Department of Geology, University of Tasmania, P.O. Box 252c, Hobart, Tasmania, Australia. Abstract. A detailed examination of the hypothesis that.
Contributions to Mineralogy and Petrology
Contrib Mineral Petrol (1987) 97:417-430
9 Springer-Verlag1987
The origin of island arc high-alumina basalts A.J. Crawford, T.J. Falloon, and S. Eggins Department of Geology, University of Tasmania, P.O. Box 252c, Hobart, Tasmania, Australia Abstract. A detailed examination of the hypothesis that high-alumina basalts (HAB) in island arcs are primary magmas derived by 50-60% partial melting of subducted ocean crust eclogite shows that this model is unlikely to be viable. Evidence suggests that the overwhelming majority of arc HAB are porphyritic lavas, enriched in A1203 either by protracted prior crystallization of olivine and clinopyroxene, or by plagioclase phenocryst accumulation in magmas of basaltic andesite to dacite composition. Experimentallydetermined phase relationships of such plagioclase-enriched (non-liquid) compositions have little bearing on the petrogenesis of arc magmas, and do not rule out the possibility that arc HAB can be derived by fractionation of more primitive arc lavas. Although models invoking eclogite-melting can match typical arc HAB REE patterns, calculations indicate that the Ni and Cr contents of proposed Aleutian primary HAB are many times lower than such models predict. In contrast, Ni vs Sc and Cr vs Sc trends for arc HAB are readily explained by olivine (+Cr-sp) and clinopyroxene-dominated fractionation from more primitive arc magmas. G E N M I X major element modelling of several HAB compositions as partial melts of MORB eclogite, using appropriate experimentally (26-34 kb)-determined garnet and omphacite compositions yields exceptionally poor matches, especially for CaO, Na20, MgO and A12Oa. These mismatches are easily explained if the HAB are plagioclaseaccumulative. Groundmasses of arc HAB are shown to vary from basaltic andesite to dacite in composition. Crystal fractionation driving liquid compositions toward dacite involves important plagioclase separation, resulting in development of significant negative Eu anomalies in more evolved lavas. Plagioclase accumulation in such evolved liquids tends to diminish or eliminate negative Eu anomalies. Therefore, the absence of positive Eu anomaly in a plagioclase-phyric HAB does not indicate that plagioclase has not accumulated in that lava. In addition, we show that plagioclase phenocrysts in arc HAB are not in equilibrium with the liquids in which they were carried (groundmass). Exceptional volumes of picrite and olivine basalt occur in the Solomons and Vanuatu arcs; the presence in lavas from these and other arcs (Aleutian, Tonga) of olivine phenocrysts to F o 9 4 , finds no ready explanation in the primary eclogite-derived HAB model. We suggest that most lavas in intra-oceanic arcs are derived from parental magmas with Offprint requests to: A.J. Crawford
> 10% MgO; fractionation of olivine ( + Cr-sp) and clinopyroxene drives liquids to basalt compositions with < 7% MgO, but plagioclase nucleation is delayed by their low but significant ( < 1% ?) H 2 0 contents. Thus evolved liquid compositions in the basaltic andesite - andesite range may achieve relatively high A1203 contents ( < 17.5%). The majority of arc basalts, however, have A12Oa contents in excess of 18%, refleeting plagioclase accumulation. We give new experimental data to show that HAB liquids may be generated by anhydrous, low-degree ( < 10%) partial melting of peridotite at P < 18 kb. Relative to arc HAB, these experimental melts have notably higher M g ~ (69-72) and are in equilibrium with olivine F O 8 7 - 8 9 Only further detailed trace element modelling will show if they might be parental magmas for some arc HAB.
