51. petrology and geochemistry of igneous rocks, … · petrology and geochemistry of igneous...

51. PETROLOGY AND GEOCHEMISTRY OF IGNEOUS ROCKS, DSDP LEG 37

M.F.J. Flower, P.T. Robinson,1 H.-U. Schmincke, and W. Ohnmacht, Institut für Mineralogie der Ruhr-Universitat, 463 Bochum, West Germany

INTRODUCTION

Leg 37 of the Deep Sea Drilling Project wassuccessful in its prime objective of deep penetrationinto acoustic basement in the mid-Atlantic Ocean. Atotal of 1477 meters of basement was drilled at foursites (332, 333, 334, and 335) with a maximum penetra-tion of 583 meters in Hole 332B. Shipboard studies ofthe drilled rocks revealed unexpected variations inlithology, chemical composition, and paleomagneticstratigraphy at three of the four sites.

We have undertaken a detailed geochemical study ofthese rocks in order to: (1) identify and characterizeprimary magmas at each site, (2) determine the extentand nature of shallow level fractionation beneath themedian rift, (3) determine the relationship betweenmagmatic evolution and the eruptive buildup of layer 2,(4) determine the geochemical and geothermal evolu-tion of the sub rift melting zone.

The drilled sections of Leg 37 also provide a uniqueopportunity to evaluate proposed large-scale physicalprocesses such as "mantle plumes" and "hot spots" byreference to secular chemical variations both withinand between holes at the four sites, representing a timespan of approximately 10-12 m.y. Major, minor, andtrace elements were determined for 133 whole-rockspecimens from each of the five drilled sections. Theseare reported in Chapters 2, 3, 4, and 5 (this volume).The main objectives are to identify coherent chemicalgroups, to determine interrelationships of groups interms of fractional crystallization and partial fusion ofthe upper mantle, and to observe secular variations inmagma compositions.

Chemical effects due to halmyrolysis and postcon-solidation alteration processes are described and dis-cussed separately (Robinson et al., Chapter 61, this vol-ume). It should be noted that the analyses reported herehave not been corrected for CaC03 and MgCθ3 intro-duced by percolating solutions.

Whole-rock analyses were carried out by X-rayfluorescence (except for FeO, Na2θ, H2O, and CO2) atImperial College, London (Holes 332B, 333A, and 334)and the Ruhr-Universitat, Bochum (Holes 332A and335). Mineral compositions were determined by elec-tron microprobe at the E.T.H., Zurich and ImperialCollege. Analyses of interlaboratory standards aregiven in Wright (Chapter 10, this volume).

'Permanent address: Department of Earth Sciences, University ofCalifornia, Riverside, California.

CHEMICAL VARIATION OF DRILLED ROCKS

Magma groups have been defined from the whole-rock chemical analyses in terms of major element ox-ides. The groups are defined on the basis of chemical af-finity and include not only rocks of similar chemistry,but also those whose compositions can be related bysimple addition or subtraction of observed phenocrystphases. The important discriminants are AI2O3, TiCh,and FeO for any given MgO content. In identifyingcompositional groups analyses reflecting the effects ofalteration (high H2O and CO2 contents) were not in-cluded. Thus, each group so defined should a priorirepresent an independent low pressure fractionationsystem, whereas variation between groups must be ex-plained by variable parental magma compositions.

These groups are described and discussed for each ofthe five major holes drilled on Leg 37 (332A, 332B,333A, 334, and 335) in terms of their major and traceelement geochemistry, phenocryst and groundmassmineralogy, and stratigraphy. Preceding this is a briefevaluation of the chemistry of fresh glass cooling rinds.Glass compositional groups have been notatedalphabetically and whole-rock groups numerically toavoid confusion. Average chemical compositions ofglass groups are given in Table 1 and whole-rockgroups in Table 2, whereas average compositions ofmineral phases in individual magma groups are given inTables 3, 4, 5, and 6.

Chemistry of Glass Cooling RindsGlass rinds are most common in Holes 332A, 332B,

Site 335, and the upper eruptive part of Site 334. Fromshipboard lithologic studies we conclude that virtuallyall of the glassy zones are associated with eruptedmagmas, especially pillow sequences. The glassanalyses are valuable for four main reasons: (1) theyreflect the lower limit of MgO content in primitive liq-uids, (2) they show none of the effects of crystal ac-cumulation evidenced by whole-rock compositions,(3) they give the most reliable indication of magmaticK2O content, (4) they provide compositional referencegroups against which whole-rock analyses may be com-pared in order to identify low pressure fractionationprocesses.

For Leg 37 samples 102 glass analyses by electronmicroprobe are available—40 from Dalhousie Univer-sity (F. Aumento) and 62 from the Smithsonian Institu-tion (W.G. Melson). For this report we have selectedthe Smithsonian data for comparison because they cor-respond most closely with associated whole-rock com-positions and give greater stratigraphic coverage.

653

M. F. J. FLOWER, P. T. ROBINSON, H.-U. SCHMINCKE, W. OHNMACHT

TABLE 1Average Compositions of Leg 37 Glass Groups Identified by T.L. Wright for Analyses by W.G. Melson

SiO 2

TiO 2

A1 2O 3

FeO a

MnO b

MgO

CaO

Na 2 O

K 2 Op

2 o 5

Total

No. ofanalyses

Core

Equiv.rockgroup

332A-a

51.28

1.03

14.82

9.79

0.20

7.62

12.49

2.07

0.16

0.10

99.56

2

15, CCto 18, CC

332A-2

332A-b

51.91

1.19

14.58

10.95

0.20

6.70

11.41

2.30

0.19

0.11

99.54

2

22-1 to25-1

332A-2

332A-C

51.67

1.13

14.66

10.48

0.20

7.47

11.97

2.12

0.17

0.11

99.97

1

40-1

332A-3

332A-d

51.10

1.25

14.81

10.61

0.20

7.46

11.91

2.10

0.22

0.13

99.79

4

26-1 to26-2

1

332A-e

50.99

1.23

14.42

11.25

0.20

7.09

11.59

2.20

0.20

0.10

99.27

1

40-2

?

332B-a

51.52

1.16

14.87

9.95

0.20

7.77

12.40

2.02

0.19

0.11

100.19

3

8-1

332B-2

332B-b

50.09

0.76

16.34

9.32

0.20

8.92

12.29

2.05

0.11

0.07

100.15

2

19-1

332B-3

332B-C

49.60

0.64

16.63

9.09

0.20

8.98

12.38

2.05

0.07

0.04

99.68

6

20-2 to25-2

332B4

332B-d

50.60

0.89

16.09

10.03

0.20

7.95

11.77

2.34

0.07

0.06

100.00

8

22-2 to25-2

332B-5

332B-e

49.75

0.67

15.77

8.74

0.20

8.70

13.85

1.90

0.07

0.04

99.69

3

36-1 to364

332B-6

334

52.03

0.83

14.53

9.86

0.20

7.91

12.72

1.81

0.08

0.07

100.04

4

16-5 to19-2

335-a

50.69

1.1716.28

9.59

0.20

6.66

12.12

2.57

0.17

0.09

99.54

1

5-2

335-b

50.49

1.13

15.67

9.39

0.20

7.99

11.87

2.39

0.17

0.10

99.40

24

5-2 to14-3

aTotal Fe-oxide.

Assumed MnO.

Normative glass compositions, along with whole-rock analyses, are plotted on the Tilley-Muir diagram(Figure 1) and show Leg 37 basalts to be exclusivelyolivine and quartz tholeiites. Chemical groupings of theSmithsonian glass analyses were made by T.L. Wrightby means of a chemical sorting program using specifiedelement tolerances (see Melson et al., in press). Thesegroups are shown with whole-rock data on MgO varia-tion diagrams in Figures 2, 3, and 4. Compositionalvariations in the glasses are greatest in Holes 332A and332B and appear to be independent of either secular orgeographic influences.

Glass analyses from Hole 332A fall into three distinctgroups (332A-a, -b, and -d); two isolated analyses mayrepresent separate groups (332A-C and -e). Oxide versusMgO variation suggests a simple two- or three-phasecotectic assemblage relating groups 332A-a and-b, whereas 332A-d, by virtue of higher contents ofTiθ2, K2O, P2O5, and (?)FeO, appears to be unrelatedto the other groups (Figure 2).

Five compositional groups are recognized in Hole332B (a-e inclusive), notated with increasing depth, anddistinguished by relative abundances of Tiθ2, K2O,CaO, and AI2O3. Most distinctive are groups 332B-a(high TiO2 and low AI2O3), 332B-b and -c (low TiO2

and high AI2O3), and 332B-e (high CaO, moderateAI2O3, and low T1O2). None of the groups can berelated by simple fractional crystallization schemes in-volving removal of olivine + plagioclase ± clinopyrox-ene. Variations in Tiθ2, AI2O3, and CaO are compatiblewith derivation of group d from group b by fractionalcrystallization, but this process is precluded by thehigher content of K2O in group d glasses. The sameargument probably holds for groups 332B-b and -c,which in other respects are nearly identical. The onlyother possible fractional crystallization sequence is

332B-a toward 332A-d, both high-Ti groups. However,332A-d is about 0.5% poorer in SiO? (Table 1) and, be-ing lower in MgO, must derive from a more Siθ2-poorparent than 332B-a (Figure 3).

Glasses from Sites 334 and 335 form discrete com-positional groups and do not correspond to any fromHoles 332A and 332B (Table 1). Site 334 glass com-positions are relatively low in Na2θ, moderately high inCaO, and high in Siθ2, whereas Site 335 glasses arenotable for high Tiθ2 contents (cf. groups 332A-d and332B-a, Table 1).

