19. chemistry of leg 45 basalts introduction · 19. chemistry of leg 45 basalts j. m. rhodes,1 d....

19. CHEMISTRY OF LEG 45 BASALTS

J. M. Rhodes,1 D. P. Blanchard,3 M. A. Dungan,4 K. V. Rodgers,2 and J. C. Brannon2

INTRODUCTION

A total of 764 meters of volcanic basement wassampled during Leg 45 at Sites 395 and 396; most ofthe material recovered consists of basaltic pillow lavas,thin flows, and more massive cooling units. This paperis concerned with the major-and trace-element chemis-try of these basalts. A companion paper by Dungan etal. (this volume) discusses the petrography, mineralchemistry, and 1-atmosphere melting experiments forthe same suite of samples. The emphasis here will beon samples recovered from Holes 395 and 395A. Datafor samples from Hole 396 are included for complete-ness, but will be reported in detail, together with dataon samples from Hole 396B, in the Leg 46 InitialReports.

Major- and trace-element analyses for 59 basaltsamples from Holes 395 and 395A are given in Table1; analyses for 8 samples from Hole 396 are given inTable 2. In these tables, the samples are listed in orderOf increasing depth within basement, reported to thenearest meter. Also indicated in Tables 1 and 2 are themajor magmatic compositional units, first identified onthe basis of shipboard X-ray fluorescence analyses, andmodified slightly in the light of the recent data. Withinthese two tables, the oxidation ratio (O.R.) is given asFe2O3/(total iron as FeO). The atomic ratio Mg/(Mg + Fe) has been calculated, after adjustment forthe oxidation state of iron, such that the ratio Fe 3 + /(Total iron as Fe 2 + ) is 0.1 (Bass, 1971). For conve-nience, the atomic Mg/(Mg + Fe) ratio is designatedthe Mg'-value in Tables 1 and 2 and throughout thepaper. The same convention for handling the oxidationstate of iron has been used in the CIPW norm calcula-tions that are referred to in the text and illustrated inFigure 1.

Table 3 presents additional trace-element data on26 samples selected from Table 1 on the basis ofpetrography and major element chemistry.

The major-element data were obtained by X-rayfluorescence analysis (XRF) on fused glass discs,prepared by fusing the sample with a lanthanum-bearing lithium borate fusion mixture (Norrish andHutton, 1969). FeO was determined titrimetricallyusing the modified cold acid digestion method ofWilson (Maxwell, 1968), and Fe2θ3 was obtained bydifference from the XRF total iron value. Na£θ was

'Lockheed Electronics Co., Inc., Houston, Texas. Present ad-dress: Dept, of Geol. and Geog., Univ. of Massachusetts, Amherst.

2Lockheed Electronics Co., Inc., Houston, Texas.'NASA, Johnson Space Center, Houston, Texas.4NASA, Research Associate, Johnson Space Center, Houston,

Texas.

APHYRIC BASALTS

A

•

•

A2

A3A4

PHYRIC BASALTS

oΔ

α

A

P2

P3

P4, P5

30

70 60 50PLAGIOCLASE

Figure 1. Normative olivine-plagioclase-pyroxenerelationships in Leg 45 basalts. The classification andinferred olivine-plagioclase cotectic is from Shido etal. (1971).

determined on a separate 20 to 30 mg aliquant byinstrumental neutron-activation analysis (INAA). Totalwater content was measured coulometrically using aDuPont moisture analyzer, in which the sample isfused with a lead oxide flux.

The trace elements (Rb, Sr, Y, Zr, Nb) were deter-mined by XRF analysis on pressed powder pellets.Corrections were made for non-linear backgrounds,tube contamination, and inter-element interferences(Norrish and Chappell, 1967). Corrections for matrixeffects were based on a modification of the Comptonscattering method (Reynolds, 1967), using the Ag-Compton peak. Additional trace-element data (Table3) (La, Ce, Sm, Eu, Tb, Yb, Lu, Hf, Cr, Se, Ni)were obtained by INAA following the methods ofJacobs et al. (in press).

BASALT CHEMISTRY

All the basalts recovered from Holes 395, 395A, and396 are olivine and hypersthene normative, containingbetween 1.8 and 14.2 per cent of normative olivine,and plotting in the olivine tholeiite field of the basalttetrahedron (Yoder and Tilley, 1962). Alkali basaltsand transitional tholeiites are conspicuously absent, asalso are quartz tholeiites. They have the overall com-

447

J. M. RHODES ET AL.

TABLE 1Chemical Composition of Basalt From Site 395

Sample(Interval in cm)

JSC No.Depth (m)Basalt typeSiC•2Tiθ2AI2O3Fe 2 O 3

FeOMnOMgOCaONa2θK2OP 2 O 5

STotal H2OTotal

Total Fe as FeOMg'-value0. R.

Trace Elements(ppm)

RbSrYZrNb


JSC No.Depth (m)Basalt typeSiO2

Tiθ2AI2O3Fe2θ3FeOMnOMgOCaONa2θK2OP2O5STotal H2OTotal

Total Fe as FeOMg'-valueO. R.

Trace Elements(ppm)

