinels in the same thin-section.

The chemical zoning of several grains of spinel isshown in Table 3 (21-22, 23-24) and in Figure 2B (shownby arrows). The result of a microprobe analysis on a eu-hedral spinel crystal in Sample 505B-6-1, 109-111 cm isshown in Figure 3. This spinel is attached to olivine andPlagioclase phenocrysts in variolitic glass. A12O3 andMgO contents decrease from core to rim. On the con-trary, FeO* and TiO2 contents increase from core to rim.Cr2O3 content is constant. The chemical zoning of mostof the analyzed spinels is typified by the trend in Sample505B-6-1, 109-111 cm (with one exception); however,zoning of spinels having skeletal rims in variolitic glassis different; Cr/(Cr + Al) increases from core to marginand Mg/(Mg + Fe 2 +) decreases.

DISCUSSION

Spinel compositions vary widely in oceanic basalts.Fisk and Bence (1980) classified spinels from FAMOUSbasalts into three types; (1) spinels enriched in A12O3

and with reaction coronas, (2) spinels enriched in Cr2O3

and without reaction coronas, and (3) spinels intermedi-ate in A12O3 and Cr2O3. Sigurdsson and Schilling (1976)divided spinels from Mid-Atlantic Ridge basalts intothree types: (1) magnesiochromite with Cr/(Cr + Al) =0.4 to 0.5, (2) titaniferous magnesiochromite, and (3)chrome spinel with Cr/(Cr + Al) = 0.23 to 0.27. Com-pared with the spinels from Mid-Atlantic Ridge basaltsand FAMOUS basalts, spinels from the Costa Rica Riftbasalts are not highly variable and are slightly poorer inMg/(Mg + Fe 2 +) (Fig. 2A). Despite this small variabil-ity, we can distinguish compositional differences, whichcorrelate with the following occurrences: (1) spinels in-

cluded in olivine, (2) spinels included in Plagioclase, and(3) isolated spinels in variolitic glass.

The Cr/(Cr + Al) ratio of spinels included in olivinephenocrysts is lower when plotted on a plane of the spi-nel compositional prism, Cr/(Cr + Al) versus Mg/(Mg+ Fe 2 + ) . The Cr/(Cr + Al) ratio of the spinel increaseswith decreasing Mg/(Mg + Fe) (Fig. 2B). From resultsof experimental crystallization of chrome spinels (Fiskand Bence, 1980), the Cr/(Cr + Al) of spinel coexistingwith olivine decreases with decreasing temperature be-tween 1250 and 1200°C at / θ 2-10- 9 5 . However, at/ θ 2-10- 8 5 the Cr/(Cr + Al) of the spinel coexisting witholivine increased from 0.413 to 0.464 as temperaturewas decreased from 1250 to 1230°C. At still higher oxy-gen fugacities (fθ2 of 10 ~7-5), the Cr/(Cr + Al) increasedfrom 0.413 to 0.454 as temperature was decreased from1250 to 1232°C. Consequently, for Cr/(Cr + Al) to de-crease with decreasing temperature, low fθ2 seems re-quired, causing reduction of Cr 3 + to Cr 2 + in the melt.The Mg/(Mg -I- Fe 2 +) of spinel decreases with decreas-ing temperature at all oxygen fugacities between 10~7 5

and I0-9-5 (Fisk and Bence, 1980). This probably re-flects the decreasing Mg/(Mg + Fe 2 +) of the melt causedby olivine crystallization (Fisk and Bence, 1980). There-fore we interpret the compositions of spinels included inolivine to decrease gradually in Mg/(Mg + Fe 2 +) andincrease in Cr/(Cr + Al) as a consequence of decreasingtemperature rather than changes i n / θ 2 . Based on com-parisons with experimental data,/ θ 2 was probably 10 ~8-5

or greater.Spinels included in Plagioclase phenocrysts have high-

er Cr/(Cr + Al) than those included in olivine (Fig. 2B).The Cr/(Cr + Al) of spinel increases markedly, oncePlagioclase starts to form, probably because Plagioclasetakes up to much Al (Fisk and Bence, 1980). The Cr/(Cr+ Al) of the spinel decreases slightly as Mg/(Mg + Fe2 +)decreases. This is caused by the increasing volume ofspinel, and consequent decrease of Cr in the melt, withdecreasing temperature (Fisk and Bence, 1980).

Thus we consider that crystallization of spinels fromCosta Rica Rift basalts depends on the following.

1) Spinel crystallized during the first stage increasesin Cr/(Cr + Al) and decreases in Mg/(Mg + Fe 2 +) withdecreasing temperature, rather than changes i n / θ 2 (Fig.2B, arrow I).

