Partial melting in the mantle wedge - the role of H2O in the genesis of mantle-derived 'arc-related' magmas

The fundamental role of H2O in the generation of supra-subduction are-relat ed mantle derived magmas has long been recognized (e.g. Yoder and Tilley, 1 962). This review of available experimental data attempts to highlight some principal factors controlling magma generation and magma chemistry that ca n be attributed to the presence of a hydrous component during partial melti ng in the mantle wedge. Arc-related igneous rocks display a typical trace e lement abundance spectrum, the so-called 'arc-signature', characterized by the enrichment of highly mobile large ion lithophile elements (LILE) relati ve to high field strength elements (HFSE). This signature can be explained by a two-component mantle source consisting of variably depleted asthenosph eric mantle and a hydrous fluid (or H2O-saturated low percentage melt) that originates from the breakdown of hydrous phases transported in the cold pa rt of the partly hydrated oceanic lithosphere. The principal effect of H2O on the partial melting is a reduction of the melting temperature by 100-150 degreesC for moderate to substantial amounts of partial melting (10-25 wt. %) compared to anhydrous melting of a lherzolite source. Such a reduction o f the melting temperature requires the presence of 0.1-0.5 wt.% H2O in the source and results in 1-7 wt.% H2O in the basaltic to picritic primary liqu id. The majority of primitive are magmas are basaltic and do not represent near-solidus H2O-saturated liquids that are highly alkalic and mostly nephe line-normative. The decrease of the melting temperature on the order of 100 -150 degreesC compared to anhydrous peridotite melting requires high meltin g temperatures of 1250-1300 degreesC at 1.5 GPa (45 km depth) and 1350-1400 degreesC at 2.5 GPa (70 km depth) that are close to the average current ma ntle adiabat (ACMA). Therefore, partial melting in the mantle wedge is conf ined to the hottest region where temperatures approach undisturbed asthenos pheric conditions. At a given pressure (depth) hydrous melts are less magne sian and more silica-rich than melts produced under anhydrous conditions. A rc magmas (and their mantle sources) are more oxidized than MORB or OIB mel ts (and their respective mantle sources). It is still controversial whether or not the oxidation state and the more silica-rich nature of hydrous arc- melts are indeed the principal factors controlling their predominantly calc -alkaline nature as opposed to the predominantly tholeiitic character of MO RB and OIB basalts. (C) 2001 Elsevier Science B.V. All rights reserved.