RbSrYZrNb

395-11-1,4244

10993

A2*49.4

1.6215.09

1.289.830.208.51

10.412.800.110.130.110.55

100.04

10.980.6050.117

1.2115

35105

2.4

395A-8-1, 70-73

120126

A 2

48.71.66

15.402.568.810.207.60

10.462.900.180.170.101.62

100.33

11.110.5750.230

1.6119

35104

2.0

395-11-1, 136-138

11094

A 2 *49.3

1.6114.84

1.369.730.198.45

10.342.920.100.110.110.43

99.50

10.950.6050.124

0.9112

35105

1.9

395A-9-1, 73-79

121135A248.5

1.6014.692.848.030.208.21

10.172.930.160.120.141.93

99.56

10.590.6060.269

1.5118

35103

1.7

395-11-2,100-105

11195

A 2 *48.5

1.5814.79

1.819.060.178.56

10.182.820.140.120.111.56

99.37

10.690.6130.169

1.7116

34102

1.9

395A-9-2, 100-104

122137A 2

48.91.63

14.922.678.200.198.15

10.293.010.150.130.141.25

99.61

10.600.6040.252

1.3114

36106

1.6

395-15-2, 68-72

112133A2*

49.31.60

14.682.618.330.188.40

10.172.890.140.130.121.42

99.94

10.680.6090.244

1.1116

35106

1.7

395A-10-1, 144-146

123145A248.8

1.6014.90

1.978.960.198.40

10.232.900.140.120.131.41

99.79

10.730.6080.184

1.3115

32103

2.1

395-16-2, 63-66

113142A2*

49.61.66

15.154.626.940.198.17

10.432.820.160.160.100.91

100.94

11.100.5930.416

1.1115

37106

1.8

395A-13-1, 142-147

124174

P249.2

1.3717.342.525.960.156.55

11.542.830.110.100.091.94

99.66

8.230.6120.306

1.1153

2994

1.7

positional characteristics typical of mid-ocean ridgetholeiites, as is indicated by their relatively constantSiO2 concentrations and low TiO2 abundances, to-gether with low total alkali content and K2O/Na2Oratios (e.g., Engel et al., 1965; Melson et al, 1976).

K2O abundances are characteristically low in most ofthe samples (<Cθ.2%), and where comparative data areavailable, do not differ significantly from the K2Ocontent of basaltic glass from the same samples. A fewsamples have higher K2O contents, not related to

448

CHEMISTRY OF BASALTS

395-16-3, 3-8

114143A2*48.91.60

14.692.208.870.188.4710.252.870.170.120.141.53

99.94

10.850.6070.203

1.1118361082.1

395A-14-2, 140-147

125185P249.11.34

17.931.576.940.136.8111.732.800.090.100.101.48

100.12

8.350.6180.188

0.915227871.2

395-18-2, 4346

115161Pi*48.81.14

16.672.905.900.158.0712.242.340.130.090.050.89

99.38

8.510.6530.341

1.511026711.1

395A-14-3, 29-34

126186P249.71.33

18.132.405.860.146.5811.742.850.070.120.121.46

100.49

8.020.6190.299

0.515227861.9

TABLE 1

395-19-1,57-62

116170p2*49.71.29

18.483.234.450.125.9912.102.680.110.100.031.97

100.27

7.360.6170.439

1.215826852.0

395A-15-1, 75-81

127192P249.01.35

17.512.695.970.136.9711.542.760.110.110.121.63

99.87

8.390.6220.321

1.015530951.4

- Continued

395 A 4 1,66-69

11798A349.51.70

15.182.867.440.177.6911.172.740.280.150.071.08

100.04

10.010.6030.286

2.6131361192.3

395A-15-4,10-15

128(1)196P249.01.36

17.651.407.250.137.0711.582.710.110.120.111.71

100.21

8.510.6220.165

1.315629911.4

395A-5-1, 6-10

118106A249.21.63

15.113.956.040.177.7411.032.790.260.170.051.51

99.64

9.600.6150.411

3.3129351131.9

395A-15-4, 10-15

128(2)196P249.51.35

17.57(1.40)7.250.147.0311.57(2.71)0.110.120.11_-

8.520.620—

1.015229931.8

395A-6-1, 130-134

119117A248.71.60

14.744.306.820.188.0810.112.970.220.120.072.48

100.45

10.690.5990.402

3.0118341042.0

395A-154, 99-107

129197P248.31.30

18.012.345.960.156.8211.512.870.080.090.081.65

99.20

8.070.6260.343

0.515428891.7

basalt type, which are presumably the result ofseawater alteration, although this is not always corre-lated with total water content or the oxidation ratio.

The minor- and trace-element abundances are, simi-larly, within the range of typical mid-ocean ridge

tholeiites (Kay et al., 1970; Schilling, 1971; Pearce andCann, 1973; Hart, 1976; Bryan, et al., 1976; Erlank andKable, 1976). This is illustrated by the chondrite-normalized rare-earth patterns, which range between 13and 24 times chondritic abundances for Sm, and which

449

J. M. RHODES ET AL.


JSC No.Depth (m)Basalt typeSiθ2Tiθ2A12O3

F e 2 θ 3FeOMnOMgOCaONa2OK 2 O

P2O5STotal H2OTotal


Trace Elements(ppm)

RbSrYZrNb


JSC No.Depth (m)Basalt typeSiθ2TiO2

AI2O3F e 2 θ 3FeOMnOMgOCaONa2θK 2 O

P2O5STotal H2OTotal


Trace Elements(ppm)

RbSrYZrNb

395A-15-5, 16-20

130198

P248.9

1.1419.16

1.476.190.136.96

11.832.750.070.090.091.63

100.41

7.510.6470.196

0.6153

2476

0.7

395A-31-1, 96-107

142337

P549.8

1.1018.62

3.284.490.126.69

12.172.890.180.080.011.77

101.23

7.440.6140.440

2.6175

2371

0.8

TABLE 1 -

395A-17-1, 46-55

131211

P348.9

1.0918.05

3.704.780.156.66

12.832.500.120.090.011.19

100.08

8.110.6190.456

1.9119

2465

0.9

395A-33-1,142-148

143355

P548.9

1.1417.554.084.000.147.11

11.852.730.180.110.011.86

99.66

7.670.6470.532

2.5151

2373

1.0

- Continued

395A-19-1, 132-136

132231

P349.1

1.0816.853.424.800.137.61

12.192.310.130.080.011.94

99.67

7.880.6570.434

2.1110

2364

1.2

395A-34-1, 140-143

144364

A349.5

1.6915.32

2.987.240.197.61

11.212.800.220.170.091.33

100.32

9.920.6030.300

1.0127

36116

2.4

395A-21-1, 87-90

133242

P349.8

1.0317.732.585.230.147.53

12.492.390.150.100.031.68

100.91

7.550.6640.342

1.7112

2260

1.5

395A-35-1, 26-32

145373

A349.3

1.671.5014.505.660.167.45

10.952.880.320.130.031.95

100.03

9.710.6030.463

4.7133

35117

2.0

395A-23-1, 51-55

134261

P449.2

1.1218.51

3.764.240.146.51

12.592.750.150.120.011.01

100.10

7.620.6290.493

1.4138

2468

1.3

395A-36-1, 46-50

146383

A348.9

1.6715.134.105.800.197.48

11.092.930.260.140.022.28

100.03

9.490.6090.432

3.3130

36117

1.9

display the light rare-earth depletion typical of "nor-mal" or type I ocean ridge basalts (Bryan, et al., 1976).Mg '-values in these rocks vary from about 0.57 to 0.67(Figure 2), which is within the range prevalent for mid-ocean ridge basalts but is lower than the values found inthe most primitive basalts so far identified (Frey et al.,