2) When temperature decreases to the temperature atwhich Plagioclase begins to crystallize, the Cr/(Cr + Al)of spinel increases markedly (Fig. 2B, arrow II).

3) Spinel crystallized during the third stage decreasesslightly in Cr/(Cr + Al) as Mg/(Mg + Fe 2 +) decreaseswith decreasing temperature (Fig. 2B, arrow III).

The chemical composition of isolated spinel in vario-litic glass is scattered on a plane of the compositionalprism, Cr/(Cr + Al) versus Mg/(Mg + Fe 2 +) (Fig. 2B).This probably reflects growth of spinels at each of thethree above-mentioned stages.

The Costa Rica Rift spinels show two types of com-positional zoning. One is characterized by increasingCr/(Cr + Al) with decreasing Mg/(Mg + Fe 2 +) fromcore to margin (Table 3, 23 and 24). Similar zoning hasbeen reported by Ridley (1977). The other is character-

807

T. FURUTA, H. TOKUYAMA

Table 3. Chemical composition and structural formulas of chromian spinels from Costa Rica Rift basalts, DSDP Site 505.

Component

SiO2

A12O3

TiO2

FeO*MnOMgOCr 2 O 3

NiO

v 2 o 3Total

SiAlTiFe3 +

Fe 2 +

MnMgCrNiVMg/(Mg + Fe 2 + )Cr/(Cr + Al)

1

0.028.46

0.4118.710.45

16.0336.790.200.19

101.24

0.00.9950.0090.1380.3260.0110.7090.8630.0040.0040.6850.464

2

0.023.24

0.4620.87

0.5214.7241.59

0.210.23

101.84

0.00.8260.0110.1520.3740.0130.6621.0160.0050.0060.6390.551

3

0.028.17

0.5518.100.20

15.4436.210.360.15

99.18

0.01.0030.0080.1280.3290.0050.6960.8650.0090.0040.6780.463

4

0.028.32

0.4817.620.22

15.7235.130.370.15

98.01

0.00.9970.0100.1690.3570.0060.7000.8300.0090.0030.6620.454

5

0.024.81

0.5318.240.71

15.0338.920.380.15

98.77

6

0.1328.67

0.2717.520.40

13.9236.160.100.24

97.41

Sample

7

0.1226.78

0.4318.200.37

13.8437.26

0.130.19

97.32

No.

