Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001pepi..127..215u&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 127, Issue 1-4, p. 215-232.
Physics
26
Scientific paper
The fundamental role of H2O in the generation of supra-subduction arc-related mantle derived magmas has long been recognized (e.g. ). This review of available experimental data attempts to highlight some principal factors controlling magma generation and magma chemistry that can be attributed to the presence of a hydrous component during partial melting in the mantle wedge. Arc-related igneous rocks display a typical trace element abundance spectrum, the so-called `arc-signature', characterized by the enrichment of highly mobile large ion lithophile elements (LILE) relative to high field strength elements (HFSE). This signature can be explained by a two-component mantle source consisting of variably depleted asthenospheric mantle and a hydrous fluid (or H2O-saturated low percentage melt) that originates from the breakdown of hydrous phases transported in the cold part 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°C for moderate to substantial amounts of partial melting (10-25wt.%) compared to anhydrous melting of a lherzolite source. Such a reduction of the melting temperature requires the presence of 0.1-0.5wt.% H2O in the source and results in 1-7wt.% H2O in the basaltic to picritic primary liquid. The majority of primitive arc magmas are basaltic and do not represent near-solidus H2O-saturated liquids that are highly alkalic and mostly nepheline-normative. The decrease of the melting temperature on the order of 100-150°C compared to anhydrous peridotite melting requires high melting temperatures of 1250-1300°C at 1.5GPa (45km depth) and 1350-1400°C at 2.5GPa (70km depth) that are close to the average current mantle adiabat (ACMA). Therefore, partial melting in the mantle wedge is confined to the hottest region where temperatures approach undisturbed asthenospheric conditions. At a given pressure (depth) hydrous melts are less magnesian and more silica-rich than melts produced under anhydrous conditions. Arc magmas (and their mantle sources) are more oxidized than MORB or OIB melts (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 MORB and OIB basalts.
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