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Crustal contamination processes traced by helium isotopes: Examples from the Sunda arc, Indonesia
EPSL ELSEVIER
Earth and Planetary Science Letters 126 (1994) 15-22
Crustal contamination processes traced by helium isotopes: Examples from the Sunda arc, Indonesia M. Gasparon a, D.R. Hilton b,1, R. Varne a a Geology Department, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia b Institut fiir Mineralogie, FR Geochemie, Freie Unicersitiit Berlin, Boltzmannstrasse 18-20, D-14195 Berlin, Germany
Received 28 March 1994; revision accepted 7 June 1994
Abstract Helium isotope data have been obtained on well-characterised olivine and clinopyroxene phenocrysts and xenocrysts from thirteen volcanic centres located between central Sumatra and Sumbawa in the Sunda arc of Indonesia. Olivine crystals in mantle xenoliths (lherzolite) from Bukit Telor basalts are primitive (Mg# = 90), and their 3 H e / 4 H e value ( R / R A = 8.8) indicates that the Sumatran mantle wedge is MORB-like in helium isotope composition. All other samples have lower 3 H e / a H e ratios ranging from 8.5R A to 4.5RA, with most (thirteen out of eighteen) following a trend of more radiogenic 3He/aHe values with decreasing Mg#. The only exceptions to this trend are phenocrysts from Batur, Agung and Kerinci, which have MORB-like 3 H e / a H e values but relatively low M g # (Mg# = 70-71), and two highly inclusion-rich clinopyroxenes which have 3 H e / a H e values lower than other samples of similar Mg#. The results indicate that crustal contamination unrelated to subduction in the Sunda arc is clearly recorded in the 3 H e / a H e characteristics of mafic phenocrysts of subaerial volcanics, and that addition of radiogenic helium is related to low-pressure differentiation processes affecting the melts prior to eruption. These conclusions may have widespread applicability and indicate that helium isotope variations can act as an extremely sensitive tracer of upper crustal contamination.
I. Introduction C o n t a m i n a t i o n o f m a n t l e - d e r i v e d m e l t s by p r e - e x i s t i n g crust is a w e l l - d o c u m e n t e d p h e n o m e n o n in b o t h t h e o c e a n i c a n d c o n t i n e n t a l e n v i r o n m e n t s [1,2]. Its r e c o g n i t i o n is i m p o r t a n t
[uc] 1 Present address: Faculty of Earth Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands.
for distinguishing c o m p o s i t i o n a l f e a t u r e s imprinted onto magmas during their passage through t h e crust f r o m g e o c h e m i c a l a n d isotopic c h a r a c teristics which a r e d i a g n o s t i c of m a g m a s o u r c e histories. I n s u b d u c t i o n - r e l a t e d e n v i r o n m e n t s in p a r t i c u l a r such r e c o g n i t i o n is o f t e n p r o b l e m a t i c as a ' c r u s t a l ' s i g n a t u r e in arc m a g m a s m a y reflect the i n f l u e n c e o f s u b d u c t e d s e d i m e n t s in t h e source r e g i o n s in a d d i t i o n to a s s i m i l a t i o n o r rew o r k i n g o f u p p e r m o s t crust. This has l e d to a c e r t a i n d e g r e e o f d e b a t e in t h e i n t e r p r e t a t i o n of the m o s t u s e d i n d i c a t o r s o f crustal c o n t a m i n a -
0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0012-821 X(94)0013 1-H
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M. Gasparon et al. / Earth and Planetary Science Letters 126 (1994) 15-22
tion, namely radiogenic isotopes (particularly Sr and Pb), oxygen isotopes, and various trace element ratios [3-5]. Following the observation of predominantly radiogenic helium isotope ratios in some Andean lavas [6], it was suggested that helium isotope variations ( 3 H e / 4 H e ) may have the potential to recognise low-pressure (upper crustal) contamination effects in arc-related rocks. This results from the observation that in spite of large variations in sediment load being subducted, age of subducting crust and thickness of crust through which arc magmas are erupted [e.g. 7], most arc magmas analysed to date are characterised by a 3 H e / 4 H e ratio (R) close to M O R B (8R A, where R A = air 3 H e / 4 H e ) [8]. However, because of the tendency of magmas to undergo pre-eruptive degassing [9,10] and as helium is both highly incompatible and atmophilic in nature, mantle-derived helium can become depleted in magmas stalled in the crust. In such cases, magmas can become susceptible to the effects of addition of radiogenic helium ( < 0.1R A) by crustal contamination. An additional advantage of helium in this respect is that at magmatic temperatures it is expected to equilibrate with virtually all phenocrysts on relatively short (1-50 yr) timescales [11]; therefore, the conditions and mechanisms of melting (equilibrium vs. disequilibrium) should have little or no effect on the final 3 H e / a H e value. In this letter, we test these assumptions by using wellcharacterised olivine and clinopyroxene crystals from a number of volcanic centres in the Sunda arc of Indonesia. We show that because 3 H e / 4 H e ratios can be related to the degree of magma evolution, helium should be considered a tracer par excellence for the shallow-level contamination of mantle-derived melts.
2. Tectonic and petrological background The samples were selected from a number of volcanic centres from central Sumatra to Sumbawa in the Sunda arc (Fig. 1). The centres are either presently active, or have been active in the Quaternary (Ratai, Bukit Mapas, Bukit Telor, Sukadana plateau, Soromundi) [13-15]. With the
exception of Bukit Telor and the Sukadana plateau, all centres are situated between 100-200 km above the Benioff zone. The tectonic environment changes from Sumatra to Sumbawa [12]: in Sumatra, oceanic lithosphere younger than 100 Ma is subducting obliquely beneath old continental crust whereas beneath Java and the east Sunda arc the lithosphere is older and subducting in a direction almost perpendicular to the arc. The arc crust is quasi-continental in composition and thickness in west Java, whereas it is oceanic in thickness and probably also in composition in the east Sunda arc. Most samples are olivine a n d / o r clinopyroxene phenocrysts from typical medium-K calc-alkaline basalts/basaltic andesites. However, we draw attention to four exceptions: (1) Basalts from Bukit Telor are aphyric but carry small lherzolite xenoliths and olivine xenocrysts from disaggregated xenoliths. The 'fertile' nature of the xenoliths (presence of clinopyroxene), the lack of primary and secondary melt, fluid and mineral inclusions in all xenolith minerals, and the low CaO contents in olivine xenocrysts (approximately 0.07% [13]) all indicate that these xenoliths are of mantle origin and were derived from the Sumatran mantle wedge. (2) The Sukadana basalts have abundant olivine phenocrysts (up to 7% modal) set in a matrix of augite, plagioclase and olivine, and lack phenocrysts of plagioclase, hypersthene and hydrous minerals. Together with the Bukit Telor basalts, these rocks differ from the other Sunda samples in that they are located behind the arc in Sumatra and have marked petrographic, geochemical and isotopic within-plate affinities [13]. (3) The volcanics from Soromundi are highly undersaturated and K-rich, and contain modal feldspathoids (leucite + nepheline) and phenocrysts of phlogopite and amphibole. Sample 48139 contains phenocrysts of clinopyroxene + amphibole + phlogopite + magnetite, set in a groundmass composed of plagioclase + clinopyroxene + nepheline + leucite + magnetite + sanidine + apatite, and has high contents of Sr and Rb (1511 and 114 ppm respec-
M. Gasparon et al. / Earth and Planetary Science Letters 126 (1994) 15-22
17
the clinopyroxene and olivine megacrysts in ankaramites from Rinjani volcano in Lombok [16]. In general, the crystals in the present study have abundant primary and secondary fluid and melt inclusions, with the clinopyroxene samples from Soromundi (sample 48139) and Sangeang Api (sample 48067) being particularly rich. Samples of Batur, Agung, and Kerinci are virtually free of secondary inclusions, and only the olivines from Bukit Telor contain no inclusions.
