GEOCHEMISTRY OF VOLCANIC ROCKS FROM MT. PELEE, MARTINIQUE

Journal of Volcanology and Geothermal Research, 26 (1985)147--165 147 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

GEOCHEMISTRY OF VOLCANIC ROCKS FROM MT. PELEE, MARTINIQUE

C. DUPUY', J. DOSTAL 2 and H. TRAINEAU 3

'Centre Gdologique et G~ophysique, Universit~ des Sciences et Techniques du Languedoc, Pl. E. Bataillon, 34060 Montpellier Cedex, France 2Department of Geology, Saint Mary's University, Halifax, Nova Scotia, B3H 3C3, Canada 3D~partement G~othermie, BRGM, B.P. 6009, 45060 OrMans Cedex, France

(Received April 20, 1984; revised and accepted December 28, 1984)

ABSTRACT

Dupuy, C., Dostal, J. and Traineau, H., 1985. Geochemistry of volcanic rocks from Mt. Pel~e, Martinique. J. Volcanol. Geotherm. Res., 26: 147--165.

The volcanic rocks of Mt. Pel~e (Martinique) are predominantly composed of andesites; basic andesites and dacites are subordinate. The Mt. Pel~e rocks have geochemical characteristics typical of island-arc calc-alkaline rocks. The chemical composition of the erupted material has not changed significantly during the past 100,000 y. This compositional uniformity with time is consistent with the model of McBirney (1980), which suggests that a magma chamber includes two zones -- an upper graded stable part and a lower conveeting part with approximately constant composition. Chemical variations within individual eruptions are ascribed to fractional crystallization, which probably was accompanied by a process leading to some homogenization of incompatible elements.

INTRODUCTION

Numerous recent geochemical studies on volcanic rocks have focused on elucidating the processes operative in the origin and evolution of the magma. Many of the investigations are, however, based upon suites of grab samples wi thout adequate knowledge of their time-stratigraphic con tex t (cf. Hildreth, 1981). The lack of this information limits the understanding of the temporal evolution of the magma systems, the original zonation of the magma bodies, and the actual mechanisms which take place in the magma chambers during "high-level" differentiation. The purpose of this paper is to present geochemical data on a suite of calc-alkaline lavas from a single volcano, where sampling was under time-stratigraphic control .

Mt. Pel6e volcano is situated at the nor thern end of the island of Mar- t inique in the Lesser Antilles, which run along the eastern margin of the Caribbean Sea and are part of an island arc formed by the underthrust ing of the small Caribbean plate by the Atlantic plate. Mt. Pel6e is a large composite

0377-0273/85/$03.30 © 1985 Elsevier Science Publishers B.V.

148

volcano that covers an area of about 120 km 2 and rises nearly 1,400 m above sea level. The geology and volcanic stratigraphy of Mt. Pel~e have been ex- tensively studied during this century (Lacroix, 1904; Perret, 1935; Grune- vald, 1965; Westercamp, 1974, 1980; Gunn et al., 1974; Roobol and Smith, 1975, 1976, 1980; Smith and Roobol , 1976; Fischer et al., 1980; Traineau, 1982) and thus only a brief summary is given below.

The volcano overlies older volcanic complexes of northern Martinique. The northern part of the volcano was built on top of the ancient volcano of Mt. Conil, which was formed by lava flows and domes of andesitic to dacitic composit ion (Westercamp, 1980). In its southern and eastern parts, the basement of Mt. Pel~e is composed of breccias and andesitic lava flows of the Morne Jacob volcanic complex, which has been extinct for more than 2 m . y .

The growth and volcanic activity of Mt. Pel~e which began about 0.4 m.y. ago (Bellon et al., 1974) can be subdivided into three stages (Westercamp, 1974; Westercamp and Traineau, 1983). Predominantly welded coarse pyroclastic flows and thick andesitic lava flows of the first stage crop out mainly on the western flank of Mt. Pel~e. Although their age is not well established, the main volcanic activity of this stage probably ended 0.2--0.3 m.y. ago. The second stage includes rocks ranging in age between 0.1 m.y. and 13,500 y.B.P. (years before present). It is characterized by explosive volcanic activity which produced air-fall deposits and pyroclastic and debris-flow deposits. At least two important voluminous St. Vincent-type eruptions occurred during this stage: one was dated at 25,700 -+ 1,200 y.B.P. (SV-1), and the other (SV-2) took place 22,300 -+ 1,300 y.B.P. The third stage ("present cone") of volcanic activity of Mt. Pel~e started 13,500 y.B.P, following a volcanic hiatus, which lasted about 6,000 y. The eruptive products of the third stage include various pyroclastic deposits, including alternating nudes ardentes deposits (Pel~ean-type activity), and pumiceous deposits (Plinian and pumice flow activity) (Roobol and Smith, 1976).

Pumices of several eruptions are texturally heterogeneous containing black and white bands; the black bands are slightly more basic. The textural and compositional variations of pumiceous material are more pronounced in voluminous eruptions such as SV-1 and SV-2.

Thirty-four separate magmatic eruptions have been identified (Westercamp and Traineau, 1983), only ten of which occurred before 5,100 y.B.P. The relatively small number of eruptions recorded between 13,500 and 5,100 y. B.P. is probably due to lack of outcrops. On the other hand, the eruptive history of Mt. Pel~e during the last 5,100 years is well-documented, and probably all important volcanic events have been recorded (Traineau, 1982; Westercamp and Traineau, 1983).