WHOLE-ROCK LITHOLOGY AND CHEMISTRY

Hole 332A

Lithology

Hole 332A penetrated 330.5 meters into acousticbasement with about 10% recovery. Seven lithologicunits were recognized by the shipboard party basedlargely on phenocryst mineralogy. The upper third ofthe section consists chiefly of aphyric and sparselyphyric basalt with one coarsely plagioclase-phyric unitnear the top; moderately to highly plagioclase- andolivine-phyric lavas predominate below about 93meters subbasement. The hole bottomed in a highlyporphyritic basalt containing abundant phenocrysts ofplagioclase, augite, and olivine. Chalk layers and glassor palagonite breccias are common in the upper part ofthe section, but decrease gradually with depth.

The rocks between Cores 30 and 35 (226-283 m sub-basement) are notably altered with abundant smectite,carbonate, and zeolite; elsewhere the rocks are relative-ly fresh, but nearly all exhibit some evidence of interac-tion with seawater.

654

PETROLOGY AND GEOCHEMISTRY OF IGNEOUS ROCKS

TABLE 2Average Compositions of Leg 37 Whole-Rock Groups (Volatile-Free), Corrected for Carbonate-Contamination

SiO2

T i O 2A12O3

Fe2°3FeOMnOMgOCaONa2OK2OP2°5Total

No. ofanalyses

CrNiBaRbSrYZrCuZn

CaO/Al2O3

P/YZr/Y100 × Mg/(Kg + Fe+ 2)

332A-1

49.27

0.4820.78

1.963.930.106.53

14.901.770.220.06

100.00

2

1888829

1.9103

11.5377541

0.71722.8

3.21

332A-2

51.43

1.0914.823.306.690.167.36

12.502.180.330.14

100.00

23

996976

5.3118

24.7727558

0.84324.7

2.91

332A-3

50.84

1.2214.83

3.906.730.157.70

12.082.100.300.15

100.00

7

1518479

5.3112

27.0816858

0.81524.2

3.00

332 AA

50.79

1.0116.412.996.000.157.26

12.972.050.240.13

100.00

15

1588568

3.6111

21.9677154

0.79025.9

3.05

332A-5

49.69

0.5616.88

1.654.980.12

10.1714.221.570.110.05

100.00

3

41114533

1.29911.6427543

0.84218.8

3.62

332A-6

50.87

1.0515.093.196.370.167.48

13.142.220.300.13

100.00

2

15878723.9

10525.4707356

0.87122.4

2.75

X

49.3

0.5017.5

1.55.750.139.5

14.21.550.050.02

100.00

0.811

74.6

F

50.0

0.717.4

8.20.19.8

12.92.20.0nd

101.5

0.741

71.6

332B-1

48.65

0.3821.87

1.043.920.107.05

15.401.510.050.03

100.00

7

26910325

1.3112

10.63882

0.70412.43.58

332B-2

50.82

1.0615.872.626.240.167.59

13.162.120.250.11

100.00

12

2017876

4.314523.66870

0.82920.4

2.88

332B-3

48.52

0.7514.692.436.720.16

13.1511.362.010.150.06

100.00

2

76030846

1.78718.44785

0.77314.22.55

332B4

48.61

0.7014.362.266.670.15

14.1011.10

1.880.120.05

100.00

1

89237242

1.78618.34383

0.773

11.92.35

332B-5

49.47

0.8016.482.716.250.169.75

11.882.300.150.05

100.00

4

43620443

3.18820.04684

0.721

13.12.30

Major Element GeochemistrySix magma groups are recognized in Hole 332A (1-6

inclusive), generally notated with increasing depth(Figure 2 and Table 2).

Group 332A-1—Group 1 comprises about 10 metersof highly plagioclase-phyric basalt (lithologic unit 2) ly-ing immediately beneath a thin aphyric sequence at thesediment-basalt contact. It is characterized by excep-tionally high AI2O3 (up to 21 wt %) and CaO (15 wt %)and low Tiθ2, total iron, SiCh, and P2O5. Theporphyritic rocks are divided by a thin aphyric layer(belonging to magma group 332A-6), but there are noglassy zones or sedimentary intercalations.

Large plagioclase phenocrysts (up to 20 modalpercent) are relatively uniform in composition (An92),but many have narrow overgrowths of more sodic com-position. Microphenocrysts of plagioclase are slightlymore sodic with an average composition of Anββ.Olivine microphenocrysts (1 modal percent) have anaverage composition of Fosβ.

Chemical variations within this group (Figure 2)suggest that it originated by addition of An-richplagioclase to a magma already poor in Tiθ2, K2O, andP2O5, and rich in CaO and AI2O3.

Group 332A-2—This is the most voluminous magmatype in Hole 332A, and it encompasses a range of com-positions between glass groups 332A-a and -b, in-cluding a few compositions more magnesian than332A-a (Figure 2). It comprises most of lithologic units3 and 4, a sequence of sparsely phyric and aphyric

basalts, respectively. Where present, phenocrysts are ofplagioclase (Anβ4), clinopyroxene, and olivine (Foes).Variation of Tiθ2, CaO, and AI2O3 indicates simplecotectic fractionation of the phenocryst phases andprobably reflects a series of closely related fractionationpaths rather than a single liquid line.

Group 332A-3—This group has higher T1O2 (1.2 wt%) and P2O5 than 332A-2 for equivalent 100 ×Mg/(Mg + Fe+2) and is closely related chemically andstratigraphically to 332A-d glass compositions (Figure2). Lavas of this group make up lithologic unit 5 and in-clude a few intercalated layers at the base of lithologicunit 6. They are sparsely phyric, with small phenocrystsof plagioclase (Anss-βs), clinopyroxene, and olivine(F083). Group 332A-3 compositions are mostly in-termediate between 332A-d and 332B-a glass com-positions. However, as already indicated, any magmaassociated with the 332A-d glasses must have beenderived from a less Siθ2-rich magma than that givingrise to 332B-a glass compositions (cf. Figures 2 and 3).

Group 332A-4—Basalts of this group form lithologicunit 6, a series of coarsely plagioclase-phyric lavas, in-tercalated near the base with rocks of group 332A-3(Figure 5). Oxide variation (Figure 2) indicatesplagioclase accumulation in a range of host liquidsmore magnesian than, and probably parental to, group332A-3 magmas. AI2O3 and CaO contents are high (upto 16.8 and 13.8 wt %, respectively), but plots of S1O2and CaO versus MgO suggest variability in the com-position of the added plagioclase (Anss-βs).

655


TABLE 2 - Continued

SiO2

TiO 2

A 1 2 ° 3F e 2°3FeO

MnO

MgO

CaO

Na2O

K2OP2°5

Total

No. ofanalyses

Cr

Ni

Ba

Rb

Sr

Y

Zr

Cu

. Zn

CaO/Al2O3

P/Y

Zr/Y

332B-6A

47.70

0.56

13.76

2.75

6.45

0.16

16.15

10.50

1.78

0.15

0.04

100.00

6

1300

499

32

3.9

93

15.7

36

90

0.763

11.1

2.29

332B-6B

48.78

0.52

19.11

2.37

4.23

0.14

8.23

14.68

1.79

0.11

0.04

100.00

4

434

131

29

1.8

109

15.8

39

85

0.768

11.1

2.47

332B-7

50.37

1.02

16.04

2.79

6.33

0.18

7.96

12.73

2.23

0.25

0.10

100.00

13

181

84

69

3.5

112

23.9

63

62

0.794

18.3

2.64

333A-1

51.08

0.96

14.80

2.22

5.91

0.16

10.24

12.52

1.94

0.17

-

100.00

2

681

225

79

2.3

136

20.7

71

77

0.846_

3.43

333A-2

51.34

1.05

15.03

3.93

5.69

0.17

8.30

11.80

2.39

0.19

0.11

100.00

2

158_

91

2.7

107

24.6

76

80

0.785

19.6

3.09

333A-3

51.00

1.44

15.48

3.50

6.16

0.15

7.73

11.79

2.34

0.27

0.14

100.00

3

281

94

111

7.2

122

29.1

90

56

0.762

21.0

3.09

333A4

49.96

0.70

16.47

2.23

5.03

0.15

8.61

14.89

1.78

0.13

0.05

100.00

3

466

119

57

2.4

147

18.0

54

62

0.904

12.1

3.00

333A-5

51.20

1.16

15.90

2.19

6.82

0.16

7.90

12.31

2.13

0.13

0.10

100.00

3

276

102

80

2.4

115

25.0

74

71

0.774

17.5

2.96

334-1

51.67

0.89

15.80

1.29

7.52

0.16

8.05

12.29

2.04

0.21

0.08

100.00

1

249

114

84

2.7

87

22.7

49

64

nd

0.778

15.4

2.16

334-2

51.44

0.77

15.17

2.69

5.67

0.16

8.94

13.12

1.86

0.12

0.06

100.00

3

168

100

66

2.4

75

20.4

41

96

nd

0.865

12.8

2.01

334-3

49.94

0.13

16.84

1.96

4.86

0.15

10.87

14.16

1.07

0.02

-

100.00

2

732

225

18

0.3

30

5.3

6.9

90

nd

0.841

• -

1.30

3344

44.82

0.07

5.47

3.89

5.21

0.10

36.01

4.27

0.15

0.01

-

100.00

2

5477

2170

6.2-

2.7

2.6

4.9

34

nd

0.781

-

1.88

335-1

50.28

1.11

15.72

3.97

5.48

0.16

8.45

11.98

2.46

0.28

0.11

100.00

7

256

154

63

4.3

106

26.2

56

68

54

0.762

16.7

2.14

aX = calculated noncumulate host composition for 332B-1.bF = Glass composition 3-14 from Frey et al. (1974).

