High hydrogen solubility in Al-rich stishovite and water transport in the lower mantle

Details High hydrogen solubility in Al-rich stishovite and water transport in the lower mantle High hydrogen solubility in Al-rich stishovite and water transport in the lower mantle

Oct 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007e%26psl.262..620l&link_type=abstract

Earth and Planetary Science Letters, Volume 262, Issue 3-4, p. 620-634.

Physics

13

Scientific paper

Stishovite is an important phase in subducting oceanic crust. The post-garnet assemblage from a precursor eclogite lithology contains up to 25% stishovite at pressures above 25 GPa. This stishovite may contain up to 5 wt.% Al2O3. We measured the hydrogen contents of stishovite samples synthesized at 20 25 GPa and 1200 1800 °C from several starting materials containing up to 10 wt.% Al2O3. FTIR spectra of stishovite show major bands at 3111 3134 cm- 1, with the frequencies increasing as H2O and Al2O3 contents increase, and several minor bands at 2659 2667, 3240, 3261, 3312 3334, and 3351 cm- 1. The H2O content of Al-free stishovite is in the range of 16 30 wt. ppm. The maximum H2O content of Al-bearing stishovite (4.4 wt.% Al2O3) synthesized at 20 GPa and 1400 °C is 3010 ± 300 wt. ppm. Most hydrogen in stishovite is associated with Al3+ substitutional defects on the octahedral (Si4+) site. The hydrogen can occupy 40% of vacancies created by incorporation of Al3+ at 20 GPa. This observation along with some anomalies in the FTIR spectra may indicate an alternative mechanism of Al3+ incorporation into stishovite via the formation of oxygen vacancies or interstitial Ali defects. We report the highest H2O concentrations in Al-stishovite to date, and argue that it is the most important carrier of water into the lower mantle post-garnet eclogitic assemblages.

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