8

0.0427.75

0.4418.870.43

15.2038.940.080.21

101.96

9

0.0726.47

0.4718.950.45

15.4039.63

0.130.18

101.78

Number of cations on the basis of 4 oxygens

0.00.9020.0070.1450.3250.0180.6910.9490.0090.0040.6800.513

0.0041.0370.0060.0800.3700.0100.6370.8770.0020.0060.6330.458

0.0040.9490.0100.1020.3700.0100.6400.9140.0030.0050.6340.483

0.0010.9680.0100.1150.3520.0110.6710.9110.0020.0050.6560.485

0.0020.9300.0110.1320.3410.0110.6840.9340.0030.0040.6670.501

10

0.0529.20

0.4518.280.37

15.7137.190.200.26

101.71

0.0021.0120.0110.1230.2360.0090.6890.8650.0050.0060.6790.461

11

0.1630.080.46

18.150.39

15.9136.410.160.18

101.90

0.0051.0350.0100.1200.3240.0100.6930.8400.0050.0040.6820.448

12

0.1427.010.55

18.530.47

15.3637.32

0.270.21

99.86

0.0040.9620.0120.1410.3270.0120.6920.8910.0060.0050.6790.481

13

0.0132.280.50

19.450.41

15.9032.700.280.20

101.73

0.001.1270.0110.1250.3420.0110.6810.7430.0060.0050.6660.397

*Total iron as FeO; ferric and ferrous iron calculated from total Fe and structural formula. 1. 505A-2-1, 45-47 cm, brown subhedral spinel included inelongated Plagioclase, diam. 50 µm. 2. 5O5A-2-1, 45-47 cm, brown subhedral spinel attached to olivine in variolitic glass, diam. 15 µm. 3. 5O5B-2-1,115-117 cm, small brown spinel attached to Plagioclase in variolitic glass, diam. 12 µm. 4. 5O5B-2-1, 115-117 cm, brown subhedral spinel attached toPlagioclase in variolitic glass, diam 20 µm. 5. 505B-2-1, 115-117 cm, brown subhedral spinel included in olivine, diam. 20 µm. 6. 505B-2-2, 81-83 cm,small brown spinel in variolitic glass, diam. 15 µm. 7. 505B-2-2, 81-83 cm, brown subhedral spinel in variolitic glass, diam. 18 µm. 8. 5O5B-3-1,114-116 cm, rim of brown rectangular spinel attached to Plagioclase in variolitic glass, diam. 25 µm. 9. Core of crystal in No. 8. 10. 5O5B-3-1,114-116 cm, rim of brown subhedral spinel attached to Plagioclase in variolitic glass, diam. 40 µm. 11. Core of crystal in No. 10. 12. 5O5B-3-1,114-116 cm, small spinel in variolitic glass, diam. 13 µm. 13. 505B-3-1, 114-116 cm, brown euhedral spinel in altered olivine, diam. 60µm. 14. 505B-3-2, 86-88 cm, subhedral spinel included in Plagioclase, diam. 15 µm. 15. 505B-3-2, 86-88 cm, subhedral spinel included in Plagioclase,diam. 20 µm. 16. 505B-4-1, 10-12 cm, brown subhedral spinel included in olivine, diam. 25 µm. 17. 505B-4-1, 10-12 cm, brown subhedral spinel in-cluded in Plagioclase, diam. 50 µm. 18. 5O5B-5-1, 31-33 cm, brown subhedral spinel included in olivine, diam. 25 µm. 19. 505B-5-1, 31-33 cm,brown subhedral spinel attached to olivine and Plagioclase in variolitic glass, diam. 15 µm. 20. 505B-5-1, 31-33 cm, subhedral spinel in varioliticglass, diam. 30 µm. 21. 5O5B-5-1, 31-33 cm, core of brown subhedral spinel with skeletal texture in variolitic glass, diam. 160 µm. 22. Rim of crystalin No. 21. 23. 5O5B-6-1, 109-111 cm, rim of brown euhedral spinel attached to olivine and Plagioclase in variolitic glass, diam. 60 µm. 24. Core ofcrystal in No. 23. 25. 505B-6-1, 109-111 cm, brown subhedral spinel in variolitic glass, diam. 20 µm. 26. 505B-6-1, 109-111 cm, brown subhedralspinel included in olivine, diam. 20 µm. 27. 505B-6-1, 109-111 cm, brown subhedral spinel in variolitic glass, diam. 50 µm. 28. 5O5B-6-2, 6-8 cm,brown subhedral spinel included in olivine, diam. 30 µm.

ized by decreasing Cr/(Cr + Al) with decreasing Mg/(Mg + Fe2 +) from core to margin (Table 3, 21 and 22).The former type may be formed at fθ2 other than10-8 5; the latter type is rarely observed and always oc-curs with skeletal texture. This type might be formed at/o 2 less than 10 ~9-5. This is supported by the fact thatthe calculated Fe 3 + of the former type of spinel is richerthan that of latter type (Table 3).

Spinels having skeletal rims occur in Sample 5O5B-5-1,31-33 cm (Fig. 1A). Fisk and Bence (1980) reported asimilar texture, which they called a corona-like rim.Their samples are characterized by high A12O3 (~40 wt.°/o) in cores and low A12O3 (-30 wt. °7o) in the rims.These authors could not synthesize such highly alumi-nous spinel; hence, they concluded that the highly alu-minous spinels are essentially xenocrysts which crystal-lized at high pressures, or from a magma enriched inA12O3, and perhaps MgO. They interpreted the corona-like rims to be reaction products formed under differentconditions. Although our spinels seem to have similartextures, they are much poorer in A12O3. Consequently,the Costa Rica Rift spinels with skeletal rims formedunder conditions different than those of the Mid-Atlan-

tic Ridge spinels with corona-like rims. The skeletal rimsmay have formed during rapid cooling (Natland, et al.,this volume).

ACKNOWLEDGMENT

The manuscript benefited from comments and suggestion by J.Natland, S. Arai, K. Hamuro, and M. Toriumi, and a critical reviewby K. Kobayashi and K. Fujioka.

REFERENCES

Dick, H. J. B., and Bryan, W. B., 1979. Variation of basalt phenocrystmineralogy and rock compositions in DSDP Hole 396B. In Dmi-triev, J., Heirtzler, J., et al., Init. Repts. DSDP, 46: Washington(U.S. Govt. Printing Office), 215-225.

Fisk, M. R., and Bence, A. E., 1980. Experimental crystallization ofchrome spinel in FAMOUS basalt 527-1-1. Earth Planet. Sci.Lett., 48:111-123.

Irvine, T. N., 1965. Chromian spinel as a petrogenetic indicator. Part1. Theory. Can. J. Earth Sci., 2:648-672.

, 1967. Chromian spinel as a petrogenetic indicator. Part 2.Petrologic applications. Can. J. Earth Sci., 4:71-103.