Group 332A-5—The rock type comprising thismagma group is unique among Leg 37 basalts. It ishighly porphyritic, with phenocrysts of clinopyroxene,plagioclase (Anββ-s<>), olivine (Foβs>), and minor spinel,and is probably the result of crystal accumulation. Itforms the lowermost drilled unit (lithologic unit 7) inHole 332A and is chemically distinctive, with MgOranging from 9.5 to 11 wt % and CaO and AI2O3 com-monly exceeding 14 and 16.5 wt %, respectively. Ex-trapolation from the bulk composition by removal ofcumulate phases suggests that the host liquid was richin CaO and AI2O3 and poor in TO2, K2O, and P2O5,similar in composition to that predicted for plagioclasecumulate magmas of groups 332A-1 and 332B-1.

Group 332A-6—Two specimens (Core 7, Section 1,20-22 cm; and Core 21, Section 1, 31-33 cm), althoughchemically similar to magmas of group 332A-2, areassigned to a separate group because of their distinctlyhigher Cr contents and 100 × Cr/(Cr + Ni) ratios(Table 2).

Trace and Minor Element GeochemistryIn Figure 6, Ti, Sr, and Y contents of 332A basalts

are plotted against Zr. Zr is used as the abscissa becauseof its "incompatible" character and its low mobilityduring post-crystallization alteration processes (Cann,1970; Pearce and Cann, 1971). Lines of equalelement/Zr ratio have been drawn to monitor anydeparture from unity in the rocks. Such differencesshould reflect magmatic source processes if they cannot

be attributed to fractional crystallization or alteration.Effects of low pressure fractional crystallization shouldreflect the observed effects on interelement ratios ofphenocryst-phase cumulates.

Good correlation between Ti and Zr is observed forall magma groups, cumulate and noncumulate, withTi/Zr ratios of 80 to 90. Sr has a high partition coeffi-cient between plagioclase and basaltic matrix (D =1-3; Philpotts and Schnetzler, 1970), clearly evidencedby the high Sr/Zr ratios of plagioclase cumulates ingroups 332A-1, -4, and -5 (Figure 6).

The Zr/Y ratio appears to be unaffected byplagioclase accumulation; however, there is a percepti-ble difference in Zr/Y between noncumulate magmas332A-2 and -3 (Table 2). The ratio of P/Y is likewiseunaffected by accumulation (or fractionation) ofplagioclase (cf. 332A-4 and -3, Table 2), and probablyolivine and clinopyroxene. P/Y and Zr/Y ratios areprobably inherent features of mantle-derived liquids,unless garnet has been fractionated at high pressure.Average values of P/Y are similar for all magmagroups, ranging from 22 to 26, except for group 332A-5(18.8).

Ba and Rb are enriched with Zr although con-siderable scatter is introduced by alteration effects (cf.Ridley et al., 1974).

Cr abundance and the ratio 100 × Cr/(Cr + Ni) arediagnostic for each group. Cr contents are similar (150to 170 ppm) in groups 332A-3, -4, and -6, all of whichhave MgO in the range of 6.5 to 8.0 wt %. Cr is distinct-

656


TABLE 3AAverage Plagioclase Compositions of the Magma Groups, Hole 332A

SiO2

A12°3CaONa2OK2O

FeOa

MgO

TiO2

TotalAn

1

44.7135.44

19.430.990.000.22

0.280.00

101.0791.6

2

46.2134.3318.99

1.500.000.270.300.00

101.6087.6

3

47.71

32.6316.92

2.270.02

0.600.390.02

100.5680.5

4

47.2733.5717.87

1.880.010.390.240.02

101.2583.9

5

49.8231.2115.33

2.980.050.63

0.450.05

100.52

73.7

6

46.0534.4218.37

1.44

tr0.350.210.00

100.84

87.6

7

46.6033.9918.06

1.71

0.010.360.22

0.03100.9885.3

8

50.4730.3815.00

3.190.08

0.760.390.05

100.3271.9

9

45.3635.30

18.351.310.000.370.200.03

100.9288.6

10

45.80

35.0217.80

1.370.000.36

0.230.02

100.6087.8

11

46.5633.8917.69

1.60

0.010.530.240.03

100.5585.9

Note: 1, Large phenocryst — magma group 1; 2, Small phenocryst — magma group 1; 3, Groundmass microlites —magma group 1; 4, Small phenocryst - magma group 2; 5, Groundmass lath - magma group 2; 6, Large phenocrysts- magma group 4 (7 crystals); 7, Microphenocrysts - magma group 4 (6 crystals); 8, Groundmass microlites -magma group 4 (5 crystals); 9, Large phenocryst - magma group 6; 10, Small phenocryst - magma group 6; 11,Groundmass microlite — magma group 6.

aTotal Iron.

TABLE 3BAverage Plagioclase Compositions of the

Magma Groups, Hole 332B

SiO2

A12O3

CaONa2O

K2OFeOa

MgOTiO2

TotalAn

1

45.46

34.8618.55

1.20

0.000.270.23

0.03

100.6089.5

2

50.03

31.63

15.102.920.04

0.720.36

0.03

100.8374.9

3

45.66

34.7118.79

1.29

0.010.330.18tr

100.9589.0

4

46.6034.14

17.831.790.02

0.380.22

0.01100.99

84.5

5

50.67

30.7014.80

3.22

0.03

0.700.29

0.05100.4671.5

Note: 1, Large phenocryst - magma group 1; 2, Ground-mass microlites — magma group 1 (2 crystals); 3, Largephenocrysts - magma group 2 (3 crystals); 4, Smallphenocrysts - magma group 2 (3 crystals); 5, Ground-mass microlites - magma group 2 (3 crystals).

aTotal iron.

ly lower in group 332A-2 magmas (85-110 ppm). Group332A-3 and -4 magmas are also characterized by .higherTi and Zr contents than groups 332A-2 and -6 magmasfor equivalent MgO contents.

Geochemical StratigraphyThe magma groups in Hole 332A have a simple

stratigraphic sequence and chemical variation withdepth shows good correspondence with lithologicvariation (Figure 5).

The highly porphyritic basalt (group 332A-5) at thebase of the section probably represents the residualcumulate of a fractionated body of CaO- and AI2O3-rich magma. The succeeding highly phyric and sparselyphyric magmas of groups 332A-4 and 332A-3, respec-tively, probably reflect a complex magma fractionationsystem. The thick (160 m) sequence of group 332A-2magmas that followed, reflects repeated episodes of

fractionation, eruption, and replenishment within high-level sub-rift magma chambers. Rare intercalations ofgroup 332A-6 magma in lithologic units 4 and 2 appearto derive from an independent system, as does the up-permost plagioclase phyric basalt (group 332A-1).

Paleomagnetic stratigraphy (Hall and Ryall, Chapter16, this volume) suggests eruptive episodes may beabout 100 yr in duration with long intervening qui-escent periods. Thus, if buildup of the drilled sectionoccurred over about 100,000 yr (a plausible estimateconsidering crustal spreading rates and width of the ac-tive median rift), complex patterns of magma fractiona-tion are to be expected.

Cyclic variation of 100 × Mg/(Mg + Fe+2), Cr, Ni,and LIL-elements with depth is observed within lavasbelonging to magma groups 332A-2, -3, and -4 (Figure7). Typical cycles begin with eruption of MgO-poor,

TABLE 3CAverage Plagioclase Compositions of the

Magma Groups, Sites 333 and 335

SiO2

A12O3

CaONa2O

K2OFeOa

MgOTiO2

Total

An

1

46.3034.7518.32

1.490.010.310.25

0.00101.4487.2

2

46.3034.5818.57

1.45tr

0.320.230.00

101.4587.5

3

49.9831.7915.552.870.030.600.290.03

101.14

74.9

4

45.9734.92

18.321.47tr0.410.210.00

101.3087.4

5

50.0631.52

15.782.090.030.520.320.05

100.85

80.5

Note: 1, Hole 333A; small phenocryst — magma group 2;2, Hole 333A; small phenocrysts - magma group 3 (3crystals); 3, Hole 333A;groundmass microlite — magmagroup 3; 4, Site 335; small phenocrysts - magmagroup 1 (2 crystals); 5, Site 335; groundmass microlite— magma group 1.

aTotal iron.

657


TABLE 4AAverage Olivine Compositions of the Magma Groups, Hole 332A

SiO2

MgOFeOa

MnO

CaO

TiO2

NiOCr2°3TotalFo

1

40.2947.9511.06

0.180.410.000.190.05

100.1388.5

2

39.7345.5714.580.200.370.000.110.00

100.5684.7

3

39.3844.3616.510.230.350.020.150.00

101.0092.7

4

39.3844.8814.820.220.33tr

0.150.03

99.8184.4

5

39.3844.6314.820.230.320.010.150.03

99.5784.3

6

40.1047.8010.880.160.310.000.210.04

99.5088.7

7

39.7646.9112.390.190.330.000.140.01

99.7387.1

Note: 1, Phenocrysts - magma group 1 (2 crystals); 2, Microphenocryst -magma group 2; 3, Microphenocrysts - magma group 3 (2 crystals); 4,Phenocrysts - magma group 4 (4 crystals); 5, Groundmass crystals -magma group 4 (4 crystals); 6, Phenocryst - magma group 6; 7, micro-phenocrysts - magma group 6.

aTotal iron.

