Rare earth and other trace elements in historic azorean lavas

Journal of Volcanology and Geothermal Research, 1 (1976) 127--147 127 o Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

R A R E E A R T H A N D O T H E R T R A C E E L E M E N T S I N H I S T O R I C A Z O R E A N

L A V A S

M.F.J. FLOWER 1, H.-U. SCHMINCKE 1 and H. BOWMAN 2

lInstitut f~r Mineralogie der Ruhr-Universit~t, Bochum (Federal Republic of Germany) 2Lawrence-Berkeley Radiation Laboratory, University of California, Berkeley, Calif. (U.S.A.)

(Received October 15, 1975; revised and accepted April 28, 1976)

ABSTRACT

Flower, M.F.J., Schmincke, H.-U. and Bowman, H., 1976. Rare earth and other trace elements in historic Azorean lavas. J. Volcanol. Geotherm. Res., 1: 127--147.

Rare earth element (REE) and other trace element composit ions of 16 lavas from all historic and 2 prehistoric eruptions on 5 islands of the Azores Archipelago show notable intra- and inter-island differences. Fe enrichment and "compat ib le" element depletion due to fractional crystallization have been superimposed on variations established in the source area. Fract ionat ion of La]Sm, U/Th, K/Na and "large ion l i thophile" (LIL) element abundances are probably related to variable fusion of a source peridoti te whose LIL element distribution cannot be exactly specified in view of its possible heterogeneity. Relative light- REE enrichment in basalt appears greatest on the "potassic" island S~'o Miguel, the more sodic island Fayal and one lava from Pico, and least in basalts from the "sodic" islands Terceira, S~'o Jorge and Pico. This variation is matched by most other LIL elements, although P shows unexpected enrichment in Terceira lavas, otherwise the least LIL element-enriched and most heavy-REE-enriched. Upper mantle phase chemistry is probably critical in es- tablishing the patterns. In particular, P--REE covariance may reflect phase stabilities of apati te and (P-bearing) garnet in the upper mantle. Distribution patterns of REE in the historic lavas are similar to those of basalts from the Atlantic median rift at the crest of the Azores "p la t form". Transition to light-REE-depleted rift-erupted basalts to the southwest is believed to be step-wise with increasing water depth, possibly indicating retention of a light- REE-rich phase in the residue from partial fusion as intersection of geotherm and peridoti te solidus occur at lower pressures. The source mantle for the Azores basalts is probably light- REE- and LIL element-enriched but we find no evidence so far to suggest its emplacement by thermal "p lume" activity.

INTRODUCTION

G e o c h e m i c a l e v i d e n c e f r o m e r u p t e d m a g m a s c a n t o s o m e e x t e n t b e u s e d t o i d e n t i f y t h e n a t u r e a n d e x t e n t o f p h y s i c a l p r o c e s s e s in r e g i o n s o f u p p e r m a n t l e p a r t i a l f u s i o n . O n e o f t h e m a j o r p r o b l e m s e m e r g i n g in r e c e n t y e a r s is t h e re- l a t i o n s h i p b e t w e e n c h e m i c a l c o m p o s i t i o n o f o c e a n i c v o l c a n i c r o c k s a n d t h e i r t e c t o n i c s e t t i n g , a n d t h e e x t e n t t o w h i c h m a g m a t i c e v o l u t i o n o f o c e a n i c i s l a n d v o l c a n o e s is r e l a t e d t o t h e e f f e c t s o f p l a t e m o v e m e n t a n d t h e r m a l a n o m a l i e s in

128

the upper mantle. For several reasons the Azores are particularly amenable to the study of these questions.

The Azores "platform" is one of the largest topographic features of the central Atlantic Ocean, and has lately received much attention (Krause and Watkins, 1970; Ridley et al., 1974; Laughton and Whitmarsh, 1975; Schilling, 1975) in view of its possible association with a rising mantle "plume" (Morgan, 1971, 1972). It is the locus of the "Azores triple junction" where the American, Eurasian and African plates come together at the junction of the Atlantic median rift and the western end of the Azores-Gibraltar fracture zone (Fig.l). The islands straddle the Mid-Atlantic rift from WNW (31°W, 40°N) to ESE (25°W, 37°N) with Flores and Corvo to the west, and Fayal, S~o Jorge, Pico, Graciosa, Terceira, S~'o Miguel and Santa Maria to the east. Graciosa, Terceira and the western part of S~o Miguel are believed to be associated with a WNW-- ESE-trending trench known as the "Terceira Rift" (Machado, 1959), although the findings of Laughton and Whitmarsh (1975) suggest the entire Azores axis to be a complex transform fracture zone (AFZ in Fig.l) now constituting the western segment of the Eurasia/African plate boundary. Krause and Watkins (1970) previously postulated this zone to be a secondary axis of crustal spreading resulting from differential spreading rates north and south of the archipelago (Pitman and Talwani, 1972; McKenzie, 1972; Dewey et al, 1973).

