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3.2 g. Phase relations and ultramafic partial melts of hydrous KLB-1 mantle peridotite (T. Kawamoto and D. C. Rubie)

Phase relations and partial melt chemistry of KLB-1 mantle peridotite (13.6 wt% H2O, 0.89 Mg/(Mg+Fe), 1.46 (Mg+Fe)/Si atomic ratios) have been determined in a pressure range from 14 to 24 GPa and a temperature range from 900 to 1400°C. Hydrous phases, such as phase E, D, G, superhydrous B, wadsleyite and ringwoodite, are found under the present experimental conditions. Phase E is stable at 14 - 17 GPa, phase D/G and superhydrous B are found at 17 - 24 GPa. At 24 GPa and 1400°C, Mg-perovskite is observed with stishovite and a quenched partial melt showing dendritic texture. (Mg,Fe)O was not observed under these experimental conditions.

The (Mg+Fe)/Si atomic ratios for partial melts in the same system increase from 1 at 5 GPa to 2 at 11 GPa. At 7.5 GPa, the (Mg+Fe)/Si ratios for partial melts increase with increasing temperature (Fig. 3.2-12). In contrast, at a given pressure greater than 10 GPa, the (Mg+Fe)/Si ratios for partial melts decrease with increasing temperature (Fig. 3.2-12). The (Mg+Fe)/Si ratios for partial melts decrease from 2.5 at 1100°C to 1.6 at 1300°C and 14 GPa, and from 3.3 at 1100°C to 1.8 at 1300°C and 17 GPa. At 20 GPa, the (Mg+Fe)/Si ratios decrease from 2.8 at 1200°C to 1.7 at 1400°C. Similar characteristics were also observed in the MgO-SiO2-H2O system at 12 and 15.5 GPa. At a given temperature, the (Mg+Fe)/Si ratios of partial melts increase with increasing pressure. A partial melt coexisting with Mg-perovskite and stishovite at 24 GPa and 1400°C has a 2.2 (Mg+Fe)/Si atomic ratio. Ultramafic hydrous melts, which are similar to those observed experimentally under uppermost lower mantle conditions, may have contributed to chemical differentiation between the upper and lower mantle.

Fig. 3.2-12: (Mg+Fe)/Si molar ratios for the partial melts of H2O�saturated KLB-1 mantle peridotite versus pressure. KLB-1 peridotite is represented by the cross. Numerics show the experimental temperature (°C). The partial melts at 1 GPa are andesitic.

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