PETROGRAPHY

The petrography of the volcanic material of Mt. Pel~e has been recently described in detail by Traineau (1982) and Traineau et al. (1982). The

149

volcanic rocks are general ly porphyr i t i c wi th a p r edominance o f plagioclase phenocrys t s . Plagioclase usual ly displays osci l la tory zoning wi th highly calcic cores (Ang0.gs) in basalts and andesites. Other phenocrys t s are olivine and augite in basalts, augite and hype r s thene in the rocks o f in te rmedia te compos i t ion , and hype r s thene in dacites. An Fe-Ti oxide occurs in all rocks. Amph ibo l e phenoc rys t s are rare and are restr icted to basic rocks.

In addi t ion to glass, the mesostasis conta ins plagioclase, c l i n o p y r o x e n e , o r t h o p y r o x e n e and Fe-Ti oxides. All the microli tes usual ly have the same compos i t i on as tha t of p h e n o c r y s t rims. Some phenocrys t s show disequi- l ibrium features such as zona t ion and resorpt ion. Reverse zona t ion in plagioclase and the occur rence of olivine in acid andesites have been observed in

TABLE 1

List of analyzed samples and their age

Eruption Age (y.B.P.)* Sample number(s)

1. Presen t cone 1929 55 5768 1902 82 6166--6184 NRP3 320 5771 NRP2 490 5772--5774 P1 650 5775--5776 P2 1670 5777 P3 2010 5778--5779, 6145--6147 NAB2 2490 5780 NAB1 2740 5781, 6152--6155 NRS2 3700 5782--5783 P5 4060 5784 NPM 4410 5785 P6 4610 5786--5788, 6148--6150 NMR 5100 6159--6161

5650 5789 6630 5790

P8 7800 5791--5792 10280 5793

N1 11340 5794--5795 N2 13500 5796--5798

2. I n t e r m e d i a t e cone 18240 5800 19500 5801--5802

SV1 22300 5803--5804 SV2 25700 5805--5814, 5799

3. A n c i e n t cone ;* 40000 5809--5814

*After Traineau (1982) and Westercamp and Traineau (1983); y.B.P. = years before present; Eruption: P = pumiceous eruption followed by the number of eruptions of this type; N -- nu6es ardentes eruption followed by the abbreviation of the type-area (RP = Rivi~re des P~res; AB = Ajoupa-Bouillon; RS = Rivi~re Seche; PM = Pointe la Mare; MR = Morne Rouge); SV = St. Vincent-type eruptions.

150

some rocks. In some volcanoes, these features have been attributed to mixing effects (e.g. Sakuyama, 1979).

SAMPLING AND ANALYTICAL NOTES

The eighty-three samples selected for analysis include (Table 1): 6 samples from eruptions older than 40,000 y.B.P. 16 samples from eruptions with ages ranging between 26,000 and 18,000

y.B.P. 13 samples from eruptions with ages between 13,500 and 5,100 y.B.P. 48 samples from eruptions with ages between 5,100 and 50 y.B.P. All eruptions of the recent period (~< 5,100 y.B.P.), except phreatic

eruptions, are represented in the analyzed suite by at least one sample. On the other hand, probably not all eruptions of the older periods have been identified in the field. However, analyzed samples were collected from all volumetrically important older eruptions. Minerals and mesostasis separated from a cognate nodule of volcanic eruption SV-1 were also analyzed. The details on the mineral separation and purity were given by Dostal et al. (1983).

The major elements and Li, Rb, Sr, Ba, V, Cr, Co, Ni, Cu and Zn were analyzed in all samples by atomic absorption. From this set, twenty-eight samples were selected for determinations of rare earth elements (REE), Th, Hf, and Sc by instrumental neutron activation. The precision and ac- curacy of the analyses can be judged from replicate analyses of two standard rocks, BR and W-l, performed during 1982--1983 (Table 2). Chemical analyses of representative samples are given in Table 3; analyses not given may be obtained on request from the authors. The details on the method of collecting the samples and the degree of lithological variation in deposits have been given by Traineau (1982).

GEOCHEMISTRY

Whole-rock composition

The analyzed rocks show a wide range of SiO: (52--65%) although andesites with SiO2 content between 58 and 62% are by farmost abundant (> 80% of the analyzed rocks). Dacites and basic andesites are subordinate. The volcanic rocks have relatively high contents of CaO and A1203, but they are low in TiO2 and K20 and have low MgO/FeOtot ratios. When plotted against an index of differentiation such as SiO2, major and trace elements display linear trends (Fig. 1). The rocks are low in Ni and Cr and have a Co/Ni ratio > 1, indicative of extensive fractionation of ferromagnesian minerals. Strontium abundances show a distinct scatter and do not correlate with SiO2 (Table 3). On the other hand, Rb and Ba have a positive correlation with SiO2 and like K, their content is comparable to those of equivalent

TABLE 2

Analyses of international reference standards BR and W-1

BR

BR

W-1

W-1

n

12

A

12

A

5 F

6 F

SiO

2 (%

) 38

.6

(0.3

) 3

8.3

9

Li

(pp

m)

14.2

(0

.7)

13

13 (

1)

14.5

L

a (p

pm

) 10

.3

(1.0

) 9.

8 A

l~O

3 9.

8 (0

.1)

10.2

5 R

b

47

.2

(0.6

) 47

20

(1)

21

C

e 22

.4

(0.2

) 23

.0

Fe2

03to

t 12

.70

(0.0

6)

12.9

4 S

r 1

36

3

(44)

1

30

0

190

(5)

190

Sm

3.

2 (0

.0)

3.6

MnO

0.

20

(0.0

0)

0.2

0

Ba

1090

(3

4)

10

50

1

70

(8)

160

Eu

0.9

9

(0.0

4)

1.11

M

gO

13.2

(0

.1)

13

.35

S

c 36

(1)

35

.1

Tb

0

.62

(0

.08

) 0

.65

C

aO

13.7

(0

.1)

13.8

7 V

23

8 (3

) 24

0 25

6 (5

) 26

4 Y

b 2.

