Sean Whitley
School of Physical and Geographical Sciences
Contact metamorphism of carbonate xenoliths from Merapi
Volcano (Indonesia)
Merapi exhibits near continuous activity, characterised by basaltic-andesite lava
dome growth, interrupted by gravitational or explosive dome collapse and associated
pyroclastic density currents; the most recent being the paroxysmal 2010 eruption.
Abundant calc-silicate xenoliths are found at Merapi, and recent work has focused
on the role of crustal contamination by assimilation of carbonate lithologies in
influencing eruption dynamics via volatile exsolution. During contact metamorphism
in the magma, the carbonate xenoliths are transformed into skarn type calc-silicate
phase assemblages. These commonly exhibit mono/multimineralic zonations that
are related to the carbonate protolith composition, extent of Mg, Fe, Al and alkali
diffusion, metamorphic P-T conditions, and X(CO2) of the fluid. The effects of
contact metamorphism on the characteristic calc-silicate mineral assemblages in the
xenoliths are the focus of this study. Our preliminary data suggest increasing fO2 as
a result of carbonate breakdown, driving Ca-Tschermak substitution in
clinopyroxene, and likely increasing oxidation of the Merapi magma near the wall
rock contact. Diffusion of magmatic Al-Mg-Fe-Si and/or varying impurity of the
carbonate protolith results in distinct zoned assemblages throughout the xenoliths.
Complementary fluid inclusion data of the calc-silicate xenoliths will be used to
constrain magmatic storage regions and provide semi-quantitative estimates of the
fluid chemical composition.
Postgraduate Conference 2016
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