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.