TABLE 4BAverage Olivine Compositions of the

Magma Groups, Hole 332B

SiO2

MgOFeOa

MnOCaOTiO2

NiOCr2O3

TotalFo

1

39.8048.2610.640.160.370.000.180.05

99.4689.0

2

39.9648.3510.610.170.370.000.180.06

99.7089.6

3

40.7247.2611.080.160.340.010.260.06

99.8988.4

Note: 1, Phenocrysts — magma group1 (2 crystals); 2, Microphenocrysts— magma group 1 (2 crystals); 3,Microphenocrysts - magma group6 (3 crystals).

aTotal iron.

TABLE 4CAverage Olivine Compositions of the


SiO2

MgO

FeOa

MnOCaONiOCr2°3TotalFo

1

39.8946.0813.530.200.330.150.00

100.1885.9

2

39.1844.2815.670.250.320.100.00

99.8083.4

3

39.0743.8415.880.220.290.110.00

99.4083.1

4

39.8648.6510.910.150.300.210.05

100.1488.8

Note: 1, Hole 333A; phenocryst - magmagroup 1; 2, Hole 333A; microphenocryst -magma group 1; 3, Hole 333A; micropheno-cryst - magma group 2; 4, Site 335; pheno-crysts — magma group 1 (3 crystals).

aTotal iron.

often plagioclase cumulative lavas, followed by moremafic types, and reflect the episodic construction oflayer 2.

The succession of group 332A-4 and 332A-3 magmasmay reflect a larger scale cyclic unit, possibly relating toa deep-seated fractionation event of more primitivemagma, on which smaller scale effects have beensuperimposed at high levels. Tectonically quiet periodsare probably shorter and hence opportunities forcrystal-liquid fractionation are less in high-level sub-riftregions than at greater depths. Intercalation of group332A-3 lavas in group 4 lavas reflects availability ofboth types over a considerable period of time, althoughgroup 3 magmas continued erupting long after cessa-tion of group 4 eruptions. It is conceivable that otherhighly phyric, cumulate magmas (e.g., groups 332A-1and 332A-5) may also represent extreme fractions oflarge cyclic processes.

The thickest paleomagnetic units, M8 and M7(Figure 7) correspond to the most massive eruptive se-quence including magmas of group 332A-5, 332A-4,and 332A-3. The base of magnetic unit M6 (at about280 m depth) coincides with the cessation of group332A-3 magmas, and the first appearance of the distinc-tive 332A-2 group. This is probably the most importantchemical break in the section at Hole 332A. The firstappearance of group 332A-6 magma corresponds withpaleomagnetic unit M5, whereas later units coincidewith more pronounced cyclic variations. Sedimentaryintercalations become more frequent at and above thechemical transition in magnetic unit M6.

Hole 332B

LithologyIn Hole 332B, 583 meters of basement were drilled

with an average recovery of 21%. Eleven majorlithologic units were defined by the shipboard party us-ing the same criteria as for Hole 332A. These units con-sist of either a relatively uniform lithologic sequence ora sequence of repetitive lithologies such as Units 6 and

658


TABLE 5AAverage Clinopyroxene Compositions of the Magma Groups, Hole 332A

SiO2

A12O3

FeOa

MnO

MgOCaONa2O

TiO2

NiOCr2O3

TotalEn

1

53.631.805.300.15

18.0820.91

0.170.28tr0.10

100.4250.1

2

53.162.065.130.14

17.9820.86

0.180.300.02

0.21100.04

50.1

3

53.591.77

6.960.20

19.4917.720.120.290.03

0.09100.2653.9

4

53.192.316.04

0.1618.0919.590.180.390.020.62

100.6050.9

5

51.603.387.54

0.1917.54

18.450.200.680.04

0.3199.94

50.1

6

51.843.067.44

0.1916.6619.510.200.680.010.17

99.7647.8

7

53.852.51

3.880.11

17.4921.57

0.180.210.05

0.61100.4649.7

8

53.452.514.04

0.1017.8921.33

0.170.190.030.43

100.1450.4

Note: 1, Phenocrysts — magma group 2 (4 crystals); 2, Microphenocrysts — magma group2 (6 crystals); 3, Groundmass microlite - magma group 2; 4, Microphenocryst -magma group 3; 5, Groundmass microlite — magma group 3; 6, Groundmass micro-lites — magma group 4 (3 crystals); 7, Microphenocryst — magma group 6; 8, Pheno-crysts — magma group 6 (2 crystals).

aTotal iron.

TABLE 5BAverage Clinopyroxene Compositions

of the Magma Groups, Hole 332B

SiO2

A12O3

FeOa

MnOMgOCaONa2O

TiO2

NiOCr2O3

TotalEn

1

51.083.847.75

0.2016.2219.150.190.580.000.21

99.2247.3

2

51.492.987.850-20

16.3019.570.190.790.000.17

99.54

46.9

3

51.384.107.750.18

13.8320.75

0.260.710.00

0.2699.2341.8

Note: 1, Groundmass microlites -magma group 1 (2 crystals); 2,Groundmass microlites — magmagroup 2 (5 crystals); 3, Ground-mass microlite — magma group 6.

aTotal iron.

9. Rhythmic lithologic variations are most common inthe lower parts of the section.

Most of the basalts encountered in this hole aremoderately to highly phyric with phenocrysts ofplagioclase, olivine, and, less commonly, augite.Aphyric and sparsely phyric units are restricted to theupper and lower parts of the section. The presence ofaphanitic or intersertal groundmass textures in mostrocks and the occurrence and distribution of vesiclesand glass rinds suggest that most of the succession con-sists of pillow basalts. Possible intrusive bodies arepresent in Cores 9, 10, 11, and 26. These are recognizedby the absence of strong vesiculation and the presenceof coarser grained, microdiabasic textures. These units

typically consist of several meters of uniformly tex-tured core bounded above and below by chilled zones.

Altered and brecciated material is more abundant inHole 332B than in 332A. The most intense alterationoccurs in a 100-meter-thick interval (Cores 40-47, in-clusive) characterized by abundant secondary car-bonate.

Major Element GeochemistrySeven magma groups have been defined from oxide

versus MgO variation diagrams (Table 2 and Figure 3).Group 332B-1—Group 332B-1 comprises a 70-

meter-thick series of coarsely plagioclase-phyric rocksat the top of the section superficially resembling group332A-1 rocks. They are, however, even more highly

TABLE 5CAverage Clinopyroxene Compositions of the


1 4

SiO2

A12O3

FeOa

MnOMgOCaONa2O

TiO2

NiO

Cr2O3

TotalEn

53.012.165.670.15

17.9019.720.190.28

0.000.52

99.6150.8

52.272.915.34

0.1617.2420.42

0.210.35

0.000.35

99.2549.4

53.022.956.580.19

17.0619.690.190.54

0.000.37

100.5948.9

52.343.433.21

0.1017.7921.12

0.190.21

0.04

1.0699.4951.3

52.893.085.220.15

18.1019.530.150.39

0.000.23

99.7451.6

49.093.30

12.350.27

13.6417.780.291.46

0.00

0.1098.2940.9

Note: 1, Hole 333A; phenocryst - magma group 1; 2, Hole 333A;microphenocrysts - magma group 1 (2 crystals); 3, Hole 333A;groundmass microlite - magma group 1; 4, Hole 333A; pheno-crysts - magma group 3 (2 crystals); 5, Hole 333A; groundmassmicrolite - magma group 3; 6, Site 335; groundmass microlite— magma group 1.

aTotal iron.

659


TABLE 6Average Spinel Compositions, Hole 332B

FeOa

MnOMgOCr2°3A12O3

Fe2O3a

TiO2

Total

Fe+2

Mn

MgCrAlFe + 3

Ti

0

100.Mg/(Mg + Fe+2)100.Cr/(Cr + Al)100.Fe+3/(Fe+3 + Cr + Al)No. of analyses

Microphenocrysts

12.490.39

15.0143.3624.17

3.390.35

99.16

2.5400.0815.4438.3406.9220.6100.064

32.000

68.254.6

3.84

6

Inclusionsin Olivine

10.97.39

15.9844.3823.85

3.07.38

99.02

2.2220.0815.7668.4966.8050.5590.070

32.000

72.555.5

3.53

3

aFe2O3 calculated from FeO; FeO value adjusted.

enriched in AI2O3 and CaO (up to 23 and 16.5 wt %,respectively) and poorer in TIO2, P2O5, and K2O forequivalent MgO contents. They also have markedlyhigher values of the ratio 100 × Mg/(Mg + Fe+2) (>77)and higher Cr abundances. Plagioclase phenocrysts areapproximately An™, and olivine microphenocrystshave compositions in the range F089-90. Oxide variationdiagrams (Figure 3) show that MgO varies from 5.6 to8.7 wt % along a plagioclase accumulation trend. Themagnesian noncumulate host liquid was exceptionallyrich in AI2O3 and CaO and low in LIL-elements,matched only by the noncumulate host of group 332A-5 magmas-at the base of the section in Hole 332A.

Group 332B-2—This group is represented by about90 meters of aphyric and sparsely phyric basalt oflithologic unit 2 and aboµt 100 meters of sparsely tocoarsely phyric basalts in lithologic unit 3. Coarselyphyric rocks are rich in plagioclase (Angç), whereas lessphyric specimens have phenocryst assemblages ofplagioclase 4- olivine or plagioclase + olivine +clinopyroxene. Tiθ2 and P2O5 contents (Figure 3) showthat these rocks are related to glass group 332B-a. Ox-ide variation diagrams suggest some plagioclase ac-cumulation to 2- or 3-phase cotectic compositions. Theratio 100 × Mg/(Mg + Fe+2) ranges from 64 to 68.

