Host: The Japanese Association of Mineralogists, Petrologists and Economic Geologists
Water plays very important role in the geodynamics of the Earth's interior. It can dramatically affect physical properties, melting temperatures and melt compositions. Recently, Litasov and Ohtani (2002) reported phase relations at 10-25 GPa for the starting composition of CaO-MgO-Al2O3-SiO2-pyrolite+2wt% H2O and found significant decrease of intensive melting temperature (defined as an apparent solidus) at the pressures of 13-16GPa near olivine/wadsleyite boundary. This observation can be connected with differences in water storage capacity in wadsleyite and olivine. It is also play very important role for possibility of hydrous origin of the volcanics rocks (like komatiites) in Archean, when temperature of mantle plumes could be enough for very deep melting. Possible release of water from hydrous wadsleyite to the melt or fluid may cause intensive melting in the bottom of the upper mantle. The possibility of hydrous origin of some Archean komatiite magmas was recently demonstrated by different studies. However, 2wt% of H2O should be very high for the natural systems. Therefore, here we tested hypothesis of hydrous melting at the bottom of the upper mantle in the pyrolite with 0.5wt% of H2O. Phase relations and melt compositions have been determined at 13.5-17.0GPa and temperatures from 1600 to 2100°C. Garnet is the first liquidus phase in the pressure range of 13.5-17GPa followed by olivine at 13-16GPa and anhydrous phase B at 16-17GPa. Clinopyroxene appears only in experiment at 13.5GPa and 1600°C. Experimental products at 13.5GPa and 1600-1900°C do not contain any quench crystals. Quench crystals of the partial melt were detected at 13.5GPa and 1950°C. Similarly, at 16-17GPa quench crystals were detected only at 2100°C, whereas they are absent at lower temperatures. Therefore, the apparent solidus temperature at pressures above 16GPa is close to that of dry solidus, however it is lower than dry solidus at 13.5GPa (1900°C and 2050°C respectively). We do not detect drastic decrease of solidus temperature along phase boundary between olivine and adsleyite in the pyrolite with 0.5wt% H2O. The solidus temperature decreases gradually. Compositions of small fraction melts of pyrolite+0.5wt% H2O fall in the range of those obtained in dry pyrolite and pyrolite+2.0wt% H2O (Litasov and Ohtani, 2002). Partial melt at 13.5 GPa has high CaO (13.8wt%). The CaO content decreases significantly with increasing pressure from 13.5 to 17GPa. The SiO2 contents of partial melts at 16-17 GPa is high (50-52wt%) relative to that at 13.5GPa (48.2wt%) due to formation of anhydrous phase B.The present results for pyrolite with 0.5 wt% H2O still support a model for hydrous origin of some ancient komatiites by dehydration melting of rising wet plumes at the base of the upper mantle. Hydrous melting of mantle plume can be started at every pressure below 15-16GPa (olivine/wadsleyite transition). The water affects on composition of the partial melts indirectly, changing melting temperature and degree of partial melting.
