Hydrous phase stability and partial melt chemistry in H 2O-saturated KLB-1 peridotite up to the uppermost lower mantle conditions
Stability of hydrous phases and partial melt chemistry in 13.6 wt.% H 2O bearing mantle peridotite are reported in a pressure range up to 24 GPa and a temperature range from 900 to 1400 °C. Various hydrous phases like phase E, phase D, superhydrous B, hydrous β and γ phases of olivine stoichiometry are observed between 14 and 24 GPa. The phase boundary between the β phase and γ phase of olivine under hydrous conditions is found to be at higher pressure than under the dry conditions. The H 2O-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 2O-undersaturated solidus between the dry and wet solidus [Science 276 (1997) 240] with respect to H 2O abundance at a constant pressure. Under H 2O-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 °C to 1.6 at 1300 °C at 14 GPa, from 3.3 at 1100 °C to 1.8 at 1300 °C at 17 GPa, and from 2.8 at 1200 °C to 1.7 at 1400 °C at 20 GPa. This is in contrast to the reported constant value of dry partial melts (1.1 between 10 and 18 GPa [J. Geophys. Res. 99 (1994) 17729]). A partial melt coexisting with Mg-perovskite at 24 GPa and 1400 °C 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.