25

(0.0

6)

2.1

Na2

0 3.

04

(0.0

3)

3.07

C

r 34

9 (7

) 38

0 1

15

(2)

114

Lu

0.31

(0

.02

) 0.

35

K20

1.

39

(0.0

2)

1.41

C

o 55

(1

) 50

44

(2)

47

T

h

2.3

(0.2

) 2.

42

TiO

2

2.62

(0

.04

) 2.

61

Ni

268

(3)

260

77 (

1)

76

Hf

2.3

(0.1

) 2.

67

P205

1.

06

(0.0

8)

1.05

C

u 79

(4

) 72

10

7 (4

) 11

0 Z

n

164

(3)

150

84 (

2)

86

n =

nu

mb

er o

t d

eter

min

atio

ns;

val

ues

in b

rack

ets

corr

esp

on

d t

o 1

sta

nd

ard

dev

iati

on

; A

= "

pre

ferr

ed"

valu

es o

f A

bb

ey

(19

83

); F

= "

pre

ferr

ed"

valu

es o

f F

lan

agan

(19

73).

t.a

152

(a)

1.4 K 2 0 %

1 .2

* m ~ * ' " ~1" : " o" 1

.s = . . ' J d ~ . . ; " 11" " ' , " t , * • •

• = = • .6

. 4 •

I I I Si02 %

2 6 0 8 a p p m *

2 4 0 •

2 2 0

200 * . ~ ~:..

1 8 0 * • , ~ &

160 * J~ "

1 4 0

S i 0 2 %

5 T h p p m

4

3 • ql~A | • r • •

2 • " S i O = %

I I

4 H f p p m

3

• S i 0 2 % I

3 2 Ce p p m

30 •

2 8 °

2 6 " :

• ¢ 2 4 *

2 2 m " t 2O

18

l e • S i O z % L I

5 5 6 0 6 6

Fig. 1. Var ia t ions of K, Ba, Th, Hf, Ce, Cr, V and Co in the rocks of Mt. Pel6e subd iv ided accord ing to the i r age (Tab le 1). Symbols : circles - - r o c k s wi th ages b e t w e e n 0 and 5 ,100 y.B.P. ; t r iangles - - rocks wi th ages be t w een 5 ,650 and 13 ,500 y.B.P. ; squares - - rocks wi th ages b e t w e e n 18 ,000 and 26 ,000 y.B.P. ; s tars - - r o c k s o lder t h a n 40 ,000 y.

153

( b ) 4 0 C r p p m

3 0

2O

** m, •

" . . . .

I 1 i i i i I r t i i i S i 0 2 %

V ppm

e.

2 0 0

t , o .

1 0 0 , %,,*

I I I i I j e • •

• e • S J 0 2 9 ~ [ l I [ I l J J ] / " 1 " [

3 0 Copprn

m.

2 0 " . . mm •

10 ~ g •

I i m I l I [ J I I I I S i 0 2 % 6 0 6,6 6 0 6 5

rocks from some other islands of the Lesser Antilles such as St. Kitts and Dominica (Brown et al., 1977). K/Rb ratios in volcanic rocks of Mt. Pel~e range between 300 and 350, characteristic of the volcanic rocks of the Lesser Antilles arc (Gill, 1981). Chondrite-normalized REE patterns of the studied rocks (Fig. 2) show a slight light REE (LREE) enrichment with La/Yb ratios around 5. These geochemical characteristics are typical of island-arc calc-alkaline series (Jake~ and White, 1972) and have been described for volcanic rocks of the other islands of the Lesser Antilles. However, as Gunn et al. (1974) have already noted, the volcanic rocks of the Lesser Antilles arc display some intra- and inter-island variation in composit ion. Compared to analyses f rom other volcanic centres on Martinique (e.g. Gunn et al., 1974) the Pel~ean rocks are similar to those from Pointe du Diamant but

TA

BL

E

3

w,

cja

Ch

em

ica

l c

om

po

siti

on

s o

f v

olc

anic

ro

ck

s fr

om

Mr.

Pel

~e

57

68

6

18

3

57

72

5

77

4

57

76

5

77

9

57

81

5

78

7

57

95

5

79

6

57

97

5

80

0

58

01

5

80

3

58

04

5

80

6

57

99

SiO

2 (

%)

