Magma-Carbonate Interaction at Merapi Volcano, Indonesia

Abstract

Merapi volcano, Indonesia, is a highly active arc volcano built upon a crustal succession that includes thick carbonate sequences. Calc-silicate (skarn) xenoliths are frequently found in Merapi’s erupted products and constitute direct evidence for magma-carbonate interaction beneath the volcano. The xenoliths show mineral assemblages that allow them to be subdivided into two main groups: (1) magmatic skarns formed within the magma system and (2) exoskarns formed of metamorphosed and metasomatised wall-rocks. Geochemical data including 87Sr/86Sr and δ18O values in erupted rocks and minerals support a model involving magma-carbonate interaction in Merapi magma evolution, while elevated (crustal-type) values in fumarole gases have been recognised during recent eruptive events. Furthermore, experiments confirm that carbonate assimilation in Merapi-type magma can rapidly produce a voluminous C–O–H gas phase, which may at times place additional pressure on the magmatic system. The detection of extremely low δ13C values in remnant calcite in Merapi xenoliths relative to unmetamorphosed limestone underscores the efficiency of carbonate and calc-silicate degassing during carbonate assimilation. Decarbonation of the arc-crust beneath Merapi may thus at times magnify the volcanic CO2 output and potentially contribute to the volcano’s notoriously erratic eruption behaviour. Magma-carbonate interaction at Merapi is therefore particularly well-studied and represents an archetypical locality for this process.