Hydrous phase stability and partial melt chemistry in H2O-saturated KLB-1 peridotite up to the uppermost lower mantle conditions

Stability of hydrous phases and partial melt chemistry in 13.6 wt.% H(2)O bearing mantle peridotite are reported in a pressure range up to 24 GPa and a temperature range from 900 to 1400 degreesC. Various hydrous phases like phase E, phase D, superhydrous B, hydrous P and gamma phases of olivine stoichiometry are observed between 14 and 24 GPa. The phase boundary between the beta phase and gamma phase of olivine under hydrous conditions is found to be at higher pressure than under the dry conditions. The H(2)O-undersaturated solidus temperature of a model mantle is estimated up to 11 GPa based on (1) the reported solubility of H in nominally anhydrous minerals and (2) linearity of H(2)O-undersaturated solidus between the dry and wet solidus [Science 276 (1997) 240] with respect to H(2)O abundance at a constant pressure. Under H(2)O-saturated conditions at pressures greater than 10 GPa, the (Mg + Fe)/Si atomic ratios of partial melts decrease with increasing temperature: from 2.5 at 1100 degreesC to 1.6 at 1300 degreesC at 14 GPa, from 3.3 at 1100 degreesC to 1.8 at 1300 degreesC at 17 GPa, and from 2.8 at 1200 degreesC to 1.7 at 1400 degreesC at 20GPa. This is in contrast to the reported constant value of dry partial melts (L I between 10 and 18 GPa [J. Geophys. Res. 99 (1994) 17729]). A partial melt coexisting with Mg-perovskite at 24 GPa and 1400 degreesC has (Mg + Fe)/Si and Ca/Al atomic ratios of 2.2 and 11, respectively. Ultramafic hydrous melts similar to those observed experimentally under uppermost lower mantle conditions may have contributed to chemical differentiation between the upper and lower mantle. (C) 2004 Elsevier B.V. All rights reserved.