tively), low HFSE, and high KzO ( K 2 0 / N a 2 0 = 1.16) contents [14,16,17]. (4) Samples 48083 and 48084 from Sangeang Api are olivine-clinopyroxenite xenoliths with idiomorphic to granoblastic texture, composed of subhedral clinopyroxene (70-95%) and anhedral olivine, and minor amounts of phlogopite. These cumulates were precipitated from hydrous alkaline basaltic magmas, presumably at relatively low pressure ( < 10 kbar) and under conditions of high f O 2 [16]. The associated volcanics range in composition from phonolitic tephrites to potassic nepheline trachyandesites, are moderately K-rich ( K 2 0 / Na20 = 1), and have high Sr and Rb (up to 1200 and 110 ppm respectively) and relatively low HFSE contents [14,16,17]. Sample 48067, also from Sangeang Api, is an olivine megacryst-bearing phonolitic tephrite with abundant large clinopyroxene and olivine crystals which are compositionally similar to
3. Results and discussion
In Table 1 we report t h e 3He/4He ratios and Mg# values (where Mg# = 100 x Mg2+/(Mg 2+ + Fe z+) for olivine and clinopyroxene phenocrysts and xenocrysts from the thirteen volcanic centres (Fig. 1). For the most part, these phases retain their inherited helium isotope signature v
0~
mn
, •K
"
~
~
Sul'awesi~'~
"'"
C
.,5 .
Indian Ocean 10
500
km
loo
,io
,,20
Fig. 1. Sketch map of the Sunda arc (after [12]), with the location of the volcanic centres discussed in the text. Location abbreviations are keyed to Table 1.
M. Gasparon et aL / Earth and Planetary Science Letters 126 (1994) 15-22
18
ing either higher or lower 3He/4He values [28] has not affected the helium isotope characteristics of this part of the mantle, and that R / R A values lower than those of Bukit Telor reflect admixture with radiogenic helium unrelated to a mantle component. Secondly, all the remaining samples have 3He/aHe values lower than Bukit Telor, ranging from 8.5R A (Agung) to 4.5R A (Soromundi). Although the higher values also fall within the MORB range, the lower ratios represent close to a 50:50 admixture of MORB and radiogenic helium. In Fig. 2, we plot the 3He/aHe values against the Mg# of the olivine and clinopyroxene crystals. Three trends emerge: (1) Irrespective of the composition of their host rocks, most samples (arrowed in Fig. 2) demonstrate a clear correlation between 3He/4He and Mg#, with the more radiogenic 3He/aHe values found in samples with lower
after eruption [11], but all samples were processed by vacuum crushing to avoid any spurious radiogenic a n d / o r spallation components. The Mg# of both minerals is used to monitor in a fairly simple way the degree of evolution of the parent melts. The first point to note is that the olivine xenocrysts from the Bukit Telor lherzolite xenoliths have the highest 3He/4He value yet reported for the Sunda arc [8,20,22], well within the (lo-) range found in MORB (Fig. 2) [23-25]. The crystals are primitive (Mg# = 90) and because they are seemingly mantle derived, represent an unambiguous indication of the MORB-Iike 3 H e / 4He nature of the Sumatran mantle wedge. This is in keeping with the general conviction that MORB-type materials are common in the source regions of arc magmatism [26,27]. Because the 3He/nile values are coincident with those of MORB, we assume that OIB-type material hav-
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Crustal Contamination
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Fig. 2. M g # - H e isotope diagram. Typical MORB value is from [25]. Error bars (2~r) from Table 1 are shown for each sample. The error bar indicates the analytical error for R / R A , and the range of analyses on multiple determinations for Mg#. Of the three He duplicate analyses, only the highest ratios have been plotted. Samples plotting within error of the arrow show a decrease in R / R A for decreasing Mg#, and indicate that a greater proportion of radiogenic helium characterises more evolved magmas. Only two samples particularly rich in secondary inclusions (clinopyroxene in 48139 and 48067) show relatively low R / R A values. Three samples plot above the arrow and appear to have crystallised in a relatively closed system (at least for He), and to have had little or no perceivable interaction with crustal material.