Group 332B-3—Group 3 comprises a sequence ofmoderately to coarsely phyric pillow lavas betweenCore 16, Section 1 and Core 20, Section 1, where theyform the upper part of lithologic unit 4. Phenocrysts arechiefly olivine (Foββ-βç>) and lesser amounts of Cr-spinel.Two whole-rock analyses are available (16-2 and 19-1)which are characterized by Tiθ2 contents of about 0.75

wt % and AI2O3 contents of 14.5 to 15.0 wt % (Table 2).These lavas are probably associated with glass group332B-b.

Group 332B-4—Immediately underlying group332B-3, 332B-4 magmas form the lower part oflithologic unit 4, down to 465 meters subbasement.They consist of coarsely olivine-phyric (picritic) pillowlavas with common phenocrysts of olivine and Cr-spinel. These lavas have slightly lower Tiθ2 (0.69 wt %)and higher AI2O3 (14.36 wt %) than those of group332B-3, as shown by one analyzed specimen (Core 20,Section 1, 91-95 cm). The associated glasses (group332B-c) are likewise distinct from those of group 332B-b (Figure 3). Both magma groups are clearly olivinecumulates, and they differ markedly from the low Tiθ2,high AI2O3 and CaO picrites of group 332B-6 lower inthe section.

Group 332B-5—Lithologic unit 5 (465-495 m sub-basement) consists of glomerophyric pillow lavasbelonging to magma group 332B-5. Prominentaggregates of olivine (Foβ<>) and plagioclase (Ans>o) areassociated with Cr-spinel microphenocrysts and ophiticclinopyroxene. The phenocryst aggregates appear torepresent incipient states of in situ crystallization, withlittle or no differential crystal-liquid movement. Oxidevariation (Figure 3) suggests these magmas lie on anolivine-control trend. Group 332B-5 magmas are richerin Tiθ2 and P2O5 than group 332B-3 or -4 magmas ofequivalent MgO content, but poorer in these elementsand richer in AI2O3 than noncumulate magmas ofgroup 332B-2. Their primitive character is reflected byhigh ratios of 100 × Mg/(Mg + Fe+2), which rangefrom 70 to 72. The distinctive composition of thesemagmas is also reflected in the associated glasses ofgroup 332B-d (Table 1, Figure 3).

Group 332B-6—Magmas of this group arerepresented by several diverse lithologies, ranging fromolivine-phyric (picritic) to coarsely plagioclase-phyrictypes, which are interbedded over an interval of morethan 200 meters with magmas of other groups. Thesebasalts occur sparsely at the base of lithologic unit 4, inunit 6, and frequently in units 7, 8, and 9. They arecumulates of either olivine (Fosβ) and Cr-spinel orplagioclase (An™), and range in MgO content from 7.8to 25.0 wt %. Average analyses in Table 2 are given forboth olivine and plagioclase phyric types (groups 332B-6A and 6B, respectively). Oxide variation for each sub-group suggests their possible derivation from a non-cumulate CaO-rich magma, poor in Tiθ2, FeO, Na2θ,and P2O5, similar to glass compositions of group 332B-e (Table 1). Glasses of this composition occur with332B-6 magmas at about 600 meters subbasement.

Group 332B-7—Major element chemistry of thisgroup is similar to that of 332B-2, but the magmas ofgroup 7 have higher values of 100 × Cr/(Cr + Ni) forequivalent values of 100 × Mg/(Mg + Fe+2) (Figure 8).Five analyzed specimens from this group have relative-ly low Tiθ2 (0.85 to 1.0 wt %) and may define a sub-group. AI2O3, CaO and Siθ2 variation in these magmas(Figure 3) indicates that they are plagioclase-cumulative types, which may in part account for theirlower Tiθ2.

660


NE

HY

Figure 1. Normative compositions of Leg 37 basalts and plutonic rocks, normalized with F^2^3 = ^ w t %' Vetted on theTilley-Muir diagram; (a) Hole 332A, and 332B, data; (b) Site 334 and 335 data. Dashed lines indicate extent of glasscompositions.

The rocks of group 7 range from aphyric to sparselyor highly phyric basalts with phenocrysts of plagioclase(Anβ5-9o), olivine (F084-89), and sparse clinopyroxene.Group 7 magmas occur in lithologic unit 6 (interbeddedwith group 332B-6 lavas) and comprise the whole oflithologic units 10 and 11 at the base of the section.

Trace and Minor Element Geochemistry

The highest LIL-element concentrations occur ingroups 332B-2 and -7 and lowest in cumulate groups

332B-1 and -6, with intermediate contents in groups332B-3, -4, and -5.

Ti/Zr ratios range from 70 to 100 and are broadlysimilar to those for 332A rocks (cf. Figures 6 and 9). Asexpected, Sr/Zr ratios are high in plagioclase cumulatemagmas (groups 1 and 6B); the lowest values occur ingroup 7 magmas which have fractionated plagioclase.Zr/Y ratios range from 3.58 to 2.29 in these rocks, be-ing highest in the plagioclase cumulates of group 1 andlowest in the picrites of group 6A.

661


1-3

TiO CαO

Nα2O

MnO

MgOFigure 2. MgO variation diagrams for Hole 332A data, with control trends of fractionating phases and Hole 332A

glass groups (332A-a to -e) indicated. Whole-rock chemical groups indicated by the following symbols: 332A-1, +;332A-2, •; 332A-3, x;j332A-4, O; and 332A-5, A Phenocryst phase control lines: solid = olivine, dot-dash = clino-pyroxene, and dash = plagioclase.

Olivine- and plagioclase-cumulate groups lie alonglines of approximately equal P/Y ratio. However, P/Yis strongly fractionated between the noncumulategroups, ranging from 10-15 in group 5 magmas to 17-25in groups 2 and 7.

Rb and Ba data reflect their mobility during altera-tion, with Ba showing less scatter than Rb. Data from

the freshest specimens suggest Ba/Zr is not greatly frac-tionated between magma groups.

Cr abundance is distinctive for each magma group,despite its sensitivity to fractionation by maficphenocryst phases. Plagioclase cumulates of groups332B-1 and -6B have Cr contents of 230-320 ppm and360-450 ppm, respectively (cf. 165-190 ppm for magma

662


CαO

Nap

MnO

4714 12

MgOFigure 3. MgO variation diagrams for Hole 332B data, with control trends of fractionating phases and Hole 332B glass

groups (332B-a to -e) indicated. Whole-rock chemical groups indicated by the following symbols: 332B-1, +; 332B-2,•; 332B-3, ±; 332B-4, Δ 332B-5, o;1332B-6a, a; 332B-6b, •; and 332B-7a, x Phenocryst phase control lines: solid =olivine, dot-dash = clinopyroxene, and dash = plagioclase. Note correspondence of composition X (*), (see Table 2).the calculated host for group 332B-1 and group 332A-5 for TiO2, A12O3, and CaO.

group 332A-1). Cr contents of the three picrite groups332B-3, -4, and -6A average 760, 890, and 1300 ppm,respectively (cf. 410-540 for the cumulate magmas ofgroup 332A-5). For the mostly noncumulate magmasCr content ranges from 180 to 525 ppm.

Geochemical Stratigraphy

Chemical variation with depth shows that eachmagma group occupies a characteristic stratigraphicposition (Figure 10). The most distinctive chemicalbreak occurs at about 400 meters depth—the base of

lithologic unit 3b—below which there is a varied seriesof cumulate and noncumulate rock types. Above about500 meters there is good correlation among chemicalgroups and lithologic units, with groups 332B-4 and -3corresponding to picrite unit 4, group 332B-2 tolithologic units 3b, 3a, and 2, and group 332B-1 tolithologic unit 1. The lithology of unit 3b suggests aseries of microdoleritic intrusions, whereas thesucceeding units 3a and 2 are probably an eruptive se-quence. Small-scale cyclic variation is evident (Figures7 and 10) but not as distinctive as that in 332A magmasor in the lower part of Hole 332B.

663


18

17 -

15h

X '

x

1 1

~.) 335-b

2 334

335-a

l ö

17

16

15

14

13

12

11

10

-

X

-

-

-

-

X

Whole rock

A 334-1

Δ 334-2

X 334-4

• 335

Δ

Δ Δ

groups:

(gabbros)

-

-

-

O 3 3 4

. 335-a

# ^ J 4 335-b

-

12 11 10 9%MgO

Figure 4. MgO variation diagrams forgroups shown.

, and CaO from Sites 334 and 335, with magma groups and glass

In the interval between 500 and 670 meters the rocksare mostly porphyritic with rapid intercalation ofolivine- and plagioclase-phyric types. This is due inpart to interbedding of magma types but largely to theextreme fractionation of group 332B-6 magmas.Lithologic unit 5 is composed of group 332B-5magmas, unit 6 is a complex interbedded unit, units 7,8, and 9 are largely of 332B-6 magma, and units 10 and11 consist mostly of 332B-7 magma. Rhythmic varia-tion with depth is most pronounced in group 332B-6magmas and less so in group 332B-7 and -5 sequences(Figures 7 and 10).