40 : : : " : : - t ' I ' ' / ' i /::,:::.:::::~:q0Rvo I o, , , ,

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J , : : : i ! : . "::::::::~ ~ : : : ~ i i i i i 1 1 1 1 1 : • /

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/ ~ d l ~ l . . . . . ID . . -' : : . . . . . . . , 111 : : . : . : . : : : : , : : : : : :~ : : ! :b'~ I / I I

; _ _ ~ _ ~ , , ~ ~ [ STA. MARIA ~ ~ . 6 , ° I ~

~ ,,,,0 ? ,, ,?, o , , 2 ' ' , - I I • • t /

# i

i I I

25 I I

Fig.1. The Azores region; simplified after Laughton and Whitmarsh (1975). The thick dashed line indicates presently active plate boundary ; solid lines are fracture zones or prominent linear topographic features; major topographic highs are stippled; thin dotted lines are palaeo- magnet ic isochrons in mill ion years. E A F Z = East Azores fracture zone; A F Z = Azores fracture (?) zone; A ffi s ta t ion A in Schilling (1975).

129

However, the magnetic anomaly pattern south of this zone trends at ca. 20°N and, although undated, is inconsistent with this hypothesis. Laughton and Whitmarsh (1975) concluded from the regional anomaly pattern and recon- structed spreading rates that the Eurasia/African plate boundary has undergone several changes of strike-slip movement and substantial modification of its to- pography and configuration.

Extracts from unpublished bathymetric charts of the Azores region (cited by Laughton and Whitmarsh, 1975) indicate great topographic complexity and suggest that "platform" is perhaps a misnomer for the area. Topographic highs coincide mostly with probable plate boundaries and inactive strike-slip fracture zones, the area between the AFZ and EAFZ (East Azores fracture zone) (Fig.l) being especially complex. The amplitude of relief is greatest near to the islands, where the "Terceira trench" attains depths in excess of 3000 m.

Our previous studies of lava composition in the Azores (Schmincke and Weibel, 1972) revealed notable inter-island differences, e.g. in K/Na ratio and SiO2-saturation, although the samples were unfortunately undated. We have therefore collected specimens from all recorded historic eruptions in the Azores with the main objective of studying chemical variation within one time horizon. The present work is an attempt to match variation of some geochemically significant parameters to an acceptable geophysical model for magma genesis in the region. We have analyzed specimens from 14 historic and 2 prehistoric eruptions from the islands Pico, Fayal, S~o Jorge, Terceira and S~'o Miguel, for rare earth elements (REE) and the elements Hf, Sc, Ta, Cr, U, Th, Ni, Co, V and Zn, in addition to major elements. This paper deals mainly with the trace element data, as major element results for whole rocks and phenocryst phases are to be published separately (Schmincke and Flower, in preparation).

Schilling (1975) has published REE data for basalts erupted within the At- lantic median rift in a zone stretching southwestwards from the Azores to latitude 33°N. These basalts are presumably very young, and in this sense com- plement our own samples in relation to the physical evolution of the region. Schilling (1975) found greatest variability of REE abundance near the Azores, and observed a progressive change from light-REE-enriched to light-REE-depleted distribution patterns away from the Azores.

ANALYTICAL TECHNIQUE

The system of analysis, neutron activation analysis, with a detailed expla- nation of accuracies attainable, has been described previously by Bowman et al. (1973) and Perlman and Asaro (1969). Briefly, 1-g amounts of each specimen were ground in agate by hand. 100-mg aliquots were mixed with cellulose, pressed into pills and irradiated along with calibrated composite standards in a triga-type reactor. The general precisions for determining REE concentrations vary considerably, with Sm being the most precise and Lu the least. For example the precisions for sample ASJ-1 for Sin, La, Eu, Ce, Yb, Tb, Dy, Nd and Lu are respectively 0.3, 0.4, 0.8, 1.2, 1.8, 3.4, 4.0, 5.5 and 6.0%. The overall accura-

130

cies for the REE measurements have been checked against the most recent REE analysis by isotope dilution mass spectrometry (unpublished report, U.S.G.S.). Eight out of nine REE agreed to within lo. One element, Yb, agreed to only 1.3o, with our result being the higher of the two.

AGE RELATIONS OF THE AZORES

Judged from their geomorphic preservation, the surface volcanics of Pico, S~'o Jorge, Graciosa and Terceira are youngest, while Corvo and Flores probably belong to an older western group, similar in age (and partly in chemical character) to the eastern group of Santa Maria and S~'o Miguel (Schmincke and Flower, in preparation). Fayal has an old core, with several historic eruptions in the western part of the island (Machado, 1967}. The oldest volcanics known are from Santa Maria, dated as 8.12--6.08 m.y. in age (Abdel- Monem et al., 1968). The Nord-Este ankaramite complex of S~o Miguel has been dated at 4.01 m.y., while dates of 4.65 and 4.0 m.y. were obtained for the Formigas islets to the north of Santa Maria (Abdel-Monem et al., 1968). Sea- floor magnetic anomalies are undated to the south of the Azores, but a 49- m.y. anomaly is recorded to the northwest of S~'o Miguel, and one of 9 m.y. north of S~o Jorge and Graciosa (see Fig.l). It appears likely that crustal spreading has proceeded both north and south of the Azores fracture zone, away from the main Mid-Atlantic median rift in an east-southeasterly direction.

