29. CHEMICAL ZONATION OF PLAGIOCLASE PHENOCRYSTS FROM LEG 51, 52,AND 53 BASALTS

Claire Bollinger and Michel Semet, Laboratoire de Géochimie et Cosmochimie, Institut de Physique du Globe de Paris,Département des Sciences de la Terre, 75230 Paris, France

INTRODUCTION

Chemical zonation of phenocryst phases present a recordof the physicochemical changes in magma from which theycrystallized. Several recent studies (e.g., Bottinga et al.,1966; Albarède and Bottinga, 1972; Kirkpatrick, 1976,1977; Sibley et al., 1976; Shimizu, 1978) have addressedthe zonation of major and trace elements in plagioclase fromthe point of view of the growth dynamics from a magma andhave shown that disequilibrium composition features of thephenocrysts should provide a better understanding of thephysicochemical evolution of the associated magma. As dif-fusion coefficients, growth rates, and chemical compositionof phenocrysts as a function of supercooling become avail-able, parameters like cooling rates and non-equilibrium par-tition of trace elements of crystallizing magmas may bemodeled quantitatively. Oceanic basalts are among the ter-restrial volcanic rocks that show relatively little differentia-tion from primitive liquids originating in the mantle and donot seem to show extensive mixing from different sources.Their relatively simple crystallization history make themwell suited for the detailed study of the chemical zonation oftheir phenocryst assemblages.

The types of zonation patterns found in plagioclasephenocrysts from volcanic rocks and their possible pet-rogenetic interpretation are reviewed by Smith (1974,chapt. 14 to 17). Four main types are present in plagioclasephyric oceanic basalts: continuous zoning, oscillatory zon-ing, patchy zoning, and sector zoning. Some phenocrystsmay show more than one type in the different growth zones,indicating that several growth-controlled phenomena maybe at work during the crystallization of a single crystal.Furthermore, correlation of zonation patterns between thegrowth faces of individual phenocrysts, or between differentcrystals of the same sample (e.g., Sibley et al., 1976), mayhelp in attributing the patterns to specific growth featuressuch as chemical changes in the liquids, cooling ratechanges, settling, floating, and convection in the magma.

Samples were selected from Legs 51, 52, and 53 ofGlomar Challenger for which a thick section of well-characterized basalts was available. The aim of our study isto obtain detailed major and trace element data for severalselected samples of the basalt pile, with the hope of recon-structing some aspects of their petrogenetic history. Thisreport is concerned primarily with zonation patterns of themajor elements (Na, Al, Si, and Ca) and is preliminary inseveral regards; major element data can provide only limitedinformation on the zonation patterns and, more importantly,the resolution in the chemical changes is limited to severalµm by electron microprobe analysis. The second part of the

study consists of detailed trace element determinations inthe samples studied by ion microprobe analysis. A com-prehensive discussion of the results, therefore, will not beattempted at this point.

ANALYTICAL PROCEDURE

Most samples studied are mino-cores. Doubly polishedthin sections were prepared by standard petrographictechniques. Chemical analyses were performed with aCAMECA Camebax electron microprobe which has a 40°take-off angle. Wavelength dispersive spectrometers and aprimary current of 0.01 to 0.04 µA were routinely used.The beam was slightly defocused (3-5 µm 0) for largezones, whereas spot analysis (= 1 µm spot 0) was carriedout for the finer zones. A modified Bence-Albee data reduc-tion program (Bence and Albee, 1968) utilizing a LakeCounty labradorite, an Amelia County albite, and an Eifel(Germany) orthoclase standard was used to obtain the majoroxide concentrations.

RESULTS

Most of the samples studied were from pillow basalts,although a few came from massive units. Preliminary pet-rographic examination showed that zonation patterns wereof three general types: (a) relatively homogeneous crystals;(b) oscillatory zoned crystals; (c) oscillatory and sectorzoned crystals.

Phenocrysts from pillow basalts may be of any of thethree types; type b is, however, the most abundant. Massiveunits, on the contrary, mostly contain type b phenocrysts.All phenocrysts have a relatively sodic rim of variablethickness ranging from 10 to 100 µm.

