H2O-rich magmas or silicate-rich H2O fluids? A perspective from high pressure and temperature experiments
Abstract
Crystalline phases coexisting with H2O fluids in a Bassett type externally heated diamond anvil cell are identified at 800 - 1000 °C and 0.5 - 5.2 GPa using synchrotron X-ray diffraction at SPring-8. Up to 3.3 GPa, forsterite (Mg2SiO4) crystallizes in enstatite (MgSiO3) - H2O system. In contrast, enstatite dissolve into the H2O fluids congruently at higher pressure conditions. This observation suggests that Mg/Si ratios of the H2O fluids become more than unity at 3.3 GPa corresponding to about 100 km depth. Chemical compositions of silicate dissolved into H2O fluids have been characterized by SiO2-rich feature at 1 - 3 GPa (Nakamura and Kushiro, 1974, Cargenie Year Book Ryabchikov et al., 1982, Contrib. Mineral. Petrol., Zhang and Frantz, 2000, Am. Mineral.). In contrast, recent quench experiments at higher pressure conditions suggested that H2O fluids coexisting with enstatite and forsterite become higher Mg/Si ratios as pressure increases from 3 to 10 GPa (Stalder et al., 2001, Contrib. Mineral. Petrol., Mibe et al., 2002, Geochim. Cosmochim. Acta). Our in-situ experimental data are consistent with the previous quench experimental studies. Partial melts of H2O-saturated peridotite up to 10 GPa (Kawamoto and Holloway, 1997, Science) show similar Mg/Si ratios to the H2O fluids compositions in the quench data by Stalder et al. (2001) and Mibe et al. (2002). It is, therefore, likely to suggest that there is no distinction between silicate-rich H2O fluids and H2O-rich silicate melts somewhere in the Earth's upper mantle.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....1670K