Introduction High-A120a basalts (HAB) are volumentrically important lavas in many intra-oceanic island arcs (e.g., South Sandwich, Mariana, Aleutian, Vanuatu, Tonga-Kermadec and New Britain arcs), and some controversy surrounds their petrogenesis. Most petrologists and geochemists presently believe that HAB are derivative following extensive fractionation dominated by olivine and clinopyrox~ne from a primary magma generated by partial melting of peridotite in the mantle wedge above the descending slab of subducted ocean crust. However, others (Marsh 1979a, 1982; Brophy and Marsh 1986; Johnston 1986; Brophy 1986; Myers et al. 1986a, b) have argued persistently that HAB are primary arc magmas generated by partial melting of subducted oceanic crust. In this paper, we critically re-evaluate the primary HAB hypothesis and provide evidence that HAB are derivative lavas (often plagioclase-accumulative) of more primitive magmas containing 1(L15% MgO, derived by partial melting of peridotite in the mantle wedge above the subducted slab.
High-A1 basalts Historical perspective
For the purposes of this discussion, we consider HAB to be lavas with 16.5% A1203, and usually 55, all three of Kuno's primary magma types in the Japan arc have close to 17% A1203, very similar to values for primitive MORB (Perfit et al. 1980). Yoder and Tilley (1962) concluded that HAB petrogenesis probably involves the concentration of plagioclase and considered plagioclase flotation to be possible but not probable. They suggested that a process suppressing the appearance of plagioclase, such as protracted history of olivine fractionation, could drive residual basaltic liquids into the HAB compositional spectrum; further, they noted that high water pressure in a magma significantly delays crystallization of plagioclase. Finally, Green et al. (1967) showed, in a series of anhydrous experiments at 0-9 kb on a number of tholeiite and alkali basalt compositions, that HAB could be derived from parental olivine tholeiite or alkali olivine basalt magmas by fractionation of dominantly olivine in the depth range 15-35 km (4-9 kb). While Tilley (1950) and Kuno (1960) defined HAB on the basis of aphyric lava (i.e., liquid) compositions, the great majority of arc lavas subsequently classified as HAB are strongly porphyritic, often with 30-60 modal% of phenocrysts, of which plagioclase is usually dominant (Ewart 1982). Compositions of such porphyritic lavas are therefore unlikely to represent liquid compositions, unless each lava sampled has fortuitously maintained the liquid composition by retaining phenocrysts in eotectic proportions. Given the generally turbulent and volatile-charged nature of eruptive processes operative in arc volcanoes (witnessed by the immense volumes and high proportion of pyroclastics in arc sequences; Miyashiro 1975), and the specific gravity contrast between heavier olivine and pyroxene and lighter plagioclase, the likelihood of a magma retaining phenocrysts in cotectic proportions seems remote. Plagioclase phenocryst compositions in HAB fall typically between An95 and An75, and these crystals often, but not always (Brophy 1986) show complex and variable compositional zoning, sieve textures and evidence of resorption and overgrowths (Ewart 1982). Olivine phenocryst compositions range from Foss to around Fo7o and individual lavas often contain phenocrysts ranging in core compositions by more than 15 tool% Fo. The majority of olivine phenocrysts in arc HAB are generally more Fe~ than predicted by olivine-liquid Kd's between 0.29 and 0.33 (Kay and Kay 1985a; Brophy 1986), an observation referred to as the "olivine problem" by Brophy (1986). Clinopyroxene phenocrysts usually show less compositional variation than olivine or plagioclase, and Mg~ ratios cluster around 85 to 75. However, many reports of complexly-zoned clinopyroxene phenocrysts in HAB suggest widespread involvement of magma mixing in the petrogenesis of these lavas (e.g., Sakuyama 1981; Kay and Kay 1985a). Orthopyroxene phenocrysts are sometimes present in more evolved HAB, and range in Mg~ between 65 and 75. Phenocrysts and groundmass grains of titanomagnetite are ubiquitous in HAB.