M. Gasparon et al. / Earth and Planetary Science Letters 126 (1994) 15-22
Mg#. The process of magma ageing whereby radiogenic helium is produced within magma stored in the crust [29] is unlikely to contribute a significant proportion of the measured 4He unless storage times for Sunda arc volcanics are unusually long or radioelement contents unusually high. As argued by Hilton
19
et al. [6], estimated transfer times from magma source to eruption and U contents of ~ 2 × 104 yr and 3 ppm respectively for arc-related lavas would be expected to give an equilibrium radiogenic 4He content of ~ 0.1 nccS T P / g phenocryst for an olivine melt partition coefficient of 0.008 [30]; assuming a two-
Table 1 M g # and He isotopic ratios in mineral phases of Sunda volcanics Volcano
Sample number $
Location
Rock type #
Phase *
Mg# t
R/RA + He (nccSTP/g)
Sumatra
Kerinci (K) Ratai (R) Sukadana (Suk) Bukit Mapas (BM) Bukit Telor (BT) Dempo (D)
75267(14) 75227(5) 75215(4) 78129-34 75378-9 75260
101°18'E 1°39'S 105°02'E 5°32'S 105%9'E 5°26'S I04°19'E 4°30'S 103°45'E 1°17'S 103°14'E 4°02'S
B BA B BA B BA
ol cpx ol ol ol (X) ol
69.87-+1.78(52) 71.93-+l.59 (51) 84.24-+1.27(85) 86.53+1.63 (181) 89.65L-0.41(26) 75.85+2.09 (38)
7.6-2-0.6 6.8iO.7 7.5_+0.6 7.8~-0.7 8.8_+0.5 6.2-+0.8
2.05 1.35 2.06 2.08 7.48 1.51
(2)
105°25'E 6°06'S
BA
cpx
75.54+1.20 (50)
6.9+1.8
0.45
75416(22)
108°05'E 7°15'S
B
ol
85.50~-5.06(29)
7.3-+0.4
7.22
(23) (24)
115°22'E 8°14'S 115°30'E 8°20'S
BA BA
ol cpx
69.85-+0.58(41) 71.39-+2.01(42)
7.4_+0.7 8.5_+0.5
2.90 3.51
Soromundi (Sor) Sangeang Api (SA) Sangeang Api (SA) Sangeang Api (SA)
48139 48067 48067 48083
118°36'E 8°18'S ll9°0I'E 8°13'S l19°01'E 8°13'S 119°02'E 8°14'S
LL PT PT OC
78.85+4.93 (41) 86.58-+2.07(55) 86.61+2.59 (50) 80.51-+0.24(61)
Sangeang Api (SA) Sangeang Api (SA)
48083 48084
119°02'E l I9°01'E
8°14'S 8°13'S
OC OC
Sangeang Api (SA)
48084
l19°01'E
8°13'S
OC
cpx ol cpx ol(X) ol (duplicate) cpx (X) ol(X) ol (duplicate) cpx (X)
4.5_+0.6 6.9-+0.9 5.8--+0.8 7.0_+0.5 6.8_+0.5 6.55--0.3 6.9_+0.6 6.5-k0.5 7.1_+0.4
1.22 0.89 0.80 3.90 3.73 9.07 8.33 7.08 31.40
Floras Ebulobo (E)
67168
121°11'E
8°48'S
BA
6.0-20.8 5.3_+0.9
0.53 0.77
Sunda Strait
Krakatau (Kra) Java
Galunggung (G) Bali
Batur (B) Agung (A) Sumbawa
cpx cpx (duplicate)
80.31+1.27 (62) 79.19+0.30 (60) 81.03-+1.18(50) 74.75+1.26 (23)
* The first n u m b e r refers to the University of Tasmania catalogue number, the n u m b e r in parenthesis to the Freie Universitat Berlin number. Additional sample details can be found in [13] (Sumatra and Java), [D.R. Hilton et al., in prep.] (Sunda Strait and Bali), [14,16,17] (Sumbawa) and [18] (Flores). # B = basalt; BA = basaltic andesite; L L = leucite lamproite; P T = phonolitic tephrite; OC = olivine clinopyroxenite. * ol = olivine; cpx ~ clinopyroxene; X = xenocryst (all others are phenocrysts), t M g # is the average 100 × M g 2 + / ( M g 2 + + FEE+), calculated assuming that Fe measured is 100% Fe 2+. M g # was measured on several mineral grains ( n u m b e r in parentheses) of the same grain fraction and mineral phase used for He analyses. T h e grains were analysed using a Cameca SX50 microprobe at the Central Science Laboratory of the University of Tasmania, using a 1 / ~ m b e a m and 15 kV and 20 nA. Primary and secondary ( U S N M 111312/444 for olivine and U S N M 122142 for clinopyroxene) standards [19] were used for calibrations and corrections. Typical analytical errors for M g # values are ± 0.35 for olivine and ± 0.25 for clinopyroxene. + Helium was released in all cases by vacuum crushing and analysed following the procedure described in [20]. The reported 3 H e / 4 H e values represent an integrated signature of the different inclusion types. 3 H e / g H e values (R) are reported relative to the atmospheric value (R A = 1.4 × 10-6), and have been corrected for a minor atmospheric helium component ( < 2%) assuming that any measured neon above the blank is air derived [21]. Blank corrections for helium exceeded 5% in only three cases: Sangeang Api 48067 ol (6.2%) and cpx (5.9%), and Krakatau (6.2%).
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M. Gasparon et al. /Earth and Planetary Science Letters 126 (1994) 15-22
component mixture between MORB-like (8R A) and radiogenic helium (0.05RA), all phenocrysts analysed in this study have considerably higher radiogenic 4He contents than this estimate. More probably, therefore, contamination by surrounding wallrock [6,25] is the source of the radiogenic helium. As differentiation processes leading to more evolved mineral compositions usually seem to be lowpressure processes which are operative in magma chambers, the observation of decreasing 3He/aHe with extent of differentiation of the mafic crystals is clear evidence of a crustal provenance for the radiogenic helium component. In this respect, the Bukit Telor sample effectively rules out a slab-derived radiogenic helium contribution at the source of the magmas. It should be pointed out, however, that because any radiogenic helium signal is enhanced by pre-eruptive degassing depleting the magmas of their original volatile inventory, 3He/aHe ratios cannot be used to model the proportion of bulk upper crustal material due to variability in both mantle and crustal end member helium concentrations. (2) The two highly inclusion-rich clinopyroxene samples (48139 and 48067) have 3He/aHe values significantly lower than other samples of similar Mg#. The relatively low R / R A values of these two samples are unlikely to be due to the presence in their source of a crust-derived component enriched in 4He: both Sangeang Api and Soromundi, like all the other volcanic centres discussed in this letter, are situated to the west of the transition zone which separates oceanic crust in the west (high R / R A) from the continental margin in the east (low R//R A) of the subducting Indian Ocean plate. Indeed, their R/R A values are considerably higher than those of east Sunda-Banda arc phenocrysts, which have suffered source contamination [20,22]. Notably, the olivines in sample 48067 do fall within the M g # - R / R A trend defined by the other Sunda arc volcanics, and only the clinopyroxenes, which have about 10 x more inclusions than the olivines in equilibrium with them, have relatively low R//RA values.
