Geochemistry of the Mariana arc (western Pacific): Source composition and processes

  • Woodhead J
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The northern Mariana islands in the western Pacific form a prime example of an intra-oceanic island arc system, free from the effects of continental crustal contamination. Radiogenic (Sr, Nd, Pb, Hf) and stable (O, H, S) isotope data are presented for a suite of lavas from these islands, together with rare-earth and other trace-element compositional data for the same samples. This geochemical synthesis provides an opportunity to address a number of outstanding questions concerning oceanic arc petrogenesis, in particular the nature of the mantle source and processes occurring within this region. Much debate surrounds the origin of the commonly observed enrichments in island arc lavas relative to mid-ocean ridge basalt (MORB), in particular whether these result from incorporation of slab-derived components into a MORB-type mantle or whether they reflect preferential melting of a source with affinities to oceanic island basalt (OIB). Although some geochemical data are ambiguous, the majority appear to favour the slab involvement hypothesis, with contributions from both subducted sediment and altered oceanic crust. The trace-element enrichment patterns of the Mariana arc volcanics suggest that the agent of mass transfer between the subducting slab and overlying mantle wedge is a fluid, which has leached elements from the slab, rather than a direct partial melt of the latter. Although the exact composition of this fluid is unknown, the data indicate that a large proportion of it may be aqueous. Elemental partitioning into this fluid is highly selective (i.e. mass transfer between the slab and mantle wedge is not an iso-chemical process), a conclusion which has considerable implications for studies of crust and mantle evolution. The commonly cited depletions of high-field-strength elements (HFSE) in island arcs relative to MORB are thought to be largely a product of the data normalization process; in reality substantial overlap exists between HFSE concentrations in arc lavas and MORB. Regional geochemical variations between the islands in the arc are small and are thought most likely to be related to heterogeneity in the nature of the subducting assemblage. © 1989.

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  • Jon D. Woodhead

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