From Fig.1 it seems probable that the western termination of the EAFZ between the 9- and 21-m.y. magnetic anomalies could correspond with trans- lation of the active plate boundary to the AFZ and initial activation of the main Azores volcanoes during that period. The probable age of Corvo and Flores would likewise be consistent with their having formed at the new triple junction some 10 m.y. ago considering their distance from the present-day spreading axis and assuming a mean spreading rate slightly greater than 1 cm/ yr.

PETROGRAPHY AND MAJOR ELEMENT VARIATION

The most common mafic lavas of the Azores are alkali-olivine and "tran- sitional" basalts. Intermediate magma types are common and belong to the hawaiite-mugearite-benmoreite association (Girod and Lef~vre, 1972; Schmincke and Weibel, 1972), while comenditic trachytes from S~'o Miguel and comendites and pantellerites from Terceira have been reported (Schmincke, 1973). The petrography indicates that olivine and clinopyroxene are the dominant phases during basaltic fractional crystallization, although plagioclase is probably at or near the low-pressure liquidi of some more evolved lavas. Subsidiary amounts of Fe--Ti oxides, apatite and kaersutitic amphibole appear as phenocrysts in hawaiitic lavas during evolution towards mugearite and trachyte. The fractionation trends are shown in Fig.2 in which major element oxides are plotted versus MgO content.

131

1.5

K1_

0.5

4

~ 3 i..--

2

II

10 o o 9

( D

8

12 u')

LLI II O

X ~:) I0 o

Z : 9 U.I 0 8

W n 5

o

- - Z

uJ 3

2

I

17

13

56

52

48

44

I ' I ' I ' I I I ' I i I ' I ' I ' I ' I ' I '

~ +

~ o

+ ~

o + +

• 4-

o • • +

• ~+~

0+..

i 3 .2 - t

0 [] + ~

o /

O +

I , I J I i I I I , I , I J I i I I i J I I I J

13 12 II I0 9 8 7 6 5 4 3 2 .

MgO

Fig. 2. P l o t o f m a j o r e l e m e n t o x i d e s v e r s u s M g O f o r 15 h i s t o r i c a n d 2 p r e h i s t o r i c l avas f r o m t h e A z o r e s . S y m b o l s as f o l l o w s : f i l l ed c i r c l e s = P i c o ; o p e n c i rc les = SEo J o r g e ; t r i a n g l e s = F a y a l ; c r o s s e s = T e r c e i r a ; s q u a r e ffi SEo Migue l . T r e n d l ines : so l id = b a s a l t s f r o m Pico , S~o J o r g e a n d F a y a l ; d a s h e d ffi b a s a l t - m u g e a r i t e t r e n d f o r Te rce i r a .

132

Grouped according to SiO2-saturation, most of the historic lavas are mildly nepheline-normative, while the K-rich basalt ASM-42 from S~o Miguel is more strongly undersaturated. However, lavas AP-2 and AP-9 from Pico are hypersthene-normative and low in K. Thus historic lavas reflect the three main trends ("hypersthene-normative", "alkalic" and "basanitic") recognized for Azores lavas in general. In contrast to this mode of grouping, the lavas, and even individual island successions may also be categorized in terms of alkalinity (as ex- pressed by K/Na ratio), probably reflecting generalized relations of "large ion lithophile" (LIL) elements. One relatively "potassic" island, Sao Miguel, is recognized (Schmincke and Weibel, 1972), while the lavas of Santa Maria and Flores are also relatively potassic, but less so. The islands Pico, Graciosa, Sao Jorge, Terceira and Fayal form a "sodic" group, whose lavas are mostly mildly nepheline- or hypersthene-normative. Fayal lavas, however, appear to be ex- clusively nepheline-normative.

INTRA-ISLAND VARIATION

The REE and other element data for historic lavas are given in Table 1, with eruptive dates and significant element ratios such as (La/Sm) e.f.*. Chondrite- normalized REE distribution patterns are given in Fig.3. These all show relative enrichment of light REE, typical of oceanic island lavas, (e.g. Schilling and Winchester, 1969; Zielinski and Frey, 1970; Flower, 1971). Light-REE relative enrichment ranges through a factor of 3 between basaltic types and the REE- enriched trachyte AT-11, while among basalts there is considerable variation in abundance of both light and heavy REE, and their relative fractionation in terms of (La/Sm) e.f. (Table 1). Most samples show a slight positive Eu anomaly, with the exception of the trachyte AT-11 and K-rich basalt ASM-42.

In Fig.4 the elements K, Ce, Th, Ta, U and Hf are plotted versus La for all samples except AT-11 (trachyte). Two straight-line trends may be identified from the variation, except for Ce where they merge into one: (i) including all basalts from Fayal, Pico and S~o Jorge, and (ii) including the basalt and hawaiites from Terceira. A steep highly fractionated trend (iii) may also exist which includes basalts from S~o Mignel, Fayal, Terceira and Pico specimen AP-11, but more data are required to confirm this. It would appear that element /La abundance ratios are fractionated most with increasing La along " t rend" (iii) and least along trend (ii). With the exception of the Terceira basalt AT-24, Fe/Mg ratios for basaltic samples lie within a relatively close range (Table 1), while hawaiites show a corresponding trend to high Fe/Mg ratios, reflecting that whereas trend (ii) could be explained by a fractional crystallization hypothesis, trends (i) and (iii) are attributable to other factors, probably ef- fective in the region of magma generation. We now consider each island indi- vidually:

*e.f. = enr ichment factor, or rock/chondrite abundance ratio; chondrite data used taken f rom

Masuda et al. (1973) .