Crystallographic evidence indicates that sample 48067 equilibrated at submagmatic temperatures ( ~ 600°C [G.-M. Molin and R. Varne, unpublished]), so that the large difference in 3He/nile values between olivines and clinopyroxenes may reflect changing fluid conditions in the magma chamber and the late-stage dominance of a crustal fluid flux. In this case, the enhanced crustal input is more likely to be recorded in clinopyroxenes because of their higher helium diffusivity [11]. (3) Phenocrysts from three volcanic centres (Batur, Agung, Kerinci) have relatively low Mg# (70-71) but MORB-Iike 3He/4He values indicating that they evolved in a system without large degrees of contamination by 4He-rich material. For Batur, there is compositional [18] and U-Th evidence [31] that the magmas underwent closed-system fractionation and that transit times from source to surface were exceedingly short (< 8 kyr) [32]. In such circumstances, shallow-level (magma chamber) degassing of the mantle-derived volatile component may have been inhibited so that, in spite of the differentiated nature of the magmas, contamination with radiogenic helium would not have led to a perceivable shift in 3He/nile values. We can only conjecture that a similar scenario applies also to Agung and Kerinci because, like Batur, their phenocrysts also have relatively high helium concentrations given that they also are virtually free of secondary melt and fluid inclusions. The important point about these samples, however, is that they show that low Mg# is not necesarily associated with low 3He/4He ratios, and that among the geochemically evolved magmas only those which have undergone a relatively closed-system fractionation may retain the He isotopic signature of their source.
4. Conclusions
Petrography, mineral chemistry and helium isotopes have been used together to identify cases where a crustal component has contaminated magmas after their generation. This approach has
M. Gasparon et al. / Earth and Planetary Science Letters 126 (1994) 15-22
great potential, therefore, in the investigation of the c i r c u m s t a n c e s a n d timing of crustal c o n t a m i n a t i o n a n d for identifying m a g m a s likely to have suffered m i n i m a l i n t e r a c t i o n e n r o u t e to the surface. Criteria such as M O R B - I i k e 3 H e / a H e ratios in d i f f e r e n t i a t e d m a g m a s a n d h e l i u m isotope e q u i l i b r i u m b e t w e e n c o g e n e t i c olivine a n d clinopyroxene phenocrysts a p p e a r most useful in this respect. It m u s t be e m p h a s i s e d that a l t h o u g h some m a g m a s may have acquired some of their ' c r u s t a l ' geochemical a n d isotopic characteristics by s u b d u c t i o n - r e l a t e d source c o n t a m i n a t i o n , the p r o v e n a n c e of the crustal (radiogenic) h e l i u m in most arc e n v i r o n m e n t s (the east S u n d a - B a n d a arc b e i n g a n o t a b l e exception [8,20,22]) is restricted to the u p p e r m o s t crust. T h e observation that p r o v e n a n c e may vary for different crustal tracers could provide the e x p l a n a t i o n for the phen o m e n o n [cf. 6] that i n f o r m a t i o n from h e l i u m isotopes is f r e q u e n t l y at odds with, or ' d e c o u p l e d ' from, that of o t h e r geochemical indicators.
Acknowledgements R.V. a n d M.G. gratefully acknowledge the fin a n c i a l s u p p o r t of the U n i v e r s i t y of T a s m a n i a a n d the A u s t r a l i a n R e s e a r c h Council. D . R . H . t h a n k s the G e r m a n Science F o u n d a t i o n ( D F G ) for f u n d i n g , Pat a n d Sue Jonklaas for hospitality in J a k a r t a a n d for a r r a n g i n g the K r a k a t a u a n d Bali trips, a n d K. H a m m e r s c h m i d t a n d H. F r i e d r i c h s e n for s u p p o r t in the laboratory. W e also t h a n k the I n d o n e s i a n I n s t i t u t e of Sciences (LIPI), the I n d o n e s i a n Geological R e s e a r c h a n d Development Centre (GRDC), Kardana Hardjadi n a t a a n d Sigit M a r y a n t o for their efforts in the field, a n d J o n D a v i d s o n a n d two a n o n y m o u s reviewers for c o m m e n t s o n the m a n u s c r i p t .
References [1] W. Hildreth and S. Moorbath, Crustal contributions to arc magmatism in the Andes of Central Chile, Contrib. Mineral. Petrol. 98, 455-489, 1988. [2] C.J. Hawkesworth, K. Gallagher, J.M. Hergt and F. McDermott, Mantle and slab contributions in arc magmas, Annu. Rev. Earth Planet. Sci. 21, 175-204, 1993.
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[3] J.P. Davidson, Crustal contamination versus subduction zone enrichment: examples from the Lesser Antilles and implications for mantle source compositions of island arc volcanic rocks, Geochim. Cosmochim. Acta 51, 21852198, 1987. [4] M. Magaritz, D.J. Whitford and D.E. James, Oxygen isotopes and the origin of high-S7Sr/S6Sr andesites, Earth Planet. Sci. Lett. 40, 220-230, 1978. [5] M.T. McCulloch and J.A. Gamble, Geochemical and geodynamical constraints on subduction zone magmatism, Earth Planet. Sci. Lett. 102, 358-374, 1991. [6] D.R. Hilton, K. Hammerschmidt, S. Teufel and H. Friedrichsen, Helium isotope characteristics of Andean geothermal fluids and lavas, Earth Planet. Sci. Lett. 120, 265-282, 1993. [7] R. von Huene and D.W. Scholl, Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust, Rev. Geophys. 29, 279-316, 1991. [8] R. Poreda and H. Craig, Helium isotope ratios in circum-Pacific volcanic arcs, Nature 338, 473-478, 1989. [9] T.M. Gerlach and E.J. Graeber, Volatile budget of Kilauea volcano, Nature 313, 273, 1985. [10] P. Sarda and D. Graham, Mid-ocean ridge popping rocks: implications for degassing at ridge crests, Earth Planet. Sci. Lett. 97, 268-289, 1990. [11] T.W. Trull and M.D. Kurz, Experimental measurements of 3He and 4He mobility in olivine and clinopyroxene at magmatic temperatures, Geochim. Cosmochim. Acta 57, 1313-1324, 1993. [12] W.B. Hamilton, Tectonics of the Indonesian region, U.S. Geol. Surv. Prof. Pap. 1078, 345 pp., 1979. [13] M. Gasparon, Origin and evolution of mafic volcanics of Sumatra (Indonesia): their mantle sources, and the roles of subducted oceanic sediments and crustal contamination, Ph.D. Thesis, Univ. Tasmania, 1993. [14] R. Varne and J.D. Foden, Geochemical and isotopic systematics of eastern Sunda arc volcanics: implications for mantle sources and mantle mixing processes, in: The Origin of Arcs, F.C. Wezel, ed., pp. 159-189, Elsevier, Amsterdam, 1986. [15] M. Neumann van Padang, Catalogue of the Active Volcanoes of the World including Solfatara Fields. Part 1: Indonesia, 271 pp., Int. Volcanol. Assoc., Naples, 1951. [16] J.D. Foden, The petrology of some young volcanic rocks from Lombok and Sumbawa, Lesser Sunda Islands, Ph.D. Thesis, Univ. Tasmania, 1979. [17] J.D. Foden and R. Varne, The petrology and tectonic setting of Quaternary-Recent volcanic centres of Lombok and Sumbawa, Sunda arc, Chem. Geol. 30, 201-226, 1980. [18] G.E. Wheller, Petrogenesis of Batur Caldera, Bali, and the geochemistry of Sunda-Banda arc basalts, Ph.D. Thesis, Univ. Tasmania, 1986. [19] E.J. Jarosewich, J.A. Nelen and J.A. Norberg, Reference samples for electron microprobe analysis, Geostand. Newslett. 4, 43-47, 1980.