61

.90

6

1.9

0

58

.25

6

0.3

6

60

.95

5

7.5

9

62

.79

5

6.5

0

57

.09

5

9.5

9

59

.79

5

9.4

3

65

.02

5

4.4

5

63

.22

5

7.0

6

52

.45

A

120

J 1

7.1

9

17

.29

1

7.6

2

17

.47

1

7.7

0

17

.85

1

7.1

8

18

.40

1

8.3

7

17

.42

1

7.3

0

18

.30

1

6.3

4

18

.35

1

6.9

0

18

.14

1

8.0

9

Fe2

03

T

6.9

7

6.8

0

8.1

0

7.6

7

7.3

0

7.5

0

6.8

2

8.4

7

7.9

6

7.4

0

7.0

7

7.0

3

4.9

7

9.0

3

5.0

5

8.0

0

9.2

7

Mn

O

0.1

8

0.1

8

0.2

0

0.1

9

0.1

9

0.1

7

0.1

8

0.1

9

0.1

9

0.1

9

0.1

9

0.1

7

0.1

4

0.1

9

0.1

4

0.1

8

0.1

8

Mg

O

2.3

1

2.2

4

3.1

1

2.4

4

2.3

4

3.0

9

1.9

7

3.2

2

2.8

4

2.9

5

2.7

2

2.4

2

1.5

3

4.0

1

1.4

9

3.4

9

4.9

2

CaO

6

.19

6

.17

7

.28

6

.52

6

.38

7

.75

5

.97

7

.77

6

.94

6

.69

6

.34

6

.07

4

.84

9

.00

4

.88

7

.36

9

.18

N

a20

3

.53

3

.49

3

.26

3

.50

3

.50

3

.00

3

.63

3

.00

3

.05

3

.27

3

.40

3

.32

3

.61

2

.97

3

.54

3

.03

2

.73

K

20

1

.00

1

.00

0

.87

0

.90

0

.90

0

.84

0

.90

0

,76

0

.81

0

.95

1

.01

0

.68

1

.46

0

.76

1

.34

0

.79

0

.68

T

iO 2

0

.45

0

.45

0

.58

0

.48

0

.48

0

.64

0

.47

0

.65

0

.55

0

.50

0

.49

0

.54

0

.39

0

.77

0

.41

0

.68

0

.83

P

20

5

0.1

8

0.1

8

0.1

8

0.1

9

0.2

0

0.1

8

0.2

3

0.2

0

0.2

0

0.2

2

0.2

3

0.1

7

0.1

6

0.1

5

0.1

5

0.1

8

0.1

3

H2

0+

0

.02

0

.09

0

.06

0

.05

0

.02

0

.15

0

.03

0

.08

0

.27

0

.13

0

.14

0

.32

0

.20

0

.08

0

.35

0

.02

0

.34

H

20

--

0.0

2

0.1

6

0.2

6

0.1

3

0.1

4

1.3

7

0.2

9

0.5

1

1.8

0

0.3

8

0.6

0

0.8

4

1.1

8

0.0

9

1.8

2

0.4

5

1.0

0

99

.98

9

9.9

7

99

.78

9

9.9

3

10

0.1

1

10

0.1

6

10

0.4

7

99

,76

1

00

.10

9

9.7

4

99

.40

9

9.3

3

99

.88

9

9.8

8

99

.34

9

9.4

1

99

,82

Li

(pp

m)

24

. 2

1.

19

. 2

2.

22

. 1

5.

22

. 1

3.

16

. 2

0.

23

. 1

6.

25

. 1

2.

24

. 1

3.

12

. R

b

28

. 2

8.

22

. 2

3.

26

. 2

5.

23

. 1

6.

21

. 2

6.

28

. 1

4.

45

. 1

8.

42

. 2

0.

15

. S

r 2

83

. 2

84

. 2

82

. 2

89

. 2

87

. 2

70

. 2

97

. 3

03

. 2

68

. 2

80

. 2

88

. 2

85

. 2

55

. 3

04

. 2

55

. 3

34

. 2

84

. B

a 2

00

. 2

08

. 1

66

. 1

78

. 1

90

. 1

61

. 1

88

. 1

38

. 1

50

. 1

92

. 1

95

. 2

10

. 2

52

. 1

66

. 2

54

. 1

72

. 1

25

. V

7

2.

72

. 1

18

. 8

6.

79

. 1

40

. 4

7.

12

2.

10

6,

88

. 8

3.

11

8.

66

. 2

48

. 6

9.

18

7.

27

0.

Cr

7.

8.

8.

6.

6.

8.

6.

7,

6.

37

. 2

3.

6.

7.

24

. 7.

1

5.

38

. C

o

12

. 1

2.

17

. 1

5.

13

. 1

8.

12

. 1

9.

17

. 1

6.

15

. 1

5.

9.

25

. 1

0.

21

. 2

8.

Ni

6.

6.

7.

7.

6.

8.

7.

8.

7.

20

. 1

4.

5.

5.

12

. 5.

1

1.

17

. C

u

31

. 1

7.

30

. 2

4.

24

. 3

7.

12

. 2

9.

30

. 3

6.

48

. 1

4.

9.

81

. 1

1.

65

. 6

8.

Zn

8

6.

89

. 9

1.

91

. 9

0.

87

. 9

3.

92

. 9

2.

88

. 8

8.

89

. 7

0.

87

. 7

2.

81

, 7

9.