The lower part of the 332B section was built up bycoeval eruption of strongly contrasting magma types—each fractionated under different conditions. Paleo-magnetic stratigraphy (Hall and Ryall, Chapter 16, thisvolume) indicates closely spaced magnetic units atdepths below about 525 meters, with no simple relationto lithology and chemistry. Above this depth, the upperpart of magnetic unit M4 corresponds to lithologic unit5, M3 to unit 4, M2 to units 3a and 3b, and Ml to unit1, each representing a chemically distinct magma type.The chemical and magnetic break at about 408 metersoccurs at the top of a continuous 100-meter-thickpillow sequence of group 332B-5, -4, and -3 magma(Figure 7). This break coincides with a sharp change inoxidation state of the rocks, as reflected by the cation-deficiency of groundmass Fe-Ti oxides (Bleil and Peter-son, Chapter 17, this volume). These features reflect a

lengthy noneruptive interval, with considerable rock-seawater interaction, massive intrusion of the over-lying sequence, or a combination of both processes.

Hole 333A

Lithology

In this hole 312 meters of basaltic basement weredrilled. On the average, coring was done at about 30-meter intervals. Recovery was very poor (8%)suggesting that a large part of the section consists ofbasaltic rubble, breccia, and interbedded sediment.Three principal basalt lithologies were identified on thebasis of phenocryst assemblages: olivine phyric, olivine+ plagioclase phyric, and olivine + clinopyroxene +plagioclase phyric. Based on varying olivine/plagio-clase ratios, the section was divided into seven litho-logic units by the shipboard party.

Major Element Geochemistry

Specimens from each of the seven were analyzed.Variation of Tiθ2, AI2O3, and CaO versus MgOsuggests the existence of several independent magmatypes and provides evidence of extensive low pressurefractional crystallization. Five distinct magma typescan be recognized and average compositions of theseare given in Table 2. Several analyzed specimens do notfall into any of the recognized groups and may repre-

664

100 —

TiO 2

0.4 0 6 0-8 10 12I I I I I

2 ^ 5

l•4 0 0 2 0 0 6 0 1 0 014 018I I I 1 I I

P>O KoO

0 0-2 0-4 0-61 I I I

Nα2O

1-4 16 l 8 20 2 2 2-4I I I I I I

CαO

II 12 13 14 15I I I I I

A1 2 O 3

14 16 18 20 22I I I I I

MgO

6 7 8 9 10 II 12I I I I I I I

FeO

6 7 8 9 10 III I I I I I

SiO2

49 50 51 52 53I I I I I

I5CH

200-

250-

300-

350-

400-

li

i i i i i i i i i i i i i i i i i i i i i

\

i i i i i i i i i i i i i i i i i i i i i i i

Figure 5. Hole 332A: major element oxides versus subbottom depth in meters indicating vertical extent of magma groups.

ozwoG

R


26

22

Y

65 70 75 80 85

Figure 6. Y, Sr, and Ti versus Zr for Hole 332A rocks; dotted straight lines show equal element; Zr ratios (quoted inparentheses); whole-rock groups are indicated with same symbols as in Figure 2.

666

Subbottom MagneticDepth (m) Units

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Figure 7. Variation of 100 x Mg/fMg + Fe z) with subbottom depth in meters along side lithology and magnetic stratigraphy for Holes 332A, 332B, and 333A.Probable vertical extent of magma groups is indicated. Horizontal dotted lines show the most important sedimentary intercalations in 332A and 332B.lithologic unit boundaries are shown by continuous horizontal lines. Polarity of magnetic units (M) in Holes 332A and 332B is indicated by + or -; sh in 333Adenotes shallow NRM inclinations.

β

wHJOO

roo>zöowonxws

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90

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OLIVINE (Fo89)

DSDP LEG 37

SITES 332 & 333

CUMULATE GROUPS SHADED

oi r cpx

60 70 80 90

100.Mg/(Mg+Fe^)

100

Figure 8. Plots of 100 x Mg/(Mg + Fe ) versus 100 x Cr/(Cr + Ni) for whole-rock and mineral datafrom Holes 332A, 332B, and 333A; magma groups indicated.

668


30

26

22

18

14

160

140

120

100

80

60

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8000

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50 55

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Figure 9. Y, Sr, and Ti versus Zr for Hole 332B rocks; dotted straight lines show equal element IZr ratios(quoted in parentheses); whole-rock groups are indicated with same symbols as in Figure 3.

669


150-

20CH

25CH

300-

350-

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450-

500-

550-

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Figure 10. Hole 332B: major element oxides versus subbottom depth in meters, indicating verticalextent of magma groups.

670


AUO2^39 II 13 15 17 19 21 2 3I I I i I I I I

\

I I I I

MgO15 17 19 II 13 15 17 19 21 23I I 1 I I I I I I I

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1 I—I L_J I i l l l l_l I I I I I I I l_l I I I I I

SiO2

45 46 47 48 49 50 51 52I I I I I I I I

Figure 10. (Continued).

671


sent separate chemical types, however, the availabledata are insufficient to define additional groups.

Group 333A-1—This comprises a series of olivine-phyric lavas in lithologic unit 1 with MgO contents ex-ceeding 10 wt %. These lavas have higher AI2O3 andCaO than picritic basalts of Hole 332B, andphenocrystic olivine has an average composition ofFoβó, notably more iron rich than olivines from otherpicritic units.

Group 333A-2—This group represents a series ofsparsely phyric and aphyric basalts in lithologic units 2and 7. One of the analyzed specimens is a cumulate typewith about 15 modal percent of plagioclase pheno-crysts; others contain sparse phenocrysts of plagio-clase, olivine, and clinopyroxene. These magmas arefairly primitive with 100 × Mg/(Mg + Fe+2) ratios of66-68. Parental magmas of this group were clearlylower in AI2O3 and TiCh than those of group 1.

Group 333A-3—Group 3 comprises a series of LIL-element enriched magmas characterized by Tiθ2 con-tents up to 1.3 wt % and P2O5 contents between 0.13and 0.18 wt %. These magmas occur in lithologic units4, 5, and 7 and are sparsely phyric with small pheno-crysts of plagioclase, clinopyroxene, and olivine. 100 ×Mg/(Mg + Fe+2) ratios are below 65, indicating thatthese magmas are fairly evolved.

Group 333A-4—Three specimens from lithologicunit 6 comprise this group. These basalts are char-acterized by high 100 × Mg/(Mg + Fe+2) ratios (72-77), relatively high AI2O3, and CaO, low TiO2, andmoderate MgO (Table 2). These rocks are moderatelyphyric with about 10 modal percent of plagioclase,clinopyroxene, and olivine phenocrysts. They aremineralogically and chemically similar to cumulatemagmas of group 332A-5, however, the high CaO con-tents (about 15 wt %) may be partially attributable tothe presence of secondary carbonate.

Group 333A-5—This group is represented by threespecimens of moderately to sparsely phyric basalt fromlithologic unit 8. Phenocrysts consist chiefly ofplagioclase with lesser amounts of clinopyroxene andolivine. For equivalent MgO contents these magmasare richer in AI2O3 and poorer in Tiθ2 than group333A-3 magmas but form a similar 3-phase cotecticseries. Although similar in composition to group 333A-2 these magmas have much higher Cr for a similarrange of 100 × Mg/(Mg + Fe+2) (Table 2).

Trace and Minor Element Geochemistry

Plots of Ti, Sr, and Y versus Zr are given in Figure11, with data for eruptive rocks from Sites 334 and 335.Ti/Zr ratios for 333A groups are approximately thesame as for 332A and 332B magmas. Marked ac-cumulation of plagioclase is apparent from Sr/Zr forone specimen from the CaO- and Abθ3-rich magmasof group 333A-4. Zr/Y ratios for 333A groups aregenerally in the same range as those for Hole 332Agroups, and some 332B groups, with no significant in-tergroup variation (cf. Figures 6 and 11). Significantvariation of P/Y is observed between some 333Agroups. Lowest values are for the cumulate group

(333A-4) whereas higher values of P/Y characterize theother groups.

Chemical Correlation at Sites 332 and 333

Holes 332A and 332B are located about 100 metersapart in a sediment-filled valley, with basement beneaththe site sloping about 6° to the west. Site 333 lies about1.8 km to the southwest in the same valley.

The main lithologic, chemical, and paleomagneticfeatures of each hole are compared in Figure 7 againstrespective subbottom depths. The lack of identitybetween sections is immediately apparent. Although afew individual samples of similar composition may beidentified in each hole, correlation of magma groups islargely unsatisfactory. Considering the ratios 100 ×Mg/(Mg + Fe+2) and 100 × Cr/(Cr + Ni) (Figure 8),groups 332B-7 and 332A-3 and -4 appear to correlate,although Zr/Y and P/Y ratios are at variance (Table2). Groups 332B-1 and 332A-5 show some chemicalsimilarities. Both are characterized by high Zr/Y andlow P/Y ratios (Table 2, Figure 6); however, the min-eralogy of these two units is quite different.

Groups 332B-2 and 332A-3 are somewhat differentin their major element compositions, but they havesimilar ratios of Zr/Y and P/Y (Table 2). The 332B-2magmas have higher ratios of 100 × Cr/(Cr + Ni).These two groups could be related by fractionation ofCr-rich phases such as pyroxene or spinel, althoughthere is little evidence of this from phenocrystassemblages, and no indication from within-groupvariation. However, pyroxene-dominated fractionationat high pressure could possibly account for this dis-crepancy.

Of the Hole 333A magma groups, 333A-5 may betentatively matched with 332B-2, and 333A-4 with332B-6 (Table 2). No other groups can be correlated inall respects, but some might be related by high pressurefractionation processes.