133

E o.

0 7

0 .I_ (D

O

:I I: Z. -L.

0 ~E

0

>-

U-

no'

W (..3 he" W s--

W O 0~ O - 7

0

ff]

O O

0_

Fig.3. Chondrite-normalized plots of REE abundance versus atomic number for 14 historic and 2 prehistoric lavas from Pico, S~o Jorge, Terceira, Fayal and S~'o Miguel; chondrite abundances taken from Masuda et al. (1973). Note slight +re Eu anomalies for each lava except that from S~'o Miguel and the trachyte (with ---re anomaly) from Terceira.

Pico

Pico basalts AP-2, -4, -7, -9 and -11 (in order of increasing La, Ce, Hf, Ta and U contents) probably do not belong to a simple low-pressure fractionation trend, as there is no progressive decrease in "compatible" elements Cr, Ni and Sc or increase in F/(F + M) {Table 1). AP-2 and -7 have virtually identical F/(F + M) ratios (0.49) as have AP-4, -9 and -11 (0.54--0.55). These two groups

TA

BL

E

1

Ele

men

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and

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62

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AP

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2

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5.

1 2

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A

P-4

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asal

t P

h 3

25

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58

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28

6.1

3

2.2

2

0.8

4

5.0

2.1

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AP

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alt

17

18

3

0,4

6

6.8

3

5

6.6

4

2.3

2

0.8

3

4.5

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P-1

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t 1

71

8

38

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60

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38

7

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2

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0

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5,

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AS

J-3

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asal

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58

0

27

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60

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33

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0.8

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56

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62

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35

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2

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5 1

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6

40

0

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2.

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9 4

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a (L

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49

2

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1

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2

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2

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9

46

2

90

1

20

2

38

2

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0

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8

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2

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95

0

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A

P-9

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1

05

4

4

31

0

12

0

30

7

2.7

9

0.2

7

9,1

40

2

35

7

0.5

5

AP

-11

4.2

8

16

8

39

1

80

1

20

3

84

3

.34

0

.26

1

3,2

90

2

26

9

0.5

4

AS

J-3

2

.74

19

7 55

3

00

1

20

2

20

2

.45

0

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8

°30

2

21

82

0

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A

SJ-

1 5

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26

3

0

25

0

16

0

37

3

2.81

0

.31

1

3,2

83

4

14

6

0.6

6

AT

-24

2

.75

8

8

44

3

50

1

40

3

95

2

.38

0

.44

7

,47

0

32

73

0

.61

A

T-1

4

4.5

4

23

25

2

50

1

50

7

85

2

.42

0

.39

1

0,7

13

5

23

7

0.6

8

AT

-15

5.

13

27

21

20

0

17

0

49

5

2.34

0

.38

1

1,6

23

5

89

1

0.71

A

T-1

1

12

.3

9 0.

3 4

18

0

13

70

3

.58

0

.34

2

6,5

66

4

36

0

.92

AF

-1

3.9

8

98

4

2

34

0

12

0

41

2

3.5

2

0.2

7

12

,43

3

21

82

0

.54

A

F-4

4

.20

1

27

3

9

35

0

11

0

44

6

3.3

5

0.2

2

12

,45

0

23

57

0

.56

A

F-1

5

4.0

8

13

2

39

3

00

1

00

3

92

3

.28

0

.26

1

1,6

23

2

18

2

0.5

5

AS

M-4

2

6.4

8

23

0

58

27

0

14

0

37

4

3.3

5

0.2

6

19

,09

5

19

64

0

.56

C'

(%)

7.0

16

2

12

7

5.0

S2(%

) 7

.5

18

3

16

15

6

.6

4Wt.

% r

atio

: F

eO e

xp

ress

es t

ota

l F

e o

xid

e.

~n

136

20000

15000

10000

5000

10

3.0

2.0

i.0

I

K

Th

(ipiJ /

ASM 42 t3

/ 1

J /

A P 1 1 ~ / O li

/

( i i i t

/ i

(iiJ) i

I I I I I

20 /-+0 60

] T I - [ - - - - T ~ - - I

Ce

/

j /

ii o J

Hf

( i i i ) 0

I I I I 1 I I 20 40 60

ppm La

20O

.4

" q,so

Y" i / j tO0

50

15

10

Fig.4. Plots of K, Ce, Th, Ta, U and I-If abundance versus La (in ppm. ) for 14 historic and 2 prehistoric lavas from the Azores (see Table 1 ), showing straight line trends (i) and (ii), with a speculat ive third trend, (iii). S ymb o l s as in Fig.2.

are also distinguished by their Cr and Ni contents, and within each there are significant differences of (La/Sm) e.f. and LIL element abundances. Calculated trends for olivine + cl inopyroxene -+ plagioclase fractionation from basaltic liquid indicate that for the range of (Yb) e.f. (8--10), (La/Sm) e.f. could not

137

possibly increase from 2.45 (AP-2) to 3.34 (AP-11) as a result of fractional crystallization.