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[20] D.R. Hilton, J.A. Hoogewerff, M.J. van Bergen and K. Hammerschmidt, Mapping magma sources in the east Sunda-Banda arcs, Indonesia: constraints from helium isotopes, Geochim. Cosmochim. Acta 56, 851-859, 1992. [21] H. Craig, J.E. Lupton and Y. Horibe, A mantle helium component in circum-Pacific volcanic gases: Hakone, the Marianas and Mt. Lassen, in: Terrestrial Rare Gases, E.C. Alexander and M. Ozima, eds., pp. 3-16, Central Academic Publ., Tokyo, 1978. [22] D.R. Hilton and H. Craig, A helium isotope transect along the Indonesian archipelago, Nature 342, 906-908, 1989. [23] H. Craig and J.E. Lupton, Helium-3 and mantle volatiles in the ocean and the oceanic crust, in: The Sea, Vol. 7, The Oceanic Lithosphere, C. Emiliani, ed., pp. 391-428, Wiley, New York, 1981. [24] J.E. Lupton, Terrestrial inert gases: isotope tracer studies and clues to primordial components in the mantle, Annu. Rev. Earth Planet. Sci. 11, 371-414, 1983. [25] D.R. Hilton, K. Hammerschmidt, G. Loock and H. Friedrichsen, Helium and argon isotope systematics of the central Lau Basin and Valu Fa Ridge: evidence of crust/mantle interactions in a back-arc basin, Geochim. Cosmochim. Acta 57, 2819-2841, 1993.
[26] J. Woodhead, S. Eggins and J. Gamble, High field strength and transition element systematics in island arc and back-arc basin basalts: evidence for mutli-phase melt extraction and a depleted mantle wedge, Earth Planet. Sci. Lett. 114, 491-504, 1993. [27] R.C. Maury, M.J. Defant and J.-L. Joron, Metasomatism of the sub-arc mantle inferred from trace elements in Philippine xenoliths, Nature 360, 661-663, 1992. [28] H. Craig, Helium isotope distribution in mantle hot spots, EOS 71, 1669, 1990. [29] A. Zindler and S. Hart, Helium: problematic primordial signals, Earth Planet. Sci. Lett. 79, 1-8, 1986. [30] B. Marty and P. Lussiez, Constraints on rare gas partition coefficients from analysis of olivine-glass from a picritic mid-ocean ridge basalt, Chem. Geol. 106, 1-7, 1993. [31] G.E. Wheller and R. Varne, Genesis of dacitic magmatism at Batur volcano, Bali, Indonesia: implications for the origins of stratovolcano calderas, J. Volcanol. Geotherm. Res. 28, 363-378, 1986. [32] K.H. Rubin, G.E. Wheller, M.O. Tanzer, J.D. MacDougall, R. Varne and R. Finkel, 238U decay series systematics of young lavas from Batur volcano, Sunda Arc, J. Volcanol. Geotherm. Res. 38, 215-226, 1989.
Earth and Planetary Science Letters 126 (1994) 15-22
Crustal contamination processes traced by helium isotopes: Examples from the Sunda arc, Indonesia M. Gasparon a, D.R. Hilton b,1, R. Varne a a Geology Department, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia b Institut fiir Mineralogie, FR Geochemie, Freie Unicersitiit Berlin, Boltzmannstrasse 18-20, D-14195 Berlin, Germany
Received 28 March 1994; revision accepted 7 June 1994
Abstract Helium isotope data have been obtained on well-characterised olivine and clinopyroxene phenocrysts and xenocrysts from thirteen volcanic centres located between central Sumatra and Sumbawa in the Sunda arc of Indonesia. Olivine crystals in mantle xenoliths (lherzolite) from Bukit Telor basalts are primitive (Mg# = 90), and their 3 H e / 4 H e value ( R / R A = 8.8) indicates that the Sumatran mantle wedge is MORB-like in helium isotope composition. All other samples have lower 3 H e / a H e ratios ranging from 8.5R A to 4.5RA, with most (thirteen out of eighteen) following a trend of more radiogenic 3He/aHe values with decreasing Mg#. The only exceptions to this trend are phenocrysts from Batur, Agung and Kerinci, which have MORB-like 3 H e / a H e values but relatively low M g # (Mg# = 70-71), and two highly inclusion-rich clinopyroxenes which have 3 H e / a H e values lower than other samples of similar Mg#. The results indicate that crustal contamination unrelated to subduction in the Sunda arc is clearly recorded in the 3 H e / a H e characteristics of mafic phenocrysts of subaerial volcanics, and that addition of radiogenic helium is related to low-pressure differentiation processes affecting the melts prior to eruption. These conclusions may have widespread applicability and indicate that helium isotope variations can act as an extremely sensitive tracer of upper crustal contamination.
I. Introduction C o n t a m i n a t i o n o f m a n t l e - d e r i v e d m e l t s by p r e - e x i s t i n g crust is a w e l l - d o c u m e n t e d p h e n o m e n o n in b o t h t h e o c e a n i c a n d c o n t i n e n t a l e n v i r o n m e n t s [1,2]. Its r e c o g n i t i o n is i m p o r t a n t
[uc] 1 Present address: Faculty of Earth Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands.