La

11

.89

1

1.3

9

10

.39

1

1.1

0

11

.10

9

.89

1

2.1

9

10

.39

1

2.1

9

9.3

9

13

.50

7

.50

1

2.8

0

9.8

9

7.3

0

Ce

26

.10

2

5.7

0

22

.89

2

4.8

9

24

.29

2

1.7

9

28

.10

2

1.7

0

26

.89

2

0.7

9

29

.70

1

7.2

0

31

.10

2

1.7

9

16

.20

S

m

3.2

3

3.1

5

3.0

7

3.1

8

3.0

7

2.7

3

3.4

4

3.0

5

3.2

6

2.6

5

2.7

9

2.6

7

2.7

0

2.7

2

2.7

5

Eu

1

.02

1

.05

1

.01

1

.05

1

.05

0

.97

1

.13

0

.98

1

.02

0

.90

0

.75

0

.86

0

.76

0

.86

0

.95

T

b

0.5

8

0.6

2

0.6

1

0.6

4

0.5

8

0.5

4

0.6

6

0.5

4

0.5

8

0.5

0

0.5

1

0.6

0

0.5

1

0.5

2

0.6

1

Yb

2

.36

2

.36

2

.32

2

.26

2

.22

2

.18

2

.38

2

.15

2

.26

1

.98

1

.98

2

.15

1

.96

2

.06

2

.17

L

u

0.4

1

0.4

1

0.4

0

0.3

9

0.3

8

0.3

7

0.4

1

0.3

3

0.3

7

0.3

2

0.3

3

0.3

5

0.3

3

0.3

4

0.3

6

Hf

2.7

1

2.7

5

2.4

8

2.6

5

2.5

2

2.3

8

2.8

8

2.2

0

2.5

9

2.7

0

3.2

0

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0

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9

2.0

9

1.7

9

Th

2

.90

2

.90

2

.50

2

.50

2

.50

2

.70

2

.70

2

,29

3

.09

3

.09

5

.40

2

.29

5

.40

3

.09

2

.09

155

2 O

10 I [ ~ I I _ _ I I

7 o

5 lo I M I I I k ,

~ 4o

3 0 ~

1 0 [ I I I I

La Ce Sm Eu Tb Yb Lu

Fig. 2. Chondrite-normalized rare-earth element abundances in representative rocks of Mt. Pe]~e. Samples: 5781-NAB1, 62.79% SiO2; 5774-NRP2, 60.36% SiO2; 5772-NRP2, 58.25% SiO2; 5779-P3, 57.59% SiO~; 5787-P6, 56.50% SiO2; 5795-N1, 57.09% SiO~; 5778-N2, 58.00% SiO~; 5801-18, 240-y.-old eruption, 65.02% SiO2; 5800-19, 500-y.-old eruption, 59.43% SiO2; 5803-SV-1, 54.45% SiO 2.

differ from Morne Jacob in having lower K, Rb and Th contents. The andesites of Mt. Pel6e are also lower in Ca, Ba, Sr, Ti and Ni than the average composition of the volcanic rocks of the Lesser Antilles (with comparable SiO2 contents) given by Brown et al. (1977). The isotopic studies (e.g. Davidson, 1983} indicate that the erupted magmas of Martinique were contaminated by subducted terrigenous sediments.

Temporal chemical variations

Compositional variations in the rocks of a volcano can occur on several different time scales. Some variations may be present within a single erup-

156

t ion. These ma in ly r ep resen t c o m p o s i t i o n a l he te rogene i t i es in a m a g m a s y s t e m be fo re an emiss ion and p r e s u m a b l y ref lect var ious processes such as f rac t iona l crys ta l l iza t ion , mix ing or c o n t a m i n a t i o n . O the r var ia t ions m a y occur over a long t ime interval , spanning the whole or pa r t o f the life o f a volcano. T h e y p r o b a b l y ref lect changes in the physica l cond i t ions in the m a g m a c h a m b e r or m a g m a genera t ion a n d / o r changes in the c o m p o s i t i o n of the source rocks .

Bo th t ypes o f t e m p o r a l c o m p o s i t i o n a l var ia t ions in the rocks of Mt. Pel6e were e x a m i n e d tak ing into a c c o u n t the fact t ha t the recen t per iod of vo lcanism (0 - -5 ,100 y.B.P.) is far b e t t e r k n o w n than the older per iods and tha t its volcanic s t ra t ig raphy , which relies ma in ly on r a d i o c a r b o n dat ing and cor re la t ion o f widespread and dis t inct units , is re l iably es tabl ished (Traineau, 1982; Gourgaud , 1982) .

The var ia t ions wi thin e rup t ions have been s tudied for several d i f fe ren t t ime per iods (1902 e rup t ion , P6 p u m i c e e rup t ion da ted at 4 ,610 -+ 50 y.B.P. and SV-1 and SV-2 e rup t ions da ted at 25 ,700 and 22 ,300 y.B.P, respec- t ively) . In each case, on ly represen ta t ive samples o f the e rup t ions have been ana lyzed; b locks wi th basic and in t e rmed ia t e c o m p o s i t i o n s which rep resen t < 0.5% of the vo lume o f e rup ted mater ia l were no t cons idered .

The SiO2 c o n t e n t of volcanic mater ia l f r o m the 1902 e rup t ion varies be tween 58.9 and 62.2%, with an average o f 60.7%. The p u m i c e e rup t ion da ted at 4 ,610 y.B.P. (P6 in Table 1) has a SiO2 c o m p o s i t i o n wi th in the 56 .2 - -58 .8% range. The th i rd e x a m p l e includes two separa te e rup t ions , SV-1 and SV-2, which are b o t h r a the r vo luminous . The rocks o f these two e rup t ions have a wide span of SiO2 con ten t s (52 .4- -63 .2%) . In all three suites, the var ia t ions of SiO2 are a c c o m p a n i e d by small bu t d is t inct var ia t ions of ma jo r and t race e lements mos t o f which display s ignif icant corre la t ions . Table 4 shows such cor re la t ions for the 1902 e rup t ion ; the o the r s tudied e rup t ions display similar in te r -e lement corre la t ions , which suggest tha t the p r o d u c t s o f a single e rup t ion are in te r re la ted by a s imple process such as f rac t iona l crys ta l l iza t ion.

TABLE 4

Correlation coefficients of elements in 14 samples from 1902 eruption

SiO~ Fe~O3 w MgO CaO TiO~ V Cr Co Ba P

SiO 2 1 Fe203 w --0.88 1 MgO --0.93 +0.79 1 CaO --0.95 +0.85 +0.98 1 TiO2 --0.78 +0.51 +0.85 +0.83 1 V --0.90 +0.75 +0.98 +0.97 +0.88 1 Cr 0.55 +0.47 +0.74 +0.72 +0.61 +0.78 1 Co --0.91 +0.87 +0.93 +0.95 +0.69 +0.92 +0.67 1 Ba +0.90 --0.87 0.88 --0.87 --0.64 0.83 --0.42 --0.89 1 P +0.66 --0.67 --0.68 --0.72 --0.47 --0.74 --0.61 --0.83 +0.58 1

157

The variations of SiO2 with the age of the eruptions are shown in Fig. 3. Together with volcanologic data (Traineau, 1982; Westercamp and Trai- neau, 1983) the plot indicates:

(1) Rocks with SiO2 contents t> 60% are present in deposits from all eruptions. However, in the recent period (0--5,100 y.B.P.) it seems that the relatively acid products (> 61% SiO2) were erupted mainly after a long repose period (e.g., eruptions dated at 2,740 y.B.P, and 5,100 y.B.P.). Similar observations were reported from Iceland (Thorarinsson, 1954).