AP-11, from the 1718 Santa Luzia eruption is virtually identical to all three specimens from the 1762 and 1958 Fayal eruptions -- in terms of La/Sm e.f., K/Na, etc., and element abundances (see Figs. 2, 4, 5 and 6, and Table 1) - -bu t shows little resemblance to contemporaneous and other historic lavas from Pico.

Sffo Jorge

Both ASJ-3 and -1 are nepheline-normative but differ in F / ( F + M), (0.53 and 0.66, respectively) suggesting that ASJ-1 (a hawaiite) has resulted from fractional crystallization of a mafic parent. This is supported by its low Cr, Ni and Sc, and high contents of LIL elements (Table 1). The chemical relationship between S~'o Jorge compositions differs from those between Pico lavas. ASJ-1 is probably not a result of olivine + cl inopyroxene removal from magma of ASJ-3 composition as the difference in (Yb) e.f. values for the two samples would have to be greater than that observed to accommodate the (La/Sm) e.f. range of 2.45--2.81, even if augite were the sole crystallizing phase (see Schil-

4.0 I I i I I I

3 .5

LI.: IJJ

E 3-0 t/')

O .J

2 ' 5

ASM 42 [3 AF 1;4 Z~O AP 11

Z~ AF 15 A

A P 9 •

AP 4 Q QAP7

ASJ 3 O • AP2

X AT24

2.01. 0 ~ I I I I I 2 .0 3-0 4.0

N a z O / K zO

Fig.5. Plots of chondrite-normalized La/Sm ratio versus Na20/K=O for basalts. Symbols as in Fig. 2.

1 3 8

a,

E & 20

5 0 , F

(a)

30

10 ,,,--,/

+

(bl

'

1000

0

3.0

2.8

13 us 2.6 E o .

2.l.

2.2

2.0

r I I

A S M 4 2 "-..a AP 11,,.=,

_ ~ e A P 9 A S J ~ o & A p 7

/ A P 4 o /

/ " A P 2 /

. /

A T 2 4

I I t I

/ /

/

o

/ o •

/

o / I I I

1500 2000 2500 pprn P

/ /

/ /

I 3000 3500

Fig.6. Plots for Azores historic and 2 prehistoric basalts of" (a) La versus P (ppm); shaded area "R" shows range of composit ions covered by basalts from the Reykjanes Ridge (Schil- ling, 1973); and (b) Eu versus P (ppm).

ling, 1973b). Apatite microphenocrysts are present in ASJ-1 but the effects of apatite removal from basalt magma are not easily predictable, as REE partition into apatite varies considerably according to its bulk chemical environment (e.g. Nagasawa, 1970; Frey and Green, 1974).

Terceira

The prehistoric basalt AT-24 is hypersthene-normative, while the hawaiites AT-14 and -15 from the 1761 eruption are both slightly nepheline-normative. However, they all have very similar REE rock/chondrite distributions (Fig.3) with negligible differences in (La/Sm) e.f., while total REE enrichment between AT-24 and -15 approaches a factor of 2. A characteristic trend for these lavas is observed for several elements versus La (Fig.4), while the uniform (La/Sm)e.f. values are strongly suggestive of a simple relationship by fractional crystallization. Increase of F / ( F + M) from basalt to hawaiite (0.61--0.68) is ac- companied by decreases in Sc, Ni and Cr, and increases in LIL element contents. In contrast to other LIL element abundances, phosphorus appears to be relatively enriched in Terceira rocks (Fig.2), with correspondingly high P/K, P/Ti, etc., ratios.

The trachyte AT-11 was also probably erupted in 1761 from a separate vent

139

some 3 km from the main eruptive centre at that time. It is hypersthene-normative and depleted in P, suggesting removal of apatite during development of the trachyte liquid. The rock/chondrite REE distribution (Fig.3) is character- ized by relative enrichment of both light and heavy REE as compared to AT-24, -14 and -15. A small negative Eu anomaly suggests plagioclase has been fractionated from the liquid. If Nagasawa's (1970) partition data for apatite are applicable, this phase may be responsible for the concave REE pattern, and for relative suppression of the Eu anomaly due to plagioclase.

Fayal

The nepheline-normative lavas from the 1672-3 (AF-1) and 1958 (AF-4 and -15) eruptions of Fayal are virtually indistinguishable in terms of (La/Sm) e.f. (2.26--2.34), F / ( F + M) ratios (0.54--0.56), Cr (210--227 ppm) and Sc (24 .... 25 ppm) contents. Respective values for the elements Hf, Ta, U and Th (Table 1) are also remarkably close (see also Fig.3), and almost certainly reflect the common derivation of these magmas with those giving rise to AP-11 on Pico.

