Supplementary Information

Supplementary Fig. shows isobaric melting curves obtained from batch and fractional melting models, at pressures of 1 GPa and 2 GPa, with a constant bulk water distribution coefficient (D_H2O=0.01 ) and different bulk water contents. Notice that adding water greatly depresses the solidus and produces a prominent "low-F tail"⁸. When water content (0.3 wt% melting curve) exceeds saturation in the melt, which is mostly a function of pressure^9-11, the melting function sharply increases just above the solidus due to the overabundance of water acting as an additional phase. When major phases, such as cpx, are exhausted from the residue, the productivity decreases discontinuously and then rises again (cpx-out criterium), because melting reactions start to consume principally opx⁹. Note that during batch melting the solid retains incompatible elements, such as water, up to high degrees of melting; thus water affects significantly the maximum extent of melting. In fact, addition of water to a peridotite system increases monotonically the degree of melting at constant temperature and pressure. In contrast, fractional melting determines rapid depletion of water in the residual solid. Melt productivity is low, and only a small percent of the melt fraction is produced before water’s complete exhaustion, when melting proceeds above a "dry solidus" with higher production rates^5,7 reaching the values of dry peridotite¹². A bulk partition coefficient for water between melt and residue, that decreases during progressive decompression melting because of the drop in pressure and in the modal abundance of pyroxenes⁵ yields a sharper wet-to-dry transition than would a constant value of 0.01 of the partition coefficient.