Poor geochemical and paleomagnetic correlationsmay reflect very localized eruptive centers and frac-tionation systems, restricted lateral flow of lava, orfaulting between sites. Lack of correlation betweenSites 332 and 333 may also be due to the very poorrecovery at the latter site.

Site 334

Lithology and Geochemistry

The drilled basement consists of about 50 meters ofbasalt overlying a melange of gabbro and peridotite atleast 67 meters thick. Recovery in the basement sectionwas about 20%.

Two lithologic units are recognized in the basalts; anupper sequence of sparsely phyric basalt withphenocrysts of plagioclase, clinopyroxene, and olivine,and a lower sequence of aphyric basalt. Sedimentary in-tercalations, breccia zones, and glass rinds are commonthroughout the basalts.

Four basalt specimens were analyzed from this site,one from the phyric sequence and three from theaphyric unit. The phyric basalts (group 334-1) are dis-

672


34-

30-

2 6 -

2 2 -

18 -

14

210

190-

170-

150-

1 30 -

110-

90-

70

9000

8000

7000

6000

5000

4000

3000

- Y

335

*

334-2/-^

- Sr /

/ /_ / 333A-5/

334-2C i^

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, 1 i 1 1 ........

30 40 50 60 70 90 100 no 120

Figure 11. Y, Sr, and Ti versus Zr for 333A, 334, and 335 lavas; dotted straight lines show equalelement/Zr ratios (quoted in paratheses); whole-rock groups are indicated with symbols for 333A asfollows: 333A-1, *; 333A-2, Δ 333A-3, G; 333A-4, ®; 333A-5, o; 555^-6, f; ö«c? J55^-7, X,for/ 334 land 335 specimens are: 334-1,®; 334-2, X; and 335, .

673


tinctly richer in LIL-elements and AhO than theaphyric types (group 334-2) (Table 2), although bothgroups are characterized by intermediate LIL-elementand relatively low AhO< abundance compared to lavasfrom Holes 332A, 332B, and 333A. The two magmagroups do not appear to be related by low pressure frac-tionation because the glasses of group 1 are lower inboth MgO and AhO than the whole-rock specimens ofgroup 2 (Figure 4). The phyric basalts were probablyderived from a separate, more alumina-rich magmathan that represented by the aphyric basalts.

Ti/Zr ratios for both groups are marginally higherthan in magmas at Holes 332A and 332B, whereas P/Yand Zr/Y ratios are both relatively low (Figure 11;Table 2). Group 334-1 magmas have higher Cr con-tents, but lower 100 × Mg/(Mg + Fe+2) ratios (68) andlower MgO contents than group 334-2 magmas.

The plutonic rocks comprise an interlayered se-quence of very coarse grained, two-pyroxene gabbro,olivine gabbro, and lesser amounts of peridotite, withnumerous intercalated breccia zones. The breccias con-sist entirely of plutonic rock fragments and may have amatrix of nannofossil chalk. There is a suggestion ofrhythmic layering in the plutonic sequence, but therelatively low recovery and the presence of numerousbreccia zones make this interpretation uncertain.

Three gabbros and two peridotites were analyzedfrom the plutonic complex. The gabbros consist ofcoarse-grained hypidiomorphic aggregates ofplagioclase, clinopyroxene ± orthopyroxene, andminor olivine with small inclusions of chrome spinel.The olivine is partly serpentinized and the pyroxenesare marginally altered to green amphibole. Primarycumulate textures are characteristic.

Primary minerals in the peridotites are chiefly olivineand clinopyroxene ± orthopyroxene, with minorchrome-spinel and plagioclase. Pyroxenes are marginal-ly altered to amphibole and olivine exhibits varyingdegrees of serpentinization, correlated largely with themodal percent of olivine in the rock. Textural evidencesuggests that the peridotites also originated ascumulates. Olivine compositions in the peridotites(Foss-89) are similar to those of phenocrysts in maficlavas (e.g., picrite cumulates at Hole 332B), but are notsignificantly different from olivine in possible harz-burgitic residua from partial melting (Green, 1970).The presence of orthopyroxene in the 4-phase (olivine+ clinopyroxene + orthopyroxene + plagioclase)assemblage may indicate crystallization pressures of upto 7.5 kbar (Kushiro and Thompson, 1972), althoughtextural evidence suggests that orthopyroxene was not aliquidus phase. Pyroxenes in both peridotites and gab-bros exhibit well-developed exsolution textures whichreflect slow cooling of the body.

Chemically the gabbros are somewhat similar to theoverlying basalts in composition (magma group 334-3,Table 2), but 100 × Mg/(Mg + Fe+2) ratios range from69 to 91, the exceptionally high values being at-tributable to accumulation of mafic phases. Their mostnotable feature, compared to the basalts, is the very lowLIL-element contents (e.g., T1O2 = 0.6-0.7 wt %; K2O= 0.01 wt %), and low values of Zr/Y and P/Y (Table

2). The peridotites are also poor in LIL-elements(magma group 334-4, Table 2) and have 100 ×Mg/(Mg + Fe+2) ratios in the range 88-91, similar tothose predicted for material of "pyrolite" composition(Green and Ringwood, 1967).

Site 335

Lithology and Geochemistry

At this site drilled basement consists of about 107meters of uniform, sparsely to moderately phyric,pillow basalt with numerous sedimentary intercalationsand glass cooling rinds. Phenocrysts range from 1 to 5modal percent and are mostly plagioclase (Am?) withminor olivine (Foss).

Seven whole-rock analyses were made from this se-quence. These comprise a single magma group con-firming the chemical uniformity of the sequence (Table2). For an average MgO content of 8.5 wt %, LIL-element contents are intermediate compared to basaltsat Holes 332A and 332B (TiO: = 1.11 wt %) but slightlyhigher than for Site 334 eruptive rocks (Figure 4). P/Yand Zr/Y ratios are relatively low (Table 2). Ratios of100 × Mg/(Mg + Fe+2) range from 62 to 69, reflectingsome removal of mafic phases prior to eruption. Crcontents are relatively high (cf. 334-1 in Table 2) com-pared to noncumulate magmas at Holes 332A and 332Bfor equivalent MgO content.

Rhythmic variation of major element oxides, 100 ×Mg/(Mg + Fe+2), Cr, and Ni with depth suggests cycliceruptions similar to those observed at Holes 332Aand -B. Due to relatively small penetration we have lit-tle idea as to how representative this 13 (?) m.y. old sec-tion is of layer 2. The apparent simplicity of lithologicand geochemical variation contrasts with other sitesdrilled on Leg 37.

SUMMARY AND DISCUSSIONBasaltic rocks drilled during Leg 37 conform in a

general way to the type Mid-Ocean Ridge Basalt(MORB) as characterized by Melson et al. (1975).However, the chemical variation at Sites 332 and 333alone is probably as great as that among all hithertosampled MORB types. At Site 334 a 50-meter-thicklava series overlies a layered plutonic complex,probably formed by crystal accumulation at con-siderable depth (up to about 20 km). Basement at theoldest site, 335, consists of a relatively uniform pillowlava sequence.

Low Pressure FractionationProjections of normative chemistry (Figure 12) in-

dicate that low pressure (up to 1 kbar) fractionation in-volved the phenocryst assemblages olivine ± spinel,olivine + plagioclase ± spinel, and olivine +plagioclase + clinopyroxene. Three-phase cotectic frac-tionation was confirmed in magma groups 332A-2, -3,-5, and -6, 332B-2, -7, and 333A-2, -4, and -5, by the oc-currence of green clinopyroxene phenocrysts togetherwith plagioclase and olivine (cf. Frey et al., 1974). Ex-cept for group 334-2, magma compositions generally

674


plot away from 01 -t- cpx and plag + cpx low pressurecotectics and appear to be largely controlled byvariations in ol, ol 4- plag, and (ol) + plag + cpx withminor spinel in the most mafic liquids.

Within-group decrease of 100 × Mg/(Mg + Fe+2) isaccompanied either by an increase of 100 × Cr/(Cr +Ni) (if olivine is the predominant fractionating maficphase) or by constant 100 × Cr/(Cr + Ni) (ifclinopyroxene accompanies olivine) (Figure 8). Thesetwo ratios would not be affected by addition or subtrac-tion of plagioclase, so that plagioclase-cumulatemagmas should retain the ratios of their noncumulatehost liquids. In this respect the close correspondence ofplots for groups 332A-5 and 332B-1 magmas is signifi-cant (Figure 8).

High Pressure FractionationMagmas similar in their LIL-element character (i.e.,

element abundances, P/Y, Zr/Y, etc. ratios), butdifferent in their 100 × Cr/(Cr + Ni) and Cr/Mgratios, might be related by small degrees of fractionalcrystallization at greater depths (cf. Kilauea; Wrightand Fiske, 1971). Melting relations established byKushiro and Thompson (1972) for a magnesian abyssalolivine tholeiite indicate 4-phase (ol + plag + cpx +opx) coprecipitation at 7.5 kbar and 1215°C. If such amagma is primary (it has 100 × Mg/(Mg + Fe+2) =68.4), and its melting relations are characteristic ofother primitive mid-ocean ridge basalts, fractionationof garnet-bearing assemblages (e.g., "eclogite,"O'Hara, 1968b, 1973) is unlikely in the pressure rangeconsidered. Fractionation of ol + plag + cpx ± opxwould be expected, resulting in cumulates of the typeencountered at Site 334. Fractional crystallizationdominated by pyroxene would reduce the ratio 100 ×Mg/(Mg + Fe+2), and the Cr and Ni contents with con-comitant decrease of the ratio 100 X Cr/(Cr + Ni) andincrease of LIL-element concentrations.