The model for primary, eclogite-derived HAB The hypothesis that H A B are primary magmas derived by partial melting o f subducted oceanic crust has had a chequered history. Recently, Marsh (1976, 1979a, b; t982) and coworkers (Hsui et al. 1983; Myers et al. 1986a, b; Brophy 1986; Brophy and Marsh 1986), and Johnston (1986) have rekindled interest in the primary H A B hypothesis by providing a set of arguments which support a quartz eclogite source for primary H A B magmas. Key arguments used by advocates o f the primary H A B arc magma hypothesis have been concisely stated and discussed by Johnston (1986) and
Brophy and Marsh (1986). It is appropriate here to review major aspects and arguments of the primary H A B model before attempting to refute it. Subducted oceanic crust, containing around 5 wt% pelagic sediment in its upper portion, is converted to quartz eclogite before it arrives beneath the arc volcanic chain (generally at depths of 100-150 kin). In this region, limited partial melting o f the eclogitic slab initiates diapirism and it commences rising into hotter, overlying mantle. Following ascent of 10-20 kin, at temperatures around 1400 ~ C, 50-60% of essentially anhydrous partial melting o f the quartz eclogite slab eliminates garnet from the residue and generates a primary H A B m a g m a with 50-52% SiOz, > 17% AlzO3, 4 - 7 % M g O and probably < 1% H 2 0 . This superheated m a g m a ascends without fractionation to around 10-20 km depth, where it pools in a m a g m a chamber and commences fractionating via the crystallization sequence plagioclase - titanomagnetite - olivine and clinopyroxene. Residual liquids are driven toward andesitic compositions. Isotopic data and rare earth element (REE) modelling by Von Drach et al. (1986), Brophy and Marsh (1986) and ourselves (REE ... see later) are broadly consistent with the primary H A B model summarized above.
A critical appraisal of pro-primary HAB arguments Arguments concerning the ultimate origin o f arc H A B reduce essentially to whether or not these rocks can be considered to represent liquid compositions (primary) or plagioclase-accumulative, evolved compositions (derivative). The most frequently cited and heavily weighted pro-primary H A B argument has been the claim that experimental petrologic studies of typical H A B compositions cannot drive olivine onto the liquidus under any reasonable conditions; thus, H A B cannot apparently be derived from either peridotitic mantle, or a more marie parent magma by olivine (or olivine and clinopyroxene) fractionation (Baker and Eggler 1983; Johnston 1986; Brophy and Marsh 1986; Brophy 1986). The veracity of this argument is entirely dependent on the existence in arcs o f H A B liquids, and whether or not the H A B used in the melting studies of Baker and Eggler (1982) and Johnston (1986) are liquid compositions. We note that AT-l, the Aleutian H A B used in melting studies by Baker and Eggler (1983) contains 30 modal% plagioclase phenocrysts, while the more marie South Sandwich arc H A B SSS.1.4, studied by Johnston (1986), has " a b o u t 10% phenocrysts of the assemblage plagioclase-olivine-augite". We claim that the great majority o f arc H A B (which usually contain 20-40 modal % plagioclase; Ewart 1982), including those studied experimentally, are not liquid compositions, but are enriched in normative plagioclase, either via direct accumulation of plagioclase phenocrysts, or suppression of plagioclase appearance due to crystallization at 5-10 kb (Green et al. 1967; Gust and Perfit 1987) or in the presence of water (Yoder and Tilley 1962). However, pro-primary H A B workers have put forward a series of arguments which they claim rule out the possibility that arc H A B have inherited their important and diagnostic chemical features (high A1203, low MgO, Ni, Cr) by plagioclase accumulation and concentration in a magma. Reconsideration of pertinent phase equilibria, and some important trace element characteristics of arc H A B eliminates both the above objection and the possibility that H A B are primary, eclogite-derived magmas.
419 [ISLANg RAC BRSALTSII
%
22 21 20 19 18 AI203 17 16 15 14 13 12
,].-..:..:.: .;!!;?9
r
9
9 ,~.ooO
o.