No systematic differences between the samples comingfrom different depths in the boreholes were found. The mostrepresentative samples were, therefore, selected for themicroprobe study (see Table 1). The phenocrysts are eithersubequant or lath-shaped. Prominent growth was along[001], [110], and [201] faces. Patchy zoning is apparentlyrestricted to the cores of the phenocrysts. Bands rich inglassy inclusions may occur anywhere in the crystal and arecontinuous around it. Oscillatory zones generally show twothickness ranges. Larger zones, tens of µm wide, aresubdivided into µm size zones. The latter cannot becompositionally resolved with the electron microprobe.Correlation of the larger zones between different growingfaces, or symmetry related faces, is perfect in most cases. Amajor proportion of the oscillatory zones are planar andparallel to growing faces. However, a few samples showwavy or contorted bands not conforming to prominentfaces. Engulfment of inclusions does not seem to be the

1055

C. BOLLINGER, M. SEMET

TABLE 1Petrographic Descriptions

Leg Hole Sample Unit Description

51 417D 33-4, - Massive basalt, groundmass is coarse12 cm grained, containing oxides; zoned

plagioclase phenocrysts up to 4 mm;crystal types b and c

52 418A 28-2, 5 Moderately porphyritic pillow basalt10 cm with olivine, pyroxene, and plagio-

clase phenocyrsts; groundmass isslightly altered; a spinel inclusionin plagioclase phenocryst; crystaltypes b and c

53

53

53

53

53

53

418A

418A

418A

418A

418A

418A

50-2,105 cm

71-2,52 cm

71-2,66 cm

80-2,55 cm

85-2,93 cm

86-3,93 cm

6b

l l f

l l f

12

12

13

Moderately porphyritic pillow basaltwith olivine and plagioclase pheno-crysts, fine-grained groundmass;plagioclase is fresh and contains al-tered glass inclusions; crystal typesa, b, c

Pillow basalt moderately porphyriticwith olivine and plagioclase pheno-crysts and aphyric matrix; crystaltypes b and c

Glassy zones from a pillow basaltwith 3 mm plagioclase phenocrysts;glass inclusions in the phenocrysts aremostly fresh; cyrstal types b and c

Massive basalt with plagioclase andolivine phenocrysts and very coarsegrained rηatrix; crystal types b and c

Massive basalt with olivine, plagio-clase, and pyroxene phenocrysts andcoarse-grained matrix; crystal type b

Pillow basalt with olivine, plagioclase,and pyroxene phenocrysts and veryfine-grained matrix; crystal types band c

general explanation for this feature for which no rationalehas been found.

Analytical data for the major elements are presented inTable 2. A summary of the results is illustrated in Figure 1which presents a plot of Ca versus Al atom number foranalyses normalized to 8 oxygens; this type of illustrationfollows Bryan (1974). Detailed analyses for K, Mg, and Feare not presented at this point because these elements will bedetermined with higher accuracy with an ion microprobe.Preliminary ion microprobe results show that K is generallyon the order of 100 ppm weight and that Mg and Fe increasefrom core to rim and are enriched in {θlθ} sectors as ob-served by Bryan (1974). The average composition of theplagioclase is An.75-80 and is relatively more calcic than thosedescribed by Bryan (1974). For those analyses where K,Mg, and Fe are reported, the total number of cations is closeto the theoretical 5 to 8 oxygens so that extensive non-stoichiometry does not seem to be present (Figure 2).

DISCUSSION

The Ca versus Al relationship of Figure 1 generallyfollows the one reported by Bryan (1974) in showingenrichment of Ca relative to the theoretical Ab-An solid-solution series. Our data lie closer to the theoretical line;solid solution of the hypothetical end-member Ca (Mg, Fe)SiβOβ, as suggested by the latter author accounts satisfac-torily for the shift. Linear fits to the data for pillow basalts

and massive flows are slightly different, but within the lim-its of error of the least-squares fitted best lines.

