Dense Hydrous Magnesium Silicate Phase Research Articles

Stability of dense hydrous magnesium silicate phase G in the transition zone and the lower mantle.

Re-examination of the run products in the Mg2SiO4–H2O system by Ohtani et al. (1995) revealed stability of dense hydrous magnesium silicate, phase G, and brucite at 20 GPa and 800°C, and stability of the assemblage with superhydrous phase B + phase G + fluid at 20 GPa and 1000°C under the water saturated conditions. We reported the Raman spectrum of phase G, which suggests existence of silicon ions in the six coordination sites. Brucite, superhydrous phase B, and phase G are stable at least up to 22.5 GPa and around 1000°C, and are the candidates for the hydrous phases in a cold slab descending into the base of the transition zone and the uppermost part of the lower mantle. The X-ray diffraction and the Raman spectra data revealed that the dense hydrous magnesium silicate, phase G, and phase D re-defined by Yang et al. (1997) and Kuroda and Irifune (1998) are identical. Phase F by Kanzaki (1991) and Gasparik (1993) is likely to be identical to phase G.

Experimental and theoretical studies of the stabilities of talc, antigorite and phase A at high pressures with applications to subduction processes

We have experimentally determined the equilibrium talc ⇄ enstatite + quartz/coesite + H 2O to 40 kbar in the system MgO SiO 2 H 2O (MSH) using both synthetic and nearly pure Mg end-member natural talc and other synthetic starting materials for the other solid phases. At 40 kbar, the equilibrium dehydration boundary lies ∼ 150°C higher than that calculated using data from the existing internally consistent thermochemical data bases. The reason for this discrepancy lies in the erroneous compressibility data of talc in the data bases. We have retrieved the compressibility of talc from the experimental phase equilibrium data, and have also calculated sereral other equilibria in the MSH system involving talc, antigorite and the dense hydrous magnesium silicate (DHMS), commonly referred to as phase A. Comparison of these equilibria with selected thermal profiles at the leading edge of young and old subducting oceanic slabs, along with the dehydration condition of basaltic amphibole and solidus of mantle peridotite, provides an explanation for the observed heights of the volcanic fronts above subducting oceanic lithosphere. Further, it is found that in cold oceanic slabs (≥ 50 Ma with subduction velocity of ≥ 10 cm/y), antigorite will transform to the DHMS phase A through a vapor conserved reaction at a depth of ∼ 200 km. Phase A will then serve as a carrier of water into the deeper mantle.