(2) Volcanic material with SiO2 < 56% is restricted to the earlier stages of the growth of Mt. Pel~e.

(3) With the exception of eruptions P3 and P6, the analyzed samples indicate a tendency for the more basic rocks not to be represented in the younger eruptions.

(4) Eruptions such as P3, P6, SV-1 and SV-2 which display a relatively large range of composit ions are cataclysmic and their volcanic products are voluminous (up to 1 km 3, SV-1, Traineau, 1982).

64 S i 0 2 %

62 • • • *

oo 6 o . o * * * l

58 * ~ • * | • •

56 P3 | SV2 ~. P6 • .

54 SV1 *

52 i • 103 Year B. P

I I I I I , ' ~ - - [ I I I I - - ~ / - - " r I I ] I " ~ } - - - - I

0 5 6 14 18 26 ) 40

Fig . 3. V a r i a t i o n s o f S iO 2 as a f u n c t i o n o f t h e age o f e r u p t i o n . Stars r e p r e s e n t e r u p t i o n s o l d e r t h a n 4 0 , 0 0 0 years .

Some long-term variations in the rocks of Mt. Pel~e are similar to those observed within a single eruption and are accompanied by significant inter- element correlations comparable to those in Table 4. Compositional simi- larities among the volcanic products of various ages are emphasized by com- paring them with rocks of different ages but equivalent SiO2 content (Table 5). The small differences among andesites of comparable SiO: content may be explained by variable proport ions of phenocryst minerals as shown on Table 6. Some mobile elements such as Ba, Rb and Cu also display some scatter values in rocks which are hydrated and 18,000 y. old (e.g. Ba in Fig. 1).

Fractional crystallization and mixing

Quantitative modelling of the observed compositional variations is hin- dered by the presence of phenocrysts in the rocks. The analyzed rocks

158

TABLE 5

Average compositions of andesites from Mt. Pel~e

n 10 9 10

age (y.B.P.) 320--2,010 2,490--4,610 5,650--13,500 SiO 2 (%) 59.9 (2) 58.8 (4) 59.4 (3) K~O 0.90 (7) 0.82 (12) 0.93 (8) Rb (ppm) 24.4 (8) 20.3 (20) 24.0 (10) Sr 286 (3) 292 (4) 286 (5) Hf 2.5 (4) 2.4 (14) 2.5 (8) La 10.8 (5) 10.9 (8) 11.6 (11) Yb 2.3 (3) 2.2 (5) 2.2 (4) V 89 (29) 99 (31) 91 (4) Li 20.8 (12) 17.6 (18) 17.7 (16)

n = number of samples; y.B.P. = years before present; values in brackets are coefficients of variations.

are porphyritic and hence their composition might not represent that of the liquid. Furthermore it is probable that some crystals are not in equilibrium with the mesostasis as demonstrated in volcanic rocks from other areas (Fujimaki, 1982; Dostal et al., 1983). The ferromagnesian mineral phases which are not in equilibrium with the groundmass were formed during an early stage of crystallization and, as shown in lavas of Soufri~re, St. Vincent, are high in Cr and Ni. Such mafic xenocrysts have been identified in three studied samples {5771, 5796, 5797).

In addition to the occurrence of mafic xenocrysts, other evidence indicating a fractional crystallization process includes the positive correlations of Ca and Mg with Cr typical of the fractionation of pyroxene and correlations of Fe and V with Ti suggesting crystallization of magnetite {Table 4). The lack of variation of Sr with increasing SiO2 implies that plagioclase played an important role during fractionation, because the Sr partition coefficient for plagioclase (concentration in crystal/concentration in liquid) is approximately 2 (Dostal et al., 1983).

Fractional crystallization has been frequently suggested to explain compositional variations within the volcanic island-arc suites of the Lesser Antilles {Lewis, 1973; Brown et al., 1977; Arculus and Wills, 1980). How- ever, a reasonable fit is not always obtained by quantitative modelling of this process (Dostal et al., 1983). The present study has also at tempted to evaluate the fractional crystallization process. Initially, the process was quantified for major elements using the least-squares calculations of Wright and Doherty (1970) to test whether the fractionation of the assemblage of olivine, pyroxenes, plagioclase and magnetite can explain the observed chemical variations {Tables 6 and 7).

The data in Table 7 agree with the petrographic observations and suggest that plagioclase was the dominant fractionating phase, that the crystalliza-

159

TABLE 6

Least-squares calculations of fractional crystallization

1 2 3

SiO 2 57.6 57.1 57.1 Al203 17.8 18.1 18.1 Fe20~ T 7.5 8.0 8.0 MgO 3.1 3.5 3.5 CaO 7.7 7.4 7.4 Na20 3 .00 3 .03 2 .98 K 2 0 0.84 0 .79 0 .78 T iO 2 0 .64 0 .68 0 .68

R 0 .001

1 = andes i t e 5 7 7 9 ; 2 = andes i t e 5806 ; 3 = ca lcu la ted c o m p o s i t i o n of andes i t e 5806 ; ca lcu la ted we igh t f r ac t ions o f c o m p o n e n t s : andes i t e 5806 = 0 .92 andes i t e 5779 + 0 .057 plagioclase + 0 .0 4 8 o r t h o p y r o x e n e + 0 .02 m a g n e t i t e + 0 .002 i lmeni te . R = s u m of squares residuals .