1974; Bryan and Moore, 1976; Rhodes et al., in press).These values, together with low Ni concentrations,moderately high magmaphile element abundances, andthe presence of multiple phenocryst phases (Dungan etal., this volume), are taken as evidence that these basaltshave all undergone a substantial differentiation history.

450


395A-24-2, 69-72

135272P449.11.15

17.262.805.110.157.9012.172.650.130.090.000.88

99.42

7.630.6720.367

1.513125721.5

395A-38-1,142-148

147393A348.61.65

14.614.155.830.177.4010.832.890.220.140.0042.00

98.53

9.560.6050.434

2.3134351142.0

395A-25-1, 96-100

136280P449.31.14

17.102.585.280.147.6711.992.620.140.110.011.54

99.58

7.600.6670.339

2.313324730.9

395A41-1, 107-111

148428A349.71.70

15.302.907.280.177.2811.142.820.220.150.081.11

99.85

9.890.5930.293

0.7129361172.1

TABLE 1

395A-26-2,125-129

137292P438.91.25

18.853.414.850.175.8712.272.840.160.130.021.26

99.96

7.920.5950.431

2.516526821.3

395A-46-1, 61-64

149476A349.31.75

15.103.906.290.197.2111.102.900.310.170.021.26

99.50

9.800.5930.227

4.5132371252.2

- Continued

395 A-27-2, 111-116

138301P449.61.14

17.131.946.030.158.0312.082.640.150.100.020.71

99.69

7.780.6710.249

2.112925731.0

395A49-1, 26-31

150504A349.51.74

15.133.147.240.197.2911.192.840.180.170.091.30

99.97

10.070.5890.312

1.5131371212.5

395A-38-1, 105-112

139309P548.81.19

19.09.307

4.970.156.3312.222.780.150.110.020.97

99.87

7.730.6190.397

2.715927821.1

395A-50-2, 143-147

151516A348.81.72

15.073.976.380.167.6711.022.850.220.160.051.76

99.84

9.950.6040.399

2.8130371211.9

395A-29-1,125-131

141317P549.81.14

17.072.655.730.158.0012.042.590.130.100.020.83

99.41

8.110.6610.327

2.013225741.3

395A-51-3, 78-82

152526A448.81.70

14.933.526.670.187.6910.872.880.260.150.071.77

99.51

9.840.6070.358

2.3129371222.1

Most samples selected for analysis were relativelyfresh, with total water contents usually less than 2 percent and oxidation ratios mostly below 0.4. It is un-likely, therefore, that the major- or trace-element chem-istry of these samples will have been significantlymodified by seawater alteration. The mobile elements

K, Rb, and S are notable exceptions. The effects ofalteration on sulfur are especially pronounced, andthere is a marked inverse correlation between theoxidation ratio and the sulfur content (Figure 3). Itwould appear from this relationship that unalteredsamples should contain about 0.14 per cent sulfur, a

451

J. M. RHODES ET AL.



TiO2

AI2O3F e 2 O 3

FeOMnOMgOCaONa2OK 2 O

P2O5STotal H2OTotal


Trace Elements(ppm)

RbSrYZrNb



TiO2

A12O3

F e 2 θ 3FeOMnOMgOCaONa2OK 2 0

P2O5STotal H 2 0Total


Trace Elements(ppm)

RbSrYZrNb

395A-52-2, 75-80

153534

A350.3

1.7315.222.947.370.197.34

11.102.760.190.160.111.38

100.75

10.020.5920.293

1.0128

37119

2.8

395A-64-4,130-135

164631

A 4

47.11.59

15.005.075.490.197.72

10.672.960.150.130.003.08

99.13

10.050.6030.505

2.3126

35108

2.7

TABLE 1

395A-56-3, 62-65

154565

A349.4

1.7514.99

3.217.120.187.82

10.822.800.190.180.071.44

100.02

10.010.6070.321

1.2127

38125

2.7

395A-64, CC, 17-21

165632

A 4

48.81.56

15.073.906.360.178.15

10.772.760.170.130.082.03

99.94

9.870.6210.395

1.9132

33107

1.8

- Continued

395A-57-1, 40-43

155570

A 4

48.81.57

14.932.328.020.198.62

10.552.800.150.150.091.50

99.51

10.110.6280.229

0.9121

34107

2.4

395A-66-1, 36-41

166645

A 4

48.11.6115.023.846.630.188.32

10.772.770.120.140.052.28

99.83

10.090.6200.381

1.3127

36112

2.0

395A-59-1, 67-70

156590

A4

48.31.58

14.932.697.650.168.45

10.712.830.130.130.051.84

99.42

10.070.6240.267

0.6125

34109

2.4

395A-66-3, 12-16

167648

A 4

47.71.59

14.764.715.870.217.99

10.672.960.180.140.002.62

99.37

10.110.6100.466

3.8126

35109

2.5

395A-60-1, 66-70

157599

A 4

48.41.59

15.123.216.940.198.01

10.772.850.140.120.071.95

99.36

9.830.6170.326

0.9125

35109

1.8

395A-67,CC, 130-135

168657

A 5

48.21.57

14.872.487.860.188.57

10.852.690.220.150.081.96

99.72

10.090.6270.246

0.9122

34107

1.4

value somewhat larger than that proposed by Mooreand Fabbi (1971) as typical for unaltered basaltsrecovered from deep water.

