Na depletion in modern adakites via melt / rock reaction within the sub-arc mantle
Introduction
Mantle metasomatism is of critical importance in the study of subduction magmatism. It is widely believed that arc volcanic rocks were predominately derived from the mantle peridotite metasomatised by a fluid or melt derived from the subducting slab. In the past 10 years, mantle metasomatism by slab-derived melts has been highlighted in discussions of the origin of some Na-rich arc magmas (see Section 6 for the details) and considerable attention has been given to the nature of the slab-derived melt and to processes that occur in the mantle wedge (Yogodzinski et al., 1995, Kepezhinskas et al., 1995, Kepezhinskas et al., 1996, Schiano et al., 1995, Kelemen, 1995, Kelemen et al., 1998, Kelemen et al., 2004, Drummond et al., 1996, Rapp et al., 1999, Sajona et al., 2000, Prouteau et al., 2001, Defant et al., 2002, Bourdon et al., 2002, Grove et al., 2002, Grove et al., 2003, Tsuchiya et al., 2005).
During subduction the oceanic crust undergoes progressive metamorphism from greenschist to amphibolite and finally to the eclogite facies. It has been argued that, in general, the slab does not melt under normal thermal conditions of subduction zones, but dehydrates, releasing large ion lithophile element (LILE)-enriched hydrous fluids that metasomatise the overlying mantle wedge and instigate its melting (e.g. Gill, 1981, Kushiro, 1990, Tatsumi and Kogiso, 1997). Scenarios may be different, however, when the subducting slab is already hot or heated up, such as in the cases of young (Defant and Drummond, 1990, Drummond and Defant, 1990), fast and oblique (Yogodzinski et al., 1995, Yogodzinski et al., 2001) or flat subduction (Gutscher et al., 2000), or when slab melting is fluxed by water from subjacent hydrous lithologies (Prouteau et al., 1999, Prouteau et al., 2001). In these special cases, the slab geotherm may intersect the wet solidus of amphibolite/eclogite and the slab may melt to produce sodic felsic melts with high Sr but low Y and heavy REE concentrations. Defant and Drummond (1990) identified andesites and dacites in Cenozoic arcs with such slab-melt compositions and termed these rocks adakites after Adak Island in the Aleutians where they were first described by Kay (1978). Other examples are found in the Cascades, Baja California, Central America, south Andes, the Philippines, SW Japan, and the Kamchatka arcs (see Defant et al., 2002 and references therein). The ever-increasing number of locations where adakites are described suggests that conditions for slab melting are realized more often than previously believed (Defant et al., 2002).
During ascent to the surface, slab melt will react with the hot mantle peridotite through which it migrates and may even be consumed entirely via metasomatism (Beard et al., 1993, Rapp et al., 1999, Kelemen et al., 2004, Killian and Stern, 2002). Kay (1978) first noted anomalously high MgO, Ni and Cr concentrations in adakites from the Aleutian arc and attributed these features to interaction with the mantle wedge. Most adakites found since then in modern arcs appear to be the case, with more or less contamination by mantle components (Yogodzinski et al., 1995, Kelemen, 1995, Kelemen et al., 2004, Drummond et al., 1996, Stern and Killian, 1996, Sajona et al., 2000, Xu et al., 2000, Xu et al., 2002, Bourdon et al., 2002, Gao et al., 2004). By comparison to experimentally produced partial melts of basalt, Sen and Dunn (1994a) also noted enrichment of CaO in addition to MgO in most adakites. Killian and Stern (2002) demonstrated that experimentally produced trondhjemitic slab melts (CaO / Na2O < 1) can become tonalitic (CaO / Na2O > 1) via selective assimilation of mainly clinopyroxene + spinel to explain the high MgO and CaO characteristics of adakitic glasses in the mantle xenoliths from Cerro del Fraile. The compositional anomalies in modern adakites indicate addition of mantle components to slab-derived melts. However, compositional contribution from slab melts to the mantle during the interaction is more essential in understanding the role of slab melt as a metasomatizing agent. Kelemen et al., 1992, Kelemen et al., 1998 noted silica enrichment in some peridotite xenoliths and attributed this to reaction of the mantle lithosphere with ascending melts. Here we have noted Na depletion in Cenozoic adakites relative to experimentally produced melts of hydrated basalt under P–T conditions most relevant to adakite generation. Average Cenozoic adakite is lower by 1–3% in Na2O content than these “primitive” slab-melts. In this paper, we relate the Na depletion in Cenozoic adakites to melt / rock reaction within the mantle wedge, and discuss the possible reaction mechanism responsible for this Na depletion and the implication to the mantle sources of arc magmas.
