The Magma Plumbing System of Merapi: The Petrological Perspective

Abstract

Merapi volcano in Central Java is one of the most active volcanoes in the Sunda arc and poses a continuous threat to the local population. Basaltic-andesites from Merapi carry a voluminous crystal cargo and varied inclusion types, suggesting the presence of a complex subvolcanic plumbing system that feeds the volcano. Merapi basaltic-andesite lavas are dominated by a crystal assemblage of plagioclase with lesser pyroxene, Fe-oxides, and yet rarer amphibole phenocrysts. Inclusions in the lavas can be separated into four main groups, comprising (1) basaltic enclaves and highly crystalline basaltic-andesite inclusions, (2) plutonic inclusions, (3) amphibole megacrysts, and (4) calc-silicate crustal xenoliths. Co-magmatic basaltic enclaves display chilled margins, whereas highly crystalline basaltic-andesite inclusions usually lack chilled margins, indicating mixing and mingling of compositionally variable magmas within the Merapi plumbing system. Holocrystalline plutonic inclusions comprise gabbro to diorite compositions and are generally coarse grained with occasional mineral layering. Mineral and isotope compositions of the plutonic inclusions largely overlap with those of recent Merapi lavas, defining them as cognate in origin. Mafic enclaves and amphibole megacrysts are isotopically more primitive and reflect deeper portions of the plumbing system. Geobarometry of plutonic inclusions, amphibole megacrysts, and pyroxene in lavas suggest a polybaric magma supply system beneath Merapi, likely with a larger basaltic storage reservoir at the base of the crust and a broadly andesitic one at mid crustal depth, plus several smaller pockets or chambers in the shallow upper crust. The shallow reservoir system is further supported by frequent contact-metamorphic calc-silicate xenoliths, which testify to intense magma-crust interaction in the upper crust. This is in line with elevated Sr, Pb, and O isotope data for the more evolved plutonic inclusions and in the rims of shallow-grown plagioclase crystals. The petrological evidence thus records crystallisation, crystal accumulation, magma mixing, mafic recharge, and assimilation processes below Merapi and reflects considerable amounts of semi-molten crystalline mushes that are stored within the multi-tiered (polybaric) Merapi magma plumbing system. This implies that a considerable amount of potentially eruptible magma is currently stored beneath the volcano.

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