Two broad basalt types, differing fundamentally inboth petrography and whole-rock chemistry, prevail atSite 395. These are, respectively, the aphyric and

phyric basalts. The aphyric basalts lack megascopicphenocrysts, but contain microphenocrysts of olivineand Plagioclase; the phyric basalts contain abundant(10 to 30%) Plagioclase phenocrysts, together withphenocrysts of olivine, rarer Cr-diopside, and minorchromian spinel. The aphyric basalts are the more

452


395A-60-3, 33-37

158602

A 4

47.21.55

15.095.025.360.188.40

10.602.770.160.130.012.17

98.61

9.880.6270.508

2.2117

34103

2.3

395A-61-1, 142-150

159608

P4'49.2

1.2116.05

3.814.920.157.84

11.712.720.160.100.011.66

99.55

8.350.6500.456

3.9128

2778

0.6

TABLE 1

395A-62-1, 10-15

160617

P4'49.3

1.0917.15

2.105.930.167.93

11.632.700.080.080.111.75

100.02

7.820.6680.269

0.8122

2369

1.1

- Continued

395A-63-l,4247

161619

P4'49.6

1.1216.47

2.685.560.178.18

11.592.540.090.090.051.78

99.92

7.970.6700.336

1.0123

2471

0.3

395A-63-4, 86-93

162624

P4'48.5

1.0617.19

3.924.160.157.56

11.772.640.120.090.002.75

99.90

7.690.6610.510

4.0130

2265

0.8

395A-64-1, 29-37

163626

P4'49.1

1.0617.21

3.274.710.167.31

12.454.210.100.080.021.31

100.87

7.650.6540.427

1.71362368

0.9

TABLE 2Chemical Composition of Basalt From Site 396


JSC No.Depth (m)Siθ2Tiθ2A12O3

F e 2 O 3

FeOMnOMgOCaONa2θK 2 O

P2O5STotal H2OTotal

Total Fe as FeOMg'-valueO.R.

Trace Elements(ppm)

RbSrYZrNb

15-1,110-115

17312749.30

1.2716.95

1.397.150.177.48

11.802.660.200.090.051.49

100.00

8.400.6380.165

1.0149

2684

2.1

16-1, 97-101

17613748.79

1.2616.86

3.644.900.158.03

11.852.870.210.080.051.52

100.21

8.170.6610.445

2.5145

2785

1.4

18-3, 147-150

17515949.27

1.2916.30

3.854.720.147.66

11.482.750.230.080.022.28

100.70

8.180.6500.471

2.7160

2887

1.6

22-2, 120-128

17119348.99

1.2316.69

3.654.680.147.60

11.662.790.240.090.022.47

100.24

7.960.6540.459

3.3145

2683

2.0

23-1, 58-61

172201

49.171.25

17.294.334.190.136.42

12.072.770.200.090.022.86

100.79

8.090.6110.535

2.9152

2779

1.1

24-2, 49-54

169211

49.811.30

16.523.925.020.177.21

11.762.720.230.090.012.10

100.86

8.550.6260.459

3.5127

2883

1.4

25-1, 134-139

170221

49.301.29

16.214.434.680.176.86

11.692.680.250.150.032.52

100.26

8.680.6100.510

abundant, and comprise about 59 per cent of the coredinterval.

Chemically, the phyric basalts are distinguishedfrom the aphyric basalts by higher A12O3 (16.5 to19.2%) and CaO (11.5 to 12.8%) concentrations, and by

lower SiO2, FeO, and MgO concentrations (Figures 4and 5). These differences are reflected in higher nor-mative Plagioclase contents and in lower Ab/(Ab + An) ratios in the phyric basalts. Although bothtotal iron and MgO are lower in the phyric basalts,

453

J. M. RHODES ET AL.

TABLE 3Trace Element Abundances (ppm) in Selected Leg 45 Samples

Sample(Interval in cm) 395-11-1,42-44 395-11-2, 100-105 395-19-1, 57-62 395A-4-1, 66-69 395A-8-1, 70-73 395A-10-1, 144-146

JSC No.Depth (m)Basalt type

SeCrNi

LaCe

SmEuTbYbLuHf

10995

A 2

36.6290120

2.919.63.381.330.923.30.503.0

111101

A 2

36.0290190

2.799.73.411.270.923.20.472.7

116173

P2

29.8252110

2.719.33.091.060.762.80.422.3

11799A 3

38.4320140

3.7712.2

3.911.401.03.50.533.0

120129

A 2

37.9300140

3.1811.1

3.721.390.953.60.523.0

123153

A 2

37.1300150

3.319.8

3.681.340.953.70.562.9

Sample(Interval in cm) 395A-25-1, 96-100 395A-27-2, 111-116 395A-31-1, 96-107 395A-34-1, 140-143 395A-41-1, 107-111 395A-49-1, 26-31

JSC No.Depth (m)Basalt type

SeCrNi

LaCeSmEuTbYbLuHf

136285

P 433.8

340150

2.418.12.771.060.722.50.402.3

138306

P 433.5

340130

2.337.22.721.040.602.70.402.2

142339

P 532.3

310140

2.488.12.821.060.672.7

0.412.2

144372

A 338.5

280110

3.8211.74.101.430.993.70.563.3

148433

A 338.7

290100

3.7012.34.091.471.053.60.553.3

150504

A 338.8

29040

4.1412.54.281.451.093.80.563.2

-

-

-

—_

N

\ FIELD FOR ABYSSAL

\ BASALTIC GLASS-

^ ° ~RCPo o

DD ^

D π Δ

Δ D Λer

APHYRIC BASALTS **«*.