Thermodynamics indicate that the most calcic composi-tions are likely to be closer to equilibrium composition forthe crystallization of the plagioclases. Had equilibrium beenmaintained, plagioclases of Anss-w composition should havecrystallized from the basaltic melt. Departure from thiscomposition in the larger oscillatory zoned bands are takento reflect different degrees of undercooling. A crude esti-mate of this parameter may be obtained by comparison ofthe plagioclase composition to that observed under con-trolled temperature conditions for magmas of similar bulkcomposition. Undercoolings of 50° to 100°C gave rise toAnβ5 plagioclases in the Makaopuhi lava lake (Wright andKinoshita, 1968), whereas Clochiatti et al. (1978) obtainedAnβ5 and Ansi plagioclases for 30° and 100°C undercool-ings , respectively. Those observations then suggest that thedegree of undercooling prevailing during crystallization ofthe phenocrysts in the oceanic basalts was less than a fewtens of degrees and possibly as low as a few degrees C.Crystallization of the sodic rims may have occurred atgreater undercoolings of 50° to 100°C, and have corre-sponded to eruption on the ocean floor. Enhanced nuclea-tion rates at this stage are in keeping with the appearance ofmicrophenocrysts of similar composition. Even thoughother phenomena may play a role in the development of thelarger oscillatory zoned bands, variations in cooling rate ofthe body of magma that crystallized the phenocrysts in astep-by-step progression into colder rocks seem to afford anadequate explanation. The variations would induce variabledegrees of constitutional supercooling in the sense of Bot-tinga et al. (1966) and Sibley et al. (1976). In this line ofreasoning, the fine µm-size zones remain inadequatelyexplained. They may be due to the build-up of "poisons" atthe growing surface owing to their slow diffusion in theliquid.

A fair conclusion from the data at hand would be thatdisequilibrium growth effects, such as those suggested byAlbarède and Bottinga (1972), are not pronounced. How-ever, our contention is that the scale of the latter phenome-non is generally below the resolution of microprobeanalysis. The mere fact that sectors have different averagecompositions and that growth zones for different crystallo-graphic faces do not always correspond strongly suggestthat growth rate is an important parameter in controlling thecomposition of the growing layer.

ACKNOWLEDGMENTS

The study has been supported in part by a grant from the CNRS(ATP "Gèochimie"). C.B. benefited from a fellowship of theCentre National pour 1'Exploitation des Oceans. The manuscriptwas improved by the thoughtful comments of Dr. S. Wilhelm andof anonymous reviewers.

REFERENCES

Albarède, F. and Bottinga, Y., 1972. Kinetic disequilibrium intrace element partitioning between phenocrysts and host lava,Geochim. Cosmochim. Ada., v. 136, p. 141-156.

Bence, A.E. and Albee, A.L., 1968. Empirical correction factorsfor the electron microanalysis of silicates and oxides, J. Geol.,v. 76, p. 382.

