Magma–Carbonate Interaction Processes and Associated CO2 Release at Merapi Volcano, Indonesia: Insights from Experimental Petrology

Abstract

There is considerable evidence for ongoing, late-stage interaction between the magmatic system at
\nMerapi volcano, Indonesia, and local crustal carbonate (limestone). Calc-silicate xenoliths within
\nMerapi basaltic-andesite eruptives display textures indicative of intense interaction between magma
\nand crustal carbonate, and Merapi feldspar phenocrysts frequently contain individual crustally
\ncontaminated cores and zones. In order to resolve the interaction processes between magma and
\nlimestone in detail we have performed a series of time-variable de-carbonation experiments in
\nsilicate melt, at magmatic pressure and temperature, using a Merapi basaltic-andesite and local
\nJavanese limestone as starting materials. We have used in-situ analytical methods to determine the
\nelemental and strontium isotope composition of the experimental products and to trace the textural,
\nchemical, and isotopic evolution of carbonate assimilation. The major processes of magmacarbonate
\ninteraction identified are: i) rapid decomposition and degassing of carbonate, ii)
\ngeneration of a Ca-enriched, highly radiogenic strontium contaminant melt, distinct from the
\nstarting material composition, iii) intense CO2 vesiculation, particularly within the contaminated
\nzones, iv) physical mingling between the contaminated and unaffected melt domains, and v)
\nchemical mixing between melts. The experiments reproduce many of the features of magmacarbonate
\ninteraction observed in the natural Merapi xenoliths and feldspar phenocrysts. The Carich,
\nhigh 87Sr/86Sr contaminant melt produced in the experiments is considered as a pre-cursor to
\nthe Ca-rich (often “hyper-calcic”) phases found in the xenoliths and the contaminated zones in
\nMerapi feldspars. The xenoliths also exhibit micro-vesicular textures which can be linked to the
\nCO2 liberation process seen in the experiments. This study, therefore, provides well-constrained
\npetrological insights into the problem of crustal interaction at Merapi and points toward the
\nsubstantial impact of such interaction on the volatile budget of the volcano.