A

•

•

A2

A3

A4

^ ^ _

JI

\\\\

\α

°Δ

~~—̂_

•PHYRIC BASALT

0 P2

Δ P3

ü P4, P5 -

(A, GLASSY~J SELVAGES

\\

\\

\N

\

.65Mg/(Mg + Fe)

Figure 2. TiO2 versus Mg '-value for Leg 45 basalts.The field for abyssal volcanic glass compositions istaken from the data of Melson et al. (1976).

Mg'-values tend to be higher, an indication that theyare more "primitive" than the aphyric basalts (Figures2 and 5). T1O2, a nd most other minor- and magmaphiletrace-element abundances are markedly lower in thephyric basalts (Figure 2). This difference extends alsoto the glassy rims of the phyric basalts, and is thereforenot simply a consequence of Plagioclase dilution.Strontium, on the other hand, is higher in the phyric

.14

.10

.06

.02

mmum

i

• ••••••

•

••

••

•

•

tm

•••

—

APHYRIC BASALT

PHYRIC BASALT -

-

-

-

-

-

-

••

i

• i«fc• •

.3 .5OXIDATION RATIO (O. R.)

.7

Figure 3. Relationship of sulfur content to oxidationratio (O.R.) in Leg 45 basalts. The juvenile sulfurcontent of the basalts is inferred to have been greaterthan 0.14 per cent.

454


TABLE 3 - Continued

395A-14-2, 140-147 395A-15-4, 10-15 395A-15-4, 16-20 395A-15-5,16-20 395A-15-5, 16-20 395A-19-1, 132-136 395A-21-1,87-90

125186

P2

31.1

22087

2.93

9.53.20

1.17

0.82

3.00.45

2.4

128(1)

199

P2

31.5

29083

2.79

9.23.05

1.16

0.71

2.70.42

2.4

128(2)199

P2

32.6

25090

2.95

9.73.26

1.20

0.76

2.80.46

2.6

130201

P2

28.2

260100

2.42

8.02.66

1.01

0.64

2.40.37

2.1

130201

P2

29.0

30070

2.40

7.82.681.030.69

2.40.36

1.9

132231

P3

32.5

377110

2.02

6.72.38

0.91

0.66

2.40.36

1.9

133244

P3

32.4

380130

2.07

6.82.450.930.62

2.40.371.9

395A-56-3, 62-65 395A-57-1, 40-43 395A-61-1, 142-150 395A-63-4, 86-93 395A-66-1, 36-41 395A-67-CC, 130-135

153539

A3

37.8280120

3.8112.14.331.361.023.90.593.2

154567A3

38.3

260120

4.24

12.64.201.511.06

3.90.58

3.2

155573A

4

36.7

340170

3.34

10.4

3.67

1.34

0.93

3.30.53

3.1

159609

P4

35.6320110

2.248.22.811.080.72

2.50.382.2

162626

P4

30.6

380140

2.30

8.42.93

1.08

0.72

2.60.43

2.4

166645A4

36.7

320140

3.1810.33.561.390.99

3.20.47

2.8

168660A

4

36.6330140

3.3710.6

3.671.330.94

3.60.50

3.3

APHYRIC BASALTS

A A2

• A3

• A4

PHYRIC BASALTS

O P2

Δ P3

D P4, P5

0 GLASSY SELVAGES

WT PER CENT MgO

Figure 4. A12O3 versus MgO for Leg 45 basalts. Theolivine and Plagioclase control lines are for illustra-tive purposes only; they are not intended as lines of"best fit," or to imply that the basalts are relatedsimply by olivine and Plagioclase fractionation.

basalts, both in absolute terms and with respect toother magmaphile elements. Consequently, Sr/Zr ra-tios are distinctly different in the two basalt types, andvary widely in the phyric basalts (1.55 to 2.46), incontrast to the lower, essentially constant values (1.02to 1.18) in the aphyric basalts.

12

10

8

6

APHYRIC

A

_•

—

- /

L $/

/ /_ //

BASALTS

A2

A3

A4

/

è

///

/

t

/

4

I

/

/

//

/

n O Δ

t

/

S

i

*

/

/

*

PHYRIC

O

Δ

D

1 i

/

/

/

/ -

BASALTS ~

P2

P3

P4, P5

1

6 8WT PER CENT MgO

10

Figure 5. MgO-FeO relationships for Leg 45 basalts.The dashed lines indicate Mg '-values from 0.5 to 0.7.

455

J. M. RHODES ET AL.

Figure 1 illustrates the normative olivine, plagio-clase, and pyroxene relationships for these basalts,relative to the inferred olivine-plagioclase cotectic ofShido et al. (1971). All the aphyric basalts plot in theolivine tholeiite field, within a tightly controlled elon-gate group sub-parallel to the inferred cotectic. Incontrast, the phyric basalts plot predominantly withinthe Plagioclase tholeiite field and are widely scattered.Those phyric basalts plotting close to the inferredcotectic, or just within the olivine tholeiite field, havethe lowest A12O3 contents. Presumably, they containfewer Plagioclase phenocrysts than the majority ofsamples, and may be closer to melt compositions. Asmight be expected from these relationships, in a one-atmosphere melting experiment on a typical phyricbasalt (#138), Plagioclase is the first phase to crystal-lize, followed by olivine, whereas in a typical aphyricbasalt (#122), olivine is the liquidus phase, followedby Plagioclase (Dungan et al., this volume).

Initial studies, based on shipboard X-ray fluores-cence analyses, recognized within the two major basalttypes 10 compositionally distinct magmatic units. Theseare, in order of increasing depth below the sediment/basement interface, Al, A2, PI, P2, P3, P4, P5, A3,A4, P4, A4, and A5, where A denotes aphyric basaltand P phyric basalt.

Our data essentially confirm these subdivisions,subject to minor revisions in light of the more recentand comprehensive data. Average values, calculatedfrom the data in Table 1, are given in Table 4. In orderof increasing depth within Hole 395A, the composi-tional units following can be identified.

TABLE 4Average Compositions of Leg 45 Basalt Types

Aphyric BasaltsA2

Major Elements (wt %'.

