Phase relations and melt compositions in CMAS-pyrolite-H2O system up to 25 GPa

Phase relations and melt compositions in the CaO-MgO-Al2O3-SO(0)2-pyrolite under hydrous (2% of H2O) and anhydrous conditions have been determined at 10-25 GPa and temperatures from 1400 to 2400 degreesC. At the pressure of 16 and 25 GPa the apparent solidus temperatures for the hydrous system are about 200 and 100degreesC lower than the solidus temperatures for the dry system, respectively. The apparent solidus temperature changes drastically from 1600 degreesC at 13.5 GPa to about 1850 degreesC at 16 GPa. Majorite is a liquidus phase in the hydrous pyrolite from 10 to 25 GPa. The liquidus phases in the dry pyrolite are olivine at 10-14 GPa, majorite at 14-20 GPa, and periclase at pressures above 20 GPa. We observed expansion of the stability field of anhydrous phase B and its complex relations with the other phases in the CMAS-pyrolite system. We suppose it is formed not only by incongruent melting of wadsleyite, but also by the subsolidus reaction of wadsleyite and majorite in the dry condition. Compositions of partial melts formed by low degree of-melting (10-20%) have Al2O3 depleted and CaO-rich compositions up to 22 GPa and enriched in Al2O3 (5-7 wt.%) and CaO at 25 GPa. At.10 GPa, dry and hydrous melts formed by low degree of melting have high SiO2 (48-51 wt.%) and relatively high Al2O3 (15-5.2 wt.%). Their compositions are generally consistent with those of aluminum-depleted komatiite. At 13-22 GPa, dry and hydrous melts have low SiO2 (44-49 wt.%) and Al2O3 (1.7-3.0 wt.%) and high CaO contents (9-15 wt.%). An essential depression of the apparent solidus temperature in the hydrous deep upper mantle compared to that in the transition zone provides the dehydration melting of the hydrous plume. The present results support a model of komatiite genesis by dehydration melting of rising wet plumes at the base of the upper mantle. (C) 2002 Elsevier Science B.V. All rights reserved.