S~o Miguel

The basalt from the 1563 S~o Miguel eruption (ASM-42) has the highest LIL element contents and ratios K/La, Th/La, U/La and K/Na (Figs.4 and 5) for equivalent Fe/Mg ratios of all basaltic lavas sampled, and reflects the potassic character of S~o Miguel lavas in general.

In summary, the only clear-cut trend of fractional crystallization is that towards hawalite (and probably trachyte) represented by the Terceim specimens, although ASJ-1 from S~o Jorge is also the fractionation product of an unidentified basaltic parent. The relations of basaltic magma chemistry both within and between islands are probably dependent on the physical conditions of melting and/or differences in the composition of the source material.

INTER-ISLAND VARIATION: PARTIAL MELTING VERSUS MANTLE HETERO- GENEITY

If inter-island chemical differences are to be interpreted in terms of magma- generating processes we must first consider to what extent the historic lavas chemically represent their respective island locations. It would thus be helpful to know the evolutionary stage of each volcano represented by the sample eruption. Unfortunately, there is not enough information to define the volcano- logical evolution of each island, and any dependence of chemical variation on secular stages of the types observed on some other oceanic islands (e.g. Mac- donald, 1969; Flower, 1973; Schmincke, 1973; Schmincke and Flower, 1974). However, as already noted, the K/Na character appears to persist through the development with time of some islands and is not merely a function of a particular sequence of evolutionary stages. This pattern is possibly matched by

140

other LIL element abundances and inter-element ratios, although as Fig.4 has revealed, the interrelations of these elements are not controlled by any one factor.

Partial melting

As most of the basaltic lavas have reached approximately equivalent states of Fe-enrichment relative to Mg content, it seems justified to relate their differences in LIL element abundances to a model of variable partial fusion of the type proposed by Treuil and Varet (1973), implying a single homogeneous mantle source. Values for the distribution coefficient (KD) between melt and solid, the weight fraction of melt produced and the identity and relative pro- portions of all solid phases present in the source region are not known with any certainty however, either for the general case or for mantle melting beneath the Azores. If garnet is stable (Green, 1971), heavy REE would be effectively buffered in primitive melts (Gast, 1968; Philpotts et al., 1972), depending on this phase remaining in the residuum during partial fusion. For light REE, KD is not necessarily dominated by any one phase and may range (e.g., for Ce) from 0.1 (for clinopyroxene) to 0.01 (for olivine), thus ensuring preferential light-REE partitioning into the melt, at least for low degrees of partial fusion. Kay and Gast (1973) have calculated that chondrite-normalized REE patterns ranging between those of ASM-42 and AP-2 would require variations in degree of partial fusion of 0.8--2.9% of a 4-phase (ol + opx + cpx + gr) upper mantle peridotite with a clinopyroxene/garnet ratio of about 3:1.

Application of such partial-melting models to our samples would suggest that with increasing degrees of fusion a spectrum from ASM-42 (S~o Miguel) to AT-24 (Terceira), AP-2 (Pico) and ASJ-3 (S~'o Jorge), with Fayal and other Pico samples in an intermediate position, is reflected by decreasing values of La/Sm with concomitant dilution of light-REE and other LIL element abundances. Normative chemistry would suggest the progression is not so simple, AP-9 (hypersthene-normative) appearing to derive from lesser degrees of melting than the nepheline-normative magmas AP-4 and -7. This is in opposition to all prediction from experimental petrology (e.g. see O'Hara, 1968). However, there is widespread confirmation for the independent variability of LIL elements and SiO~-saturation in oceanic island magmas (Schmincke and Flower, 1974) and the phenomenon must relate to a complex interplay of factors. These might include variations in total pressure, temperature, PH20 and PCO2 between different sites of partial fusion (Kushiro, 1974; Eggler, 1974).

To accept any single-mantle variable partial fusion model it would therefore be necessary to postulate long-term differences in (at least) partial fusion degree

and/or depth of melting along the Azores chain over the last 8 m.y., at the same time remembering that the positions of islands south of the main fracture zone (e.g. Fayal and Pico) may have changed relative to (e.g.) Terceira in the north, due to faster spreading rate of the northern limb (Laughton and Whit- marsh, 1975).

141

Disequilibrium melting and the role of LIL-element-rich phases

If we postulate disequilibrium partial fusion (e.g., according to the model of O'Nions and Pankhurst, 1974), and consider the relations of minor phases that concentrate LIL elements in the mantle, single-source partial fusion models may be more applicable and tolerate larger degrees of partial fusion. The observed discrepancies of magma chemistry would not be dependent on rigorous application of the solid/liquid partition requirements of a 4-phase peridotite in equilibrium with its melt. The step-like transition of SVSr/86Sr ratios with depth below sea level in basalts along the Reykjanes Ridge (Hart et al., 1973) has been interpreted by Flower et al. (1975) to reflect breakdown of phlogopite, a phase that may remain in the residuum during low-pressure partial fusion of plagioclase lherzolite (Forbes and Flower, 1974). Phlogopite fusion would not account for variation of St, P, U, Th and the light REE, but it is possible that apatite may concentrate these elements and behave in an analogous manner to phlogopite during partial fusion. The effects of such processes may be evident in the observation by Frey and Green (1974) that in several lherzolite xenolith suites LIL elements increase in abundance with increasing refractory character, as shown by Mg/Fe ratio.