Variation of 100 × Mg/(Mg + Fe+2) versus 100 ×Cr/(Cr + Ni) for Hole 332A and 332B magmas (Figure8) indicates that such a model is plausible in relatingmagma groups 332B-2 and 332A-3, the latter beingmore enriched in LIL-elements. In fact, the immediatehigh pressure derivative from group 332B-2 would bethe parent to group 332A-3, from which the group332A-4 plagioclase-cumulates were derived. Othergroups cannot be related by fractionation of pyroxene-bearing assemblages if they do not show consecutive in-crease in LIL-elements with decrease of 100 × Cr/(Cr+ Ni) and 100 × Mg/(Mg + Fe+2) ratios.

Adiabatic cooling of high pressure cotectic liquidswould bring them well within the olivine primary phasevolume at low pressures (O'Hara, 1968b), assumingthey were not formed under hydrous conditions(discussed below). The preliminary models presentedhere will be tested by petrologic mixing calculations(Wright and Doherty, 1971).

Primary Magmas

The most primitive (i.e., unfractionated) magmas atHoles 332A and 332B are the noncumulate type groups332B-2 and -5 (Table 2), together with host magmas to

cumulate groups 332B-1, -3, -4, -6 and 332A-1 and -5.Disregarding olivine cumulate compositions, the rangeof 100 × Mg/(Mg + Fe+2) for these magmas is about68-75, which corresponds with the range predicted byGreen (1970) for liquids which equilibrated withperidotite of "pyrolite" composition, and observedolivine phenocryst compositions Fos-i-90 (Green, 1970;Roeder and Emslie, 1970).

O'Hara (1968a) concluded that most "abyssaltholeiites" do not lie on the boundary of the primaryphase volume of orthopyroxene at any pressure, and socannot be derived from an enstatite-bearing uppermantle if they are primary liquids. Nevertheless,Kushiro and Thompson (1972) showed that abyssalolivine tholeiite could have equilibrated withplagioclase lherzolite at depths up to about 26 km.Consequently, we believe that the most magnesianmagmas of some groups (e.g., 332B-2, -5) were eruptedto the surface without significant fractional crystalliza-tion. However, some portions of these and othermagma batches were trapped and subsequently frac-tionated to a greater or lesser extent. It is significantthat compositional extremes (i.e., high AhO.i, low LIL-element- and low AbO•i, high LIL-element- types) oc-cur at one site (332) in sections that were probably builtup in less than 200,000 yr. The highly aluminous andLIL-depleted parent(s) to magma groups 332A-5 and332B-1 are possibly the most characteristic of"oceanic" tholeiite (cf. Engel et al., 1965; Nicholls,1965) of Leg 37 basalts. Unfortunately we have no datafor glassy (liquid) representatives, although Frey et al.(1974) report a highly aluminous and mafic glass fromthe South Atlantic (age about 40 m.y.), with A1:OJ con-tent of 17.4 wt % and MgO = 9.81 wt % (F in Table 2).The origin of such liquids has been a subject of somespeculation (Engel et al., 1965; Nicholls, 1965; Greenand Ringwood, 1967).

Anhydrous experimental melting studies of aphenocryst-free high-alumina tholeiite from the MAR(Tilley et al., 1972) show plagioclase on the liquidus at 1atm. Experiments on the same specimen at highpressure (Kushiro and Thompson, 1972) show plagio-clase to be replaced by clinopyroxene on the liquidus,without any appearance of olivine. Thus olivine frac-tionation under anhydrous conditions could not pro-duce this liquid composition. Experiments underhydrous conditions indicate olivine together withplagioclase and clinopyroxene is on or near the liquidusat pressures between 5 and 8 kbar, suggesting this liquidcould have formed from more primitive magmas orfrom peridotite at 15-25 km depth in the presence ofsmall (about 2%) amounts of H:O. This leads to thepostulate that exceptionally aluminous primary basalticliquids reflect partial fusion of plagioclase lherzolite un-der hydrous conditions. Rapid ascent, with degassing,would bring these liquids into the plagioclase primaryphase volume, the resulting magmas being very coarse-ly phyric.

Other controlling factors on primary magma com-positions are the depth and degree of partial fusion,which in turn depend on the geothermal gradient. Anaverage gradient of 45°C/km may be calculated from

675


CPX

(a) 332A 332A-1 +

332A-2

332A-3 Δ

332A-4 O

332A-5 X

CPX

(c) 333A

Figure 12. Normative compositions of Leg 37 basalts, showing individual magma groups, projected onto the plane olivine-plagioclase-clinopyroxene in the "basalt" tetrahedron ol - cpx - plag - qz; low pressure cotectic boundaries are projected inaccordance with petrographic evidence for liquidus phases and by analogy to experimentally melted basalts by Tilleyet al (1963); (a) 332A, (b) 332B, (c) 333A, and (d) 334 and 335.

676


CPX

(b) 332B

CPX

(d) 334 and 335

334-1 A

334-2 Δ

335

Figure 12. (Continued).

677

M. F. J. FLOWER, P. T. ROBINSON, H.-U. SCHMINCKE, W. OHMMACHT

the pressure/temperature conditions (1215°C, 7.5 kbar)of the 4-phase point for abyssal tholeiite (Flower et al.,1975). Given a constant gradient with time, the degreeof partial fusion would depend on fluctuations involatile content (H:O would reduce the P and T of theperidotite solidus), or other changes in bulk composi-tion of the source material. However, small changes inthe gradient could result in significant changes indegree of fusion and might be attributable to randomor intermittent sub-rift tectonic processes. We find noevidence of consistent long-term secular changes ofmagma composition (cf. Frey et al., 1974) between Sites332 and 335.

Origin of the Plutonic Rocks

Textural and chemical criteria suggest the plutonicrocks belong to a rhythmically layered cumulate com-plex, unless faulting has affected their relative jux-tapositions. Presence of nannofossil chalk in the brecciaand the absence of basaltic lava clasts suggest that thecoarse-grained complex was tectonically uplifted to thefloor of the median rift prior to eruption of the overly-ing basalts.

Although emplaced diapirically, the plutonic rocksare unlikely to be mantle material. Their inferredaverage sonic velocity of 6.88 km/sec (Hyndman,Chapter 11, this volume) is appropriate for layer 3materials. Indications of their relatively deep-seatedformation (e.g., presence of orthopyroxene) andprobable emplacement in the median rift zone suggestthat the high level position of the body is anomalous.More usually, plutonic bodies of deeper origin(possibly at magma staging areas near the fusion zone)would rise to the base of layer 2 and contribute to acontinuously accreting layer 3 together with othercrystallized magma bodies occupying shallow-level sub-rift chambers. The depth of the layer 3/layer 2 interfaceand the frequency of solid intrusions reaching the sur-face would depend on the physical properties ofcrystallizing magma (density, viscosity, gas content,etc.) and this overlying crust (permeability, strength,overall density, etc.) as evaluated for Iceland by Walker(1975).

CONCLUSIONS

1. Crustal construction along the median rift in-volves coeval eruption and intrusion of several magmatypes, characterized by different contents of Al, Ca, andLIL-elements, and formed by differential fusion ofplagioclase lherzolite at depths up to about 25 km.

2. The most primitive magmas have 100 X Mg/(Mg+ Fe+ 2) of 68-75 and, if olivine-phyric, phenocrystcompositions are Fθ84-9o. Some have probably beenerupted without significant fractionation, but mosthave been delayed at different levels beneath the rift,with development of chemical trends related to lowpressure crystal fractionations.

3. High-alumina mafic magmas, with plagioclase onthe anhydrous liquidus at 1 atm may have formed bypartial fusion of plagioclase lherzolite under slightlyhydrous conditions (about 2% H2O) at depths up to 20km.

4. Buildup of layer 2 was episodic, with cycles oferuption and high-level fractionation being related tospasmodic tectonic development of the rift.

5. No simple correlation is possible between Sites332 and 333. This may be due to extreme localization oferuption and magma flow, and/or to vertical displace-ment by faulting.

6. The plutonic complex at Site 334 is a cumulatebody. It reflects: (a) fractional crystallization oftrapped primitive LIL-depleted basaltic magma near (?)the zone of partial fusion, (b) slow cooling of thecrystalline body, and (c) solid intrusion to the floor ofthe median rift.

7. The layer 3/layer 2 interface may represent thelevel of compensation between newly crystallinemagma, prevented from reaching the surface, and thatmagma erupted to the surface before final consolida-tion. Relative thickness of the two layers would thus bedependent on the physical properties of crystallizingmagma and overlying crust.

ACKNOWLEDGMENTS

Thanks are due to G. Bullen and P. Watkins (ImperialCollege, London), H. Henning, P. van den Bogaard, R. Pier,and R.J. Starmer (Ruhr-Universitat, Bochum) for assistancewith X-ray fluorescence and wet chemical analysis; P. Sud-daby (Imperial College), Drs. M. Engl and J. Sommerauer(E.T.H., Zurich) for help with electron microprobe work; andProf. H. Puchelt (Universitat, Karlsruhe) for CO2 analyses.P.T.R. received financial support from the Humboldt Stif-tung (BRD), and M.F.J.F., H.-U.S., and W.O. were sup-ported by Deutsche Forschungsgemeinschaft. M.F.J.F. isgrateful to Prof. J. Sutton for use of laboratory facilities atImperial College. We thank Prof. W. Schreyer (Bochum) forhis continued interest and support for the project.

REFERENCES

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