Dehydration melting of nominally anhydrous mantle: The primacy of partitioning

The onset of dehydration melting of nominally anhydrous peridotite can be calculated by combination of appropriate mineral/melt partition coefficients for H2O D-H(min/liq), and a parameterization of the influence of the H2O content of melt on the solidus of peridotite. Thermodynamic models predict that olivine/melt partitioning, D-H(ol/liq), should increase with pressure, and though direct experimental determinations of D-H(ol/liq) from 0.5 to 3 GPa do not show the predicted pressure dependence, storage capacity experiments suggest increases in D-H(ol/liq) at pressures above 8 GPa and particularly at 12-14 GPa, near the base of the upper mantle. Calculations using experimental values of D-H(ol/liq) and ignoring the likely effect of pressure on D-H(ol/liq) indicate that D-H(period/liq) increases from 0.006 at 1 GPa up to 0.009 at the onset of garnet stability at 2.8 GPa and then diminishes with further increases in pressure owing to decreasing pyroxene mode and decreasing Al in pyroxene. Because these calculations ignore the likely pressure effect on D-H(ol/liq), they represent minima. Incipient partial melts of mantle with 100 ppm H2O have 1-2 wt.% H2O from 1 to 5 GPa, and this modest H2O concentration limits the stability of hydrous partial melts to temperatures approaching the dry solidus. The influence of H2O on the melting behavior of periclotite can be quantified using a simple cryoscopic approach benchmarked against experiments on hydrous peridotite. Along a mantle adiabat with a potential temperature of 1323 degrees C, calculations indicate that dehydration partial melting of peridotite with 100 ppm H2O begins at 80 km, or about 15 km deeper than would be the case for truly dry periclotite. However, decreases in D-H(period/liq) related to the onset of the stability of garnet mean that mantle modestly enriched in H2O will begin melting significantly deeper, i.e., at 104 km for 200 ppm H2O. In the low velocity zone (LVZ) beneath mature (50 Ma) oceanic lithosphere, incipient partial melting at 110 km requires 300 ppm H2O and generation of small finite (>= 0.1%) melt fractions across the entire LVZ from 90 to 200 km requires 600 ppm H2O. The minimum concentration, 300 ppm H2O, is 2-3 times that of typical convecting oceanic (MORB-source) mantle, so it is not likely that pervasive hydrous partial melting is responsible for the seismic properties of the LVZ. Extrapolation of low pressure partition coefficients to the base of the upper mantle indicates that at least 500 ppm H2O is required to induce partial melting at depths of 300-400 km along a normal mantle geotherm. This argues that typical upper mantle with similar to 100 ppm H2O is not produced by partial melting above the 410 km discontinuity. Furthermore, the 500 ppm H2O concentration is likely to be an underestimate, as it does not take into account probable enhancement in D-H(min/liq) at high pressure. (C) 2009 Elsevier B.V. All rights reserved.