The good correlation of light REE with P in the Victorian lherzolites (Frey and Green, 1974) indicates that the major light-REE contribution is from minute amounts of apatite. Thompson (1975) recently showed that P may also be present in upper mantle garnet, substituting for grossular as the component Na3AI2P3012. As garnet is notably enriched in heavy REE, especially Eu and Gd (Philpotts et al., 1969; Shimizu, 1975), the covariance of P with REE might be strongly indicative of apatite and/or garnet control on melt composition, if the effects are not masked by the REE contribution from other phases such as clinopyroxene. The strong correlation of (La/Sm) e.f., Th and U with P and ratios such as P/Ti in ocean rift-erupted basalts (e.g., Schilling, 1973a) might thus reflect that apatite is the major host for P in the upper mantle at the relatively low pressures of formation for these magmas.

Of the historic Azorean basalts analyzed, the Terceira basalt AT-24 is richest in P, but also has the highest content of heavy REE (Eu--Lu) and lowest value of (La/Sm) e.f.. In Fig.6a a positive correlation is observed between La and P for all Pico basalts (except AP-11) and the S~o Jorge basalt ASJ-3, which is continuous with that observed for abyssal tholeiites from the Reykjanes Ridge (Schilling, 1973a). However, plots for AT-24, AP-11, AF-1, -4 and -15 and ASM-42 show a sharply divergent trend of negative correlation between these elements. In contrast, plots of Eu versus P in Fig.6b show a close positive correlation for all basaltic compositions. It is thus tempting to suggest a greater in- volvement of garnet during formation of the Terceira magmas, although it is not possible to speculate further on the light-REE enrichment and relative P depletion in ASM-42 as we know little of apatite stability and its possible reaction relations with P-bearing garnet.

142

Mantle heterogeneity

It is likewise difficult to evaluate the extent to which variations of magma chemistry reflect real differences of the source bulk composition.

Trend (i) in Fig.4 probably relates primitive basalt compositions from Fayal, Pico and Silo Jorge to increasing degrees of partial fusion, as reflected by decreasing K/La, Th/La and U/La, and increasing P/La, Ta/La and Hf/La, of a similar or uniform mantle source. This source composition may, however, lie on a compositional trend that in turn gives rise to a possible magma "trend" (iii) including ASM-42, AF-15, -1 and -4, AP-11 and AT-24 (Figs.4 and 6), reflected by sharply decreasing K/La, Th/La, U/La, and also [in contrast to trend (i)] ,decreasing Ta/La and Hf/La and very sharply decreasing La/P with increasing P (Fig.6a). It is extremely unlikely therefore that magma compositions on this trend are related to a single source composition by variable degrees of partial fusion.

If the Fayal and Pico magmas derive from a source of intermediate composition between that for Terceira and that for S~o Miguel magmas, this may reflect two possibilities: (1) a simple increase in heterogeneity with distance from the median rift -- the simple progression now dislocated by differential plate movement along the AFZ (Fig.l), or (2) heterogeneity with depth where- by changing stress conditions along the fracture zone(s) induce production of distinct magma types at different conditions of pressure and temperature. In either case the effects of differential partial fusion would be superimposed on those of mantle heterogeneity to account for the diversity of primitive magma chemistry.

Variation of initial STSr/86Sr ratios in basalts from the Azores (White et al., 1975) is between 0.70332 and ca. 0.70500, and tends to match that of K/Na, Ba/K, etc., between islands. It is also independent of intra-island age differences. 87Sr/86Sr ratios for the islands Fayal, Pico and (especially) S~o Miguel are markedly higher than those of the adjacent median rift. Together with the observed enrichment of radiogenic Pb (Sun and Hanson, 1975) the Sr isotope data lend support to the postulate of mantle heterogeneity beneath the Azores {White et al., 1975).

REGIONAL VARIATION: IMPLICATIONS FOR THE AZORES "PLUME" MODEL

Schilling (1975) interprets the progressive decrease in (La/Sm)e.f. in basalts from the median rift with distance from the Azores "platform" by a model re- quiring two distinct mantle sources: (1) LIL element- and radiogenic isotope- enriched primordial "plume" material rising from great depth, and (2) LIL element- and radiogenic isotope-depleted material comprising the seismic low- velocity layer; with an intermediate zone of mixing to produce a spectrum of derivative magma chemistry.

As White et al. (1975) note: "geochemical studies alone cannot prove the existence of plumes, which are essentially dynamic concepts". However, taken

143

together, chemical data can be shown to place several constraints on the "p lume" interpretation. For instance: values for (La/Sm) e.f. and STSr/S6Sr show a wide range of variation at any one latitude on a longitudinal traverse along the median rift across the Azores, while very large differences in these ratios exist between and even within single islands (especially Pico and S~'o Miguel). This represents a serious obstacle for the plume hypothesis, at least in its simplest form, unless we conjecture that individual volcanic centres are the surface re- flection of distinct plumes of unrelated chemistry.

