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


INOUE, T., T. IRIFUNE, H. YURIMOTO and I. MIYAGI,

Physics of the Earth and Planetary Interiors

Vol. 107, 221-231 (1998)

please contact the first author to get an offprint

Abstract:
  The stability of K-amphibole has been studied at pressures 12.6-16.5 
GPa, and at temperatures 940-1450 deg-C. K-amphibole decompreses into
an assemblage of clinoenstatite + diopside + an unknown phase (X) +
stishovite + fluid at pressures above 16 GPa and temperatures below
1200 deg-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 \pm
0.1 wt. % H2O as determined by SIMS measurements, leading to a formula 
of K4Mg8Si8O25(OH2). 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 would not cause any \beta -phase to form hydrous \beta
-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.