1056

TABLE 2Analytical Data for Major Elements

SiO 2

A 1 2 O 3

CaO

N a 2 O

K 2 0

MgO

FeO

Total

Nb cations

per 8 oxygens

Si

Al

Ca

Na

K

Mg

I•e

Total

%An

SiO 2

A 1 2 O 3

CaO

N a 2 O

K 2 O

MgO

FeO

Total

Nb cations

per 8 oxygens

Si

Al

Ca

NaK

Mg

Fe

Total

% A n

1

51.7930.0213.64

3.680.09

0.85

100.07

2.359

1.6120.6660.3250.005

0.032

5.000

64.72

24

54.6127.3610.55

5.120.08

1.07

98.79

2.4991.4760.517

0.4540.005

0.041

4.922

50.84

2

48.44

33.4016.61

1.890.04

0.41

100.69

2.204

1.7910.8050.1670.002

0.016

4.985

81.31

25

49.71

32.2115.512.480.05

0.50

100.46

2.2621.7270.756

0.2190.003

0.019

4.986

75.83

3

47.9933.5116.49

1.860.03

0.41

100.29

2.193

1.8050.8070.1650.002

0.016

4.988

81.52

26

50.6831.8215.502.64

0.06

0.49

101.19

2.2881.6930.750

0.2310.003

0.018

4.983

74.85

4

48.1933.4716.73

1.840.03

0.35

100.61

2.1961.7970.8170.1630.002

0.013

4.988

82.11

27

48.3133.3516.52

2.090.01

0.33

100.61

2.2011.7910.806

0.1850.001

0.013

4.996

80.36

5

49.0332.6816.52

2.100.06

0.34

100.73

2.230

1.7520.8050.1850.003

0.013

4.988

80.02

28

52.5429.9114.243.310.04

0.76

100.80

2.371

1.5930.689

0.2900.002

0.025

4.976

68.22

6

48.8532.8316.182.400.04

0.37

100.67

2.224

1.7610.7890.2120.002

0.014

5.003

77.58

29

49.4332.1215.95

2.240.02

0.48

100.24

2.2561.7270.780

0.1980.001

0.018

4.980

78.24

7

50.57

31.5114.89

2.84

0.05

0.52

100.38

2.298

1.6880.7250.2000.003

0.020

4.984

72.65

30

50.2631.94

15.262.450.05

0.46

100.42

2.2831.7100.743

0.2160.003

0.017

4.972

75.89

8

46.6034.6918.44

1.190.03

0.33

101.28

2.121

1.8610.8990.1050.002

0.013

5.001

88.22

31

47.3133.5818.00

1.390.03

0.41

100.72

2.1631.8090.882

0.1230.002

0.016

4.995

86.3

9

50.6931.2314.493.270.07

0.63

100.38

2.3061.6740.7060.2880.004

0.024

5.003

69.08

32

47.9532.8017.54

1.760.03

0.41

100.49

2.1951.7700.860

0.1560.002

0.016

4.999

83.17

Sa

10

46.3234.64

17.801.170.06

0.38

100.37

2.124

1.8720.8750.1040.004

0.015

4.993

87.67

nple51-417D-334

11

46.6834.5417.62

1.320.03

0.45

100.64

2.134

1.8610.8630.1170.002

0.017

4.995

86.39

12

48.15

33.6016.91

2.030.04

0.42

101.15

2.187

1.7980.8230.1790.002

0.016

5.005

80.69

Sample 5 3-418 A-71-2

33

53.2928.6113.523.77

0.06

0.78

100.03

2.4221.5330.658

0.3330.003

0.030

4.979

64.32

34

52.2030.3613.963.190.05

0.59

100.36

2.364

1.6200.678

0.2800.003

0.022

4.967

68.97

35

51.42

30.3614.09

3.13

0.05

0.63

99.68

2.3481.6340.689

0.2770.003

0.024

4.975

69.39

13

47.74

34.0317.13

1.630.04

0.41

100.98

2.171

1.8240.8350.1440.002

0.016

4.990

83.75

36

49.4331.7715.82

2.370.03

0.46

99.88

2.264

1.7150.7760.2100.002

0.018

4.985

77.14

14

47.98

33.7016.94

2.04

0.(12

0.41

101.19

2.1811.8050.7250.1800.001

0.016

5.007

80.72

37

51.5530.8414.442.950.01

0.59

100.38

2.3371.6480.7010.2600.001

0.022

4.969

71.24

15

48.1533.5316.77

2.090.04

0.34

100.92

2.1901.7970.8170.1840.002

0.013

5.004

80.41

38

50.4031.2314.122.720.05

0.41

98.93

2.3151.6910.6950.242

0.003

0.016

4.962

72.70

16

51.7630.95

14.463.380.04

0.58

101.17

2.3331.644

0.6980.2950.002

0.022

4.994

68.63

39

49.4732.0614.98

2.370.03

0.31

99.22

2.2711.7350.7370.2110.002

0.012

4.968

76.61

17

56.41

27.2810.685.650.06

0.98

101.06

2.5241.4380.5120.4900.003

0.037

5.004

49.13

40

47.0634.0117.66

1.240.04

0.34

100.35

2.155

1.8360.867

0.1100.002

0.013

4.983

87.40

18

51.24

30.3314.263.110.08

0.75

99.77

2.341

1.6330.6980.2760.005

0.029

4.982

69.25

41

47.1633.6217.21

1.300.04

0.28

99.61

2.1721.8250.8490.116

0.002

0.011

4.975

86.8.1

19

47.50

34.2117.19

1.480.04

0.55

100.97

2.1611.8350.8380.1310.002

0.021

4.988

84.48

42

53.3228.2513.063.92

0.08

0.96

99.59

2.434

1.5200.6390.3470.005

0.037

4.982

62.16

20

54.73

28.3511.475.150.06

0.93

100.85

2.4641.504

0.5530.4500.003

0.035

5.010

53.12

43

52.0330.2814.583.280.05

0.59

100.81

2.3531.614

0.7060.288

0.003

0.022

4.986

69.28

21

46.62

34.5118.03

1.27

0.04

0.38

100.96

2.1291.858

0.8820.1120.002

0.015

4.999

87.24

44

52.3929.7414.113.37

0.05

0.54

100.20

2.3781.591

0.6860.2970.003

0.021

4.976

68.12

22

47.0?