for distinguishing c o m p o s i t i o n a l f e a t u r e s imprinted onto magmas during their passage through t h e crust f r o m g e o c h e m i c a l a n d isotopic c h a r a c teristics which a r e d i a g n o s t i c of m a g m a s o u r c e histories. I n s u b d u c t i o n - r e l a t e d e n v i r o n m e n t s in p a r t i c u l a r such r e c o g n i t i o n is o f t e n p r o b l e m a t i c as a ' c r u s t a l ' s i g n a t u r e in arc m a g m a s m a y reflect the i n f l u e n c e o f s u b d u c t e d s e d i m e n t s in t h e source r e g i o n s in a d d i t i o n to a s s i m i l a t i o n o r rew o r k i n g o f u p p e r m o s t crust. This has l e d to a c e r t a i n d e g r e e o f d e b a t e in t h e i n t e r p r e t a t i o n of the m o s t u s e d i n d i c a t o r s o f crustal c o n t a m i n a -
0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0012-821 X(94)0013 1-H
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M. Gasparon et al. / Earth and Planetary Science Letters 126 (1994) 15-22
tion, namely radiogenic isotopes (particularly Sr and Pb), oxygen isotopes, and various trace element ratios [3-5]. Following the observation of predominantly radiogenic helium isotope ratios in some Andean lavas [6], it was suggested that helium isotope variations ( 3 H e / 4 H e ) may have the potential to recognise low-pressure (upper crustal) contamination effects in arc-related rocks. This results from the observation that in spite of large variations in sediment load being subducted, age of subducting crust and thickness of crust through which arc magmas are erupted [e.g. 7], most arc magmas analysed to date are characterised by a 3 H e / 4 H e ratio (R) close to M O R B (8R A, where R A = air 3 H e / 4 H e ) [8]. However, because of the tendency of magmas to undergo pre-eruptive degassing [9,10] and as helium is both highly incompatible and atmophilic in nature, mantle-derived helium can become depleted in magmas stalled in the crust. In such cases, magmas can become susceptible to the effects of addition of radiogenic helium ( < 0.1R A) by crustal contamination. An additional advantage of helium in this respect is that at magmatic temperatures it is expected to equilibrate with virtually all phenocrysts on relatively short (1-50 yr) timescales [11]; therefore, the conditions and mechanisms of melting (equilibrium vs. disequilibrium) should have little or no effect on the final 3 H e / a H e value. In this letter, we test these assumptions by using wellcharacterised olivine and clinopyroxene crystals from a number of volcanic centres in the Sunda arc of Indonesia. We show that because 3 H e / 4 H e ratios can be related to the degree of magma evolution, helium should be considered a tracer par excellence for the shallow-level contamination of mantle-derived melts.
2. Tectonic and petrological background The samples were selected from a number of volcanic centres from central Sumatra to Sumbawa in the Sunda arc (Fig. 1). The centres are either presently active, or have been active in the Quaternary (Ratai, Bukit Mapas, Bukit Telor, Sukadana plateau, Soromundi) [13-15]. With the
exception of Bukit Telor and the Sukadana plateau, all centres are situated between 100-200 km above the Benioff zone. The tectonic environment changes from Sumatra to Sumbawa [12]: in Sumatra, oceanic lithosphere younger than 100 Ma is subducting obliquely beneath old continental crust whereas beneath Java and the east Sunda arc the lithosphere is older and subducting in a direction almost perpendicular to the arc. The arc crust is quasi-continental in composition and thickness in west Java, whereas it is oceanic in thickness and probably also in composition in the east Sunda arc. Most samples are olivine a n d / o r clinopyroxene phenocrysts from typical medium-K calc-alkaline basalts/basaltic andesites. However, we draw attention to four exceptions: (1) Basalts from Bukit Telor are aphyric but carry small lherzolite xenoliths and olivine xenocrysts from disaggregated xenoliths. The 'fertile' nature of the xenoliths (presence of clinopyroxene), the lack of primary and secondary melt, fluid and mineral inclusions in all xenolith minerals, and the low CaO contents in olivine xenocrysts (approximately 0.07% [13]) all indicate that these xenoliths are of mantle origin and were derived from the Sumatran mantle wedge. (2) The Sukadana basalts have abundant olivine phenocrysts (up to 7% modal) set in a matrix of augite, plagioclase and olivine, and lack phenocrysts of plagioclase, hypersthene and hydrous minerals. Together with the Bukit Telor basalts, these rocks differ from the other Sunda samples in that they are located behind the arc in Sumatra and have marked petrographic, geochemical and isotopic within-plate affinities [13]. (3) The volcanics from Soromundi are highly undersaturated and K-rich, and contain modal feldspathoids (leucite + nepheline) and phenocrysts of phlogopite and amphibole. Sample 48139 contains phenocrysts of clinopyroxene + amphibole + phlogopite + magnetite, set in a groundmass composed of plagioclase + clinopyroxene + nepheline + leucite + magnetite + sanidine + apatite, and has high contents of Sr and Rb (1511 and 114 ppm respec-
M. Gasparon et al. / Earth and Planetary Science Letters 126 (1994) 15-22
17
the clinopyroxene and olivine megacrysts in ankaramites from Rinjani volcano in Lombok [16]. In general, the crystals in the present study have abundant primary and secondary fluid and melt inclusions, with the clinopyroxene samples from Soromundi (sample 48139) and Sangeang Api (sample 48067) being particularly rich. Samples of Batur, Agung, and Kerinci are virtually free of secondary inclusions, and only the olivines from Bukit Telor contain no inclusions.
tively), low HFSE, and high KzO ( K 2 0 / N a 2 0 = 1.16) contents [14,16,17]. (4) Samples 48083 and 48084 from Sangeang Api are olivine-clinopyroxenite xenoliths with idiomorphic to granoblastic texture, composed of subhedral clinopyroxene (70-95%) and anhedral olivine, and minor amounts of phlogopite. These cumulates were precipitated from hydrous alkaline basaltic magmas, presumably at relatively low pressure ( < 10 kbar) and under conditions of high f O 2 [16]. The associated volcanics range in composition from phonolitic tephrites to potassic nepheline trachyandesites, are moderately K-rich ( K 2 0 / Na20 = 1), and have high Sr and Rb (up to 1200 and 110 ppm respectively) and relatively low HFSE contents [14,16,17]. Sample 48067, also from Sangeang Api, is an olivine megacryst-bearing phonolitic tephrite with abundant large clinopyroxene and olivine crystals which are compositionally similar to
3. Results and discussion
In Table 1 we report t h e 3He/4He ratios and Mg# values (where Mg# = 100 x Mg2+/(Mg 2+ + Fe z+) for olivine and clinopyroxene phenocrysts and xenocrysts from the thirteen volcanic centres (Fig. 1). For the most part, these phases retain their inherited helium isotope signature v
0~
mn
, •K
"
~
~
Sul'awesi~'~
"'"
C
.,5 .
Indian Ocean 10
500
km
loo
,io
,,20
Fig. 1. Sketch map of the Sunda arc (after [12]), with the location of the volcanic centres discussed in the text. Location abbreviations are keyed to Table 1.
