Decomposition of K-amphibole at high pressures and implications for subduction zone volcanism

Details Decomposition of K-amphibole at high pressures and implications for subduction zone volcanism Decomposition of K-amphibole at high pressures and implications for subduction zone volcanism

Apr 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998pepi..107..221i&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 107, Issue 1-3, p. 221-231.

Physics

9

Scientific paper

The stability of K-amphibole has been studied at pressures 12.6-16.5 GPa, and at temperatures 940-1450°C. K-amphibole decomposes into an assemblage of clinoenstatite+diopside+an unknown phase (X)+stishovite+fluid at pressures above 16 GPa and temperatures below 1200°C. The phase boundary has a negative Clapeyron slope, and the high temperature assemblage is clinoenstatite+diopside+X+wadeite-type K2Si4O9+fluid at pressures 14-16 GPa. The X phase has a cation ratio of approximately K:Mg:Si=1:2:2, and contains 1.7+/-0.1 wt.% H2O as determined by SIMS measurements, leading to a formula of K4Mg8Si8O25(OH)2. The present results suggest that the decomposition of K-amphibole in the dragged hydrous peridotite layer at the base of the mantle wedge may produce certain amounts of H2O-rich fluid at 14-16 GPa (~450 km depth), while some of H2O is trapped in the new hydrous phase X and is further carried into deeper regions of the mantle. The aqueous fluid released by the decomposition of K-amphibole should react with β-phase to form hydrous β-phase in the mantle transition region. Thus, the dehydration of K-amphibole would not cause any volcanic activities in the back arc regions, in contrast to the dehydrations of amphibole, chlorite and phlogopite, which are presumably responsible for the first and the second volcanic chains. However, some of the volcanic activities such as in Muriah, Indonesia, may be related to the dehydration of K-amphibole in unusually hot regions above the subducting slab.

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