"go
9
o
i
i
t
5
10
15
i
t
20 25 Modal % Flag
I
I
i
i
30
35
40
45
Fig. 1. Modal % plagioclase phenocrysts vs. wholerock A1203 contents for 115 arc basalts from intra-oceanic island arc volcanoes. Sources of data are papers referenced in text, plus Baker (t978), Carney and Macfarlane (1979), Dixon and Batiza (1979), Heming (1974), Johnson and Arculus (1978), and Johnson and Chappell (1978), Meijer and Reagan (1981), Stern (1979) and Stern and Bibee (1984)
I Correlation between modal % plagioclase and wholerock Al203
A compilation of available data for intra-oceanic arc basalts (Fig. 1) shows a significant correlation between wholerock A1203 contents and modal % plagioclase phenocrysts. The same correlation is observed for basalts erupted in continental margin arcs (Myers et al. 1984; Walker and Carr 1986). A few arc basalt suites (HAB from Adak Is. in the Aleutians; Myers et al. 1986a) do not show this correlation. Those HAB with exceptionally high A1203 contents ( > 19%) almost always contain >25 modal % plagioclase phenocyrsts. That this is not due to their having crystallized from a high A1203 parent magma is clearly shown by the fact that basalts with < 5 modal % plagioclase (i.e., nearliquid compositions) never have > 17.5% AlzO3. One possible explanation of the poor correlation between A1203 contents and modal amounts of plagioclase in some (e.g., Adak) suites of highly plagioclase-phyric lavas, is that they represent plagioclase accumulation in series of residual liquids bearing varying proportions of clinopyroxene, olivine and F e - T i oxide phenocrysts, rather than in a single liquid composition. This is clearly demonstrated by analyses of groundmass compositions of a suite of comagmatic HAB (see later). 2 Does crystalfractionation occur in H A B magmas ?
Marsh (1981) suggested by plotting the modal amount of each phenocryst phase against modal % groundmass (melt), that plagioclase is the liquidus phase of Aleutian HAB. The validity of this approach (and its conclusions) is totally negated, however, if any differential movement (fractionation) between phenocrysts phases and melt has occurred. Abundant evidence suggests that fractionation is more the rule than the exception for arc magmatic systems. Firstly, the frequent occurrence of cognate monomineralic (e.g., dunite, clinopyroxenite, hornblendite) or bimineralic (olivine clinopyroxenite, gabbro) inclusions in arc lavas (e.g., Arculus and Wills 1980; Conrad and Kay 1984; Foden 1983), and the striking cumulate layering in arc-related Alaskan-type ultramafic complexes (neither of which suggest that plagioclase is a near-liquidus phase in arc basalts)
indicate that effective crystal-liquid fractionation occurs in arc magmatic systems. Secondly, the numerous, well documented occurrences of temporally variable lava composition during the course of a single eruptive event in arc volcanoes strongly implies the existence of crystal-liquid fractionation in sub-arc magma chambers. For example, the 1974 Fuego eruption (Rose et al. 1978) yielded initially plagioclase-rich, mafic phenocryst-poor lavas, while subsequent continuing eruptions became progressively more mafic crystal-rich, indicating a magma chamber zoned with respect to phenocryst concentrations in a subvolcanic magma chamber. Rose et al. (1978) considered that plagioclase may have become enriched in the upper portion of the magma chamber by flotation. 3 The significance o f H A B groundmass compositions
