Melting of subducting slab supercritical liquids

In cases where arc magmas have high abundances of slab-related trace elements considered as fluid-immobile (at moderate PTconditions), such as Th and the REEs, and enhanced Th/La ratios identical to those in the subducting sediment column, this points to the mobilization and transfer of the elements into the mantle wedge by partial melts or supercritical fluids (800 °C < T < 1200 °C and 4 GPa < P < 6 GPa). In some arc magmas, however, high [Th] concentrations, ~ 1 ppm, are accompanied by low 87Sr/86Sr and high 143Nd/144Nd ratios (< 0.7030 and > 0.5130, respectively), which are characteristic of MORBs rather than sediments, implying the partial melting of subducting basaltic crust. If a hydrous fluid is present, the melting temperatures for metapelites and metabasalts are similar. At the above PT conditions both fluid-saturated, or "flush", melting and solid-fluid interactions significantly increase the mobilities for most elements, even those that are fluid-insensitive at moderate conditions (including Th and the LREEs). Therefore a large fraction of Be, B, Rb, Cs, Sr, Ba, the LREEs, Pb, Th and U could be removed effectively from the metasediments and metabasalts by fluid-saturated melts and/or supercritical fluids (Kelemen et al., 2003; Schmidt et al., 2004; Kessel et al., 2005).

Such slab processing is made possible by the thermal gradients that are established in slabs as they sink into a hotter mantle: the middle part is heated up less fast than the boundaries. Modelling predicts up to 400 °C differences in temperature between the slab-mantle-wedge interface and the base of the oceanic basaltic crust (Peacock et al., 2005). Thus fluids arising from dehydration within the relatively cold segment in the middle of the slab can develop the hotter domain close to the upper boundary. In some cases the residual eclogitic assemblages indicate the involvement of a large amount of fluid phase in their development (John et al., 2004). Under enhanced P and T, the processing of outer segments of the slab by these deeply sourced fluids could initiate fluid-saturated or flush melting or generate supercritical fluids. In most cases the ratios of incompatible trace elements in the resulting arc melts (such as Th/La in the above example) are similar to those inferred for the basaltic or sedimentary sources, which points to large melt/solid and fluid/solid ratios.

At higher pressures (~ 6.5 GPa), melting in the strict sense does not take place. Instead, the amount of H2O still present in the slab is apparently sufficient to dissolve phengite entirely, near 1050 °C. Extraction of these supercritical solute-rich melts, which could contain about 30%-40% H2O, leaves an anhydrous garnet + clinopyroxene ± coesite ± kyanite ± rutile residue (Schmidt et al., 2004). This fluid-melt phase transfers almost all K (and most B, Be, Rb, Ba and other phengite-hosted trace elements) into the mantle wedge without substantial fractionation. Thus the concepts of dehydration and partial melting are no longer separate: solid assemblages progressively dissolve in a non-solid phase that is initially volatile-rich and becomes more like a silicate melt with increasing temperature.

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