Section snippets
Constraints of geochemisty and phase relation on the depth for adakite production
According to Defant and Drummond, 1990, Defant and Drummond, 1993 and Drummond et al. (1996), the distinctive geochemical characteristics of adakite include SiO2 > 56 wt.%, Al2O3 > 15 wt.% (rarely lower), high Sr (> 400 ppm and Sr positive anomaly), low Y (< 18 ppm) and HREE (Yb < 1.8 ppm) and thus high Sr / Y (> 20–40) and La / Yb (> 20) ratios, low HFSE (negative Nb–Ta and Ti anomalies), and small or negligible Eu anomaly. These geochemical characteristics provide the best constraint on the mineralogy of
Na depletion in adakites
Fig. 2(A), (B) and (C) show the compositions of adakites, experimentally produced partial melts of hydrated or hydrous basalt (referred to as experimental slab melts hereafter) and adakitic glasses preserved in mantle xenoliths (referred to as natural slab melts hereafter), respectively, in ternary feldspar diagrams. The adakites (13 localities, 202 samples drawn from the literature) are those found in modern (Cenozoic) arc settings. The experimental slab melts (83 analyses of quenched glasses)
Effect of protolith composition and P–T conditions on the Na2O content of slab melt
Experimental workers have used many natural and synthetic materials compositionally representative of variably altered and metamorphosed MORB to study the high-pressure melting behavior of basalt under fluid-absent or fluid-present conditions. Table 2 gives the protolith compositions for the experimental slab melts in Table 1 and Fig. 2, Fig. 3. Most of these starting materials (Table 2) are compositionally close to the average N-MORB (Hofmann, 1988), exceptions are the alkali-rich basalt from
Melt / rock reaction and the “fate” of slab melt in the mantle wedge
Slab-derived siliceous melts strongly contrast to ultramafic mantle peridotite in composition and they will be in chemical disequilibrium with hot peridotite during their passage through or residence in the mantle wedge (Beard et al., 1993). Extensive interaction between these melts and peridotite is expected and few will reach the surface unchanged; some even may be consumed entirely via reaction, depending on the thermal regime, melt mass and melt / rock (reactant) ratio in the mantle wedge.
Implication for mantle metasomatism and sources of arc magmas
Metasomatism of mantle wedge via slab melt is of particular importance in subduction zones. Ringwood and coworkers (Green and Ringwood, 1972, Nicholls and Ringwood, 1973; reviewed by Ringwood, 1974, Ringwood, 1975) early developed a hypothesis on slab–mantle interaction that hydrous siliceous melts, generated by partial melting of subducted crust, rise and react with the overlying peridotite, and the fertilized mantle region then melts to produce arc basalts and andesites. Wyllie and Sekine
Conclusions
Although adakites are sodic, here we have demonstrated that they are generally Na-depleted relative to their experimental equivalents. Average adakite is lower by 1–3 wt.% Na2O than experimental slab melts. The Na depletion in adakites can be attributed to the melt / rock reaction as the magmas passed through the hot mantle wedge. The process added mantle components MgO and CaO to adakite magmas, with melt components Na2O, SiO2 and perhaps Al2O3 and K2O being transferred to the mantle probably
Acknowledgements
We sincerely thank J. Brenan, M. Barth and the editor S.L. Goldstein for reviews and evaluations on the manuscript and R.H. Smithies, T. Rushmer and T.H. Green for comments on an early version of this manuscript. These have led to substantial improvement of the manuscript. This work is supported by grants from National Nature Science Foundation of China (40373035, 40573043) and Chinese Academy of Sciences (KZCX3-SW-152, KZCX2-SW117, ZCX3-SW122, GIGCX-03-04 and GIGCX-04-03). [SG]
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