Brophy (1986) attempted to show, using the projection in Fig. 2, that Aleutian HAB are not plagioclase-accumularive. He argued that plagioclase addition to primitive arc basalts should generate HAB compositions plotting on a mixing line between primitive lavas and the plagioclase apex (Fig. 2c), whereas Aleutian HAB plot well away from this trend. We have plotted in the same projection all available analyses of aphyric lavas from intra-oceanic arcs (Fig. 2a). These lavas define a trend parallel to, and between, the experimentally-determined anhydrous 1 atm (Walker et al. 1979) and 8 kb (Baker and Eggler 1983) cotectics; this trend also plots parallel to and on the lower pressure, lower H20 content side of the 0.5-2 kb, 2-3% H20 boundary curve determined by Baker and Eggler (1983). We believe that this trend defined by liquid compositions erupted from arc volcanoes is best explained as a low pressure ( < 5 kb), low H20 content ( < 2%) cotectic, marking a broad liquid line of descent for arc magmas. We have also plotted carefully determined groundmass compositions (Table l) of some typical arc HAB (Fig. 2b), and groundmass compositions of the Cold Bay and Hakone HAB (Fig. 2c) calculated from published average phenocryst compositions and modal data. We are aware that the latter, calculated liquid compositions must be, at best, broad approximations to the real liquid (groundmass) compositions. However, in the absence of measured groundmass compositions for these lavas, we have used these calculated liquid compositions in our projections, and significantly, the implications of the projected (Fig. 2 c) calculated liquid compositions are identical to those of the accurately measured liquid compositions. Importantly, measured and calculated HAB groundmass compositions are not like relatively primitive arc lavas, but rather, are evolved compositions varying from basaltic andesite to dacite. Brophy (1986) suggested that the elongate field defined by Cold Bay HAB compositions in this projection (Fig. 2c) may represent removal of plagioclase, olivine and clinopyroxene from a more primitive magma along a high-P (8 kb) cotectic. Rather, we suggest this elongate trend is an artifact of addition of subequal amounts of plagioclase to variable liquid compositions (Table 1) which fall along the natural arc lava cotectic in Fig. 2a. Groundmass (liquid) - wholerock tieIines clearly show that H A B compositions are controlled by plagioclase (plus subordinate cpx or olivine) addition to these evolved melt compositions.
420 Qtz/Or
Qtz/Or
Otz/Or
a /
Rhyolites
I "'~.\
b
// Cold Bay Hakone
9 0 9
Basalts
.#/ Vanuatu Indonesia Strornboli Fuego
Intra-oceanic Arc Aphyric Lavas
Di
9 o * 9 9 0
Plag - - - ~ - /
Di
/
~/8
~
' ~ - - - ~ ~
a'Plag
Plag
Fig. 2a--e. Pseudoternary (tool%) system clinopyroxene-plagioclase-quartz+orthoclase projected from olivine (+magnetite) (Baker and Eggler 1983), showing trend of compositions of aphyric, intra-oceanic island arc lavas (basalt to rhyolite), and the I atm (Walker et al. 1979) and 8 kb anhydrous, and 0.5-2 kb, 2-3% H20 (Baker and Eggler 1983) cotectic curves. Fe203/FeO calculated as 0.15. Data sources are in references, and include samples from the Mariana, New Britain, Vanuatu, South Sandwich, Tonga and Aeolian arcs. b Compositions of wholerock are HAB and their carefully-determined groundmass compositions (Table 1) plotted in same projection as Fig. 2a. Note that wholerock compositions correspond to basaltic andesite to dacite liquids to which have been added variable but large (30-60%) amounts of plagioclase-dominated phenocryst populations, e Same projections as in Fig. 2a, showing wbolerock and calculated groundmass compositions (Table 1) for Cold Bay, Aleutian arc (Brophy 1986) and Hakone volcano, Japan (Kuno 1950) HAB, and fields of Atka (Aleutian) HAB (Myers et al. 1986a) and some primitive Aleutian arc lavas from Okmok (Nye and Reid 1986). The apparent trend of the Cold Bay HAB wholerock compositions close to the 8 kb cotectic in this figure and Fig. 2b is clearly due to addition of subequal amounts of plagioclase-dominated phenocryst assemblages to groundmass (liquid) compositions which vary from basaltic andesite to evolved andesite compositions