SiO2TiO2A12O3FeO*MnOMgOCaONa2OK2OP2O5SMg'-value

48.961.61

14.9010.810.198.27

10.282.890.150.130.110.602

Trace Elements (ppm)

RbSrYZrNbLaCeSmEuTbYbLuHfSeCrNiLa/SmLa/YbSm/EuSr/ZrZr/NbZr/YTi/ZrZr/Hf

1.411634.9

1051.93.05

10.13.551.330.943.50.512.9

36.9295150

0.860.872.671.10

553.01

9236

A3

1

49.281.71

15.079.840.187.48

11.032.850.230.160.060.601

2.313036.5

1192.23.94

12.24.201.441.043.80.573.2

38.4280

980.941.042.921.09

543.26

8637

A4

48.071.58

14.9810.010.188.36

10.712.820.160.130.040.623

1.6125

34.4108

2.13.30

10.43.631.350.953.40.503.1

36.7330150

0.910.972.691.16

513.14

8835

P2

49.161.31

17.988.110.146.75

11.682.780.100.110.090.622

0.915427.788

1.52.708.92.991.110.732.70.412.3

30.4262

900.901.002.691.75

593.2

8938

Phyric BasaltsP3

49.271.07

17.547.850.147.27

12.502.400.130.090.020.647

1.911423.063

1.22.056.72.420.920.642.40.361.9

32.5379120

0.850.852.631.81

532.7

10233

P4

49.221.16

17.777.710.157.20

12.222.700.150.110.010.649

2.013924.874

1.22.377.72.751.050.762.60.402.3

33.7340140

0.860.912.621.88

623.0

9432

P4SU1

49.151.11

16.817.900.167.76

11.832.650.110.090.040.661

2.312923.870

0.72.278.32.871.080.722.50.402.3

33.1350125

0.790.912.661.84

1002.9

9530

P5

49.131.14

18.087.740.147.03

12.072.750.160.100.020.643

2.515424.575

1.12.488.12.821.060.672.70.412.2

32.3310140

0.880.922.662.05

683.1

9134

Aphyric Unit A2 (106 to 173 m)—This is the upper-most aphyric unit, and occurs in the upper 29 meters ofHole 395, as well as in Hole 395A. It consists of a thicksequence of chemically homogeneous, rapidly chilledpillow basalts, and has a characteristic variolitic texturecontaining microphenocrysts of olivine. This basalttype is distinguished from the other aphyric basaltsprincipally on the basis of higher iron concentrations(10.6 to 11.1%) and by slightly lower abundances ofstrontium (Table 4). The compositional variation issmall, and is commensurate with minor (<5%) olivinefractionation.

In the initial shipboard studies, the magmatic UnitAl was based on a single sample (11-1, 103-108 cm)from near the top of basement in Hole 395. It wasmore iron-rich than underlying members of the A2basalts. In order to locate the A1/A2 boundary wehave analyzed basalts immediately above and belowthis sample, and find that they have typical A2 compo-sitions (Table 1, #109, 110). From this we concludethat Unit Al is not a distinct magmatic unit, and thatthe iron-rich sample analyzed during shipboard studiesis a more evolved variant of the A2 basalt type.

Phyric Unit P2 (173 to 201 m)—This unit occurs atthe top of the thick 181-meter sequence of phyricbasalts that were cored in Hole 395A, and also occursin the bottom 10 meters of Hole 395. In Hole 395A,almost all of this unit is composed of a single thick (25m), massive cooling unit. Except for the lowermost sam-ple in the cooling unit (Table 1, #130), the P2 basaltsare uniform in composition, and are more evolved thanthe other phyric basalts. This is reflected in lower Mg '-values and higher abundances to T1O2 and othermagmaphile elements (Figure 2). The lowermost samplein the cooling unit is higher in AI2O3, has a higher Mg '-value, and has correspondingly lower abundances ofmagmaphile elements. In many respects it resemblesother, more "primitive" phyric basalts lower in thestratigraphic sequence. We attribute this difference toeither preferential accumulation of Plagioclase andolivine phenocrysts at the base of the cooling unit, orupward migration of interstitial magma, caused by filterpressing.

Phyric Unit P3 (201 to 260 m)—This unit is distin-guished from the other phyric units by lower TiO2 andmagmaphile element abundances (Figures 2 and 6).Strontium is also low in this unit (110 to 119 ppm),relative to all other phyric basalts. The plagioclase-olivine phyric basalt associated with the brecciatedserpentinized periodotite zone in Hole 395 is also lowin strontium (Table 1, #115), and tends to be lower inmagmaphile elements than most other phyric basalts.In many respects, it closely resembles the P3 basalttype in Hole 395A, and may simply be fragments ofthis basalt type incorporated into the breccia zone,rather than a separate magmatic unit (PI) as wassuggested by the shipboard studies.

Phyric Units P4 and P5 (260 to 356 m)—Initialshipboard analyses indicated that these two basalttypes are essentially similar in bulk chemistry, but

456


4.0

3.0

2.0

1.0

APHYRIC BASALTSA A2

• A3• A4

PHYRIC BASALTSo P2

P3π P4, P5

/

• /

s La/Sm

A•

O •/°

Z r / y ^ ^ ^ - ^ _

I I I ,

401.0 2.0 3.0 4.0 5.0 Sm

60 80 100 120 ZrSm & Zr CONCENTRATION IN ppm

Figure 6. Lanthanum-Samarium and Zirconium-Yttrium relationships in Leg 45 basalts. The linesshown are for constant La/Sm and Zr/Y ratios.

differ in strontium content. Additional analyses re-ported here (Table 1) show that both units containstrontium in a wide range of values: strontium in P4varies between 122 and 165 ppm, and in P5 between132 and 175 ppm. Consequently, we believe that theinitial distinction between these two basalt types is nolonger tenable.

As a group these basalts are highly variable both inAI2O3 content, which varies from 16.1 to 19.1 percent, and in Mg'-values (between 0.59 and 0.67 percent). Much of this variability must be attributed tolocalized variation in Plagioclase and olivine pheno-cryst content.