M. Gasparon et aL / Earth and Planetary Science Letters 126 (1994) 15-22
18
ing either higher or lower 3He/4He values [28] has not affected the helium isotope characteristics of this part of the mantle, and that R / R A values lower than those of Bukit Telor reflect admixture with radiogenic helium unrelated to a mantle component. Secondly, all the remaining samples have 3He/aHe values lower than Bukit Telor, ranging from 8.5R A (Agung) to 4.5R A (Soromundi). Although the higher values also fall within the MORB range, the lower ratios represent close to a 50:50 admixture of MORB and radiogenic helium. In Fig. 2, we plot the 3He/aHe values against the Mg# of the olivine and clinopyroxene crystals. Three trends emerge: (1) Irrespective of the composition of their host rocks, most samples (arrowed in Fig. 2) demonstrate a clear correlation between 3He/4He and Mg#, with the more radiogenic 3He/aHe values found in samples with lower
after eruption [11], but all samples were processed by vacuum crushing to avoid any spurious radiogenic a n d / o r spallation components. The Mg# of both minerals is used to monitor in a fairly simple way the degree of evolution of the parent melts. The first point to note is that the olivine xenocrysts from the Bukit Telor lherzolite xenoliths have the highest 3He/4He value yet reported for the Sunda arc [8,20,22], well within the (lo-) range found in MORB (Fig. 2) [23-25]. The crystals are primitive (Mg# = 90) and because they are seemingly mantle derived, represent an unambiguous indication of the MORB-Iike 3 H e / 4He nature of the Sumatran mantle wedge. This is in keeping with the general conviction that MORB-type materials are common in the source regions of arc magmatism [26,27]. Because the 3He/nile values are coincident with those of MORB, we assume that OIB-type material hav-
10
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.
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.
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Crustal Contamination
I
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.
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95
Fig. 2. M g # - H e isotope diagram. Typical MORB value is from [25]. Error bars (2~r) from Table 1 are shown for each sample. The error bar indicates the analytical error for R / R A , and the range of analyses on multiple determinations for Mg#. Of the three He duplicate analyses, only the highest ratios have been plotted. Samples plotting within error of the arrow show a decrease in R / R A for decreasing Mg#, and indicate that a greater proportion of radiogenic helium characterises more evolved magmas. Only two samples particularly rich in secondary inclusions (clinopyroxene in 48139 and 48067) show relatively low R / R A values. Three samples plot above the arrow and appear to have crystallised in a relatively closed system (at least for He), and to have had little or no perceivable interaction with crustal material.
M. Gasparon et al. / Earth and Planetary Science Letters 126 (1994) 15-22
Mg#. The process of magma ageing whereby radiogenic helium is produced within magma stored in the crust [29] is unlikely to contribute a significant proportion of the measured 4He unless storage times for Sunda arc volcanics are unusually long or radioelement contents unusually high. As argued by Hilton
19
et al. [6], estimated transfer times from magma source to eruption and U contents of ~ 2 × 104 yr and 3 ppm respectively for arc-related lavas would be expected to give an equilibrium radiogenic 4He content of ~ 0.1 nccS T P / g phenocryst for an olivine melt partition coefficient of 0.008 [30]; assuming a two-
Table 1 M g # and He isotopic ratios in mineral phases of Sunda volcanics Volcano
Sample number $
Location
Rock type #
Phase *
Mg# t
R/RA + He (nccSTP/g)
Sumatra
Kerinci (K) Ratai (R) Sukadana (Suk) Bukit Mapas (BM) Bukit Telor (BT) Dempo (D)
75267(14) 75227(5) 75215(4) 78129-34 75378-9 75260
101°18'E 1°39'S 105°02'E 5°32'S 105%9'E 5°26'S I04°19'E 4°30'S 103°45'E 1°17'S 103°14'E 4°02'S
B BA B BA B BA
ol cpx ol ol ol (X) ol
69.87-+1.78(52) 71.93-+l.59 (51) 84.24-+1.27(85) 86.53+1.63 (181) 89.65L-0.41(26) 75.85+2.09 (38)
7.6-2-0.6 6.8iO.7 7.5_+0.6 7.8~-0.7 8.8_+0.5 6.2-+0.8
2.05 1.35 2.06 2.08 7.48 1.51
(2)
105°25'E 6°06'S
BA
cpx
75.54+1.20 (50)
6.9+1.8
0.45
75416(22)
108°05'E 7°15'S
B
ol
85.50~-5.06(29)
7.3-+0.4
7.22
(23) (24)
115°22'E 8°14'S 115°30'E 8°20'S
BA BA
ol cpx
69.85-+0.58(41) 71.39-+2.01(42)
7.4_+0.7 8.5_+0.5
2.90 3.51
Soromundi (Sor) Sangeang Api (SA) Sangeang Api (SA) Sangeang Api (SA)
48139 48067 48067 48083
118°36'E 8°18'S ll9°0I'E 8°13'S l19°01'E 8°13'S 119°02'E 8°14'S
LL PT PT OC
78.85+4.93 (41) 86.58-+2.07(55) 86.61+2.59 (50) 80.51-+0.24(61)
Sangeang Api (SA) Sangeang Api (SA)
48083 48084
119°02'E l I9°01'E
8°14'S 8°13'S
OC OC
Sangeang Api (SA)
48084
l19°01'E
8°13'S
OC
cpx ol cpx ol(X) ol (duplicate) cpx (X) ol(X) ol (duplicate) cpx (X)
4.5_+0.6 6.9-+0.9 5.8--+0.8 7.0_+0.5 6.8_+0.5 6.55--0.3 6.9_+0.6 6.5-k0.5 7.1_+0.4
1.22 0.89 0.80 3.90 3.73 9.07 8.33 7.08 31.40
Floras Ebulobo (E)
67168
121°11'E
8°48'S
BA
6.0-20.8 5.3_+0.9
0.53 0.77
Sunda Strait
Krakatau (Kra) Java
Galunggung (G) Bali
Batur (B) Agung (A) Sumbawa
cpx cpx (duplicate)
80.31+1.27 (62) 79.19+0.30 (60) 81.03-+1.18(50) 74.75+1.26 (23)
* The first n u m b e r refers to the University of Tasmania catalogue number, the n u m b e r in parenthesis to the Freie Universitat Berlin number. Additional sample details can be found in [13] (Sumatra and Java), [D.R. Hilton et al., in prep.] (Sunda Strait and Bali), [14,16,17] (Sumbawa) and [18] (Flores). # B = basalt; BA = basaltic andesite; L L = leucite lamproite; P T = phonolitic tephrite; OC = olivine clinopyroxenite. * ol = olivine; cpx ~ clinopyroxene; X = xenocryst (all others are phenocrysts), t M g # is the average 100 × M g 2 + / ( M g 2 + + FEE+), calculated assuming that Fe measured is 100% Fe 2+. M g # was measured on several mineral grains ( n u m b e r in parentheses) of the same grain fraction and mineral phase used for He analyses. T h e grains were analysed using a Cameca SX50 microprobe at the Central Science Laboratory of the University of Tasmania, using a 1 / ~ m b e a m and 15 kV and 20 nA. Primary and secondary ( U S N M 111312/444 for olivine and U S N M 122142 for clinopyroxene) standards [19] were used for calibrations and corrections. Typical analytical errors for M g # values are ± 0.35 for olivine and ± 0.25 for clinopyroxene. + Helium was released in all cases by vacuum crushing and analysed following the procedure described in [20]. The reported 3 H e / 4 H e values represent an integrated signature of the different inclusion types. 3 H e / g H e values (R) are reported relative to the atmospheric value (R A = 1.4 × 10-6), and have been corrected for a minor atmospheric helium component ( < 2%) assuming that any measured neon above the blank is air derived [21]. Blank corrections for helium exceeded 5% in only three cases: Sangeang Api 48067 ol (6.2%) and cpx (5.9%), and Krakatau (6.2%).