4 Plagioclase phenocryst- liquid disequilibrium in HAB Plagioclase phenocryst compositions in the Cold Bay and other HAB (typically AnTs- 98) are not in equilibrium with, and could not have crystallized from, liquids with the Ca/ (Ca + Na) ratios of their host groundmasses (Drake 1976; Falloon and Green 1986). Plagioclase phenocrysts in equilibrium with groundmass liquid compositions of Cold Bay HAB should be 1200 ~ C) conditions to yield a typical HAB composition. We have collated garnet and omphacite compositions from basalt-eclogite melting studies from 25 to 34 kbar (Table 3), and found that with the exception of M g ~ (which is directly proportional to the starting material Mg ~), both garnet and clinopyroxene show relatively limited compositional variation in this pressure range. We chose phase compositions with Mg ~ ratios at the low M g ~ (~0.6) end of the range for MORB-derived eclogite, and used the G E N M I X program (Le Maitre 1980) to test if mixes of these garnet and omphacite compo-
Most recent syntheses conclude that arc magma petrogenesis is a multistage, multi-source process, requiring at least two, and probably more, source components. The sources for the primary eclogite-derived HAB hypothesis are altered MORB and pelagic sediments, while for the mantle-derived model, depleted suboceanic mantle peridotite provides most of the magma volume. However, LILE (e.g., Ba, K, Rb, Sr and LREE) and isotopic constraints demand some contribution from subducted, altered ocean crust plus minor amounts of oceanic sediments (Gill 1981 ; Green 1980). In the latter model, dehydration and possibly partial melting (Nicholls and Ringwood 1973) of altered oceanic crust provide fluids which metasomatize overlying mantle peridotite and probably initiate diapirism and partial melting. Clearly, an elusive but ubiquitous LILE-enriched crustal component plays an important role in arc magma genesis, whether the preferred model requires an eclogitic or a peridotite source. For this reason, we believe that LILE contents and N d S r - Pb isotopic systematics of arc HAB are non-diagnostic and of little value in determining which source is more likely for HAB. One of the most common criteria used to rule out an eclogite source for arc HAB is their REE patterns, which typically show virtually flat chondrite-normalized HREE. However, partial melting of eclogite leaving residual garnet would produce significant H R E E depletions (Gill 1974). The primary eclogite-derived HAB model of Brophy and Marsh (1986) circumvents this problem by invoking extensive (50-60%) partial melting of eclogite in a rising diapir, to the point where garnet is eliminated from the residue. We have used recently published REE Kd's and the nonmodal melting equation of Shaw (1970) to test the feasibility of this model. We find that REE patterns broadly similar
424 Table 3. Representative analyses of garnet and clinopyroxene crystallized from basaltic compositions at pressures greater than 26 kb No.
1
2
3
4
5
Garnet Pressure Temp. SiO2 TiO2 A1203 FeO MnO MgO CaO Na20
34 1450 40.3 1.1 21.1 15.7 0.4t 12.5 8.31 0.11
36 1520 40.0 1.2 22.8 13.8 13.3 8.9 -
36 1490 39.0 1.3 22.7 13.8 11.4 8.3 -
30 1300 40.7 2.2 21.2 14.5 0.2 13.5 7.4 0.2
30 1200 40.4 1.8 21.3 15.7 13.0 7.4 0.2
Charge M g ~
0.47
Clinopyroxene Pressure 34 Temperature 1350 SiO2 50.7 TiOz 1.47 A12Oa 11.4 FeO 10.0 MnO 0.16 MgO 9.57 CaO 13.2 Na20 3.42 Charge Mg #
0.47
0.61
0.6
0.55
6
7
8
9
10
30 1400 40.2 0.76 23.0 16.3 0.41 11.0 9.7 .
30 1200 40.4 0.3 22.9 12.3 13.0 10.4 . .