Variation of some chemical parameters could clearly reflect the height of the eruptive pile at a particular locality, or more specifically, factors such as the potential for fractional crystallization in subvolcanic chambers (e.g. O'Hara, 1975) and of magma product ion and/or extraction rates. If we consider the (La/Sm)e.f. in basalts from the Azores and Atlantic median rift axes by ref- erence to their distance from the arbitrary datum of sea level (Fig.7), the va-

E 2

/ + , , i I 1 i T

- +

It-

l-- I

I I i i i

+1000 0 - 1000 -2000 -3000

DEPTH / HEIGHT (m) FROM SEA-LEVEL

t - - ~ - t -

&

-4000

Fig. 7. Plots of chondrite-normalized La/Sm ratio versus vertical distance in metres from sea level for: (a) dredged basalts from the Atlantic median rift southwest of the Azores (data from Schilling, 1975), indicating sampling depth uncertainty; and (b) subaerial historic and prehistoric basalts from the Azores (this work), showing heights of present-day eruptive centres. A.A. is the (La/Sm) e.f. range covered by submarine basalts from station A at the crest of the Azores "p la t form", water depth 1700--2100 m. Symbols for subaerial lavas as in Fig. 2.

riation is seen to be markedly step-like, in an analogous manner to the STSr/S~Sr " s t ep" in the North Atlantic (Flower et al., 1975). We suggest this could simi- larly reflect transgression of the pressure-temperature breakdown curve of a

144

light-REE-rich phase (e.g. apatite) which at low pressures persists into the supra- solidus region during partial fusion while at higher pressure breaks down at the solidus.

Such a model would not necessarily require isotopic disequilibrium during melting in the sub-median rift environment, although disequilibrium melting would allow a dramatic reduction in the apparent age of the mantle fractionation event as deduced from Rb--Sr isochrons. The "depleted" magma (Fig.7) is relatively uniform in its isotopic character (White et al., 1975), whereas in contrast, those magmas enriched in radiogenic components are sufficiently varied with regard to this and other characteristics to suggest the source material to result from a variable differentiation process. The evidence from magma chemistry is that enrichment of the Azores mantle is distinctly patchy over short vertical and/or horizontal intervals. "Second-stage" melting of this relatively refractory material to produce the Azores magmas could be a function simply of the anomalous stress conditions in the vicinity of the triple junction and the opportunity at an earlier stage for large-scale extraction of LIL element-depleted "abyssal" tholeiite magma.

CONCLUSIONS

There is still no direct geothermal evidence of a thermal plume beneath the Azores. Geochemical data for recent eruptions, both in the archipelago and in distal regions of the Azores topographic high, do not unequivocally support the plume model in spite of the indications of anomalous degrees of magma eruption in this part of the Atlantic crust. In particular we consider it likely that inter- and intra-island variation of parameters such as La/Sm, K/Na and La/P is related to the control of specific phases in the source paragenesis during partial fusion.

We conclude that at any one time the variation of primitive magmas in the Azores relates to two important factors: (1) mantle heterogeneity, and (2) variable partial fusion. The first of these is related to the speculative postulate that pressure-temperature stabilities of minor LIL element-rich phases control the chemical differentiation of source material by enabling the extraction of an early-formed LIL element-depleted melt fraction (equivalent to "abyssal" tholeiite). We further believe that the intriguing phosphorus anomaly in Ter- ceira magmas could possibly reflect a phase transition in the source assemblage that brings garnet in as a subsolidus phase during partial fusion. Such effects are of great potential significance in explaining LIL element distribution in magmas and in placing constraints on the pressure-temperature conditions of generation.

We feel that it is as plausible to suggest the Azores triple junction is the remdt of a "weak spot" as of a "hot spot" (cf. Dittmer et al., 1975). Formation of the LIL element~euriched mantle beneath the Azores may be reasonably associated with abyssal tholeiite production at the Atlantic median rift. Renewed melting of this may be reactivated by the geophysical instabilities imposed by a

145

c o m p l e x o f deep t r an s fo rm f rac ture zones and addi t ional hea t f lux prov ided by relat ively more rapid man t l e c reep (Shaw and Jackson, 1973) and viscous hea t (Anderson and Perkins, 1974) . E x t r e m e f rac t iona t ion o f the mant le coup led with low degrees o f part ial fus ion would thus lead to the excep t iona l ly high values o f (La /Sm) e.f., STSr/s6Sr, K /Na in (especially) S~o Miguel magmas. This schemat ic mode l is cons i s ten t wi th the secular variat ions in spreading rate and the observed free-air posi t ive gravity anoma ly (Kaula, 1972) , par t icular ly if magma ex t r ac t ion is faci l i ta ted u n d e r these condi t ions . The analogy might be e x t e n d e d to o the r in tersec t ing spreading axes a n d / o r t r ans fo rm fractures.

ACKNOWLEDGEMENTS

We t h a n k F.A. Frey , J. Her togen , R.N. T h o m p s o n and T.L. Wright fo r re- viewing earlier draf ts o f the manuscr ip t . The w o rk was suppor t ed b y the Deutsche Forschungsgemeinschaf t .

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Rare earth and other trace elements in historic azorean lavas