Deeper in the section (608 to 629 m), there is amassive (21 m) cooling unit chemically equivalent tothe P4-P5 phyric basalts. We believe this to be a sillthat intruded underlying aphyric basalts peneeontem-poraneously with extrusion of the P4-P5 basalts.

Aphyric Unit A3 (356 to 570 m)—This exceptionallythick sequence of compositionally uniform pillow ba-salts is more evolved than the other aphyric units.These basalts are multiply saturated with micropheno-crysts of both olivine and Plagioclase (Dungan et al.,this volume), and plot towards the low-temperatureend of the inferred cotectic in Figure 1. Although theMg'-values compare closely with those of the A2basalts (e.g., 0.59 to 0.61), they are distinctly lower inboth FeO and MgO (Figure 5), and on the basis of fig.7 in Roeder and Emslie (1970), can be inferred tohave lower liquidus temperatures. In keeping with theirevolved nature, these basalts have the highest litho-phile element abundances, and are characterized by amore distinct Eu anomaly than is observed for theother aphyric basalts. The unit as a whole exhibits littlecompositional variation, and there are no obviousfractionation trends.

At the sediment/basement interface in Hole 395A isa rubble zone containing fragments of aphyric basalt,serpentinized peridotite, and gabbro (Cores 4 and 5).

Two of these basalts are indistinguishable from the A3basalts (Table 1, # 117, 118); this suggests that thisbasalt type is exposed at the surface of the basementwithin close proximity to the drill site.

Aphyric Unit A4 (570 to 608 m and 629 to 657 m)—Thisunit is intruded by a 21-meter sill compositionallyequivalent to the P4-P5 phyric basalts. There is no ap-preciable difference in chemistry between the basaltsabove the sill (Table 1, #155-160) and those beneath(Table 1, #161-168). All are aphyric basalts contain-ing olivine microphenocrysts. They are both texturallyand chemically similar to the A2 basalts, but can bedistinguished from them by a tendency toward highernormative olivine contents (Figure 1), higher Mg'-values (0.60 to 0.63), and lower total iron concentra-tions for a given value of MgO (Figure 5). TiO2, Sr, andthe magmaphile element abundances are essentiallyidentical to those of the A2 basalts.

DISCUSSIONThe chondrite-normalized rare-earth abundances for

the Leg 45 samples (Figure 7) show them to be"normal" LIL-element-depleted basalts. They arelight-rare-earth depleted, and have La/Sm and La/Ybratios less than 1.0 (Table 4), similar to the Type Ibasalts of Bryan et al., (1976). The Nb content of thesebasalts is low, resulting in high Zr/Nb ratios (mean =58) similar to those in other LIL-element-depletedbasalts (Pearce and Cann, 1973; Erlank and Kable 1976;Rhodes et al., 1976), and considerably higher than theZr/Nb ratios (<IO) of oceanic islands and"anomalous" Type II basalts sampled on the Mid-Atlantic Ridge at 45 °N and in the FAMOUS area(Erlank and Kable, 1976; Rhodes et al., unpublisheddata). Two 87Sr/86Sr values of 0.70276 ± 10 and0.70280 ± 8, obtained for Samples 117 and 153, respec-tively, are also characteristic of "normal," Type I LIL-element-depleted basalts (Hart, 1976).

PHYRIC BASALTS

La Ce Sm Eu Tb

REE ATOMIC NUMBER

Yb Lu

Figure 7. Chrondite-normalized abundances in Leg 45basalts.

457

J. M. RHODES ET AL.

Inspection of Figures 6 and 7 and Table 4 showsthat most magmaphile element ratios are very similarfor the various basalt types, and do not differ substan-tially between the phyric and aphyric basalts. Anobvious exception is the ratio of strontium to othermagmaphile elements. This is illustrated by the Sr/Zrratio, which is constant in the aphyric basalts, where Srbehaves as a magmaphile element, but is both dis-tinctly different and highly variable in the phyricbasalts, where Plagioclase influence is prevalent. Sincemany magmaphile element ratios and the characteristicREE patterns are not substantially changed by frac-tional crystallization involving the observed phenocrystphases, olivine, and Plagioclase (Kay et al., 1970;Schilling 1971; Frey et al., 1974; Schilling, 1975), wededuce that these ratios reflect the characteristics of themantle source region, and that all of the various basalttypes were derived from an essentially similar andhomogeneous LIL-element-depleted mantle source.Similar ratios pertain to basalts from Holes 396 (Table2) and 396B (Dungan et al., Initial Reports of theDeep Sea Drilling Project, v. 46, in preparation); thisimplies a widespread and homogeneous mantle sourcefor basaltic vulcanism along this section of the Mid-Atlantic Ridge. This contrasts markedly with the widevariations in magmaphile element ratios found in Leg37 basalts, both within and between holes (Blanchardet al., 1976), and the consequent necessity to postulatea complex and heterogeneous mantle source for thebasalts in that region.

Although these basalts appear ultimately to havebeen derived by partial melting of a common source,all are evolved and have undergone moderate amountsof differentiation. Evidence of this is provided by thelow Mg'-values (0.57 to 0.67), low Ni concentrations(<190 ppm), moderate magmaphile element abun-dances (e.g., TiO2 = 1.0 to 1.7%), and the presence ofmultiple phenocryst phases in all but the A2 and A4basalts. Since the compositional variation within theindividual basalt groups is small, and there are noclearly defined internal fractionation trends, we suggestthat magma compositions were established by differ-entiation within shallow magma chambers, followed byepisodic eruption onto the sea floor, with little or nofractionation at the surface or enroute to the surface.

