DECOMPOSITION OF K-AMPHIBOLE AT HIGH-PRESSURES AND IMPLICATIONS FOR SUBDUCTION ZONE VOLCANISM

The stability of K-amphibole has been studied at pressures 12.6-16.5 G Pa, and at temperatures 940-1450 degrees C. K-amphibole decomposes int o an assemblage of clinoenstatite + diopside + an unknown phase (X) stishovite + fluid at pressures above 16 GPa and temperatures below 12 00 degrees C. The phase boundary has a negative Clapeyron slope, and t he high temperature assemblage is clinoenstatite + diopside + X + wade ite-type K2Si4O9 + fluid at pressures 14-16 GPa. The X phase has a cat ion ratio of approximately K:Mg:Si = 1:2:2, and contains 1.7 +/- 0.1 w t.% H2O as determined by SIMS measurements, leading to a formula of K4 Mg8Si8O25(OH)(2). The present results suggest that the decomposition o f K-amphibole in the dragged hydrous peridotite layer at the base of t he mantle wedge may produce certain amounts of H2O-rich fluid at 14-16 GPa(similar to 450 km depth), while some of H2O is trapped in the new hydrous phase X and is further carried into deeper regions of the man tle. The aqueous fluid released by the decomposition of K-amphibole sh ould react with beta-phase to form hydrous beta-phase in the mantle tr ansition region. Thus, the dehydration of K-amphibole would not cause any volcanic activities in the back are regions, in contrast to the de hydrations of amphibole, chlorite and phlogopite, which are presumably responsible for the first and the second volcanic chains. However, so me of the volcanic activities such as in Muriah, Indonesia, may be rel ated to the dehydration of K-amphibole in unusually hot regions above the subducting slab. (C) 1998 Elsevier Science B.V. All rights reserve d.