20
M. Gasparon et al. /Earth and Planetary Science Letters 126 (1994) 15-22
component mixture between MORB-like (8R A) and radiogenic helium (0.05RA), all phenocrysts analysed in this study have considerably higher radiogenic 4He contents than this estimate. More probably, therefore, contamination by surrounding wallrock [6,25] is the source of the radiogenic helium. As differentiation processes leading to more evolved mineral compositions usually seem to be lowpressure processes which are operative in magma chambers, the observation of decreasing 3He/aHe with extent of differentiation of the mafic crystals is clear evidence of a crustal provenance for the radiogenic helium component. In this respect, the Bukit Telor sample effectively rules out a slab-derived radiogenic helium contribution at the source of the magmas. It should be pointed out, however, that because any radiogenic helium signal is enhanced by pre-eruptive degassing depleting the magmas of their original volatile inventory, 3He/aHe ratios cannot be used to model the proportion of bulk upper crustal material due to variability in both mantle and crustal end member helium concentrations. (2) The two highly inclusion-rich clinopyroxene samples (48139 and 48067) have 3He/aHe values significantly lower than other samples of similar Mg#. The relatively low R / R A values of these two samples are unlikely to be due to the presence in their source of a crust-derived component enriched in 4He: both Sangeang Api and Soromundi, like all the other volcanic centres discussed in this letter, are situated to the west of the transition zone which separates oceanic crust in the west (high R / R A) from the continental margin in the east (low R//R A) of the subducting Indian Ocean plate. Indeed, their R/R A values are considerably higher than those of east Sunda-Banda arc phenocrysts, which have suffered source contamination [20,22]. Notably, the olivines in sample 48067 do fall within the M g # - R / R A trend defined by the other Sunda arc volcanics, and only the clinopyroxenes, which have about 10 x more inclusions than the olivines in equilibrium with them, have relatively low R//RA values.
Crystallographic evidence indicates that sample 48067 equilibrated at submagmatic temperatures ( ~ 600°C [G.-M. Molin and R. Varne, unpublished]), so that the large difference in 3He/nile values between olivines and clinopyroxenes may reflect changing fluid conditions in the magma chamber and the late-stage dominance of a crustal fluid flux. In this case, the enhanced crustal input is more likely to be recorded in clinopyroxenes because of their higher helium diffusivity [11]. (3) Phenocrysts from three volcanic centres (Batur, Agung, Kerinci) have relatively low Mg# (70-71) but MORB-Iike 3He/4He values indicating that they evolved in a system without large degrees of contamination by 4He-rich material. For Batur, there is compositional [18] and U-Th evidence [31] that the magmas underwent closed-system fractionation and that transit times from source to surface were exceedingly short (< 8 kyr) [32]. In such circumstances, shallow-level (magma chamber) degassing of the mantle-derived volatile component may have been inhibited so that, in spite of the differentiated nature of the magmas, contamination with radiogenic helium would not have led to a perceivable shift in 3He/nile values. We can only conjecture that a similar scenario applies also to Agung and Kerinci because, like Batur, their phenocrysts also have relatively high helium concentrations given that they also are virtually free of secondary melt and fluid inclusions. The important point about these samples, however, is that they show that low Mg# is not necesarily associated with low 3He/4He ratios, and that among the geochemically evolved magmas only those which have undergone a relatively closed-system fractionation may retain the He isotopic signature of their source.
4. Conclusions
Petrography, mineral chemistry and helium isotopes have been used together to identify cases where a crustal component has contaminated magmas after their generation. This approach has
M. Gasparon et al. / Earth and Planetary Science Letters 126 (1994) 15-22
great potential, therefore, in the investigation of the c i r c u m s t a n c e s a n d timing of crustal c o n t a m i n a t i o n a n d for identifying m a g m a s likely to have suffered m i n i m a l i n t e r a c t i o n e n r o u t e to the surface. Criteria such as M O R B - I i k e 3 H e / a H e ratios in d i f f e r e n t i a t e d m a g m a s a n d h e l i u m isotope e q u i l i b r i u m b e t w e e n c o g e n e t i c olivine a n d clinopyroxene phenocrysts a p p e a r most useful in this respect. It m u s t be e m p h a s i s e d that a l t h o u g h some m a g m a s may have acquired some of their ' c r u s t a l ' geochemical a n d isotopic characteristics by s u b d u c t i o n - r e l a t e d source c o n t a m i n a t i o n , the p r o v e n a n c e of the crustal (radiogenic) h e l i u m in most arc e n v i r o n m e n t s (the east S u n d a - B a n d a arc b e i n g a n o t a b l e exception [8,20,22]) is restricted to the u p p e r m o s t crust. T h e observation that p r o v e n a n c e may vary for different crustal tracers could provide the e x p l a n a t i o n for the phen o m e n o n [cf. 6] that i n f o r m a t i o n from h e l i u m isotopes is f r e q u e n t l y at odds with, or ' d e c o u p l e d ' from, that of o t h e r geochemical indicators.
Acknowledgements R.V. a n d M.G. gratefully acknowledge the fin a n c i a l s u p p o r t of the U n i v e r s i t y of T a s m a n i a a n d the A u s t r a l i a n R e s e a r c h Council. D . R . H . t h a n k s the G e r m a n Science F o u n d a t i o n ( D F G ) for f u n d i n g , Pat a n d Sue Jonklaas for hospitality in J a k a r t a a n d for a r r a n g i n g the K r a k a t a u a n d Bali trips, a n d K. H a m m e r s c h m i d t a n d H. F r i e d r i c h s e n for s u p p o r t in the laboratory. W e also t h a n k the I n d o n e s i a n I n s t i t u t e of Sciences (LIPI), the I n d o n e s i a n Geological R e s e a r c h a n d Development Centre (GRDC), Kardana Hardjadi n a t a a n d Sigit M a r y a n t o for their efforts in the field, a n d J o n D a v i d s o n a n d two a n o n y m o u s reviewers for c o m m e n t s o n the m a n u s c r i p t .
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