There can be little doubt that the compositions ofthe glassy and variolitic aphyric basalts are close tothose of magmatic liquids. The compositions of thephyric basalts are more enigmatic. Several lines ofevidence indicate that these rocks are far removedfrom liquid compositions: (a) Compilations of basalticglass compositions (Melson et al, 1976) indicate thatmagmatic liquids with more than 17 per cent A12O3are extremely rare. Similarly, the glassy selvages on thephyric basalts (unpublished data) are much lower inA12O3 and not substantially different from the aphyricbasalts (Figure 4). (b) The wide scatter of composi-tions in the normative olivine-plagioclase-pyroxenediagram (Figure 1) suggests Plagioclase and olivineaddition to liquids close to the inferred olivine-plagio-

clase cotectic. Plagioclase addition trends are alsoevident in several other variation diagrams (e.g., Fig-ures 4 and 5). (c) Dungan et al. (this volume) presentexperimental, textural, and compositional evidenceindicating that many of the Plagioclase and olivinephenocrysts are too anorthitic or forsteritic to havecrystallized from melts having the compositions of thephyric basalts, but derive instead from more "primi-tive" basalts.

From this evidence, we think that the phyric basaltshave a mixed heritage, resulting from mixing of con-sanguineous "primitive" and evolved magmas andtheir attendant phenocrysts. If this interpretation iscorrect, the higher Mg'-values, normative An/(Ab + An) ratios, and lower magmaphile elementabundances of the phyric basalts relative to the aphyricones result in part from the addition of up to 20 percent Plagioclase and 5 per cent olivine phenocrysts.The glassy rims on phyric basalt pillows probablyprovide the most reliable estimate of the melt composi-tions. They have Mg '-values comparable to those of themore evolved aphyric basalts (<0.60), but are distin-guished from them by lower TiO2 values (Figure 2)and by higher CaO/Al2O3 ratios. Thus, the phyricbasalts are not simply related to any of the aphyricbasalts types by processes of Plagioclase and olivineaccumulation.

An understanding of the relationships between thevarious phyric basalt units is also frustrated by uncer-tainty concerning melt compositions. In terms of in-creasing magmaphile element abundances, the basalttypes are ranked as follows: P3, P4-5, P2. This is notsimply a function of decreasing Plagioclase and olivineaccumulation, but reflects a fundamental property ofeach of the basalt types. The increase in magmaphileelement abundances from P3 to P2 is too large andvaried, however, to be accounted for simply by crystalfractionation of the dominant phenocryst phases, Plagio-clase and olivine, and cannot readily be reconciled withthe absence of systematic changes in major elementchemistry. We attribute these differences in themagmaphile element abundances in the phyric basalts todiscrete magma types, containing varying proportionsof admixed Plagioclase, olivine, and rarer clinopyroxenephenocrysts.

The relationships between the three aphyric basaltunits are less complicated. The A2 and A4 basalts arebroadly similar in major- and trace-element chemistry,and both contain only olivine as the microphenocrystphase. The A2 basalts are higher in total iron and tendto have lower Mg'-values than the majority of the A4basalts (Figure 5). Consequently, these two basalttypes cannot be related by olivine fractionation, theonly observed phenocryst phase, and the liquidusphase in equilibrium melting experiments (Dungan etal., this volume). The A3 basalts contain micropheno-crysts of both olivine and plagioclase, and are moreevolved than the stratigraphically lower A4 basalts,from which they may have been derived by olivinefractionation. The magmaphile element abundances,

458


however, are at variance with this interpretation. Someelements, such as Sr, Y, and Se are increased in the A3basalts relative to the A4 basalts, commensurate withthe 5 per cent olivine and Plagioclase fractionationrequired by the major-element data. The differences inother elements (e.g., Ti, Zr, La, Sm, Yb) are, however,too large to have resulted from such a small amount offractionation. Further, the amount of fractionationestimated from the reciprocal of the ratio of magma-phile elements in the A3 to the A4 basalts (Andersonand Greenland, 1969) is not constant for the differentelements, and varies from as little as 4 per cent for Srto as much as 20 per cent for Sm. Clearly, thesemagma types are not related directly by crystal frac-tionation processes.

CONCLUSION

We have, at the Leg 45 site, several discrete magmatypes, both phyric and aphyric, not related directly bycrystal fractionation, which appear to have been de-rived from a common mantle source. The prevailinginterpretation of such relationships is that each basalttype represents an evolved derivative of more "primi-tive" basaltic magmas, resulting from varying degreesof melting of a common mantle source. The evidencepresented here, of magma mixing in the phyric basalts,suggests that such an interpretation may be too simple.We suggest instead that magma mixing of consanguin-eous evolved and "primitive" basalts, together withtheir attendant phenocrysts, may better explain manycharacteristics of the basalts. A subsequent publicationdocumenting the role of magma mixing in these and

other ocean floor basalts is in preparation (Rhodes etal., in press).

REFERENCES

Anderson, A. T. and Greenland, L. P., 1969. Phosphorusfractionation diagram as a quantitative indicator of crys-tallization differentiation of basaltic liquids, Geochim.Cosmochim. Acta, v. 33, p. 493-505.

Bass, M. N., 1971. Variable abyssal basalt populations andtheir relation to sea-floor spreading rates, Earth Planet.Sci. Lett., v. 11, p. 18-22.

Blanchard, D. P., Rhodes, J. M., Dungan, M. A., Rodgers, K.V., Donaldson, C. H., Brannon, J. C, Jacobs, J. W., andGibson, E. K., 1976. The chemistry and petrology ofbasalts from Leg 37 of the Deep Sea Drilling Project, J.Geophys. Res., v. 81, p. 4231-4246.

Bryan, W. B. and Moore, J. G., 1976. Compositional varia-tions of young basalts in the Mid-Atlantic Ridge rift valleynear lat. 46°49'N, Geol. Soc. Am. Bull, v. 88, p. 556-570.

Bryan, W. B., Thompson, G., Frey, F. A., and Dickey, J. J.,1976. Inferred settings and differentiation in basalts fromthe Deep Sea Drilling Project, J. Geophys. Res., v. 81, p.4285-4304.

Engel, A. E., Engel, C. G., and Havens, R. G., 1965.Chemical characteristics of oceanic basalts and the uppermantle, Geol. Soc. Am. Bull, v. 76, p. 719-734.

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