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Frances Deegan, Trinity College Dublin (Ireland)
Valentin Troll, Uppsala University (Sweden)
Carmela Freda, Istituto Nazionale di Geofisica e Vulcanologia (Italy)
Valeria Misiti, Istituto Nazionale di Geofisica e Vulcanologia (Italy)
Jane Chadwick, Vrije Universiteit (Netherlands)
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At Merapi volcano, Indonesia, frequent calc-silicate xenoliths within recently erupted basaltic-andesite display reaction rims with evidence of intense interaction between magma and carbonate crust. Further evidence for late-stage interaction between limestone and the magmatic system has also recently been identified through crystal isotope stratigraphy.
In order to resolve the actual interaction processes in detail, we have performed piston cylinder de-carbonation experiments using Merapi basaltic-andesite glass and local limestone at magmatic pressure and temperature (fixed at 0.5GPa and 1200°C, respectively). The dwell time for the experiments was varied, with experiments carried out at 0, 60, 90, 150, and 300s. All runs were carried out using both nominally anhydrous and hydrous (glass doped with 2.23wt% H2O) starting glass. Microprobe line traverses were carried out through the reaction interfaces to assess major element behaviour across these zones. Consequently, we have constructed a high-resolution time-sequence of magma-carbonate interaction in our experimental products.
Several processes of magma-carbonate interaction are observed in the experiments:
1. The development of a zone of Ca-rich, Si-poor glass surrounding the carbonate.
2. Severe de-gassing of the carbonate, promoting the development of CO2 bubbles, with several generations of bubbles frequently identified. 3. Partial melting and disaggregation of the carbonate xenoliths. 4. Magma mingling and subsequent mixing. A transition zone of hybrid composition between the Ca-rich and Ca-normal glass domains is often observed. 5. Complete dissolution of the carbonate in some cases (examples from both dry and wet experiments).
In addition to the experimental work carried out, we have studied natural samples of Merapi calc-silicate (skarn-type) xenoliths in detail. The natural samples are considerably more complex than the experimental products; notable features include:
1. The presence of an amorphous fringe zone, strongly enriched in CaO, at the margins of the xenoliths.
2. Rare carbonate inclusions and abundant micro-vesicular textures within the xenoliths and their pyrometamorphic reaction rims.
These experiments contribute considerably to our understanding of magma-carbonate interaction processes, and provide important insights into the progression of such interaction with time. Both the experimental work and observations of the natural xenoliths raise several issues of relevance for hazard assessments at Merapi, such as: the potential impact of magma-carbonate interaction on the volatile budget of the volcano and the effect that a relatively low-viscosity Ca-rich contaminant melt will have on the rheology of the Merapi system.
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