Ridley, W. I., 1977. The crystallisation trends of spinels in Tertiary ba-salts from Rhum and Muck and their petrogenetic significance.Contr. Mineral. Petrol., 64:243-255.

Sigurdsson, H., and Schilling. J. G., 1976. Spinels in Mid-AtlanticRidge basalts: chemistry and occurrence. Earth Planet. Sci. Lett.,29:7-20.

808

CHROMIAN SPINELS IN COSTA RICA RIFT BASALTS

Table 3. (Continued).

14

0.026.180.36

17.900.19

15.9438.890.150.17

15

0.027.160.28

17.360.21

16.0638.680.160.24

16

0.028.630.43

17.700.23

16.2836.470.140.14

17

0.0726.660.43

18.040.20

14.2138.%0.140.18

18

0.0432.220.45

18.290.37

15.2133.590.050.24

19

0.027.590.51

18.300.35

14.8036.650.150.22

20

0.027.130.34

18.370.41

14.7338.630.100.19

Sample No.

21

0.031.660.32

16.660.35

15.7136.190.140.22

22

0.031.590.43

18.340.32

14.7032.670.140.12

23

0.029.550.51

20.620.09

14.2834.640.130.20

24

0.032.730.44

16.940.10

16.0333.570.150.21

25

0.030.270.50

19.720.09

15.2634.110.190.23

26

0.028.930.52

19.140.10

14.8636.310.160.23

27

0.030.640.40

17.150.06

15.6735.860.150.20

28

0.0335.210.26

17.170.32

16.2432.14

——

99.87 100.15 100.11 98.80 100.46 98.57 100.44 101.15 98.39 100.02 100.17 99.37 100.25 100.13 101.37

Number of cations on the basis of 4 oxygens

0.00.9340.0080.1340.3190.0050.7190.9310.0040.0050.6930.499

0.00.9600.0060.1170.3190.0050.7180.9170.0040.0060.6930.488

0.01.0050.0090.1310.3120.0060.7230.8590.0060.0050.6990.461

0.0020.9610.0100.0920.3690.0050.6440.9420.0030.0040.6350.495

0.0011.1150.0100.0990.3510.0090.6660.7800.0010.0060.6550.412

0.00.9930.0120.1180.3490.0090.6730.8840.0040.0050.6580.471

0.00.9670.0080.1050.3600.0100.6640.9240.0020.0040.6480.458

0.01.0870.0070.0760.3300.0090.6820.8340.0030.0030.6740.434

0.01.1190.0100.1000.3610.0080.6580.7760.0030.0050.6430.410

0.01.0470.0120.1250.3940.0020.6400.8240.0030.0050.6190.440

0.01.1270.0100.0920.3220.0020.6980.7760.0030.0050.6840.408

0.01.0680.0110.1190.3490.0020.6810.8070.0040.0060.6610.430

0.01.0200.0120.1150.3640.0030.6630.8590.0040.0050.6450.457

0.01.0670.0090.0910.3320.0020.6900.8370.0040.0050.6750.440

0.0011.1880.0060.0840.3270.0080.6940.728——0.6800.380

0.55

0.50 -

<t 0.45

0.40 -

0.9 0.8 0.7

Mg/(Mg+Fe2+)

0.6

0.35

•o\

V×o

oo

iiii

o

X?

°o §^•Q

II

i « h

* Tio ,/

21O / j .

'^"Ni /-

//

//

• σu

O π

in "b*

////

'/ ^ *

— /

I

o

N

^ \

XO23

-

0.75 0.70 0.65

Mg/(Mg + Fe2 +)

0.60

Figure 2. A. Cr/(Cr + Al) versus Mg/(Mg + F e 2 + ) spinels for Mid-Atlantic Ridge and Costa Rica Rift basalts:(i) Sigurdsson and Schilling (1976); (ii) Dick and Bryan (1979); (iii) this study. B. Plots of composition ofspinels from the Costa Rica Rift basalts: circles = spinels in variolitic glass; crosses = spinels in olivine;squares = spinels in Plagioclase. See text for discussion of arrows.

809

T. FURUTA, H. TOKUYAMA

A I 2 O 3

FeO<

^ ^ C r2°3

MgO

v w ‰ ^ ^ ^ NiO

K Λ M Λ / W Λ Λ ^ ^ M n 0

V2°3

TiO,

65 µm

Figure 3. Compositional variation across a euhedral spinel in Sample505B-6-1, 109-111 cm. The numbers refer to analyses listed inTable 3 and Figure 2. A photomicrograph of this grain is shown inFigure ID.

810