34.1817.75

1.51

0.03

0.47

100.85

2.145

1.838

0.8680.1340.002

0.018

5.004

84.93

45

48.0032.8816.94

1,65

0.01

0.45

99.93

2.2041.7790.833

0.1470.001

0.017

4.981

83.47

23

47.11

33.7917.221.48

0.05

0.78

100.43

2.159

1.8 25

0.8460.1320.003

0.020

4.995

83.68

46

48.8331.5016.352.02

0.05

0.54

99.29

2.2541.7140.809

0.1810.003

0.021

4.981

79.86

1>NOz>δaso

a

o00

TABLE 2 - Continued

SiO2

A12O3

CaO

Na2OK 2 O

MgO

FeO

Total

Nb cationsper 8 oxygens

Si

Al

Ca

Na

K

Mg

Fe

Total

% An

SiO2

A12O3

CaO

Na2OK 2 O

MgO

FeO

Total

Nb cationsper 8 oxygens

Si

Al

Ca

Na

KMg

Fe

Total

% An

47

48.2231.9217.13

1.880.03

0.50

99.68

2.2241.7350.8460.1680.002

0.019

4.994

81.74

70

49.2531.3815.90

2.390.03

0.34

99.29

2.2691.7040.7850.2140.002

0.013

4.986

77.49

48

50.3731.1215.842.710.06

0.37

100.47

2.2931.6700.7730.2390.003

0.014

4.993

75.05

71

47.9533.2417.052.040.06

0.62

100.94

2.1721.8010.8400.1820.004

0.024

5.021

80.15

Sam

49

49.3632.0916.57

2.170.04

0.44

100.67

2.2471.7220.8080.1920.002

0.017

4.988

79.29

72

47.6033.6217.47

1.790.04

0.44

100.96

2.1701.8060.8530.1790.004

0.017

5.007

82.73

ple52-418A-71-2

50

52.2329.2113.283.410.06

0.83

99.02

2.3961.5800.6530.3030.004

0.032

4.967

65.89

73

47.9232.5416.92

2.210.05

0.43

100.07

2.2031.7630.8330.1970.003

0.017

5.016

79.33

51

48.3831.8315.73

2.190.05

0.51

98.69

2.2441.7400.7820.1970.003

0.020

4.986

78.04

74

54.8928.4211.664.990.12

0.97

101.05

2.4641.5030.5610.4340.007

0.036

5.005

54.05

52

48.6331.9416.582.060.04

0.49

99.74

2.2371.7320.8170.1840.002

0.019

4.990

80'.02

75

51.1431.2314.73

3.430.05

0.59

101.17

2.3101.6620.7130.3000.003

0.022

5.011

68.69

53

48.5432.4216.78

1.840.08

0.36

100.02

2.2251.7520.8240.1640.005

0.014

4.983

81.91

76

50.9631.1514.833.380.07

0.71

100.57

2.3061.6610.7190.2970.004

0.027

5.004

68.67

Sample71-2,

54

50.5530.4514.29

2.980.050.250.54

99.11

2.3251.6500.7040.2660.0030.0170.021

4.986

69.63

77

50.4531.5314.622.880.05

0.37

99.90

2.3011.6950.7140.2550.003

0.014

4.981

72.41

52418A-60-69 cm

55

47.6631.9915.902.060.010.170.42

98.21

2.2231.7580.7940.1870.0010.0120.017

4.991

78.61

78

49.8332.2815.322.520.03

0.47

100.45

2.2651.7290.7460.2220.002

0.018

4.982

75.51

Sample 52418A-80-2, 55-58 cm

56

49.6831.5215.432.670.01

99.31

2.2821.7060.7590.2380.001

4.985

76.05

57

46.1233.2117.50

1.810.02

98.66

2.1521.8260.8750.1640.001

5.018

84.13

58

53.5928.7411.894.730.06

1.02

100.03

2.4221.5380.5780.4160.003

0.039

5.008

55.73

Sample53417A-86-3

79

48.4333.2516.17

1.960.04

0.32

100.04

2.2121.7900.7910.1740.002

0.012

4.981

80.80

80

48.5332.7616.702.240.06

0.50

100.79

2.2121.7600.8160.1980.003

0.019

5.008

78.76

81

51.8029.1213.49

3.990.06

0.5 7

99.03

2.3831.5790.6650.3560.004

0.022

5.008

63.51

59

53.3029.1112.864.320.10