C o m p o s i t i o n o f mine ra l s u sed in ca lcu la t ions

Olivine Plagioclase C l i n o p y r o x e n e O r t h o p y r o x e n e Magne t i t e I lme n i t e (1) (1) (2) (1) (2) (2) (1) (2)

SiO 2 41 .0 51 .0 52.5 49 .0 51 .4 53.0 - - - - - - Al203 - - 31 .0 30 .4 6.3 2.1 0.7 4 .2 3.0 - - Fe2Oa T 19.4 - - - - 6.4 10 .0 23.7 85 .3 84 .8 57.2 MgO 42 .0 - - - - 14.1 15 .2 22.5 3.3 2.7 2.7 CaO - - 14 .0 13 .0 22.5 20.1 1.4 - - - - - - Na20 - - 3.4 3.9 0.2 0 .2 . . . . K20 - - 0.1 0.1 . . . . . . . TiO 2 - - - - - - 0.7 0.4 0 .15 10.7 12 .4 43 .0

(1) = c o m p o s i t i o n o f p h e n o c r y s t s f r o m basal ts (Tra lneau , 1982) (2) = c o m p o s i t i o n o f p h e n o c r y s t s f r o m andas i t e s (Tra ineau , 1982 )

T A B L E 7

Leas t - squares ca lcu la t ions o f f rac t iona l c rys ta l l i za t ion

Paren t : 5799 5806 5805 D a u g h t e r : 5806 5805 5804 R: 0 .31 0 .03 0 .10

Proportions of phases Olivine 0 .03 C l i n o p y r o x e n e 0 .09 0 .06 O r t h o p y r o x e n e 0.07 0.01 Magne t i t e 0 .04 0.01 0 .03 Plagioclase 0 .18 0 .14 0 .17 F 0 .66 0 .78 0.73

R = s u m o f squa res res iduals ; F -- f r ac t ion o f res idual l iquid.

160

V ppm Th ppm .4 ..A

4 .6 ~ - k " ~

.8 ~. ~ ' ~ ' ~

3 0 0 2 ° - ' ~ * " ' ~ * % * " ° • 4 , **

S i 0 2 % ¶ J i i \ \ 3e ppm \

. 9 ) ( ~ .4~, /

\ / 2 0 0 \ /

.8'~\. .. / ~ \ 3 0 " 5 ~ / * "7X'~ t . I I ~ "

.% x~ / . 6 , ( / - . :

100 • " * ~ " .7 / / % ° *

" s "~ . i t • ; ~ . : . • l ~ i i , o .9X/

S i O 2 % S i O 2 % I ] I I I I

55 60 65 55 60 66

Fig. 4. Fractional crystallization model for the rocks of Mt. Pel~e compared to observed data plotted against SiO 2. The calculation has been done for F (degree of remaining liquid) ranging from 1 to 0.4. The parent magma has the composition of sample 5799. Mineral phases crystallized in the proportions given in Table 7, col. 1. These proportions were fixed from the least-squares calculations for major elements. For V, partition coefficients were determined from mineral phases separated from a cognate cumulate nodule (Table 8). The bulk partition coefficients of Th and Ce were taken as 0.01 and 0.1 respectively (Dostal et al., 1983). Variation of SiO~ in the residual liquid was established from the least-squares calculations for major elements. For Th and Ce, in order to have agreement between the observed and calculated values, the partition coefficients would have to be anomalously high (~ 0.6).

TABLE 8

Least-squares calculations of mixing

1 2 3 4

SiO 2 (%) 57.70 57.60 Li (ppm) 17 15 Al~O~ 17.65 17.85 Rb 22 25 Fe20~ T 7.90 7.50 Ba 170 161 MgO 3.44 3.10 La 9.5 9.9 CaO 7.52 7.75 Yb 2.28 2.18 Na20 3.15 3.00 Hf 2.37 2.38 K20 0.86 0.84 V 160 140 TiO2 0.62 0.64 Co 19 18 P205 0.16 0.18 R 0.4

1 = calculated least-squares estimate of andesite 5779 obtained by mixing of 0.446 basalt 5799 with 0.554 dacite 6183; 2 = actual composition of andesite 5799; 3 = calculated trace element abundances in andesite generated by mixing of basalt 5799 with dacite 6183 in proportions obtained from the least-squares calculations for the major elements; 4 = actual trace-element concentrations of andesite 5779; R = sum of squares residuals.

161

t ion of magnetite spanned the whole composit ional range of rocks and that the precipi tat ion of olivine was limited to the basic rocks. Together with similar expected and observed relative abundances of the mineral phases, the results indicate that fractional crystallization is a viable mechanism to explain the major-element variations in the analyzed volcanic rocks of Mt. Pel~e. The process was fur ther tested by model calculations for trace elements (Fig. 4, see Dostal et al. 1983 for details). For these calculations, the propor- t ions of individual mineral phases were taken from the least-squares results. The calculated concentrat ions of transition elements agree with the observed abundances (Fig. 4). On the other hand, several incompatible elements such as Th and Ce show significant discrepancies. Similar differences have been repor ted from some other island-arc volcanic series (Johnson and Arculus, 1978; Stern, 1979; Gust and Johnson, 1981; Dostal et al., 1983). Thus from trace-element data, it appears that the studied rocks of Mt. Pel~e cannot be related by simple fractional crystallization alone. The geochemical calculations of this process using the Rayleigh equation or the open-magma chamber model of O'Hara and Mathews (1981) show that fractional crystallization alone cannot explain the large variations of major elements, particularly of SiO2, accompanied by narrow ranges of incompatible element contents observed in the rocks of Mt. Pel~e and also in many other calc-alkaline volcanic suites (e.g. Dupuy et al., 1979). It seems that fractional crystalliza- t ion was accompanied by a process which led to a certain homogenizat ion of incompatible elements.