30 1200 40.9
26 1380 40.0 0.9 22.2 14.2 0.39 14.1 7.39
35 1460 39.9 1.1 22.4 16.4 0.4 10.5 9.2
0.60
0.58
0.55
36 1520 48.6 0.9 14.6 6.0 . 10.2 14.3 2.9
36 1490 50.5 0.9 14.3 5.6 . . 10.3 14.4 2.8
30 1300 51.9 2.2 13.3 6.0 . 9.2 13.5 4.0
30 t200 51.9 2.9 12.4 4.9
0.61
0.60
0.55
0.55
9.1 13.9 4.7
30 1400 48.9 0.37 16.6 7.74 0.17 9.68
21.8 16.6 12.3 7.86 . . 0.70
1.81
30 1200 46.9 1.3 12.7 8.4 10.2 17.9 1.5
27 1400 50.5 1.2 13.0 6.2 11.0 15.3 2.3
0.60
0.58
0.60
15.1
0.58 26 1325 48.6 1.75 12.5 11.6 0.2 10.1
13.5 2.39 0.58
0.52
Aver. 40.2 1.2 22.1 14.9 0.36 12.5 8.5 17%) basalt parent? Composition of highA1, low degree ( < 10%) equilibrium partial melts from anhydrous melting experiments on MORB pyrolite at P < 18 kbar (Falloon and Green, in preparation), spinel lherzolite KLB-1 (Takahashi 1986) and spinel lherzolite PMM-1 (Fujii and Scarfe 1985) are given in Table 5. In most respects, such as high AlzO3 ( > 17%) contents and low CaO/ Na20 ( < 5 ) r a t i o s , these low degree, equilibrium partial melts of peridotite are compositionally very similar to HAB. However, a major compositional difference between typical arc HAB and the experimental melts is the notably higher Mg :~ of the latter (69-72), which correspond to a residual olivine composition of Fo87 89. It is possible that low Mg :~ HAB might be fractionation products following extraction of olivine ( < Fo89), clinopyroxene and perhaps plagioclase from a primary HAB magma of composition similar to those listed in Table 5. The exact crystallization sequence of such high-Mg:~, high-A1 HAB parental magmas can only be determined via a series of liquidus experiments under appropriate conditions, which we are presently investigating.
Table 5. Experimentally produced high-A1 basaltic liquids generated by anhydrous partial melting of peridotite compositions No
1
2
3
4
SiO2 TiOz A1203 FeO MgO CaO Na20
50.9 0.81 19.2 6.8 8.5 10.5 3.3
50.4 0.77 17.1 7.5 10.4 11.5 2.2
49.2 0.60 17.7 6.7 9.5 11.4 2.9
48.8 0.88 17.5 7.8 9.8 10.8 3.l
Mg~
0.69
0.71
0.72
0.69
Liquid compositions are all in equilibrium with a spinel lherzolite residue. 1 and 2 respectively. 10 kb, 1300~ C MORB Pyrolite and 10 kb 1350~ C MORB Pyrolite (both MPY-87, Falloon and Green in preparation). 3 10 kb 1325~ C KLB-1 (Takahashi 1986). 4 10 kb 1250~ C PMM-I (Fujii and Scarfe/985)
Conclusion We believe that the most viable model for magma generation in intraoceanic is that of Nicholls and Ringwood (1973) and Ringwood (1975). In this model, dehydration fluids and hydrous partial melts of slab eclogite ascend into, and react with, peridotite in the overlying mantle wedge, which partially melts to generate picritic or magnesian basaltic parental arc magmas. Extensive fractionation during ascent of these primitive magmas, dominated initially by olivine and clinopyroxene and later, by clinopyroxene, plagioclase and F e - T i oxides produces a series of residual liquid compositions from basalt to rhyolite. Accumulation of plagioclase phenocrysts in such residual liquids yields typical arc high-alumina basalt compositions. We conclude on the basis of petrographic, geochemical and experimental petrological evidence that high-A1 basalts in island arcs are not primary, eclogite-derived magmas. Acknowledgements. We thank Drs. Mike Perfit and James Brophy for detailed, constructive reviews of the first draft of this paper, and Dr. Tim Grove for efficient editorial handling. The senior author acknowledges financial support from a Queen Elizabeth II Post-doctoral Research Fellowship.
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