0.89

100.58

2.4111.5520.6230.3790.006

0.034

5.005

59.78

82

51.1229.6614.303.400.03

0.37

98.88

2.3551.7110.7060.3040.002

0.014

4.992

68.81

60

50.2530.9114.86

2.890.06

0.62

99.59

2.3051.6710.7300.2570.004

0.024

4.990

72.00

83

50.8930.4314.36

3.220.06

0.39

99.35

2.3341.6450.7060.2860.004

0.015

4.989

69.83

61

47.1734.1117.90

1.530.03

0.39

101.13

2.1481.8310.8730.1350.002

0.015

5.005

85.93

84

49.2431.7816.34

2.270.04

0.42

100.09

2.2541.7150.8010.2010.002

0.016

4.990

78.53

62

48.4232.6316.332.350.07

0.52

100.32

2.2161.7600.8010.2090.004

0.020

5.010

77.47

85

48.9631.9316.182.440.07

0.48

100.06

2.2441.7250.7950.2170.004

0.018

5.004

76.89

Sample 53-417A-86-3

63

48.9832.5816.522.330.07

0.5 7

101.05

2.2261.7450.8040.2050.004

0.022

5.006

77.68

86

49.7131.8115.042.710.040.290.54

100.14

2.2691.7110.7360.2400.0030.0200.021

4.998

72.25

64

53.8328.5311.835.060.10

1.04

100.39

2.4381.5230.5740.4440.006

0.039

5.025

53.95

87

49.9431.4914.64

2.430.080.430.59

99.60

2.2861.6990.7180.2160.0050.0300.023

4.975

72.52

65

51.9730.7214.133.490.05

0.70

101.06

2.3441.6330.6830.3050.003

0.026

4.994

67.15

88

49.9031.5014.762.830.020.250.44

99.70

2.2841.6990.7240.2510.0010.0170.017

4.993

71.70

66

55.1327.6811.415.310.13

0.83

100.49

2.4871.4720.5510.4640.007

0.031

5.013

52.28

89

49.5731.6915.002.790.030.240.46

99.78

2.2701.7110.7360.2480.0020.0170.018

5.000

72.19

67

50.9930.4415.053.170.06

0.62

100.33

2.2341.6350.7350.2800.003

0.024

5.001

70.54

90

48.7032.2415.39

2.360.050.210.43

99.38

2.2411.7480.7590.2110.0030.0150.016

4.993

75.62

68

47.363.03

17.041.780.04

0.32

99.57

2.1851.7960.8420.1590.002

0.012

4.998

82.79

91

49.4532.4515.28

2.550.030.240.47

100.47

2.2491.7390.7440.2210.0010.0160.016

4.998

74.04

69

47.5832.4817.03

1.870.03

0.41

99.40

2.2001.7700.8440.1680.002

0.016

4.999

82.02

92

50j)631.7715.002.790.030.210.4 3

100.29

2.2781.7040.7310.2460.0010.0140.018

4.990

72.46

TABLE 2 - Continued

SiO2

A 1 2°3CaO

N a 2 °K 2 O

MgO

FeO

Total

Nb cations

per 8 oxygens

Si

Al

Ca

Na

K

Mg

Fe

Total

% An

93

50.41

32.10

14.90

2.85

0.04

0.25

0.43

100.98

2.278

1.700

0.722

0.250

0.003

0.017

0.016

4.995

71.68

94

48.50

32.97

16.15

2.08

0.03

0.20

0.41

100.34

2.214

1.774

0.790

0.184

0.002

0.014

0.016

4.992

78.65

95

48.28

32.68

15.96

2.41

0.03

0.22

0.46

100.04

2.214

1.766

0.784

0.214

0.002

0.015

0.018

5.012

75.93

96

48.00

32.68

16.10

2.03

0.03

0.18

0.43

99.45

2.211

1.774

0.795

0.181

0.002

0.013

0.016

4.992

78.91

Sa

97

49.08

32.07

15.27

2.41

0.03

0.27

0.48

99.61

2.251

1.734

0.750

0.214

0.002

0.018

0.018

4.991

74.75

nple53-418A-86-3

98

48.77

32.37

16.21

2.10

0.05

0.19

0.4 1

100.10

2.232

1.746

0.794

0.187

0.003

0.013

0.016

4.990

78.38

99

49.71

31.65

15.47