Grove et al. (1982) have argued that the failure of fractional crystalliza- t ion models to account for the observed composit ional variations in the

TABLE 9

Compositions of minerals and mesostasis separated from cognate cumulate nodule of volcanic eruption SV-1

Mesostasis Cpx 01 Hb Mag

SiO~ (%) 47.3 41.3 Al20 ~ 20.6 6.53 0.45 14.83 10.70 Fe203 w 10.53 8.70 22.24 11.33 75.54 MnO 0.18 0.13 0.32 0.10 0.28 MgO 4.99 14.96 39.85 14.79 5.42 CaO 11.79 21.68 0.35 12.56 0.93 Na20 2.48 0.11 2.42 0.16 K20 0.50 0.04 0.01 0.39 0.02 TiO2 1.08 0.97 0.03 2.23 5.60

V (ppm) 436 492 20 900 4,000 Cr 26 396 13 353 185 Co 31 41 200 58 215 Ni 10 44 199 71 46 Cu 97 11 17 6 178 Zn 84 28 104 34 520

162

calc-alkaline volcanic series is mainly due to the mixing of magmas of basaltic and rhyolitic composition. In fact, there is some evidence that magma mixing is commonly intimately associated with fractional crystallization during differentiation of calc-alkaline volcanic suites (e.g. Eichelberger, 1975, 1978) and mixing of high alumina basalt and rhyolite magmas can produce hybrid intermediate lavas (Grove et al., 1982; Gerlach and Grove, 1982). The possibility that some rocks of intermediate composition from the Mt. Pel~e suite were formed by such a process was tested by the least- squares calculations. Table 9 shows that the composition of andesite 5779 can be generated by the mixing of magmas of basaltic and dacitic composition. However, a random process of mixing cannot readily explain the distinct predominance of andesitic rocks with relatively uniform compositions and also the origin of the end members of magma mixing, particularly dacites, which are rare in Mt. Pel~e.

DISCUSSION AND CONCLUSIONS

The volcanic rocks of Mt. Pel~e are predominantly composed of andesites (58--62% SiO2) with only subordinate amounts of basic andesites and dacites, all of which have geochemical characteristics typical of island- arc calc-alkaline suites. There is an overall trend of decreasing proportions of basic rock-types in eruptions with decreasing age. The large, voluminous and cataclysmic eruptions of Mt. Pel~e contain a wider range of compositions than the smaller ones.

A petrogenetic model for the evolution of the volcanic rocks of Mt. Pel~e should account for the following geochemical observations:

(1) Chemical variations within a single eruption imply the presence of a zoned magma chamber. In fact, the more basic rocks are present only in voluminous eruptions.

(2) Overall the volcanic material does not show any systematic variations with the age of eruptions. Rocks of a given SiO2 content with different ages have nearly identical contents of most major and trace elements. This large-scale uniformity of composition over a long time interval suggests the repetition of a petrogenetic process without a change in magma composition or physical conditions of differentiation.

(3) Although most rocks are strongly porphyritic (plagioclase particularly is abundant), they do not show any obvious cumulate compositional features such as a positive Eu anomaly.

(4) Fractional crystallization played an important role in the differentiation of the volcanic rocks of Mt. Pel~e, but it cannot explain some chemical variations such as the small differences in the contents of incompatible elements between basalts and dacites.

A model which seems to explain most features satisfactorily is the magma- chamber model proposed by McBirney (1980). According to this model,

163

the magma chamber is composed of two distinct parts: (a) An external reaction zone (an upper graded stable part), where crystal-

lization is caused by heat loss through the roof and walls of the magma chamber.

(b) An internal zone with generally constant composit ion where magma convects and brings fresh magma into the reaction zone and contributes to the extraction of the formed crystals.

The model may account for both the existence of a zoned magma chamber in its upper part and the constancy of composit ion over time. Differen- tiated liquid rises up because of gravity and accumulates under the roof of the magma chamber. It may scavenge some xenocrysts and/or phenocrysts during its ascent. The liquid continues to cool and starts to crystallize under the conditions of the closed system (McBirney and Noyes, 1980). Its composition could be preserved despite the abundance of phenocrysts. Com- parison of the rocks of similar SiO2 contents shows certain catter in the abundances of transition elements, indicating small heterogeneity in the distribution of the mineral phases. This heterogeneity may be due to sampling and/or to small gravitational effects within the individual layers of the magma chamber.

Two hypotheses may be suggested to account for the discrepancy in the abundances of major and incompatible elements: (1) Mixing or contamina- tion of magma during fractional crystallization with liquids enriched in SiO2 and depleted in incompatible elements. Such liquids are compositionally very similar to those formed by liquid immiscibility (Watson, 1976; Phil- potts, 1982). (2) Partial chemical rehomogenization by trace-element dif- fusion between two neighbouring layers within the magma chamber.

ACKNOWLEDGEMENTS

We thank Drs. D. Westercamp, J. Lewis, A.L. Smith and R. Maury for their constructive discussion and critical comments. The s tudy was financial- ly supported by PIRPSEV (France) and NSERC (operating grant A3782).

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GEOCHEMISTRY OF VOLCANIC ROCKS FROM MT. PELEE, MARTINIQUE

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