Constraints on the speciation of hydrogen in earth's transition zone

Details Constraints on the speciation of hydrogen in earth's transition zone Constraints on the speciation of hydrogen in earth's transition zone

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003pepi..136...93f&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 136, Issue 1-2, p. 93-105.

Physics

2

Scientific paper

The thermodynamic stability of hydrous B-type phases relative to hydrous β- and γ-phases is examined. The stability of phase B (Mg12Si4O19(OH)2) and superhydrous B (Mg10Si3O14(OH)4) relative to hydrous β-phase (with an end-member formula of Mg7Si4O14(OH)2) appears to be controlled by reactions that depend on whether coexisting MgSiO3 resides in the garnet structure, or is present as a pyroxene. In majorite garnet-bearing systems, the volume of B-bearing assemblages is smaller than those containing hydrous β-phase the corresponding volume change between B-assemblages relative to hydrous γ-phase-bearing assemblages is ambiguous. The increase in volume between superhydrous B-bearing assemblages and hydrous β-phase-bearing assemblages produces a free energy difference that is significantly greater than that expected from thermal contributions to the free energy. Phase B-bearing assemblages also have a smaller volume than hydrous β-phase assemblages, but the difference in volume is a factor of two smaller than that associated with superhydrous B: this may account for the greater stability range of superhydrous B compared to phase B. In contrast, pyroxene-bearing assemblages clearly favor the formation of hydrated β-phase. In accord with these expectations, heavily hydrated β- and γ-phases (those with greater than ~1wt.% H2O) have been synthesized in the past primarily from melt-rich or pyroxene-bearing assemblages. To confirm these trends, we conducted synthesis experiments on samples at pressures between 15 and 25GPa, utilizing the laser-heated diamond cell with chrysotile, San Carlos olivine, Bamble enstatite and a garnet lherzolite as starting compositions, with several weight percent water added to the anhydrous materials. At transition zone pressures, each of these materials yields a B-phase as the main crystalline hydrous phase. The combination of our calculations and experiments thus indicates that the B-phases are probably the main repository for hydrogen in the mantle between 400 and at least 520km depth, particularly as their thermal stability is expected to be enhanced through the incorporation of levels of fluorine similar to those present in xenolithic phlogopites and amphiboles.

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