2.46

0.04

0.21

0.43

99.97

2.272

1.705

0.757

0.218

0.003

0.017

0.015

4.986

74.95

100

51.72

30.89

14.32

2.92

0.02

0.34

0.56

100.70

2.337

1.641

0.693

0.256

0.001

0.023

0.021

4.972

69.72

101

48.98

31.50

15.29

2.40

0.04

0.21

0.41

98.83

2.265

1.716

0.757

0.215

0.003

0.015

0.016

4.987

75.25

102

47.90

32.24

16.18

2.03

0.03

0.19

0.51

99.28

2.213

1.766

0.801

0.182

0.002

0.013

0.020

4.996

78.68

103

49.87

31.24

15.13

2.64

0.02

98.90

2.296

1.695

0.746

0.236

0.001

4.975

75.89

Sampl

104

52.36

30.11

13.05

3.82

0.11

0.23

0.59

100.27

2.373

1.608

0.634

0.336

0.007

0.016

0.023

4.994

62.40

45-395A-15-1

105

47.21

34.02

16.99

1.65

0.04

0.28

0.29

100.48

2.158

1.808

0.832

0.146

0.003

0.019

0.011

4.977

82.30

owSo>t - 1

Noz>δ

5

t-<>

C. BOLLINGER, M. SEMET

[-• • —

T

s

9

Ss

s/ ®

/

1

~ S 4y v

S

9 /

/

i— — i

T f (

®T ® /®* » /

® Phenocryst from mass

v Phenocryst from pillo

yr

ve basalts

w basalts

/ I

Figure 1. Number of Ca versus number of Al atoms for 8 oxygens in the formula unit.The theoretical Ab-An solid solution is shown as a heavy line. The average line ofBryan s (1974) analyses is shown as a dashed line.

Bottinga, Y., Kudo, A.M., and Weill, D., 1966. Some observa-tions on oscillatory zoning and crystallization of magmaticplagioclase, Am. Mineralogist, v. 51, p. 792-806.

Bryan, W.B., 1974. Fe-Mg relationships in sector zoned sub-marine basalt plagioclase, Earth Planet. Sci. Lett., v. 24, p.157-165.

Clocchiatti, R., Havette, A., Weiss, J., and Wilhelm, S., 1978.Les Bytownites du Rift d'Asal. Premiere partie. Etude desverres basaltiques inclus dans les mégacristaux: nouvelle ap-proche pour la connaissance de certains processus pét-rogénétiques, Bull. Soc. Franc. Mineral. Cryst., v. 101, p.66-76.

Kirkpatrick, R. J., 1976. Towards a kinetic model for the crystalli-zation of magma bodies, J. Geophys. Res., v. 81, p. 2565-2571.

, 1977. Nucleation and growth of plagioclase, Maka-opuhi and Alae lava lakes, Kilauea Volcano Hawaii, Geol.Soc. Am. Bull., v. 87, p. 78-84.

Shimizu, N., 1978. Analysis of zoned plagioclase from differentmagmatic environment a preliminary ion microprobe study,Earth Planet. Sci. Lett., v. 39, p. 398-406.

Sibley, D.F., Vogel, J.A., Walker, B.M., and Byerly, G., 1976.The origin of oscillatory zoning in plagioclase: a diffusiongrowth controlled model, Am. J. Sci., v. 276, p. 275-284.

Smith, J.V., 1974. Feldspar minerals: Berlin (Springer Verlag),v. 2(1974).

Wright, T.L. and Kinoshita, W.T., 1968. March 1965 eruption ofKilauea Volcano and the formation of Makaopuhi Lava Lake,J. Geophys. Res., v. 73, p. 3181-3205.

1060

CHEMICAL ZONATION OF PLAGIOCLASE PHENOCRYSTS

80

< 75

70scale 150 µm

distance

Figure 2. Sketch of the crystal represented by Analyses 86to 97, Table 2, and variation of its chemical compositionalong the traverse shown by crosses.

1061