Arc magmas typically display chemical and petrographic characteristics indicative of crustal input. Crustal contamination can take place either in the mantle source region or as magma traverses the crust (e.g. ). While source contamination is generally considered the dominant process (e.g. [2, 3, 4]), crustal contamination in high level magma chambers has also been recognised at volcanic arcs (e.g. [5, 6]). In light of this, we aim to test the extent of upper crustal versus source contamination along the Java segment of the Sunda arc, which, because of its variable upper crustal structure, is ideal for the task.
We present a detailed geochemical study of 7 volcanoes along a traverse from Anak-Krakatau in the Sunda strait through Java (Gede, Slamet, Merapi, Kelut, Kawah-Ijen) and Bali (Batur). Using rock and mineral elemental geochemistry and radiogenic (Sr, Nd and Pb) and, stable (O) isotopes, we show a correspondence between changes in composition of the upper crust and the apparent degree of upper crustal contamination. There is an increase in 87Sr/86Sr and δ18O, and a decrease in 143Nd/144Nd from Krakatau towards Merapi, indicating substantial input from the thick quasi-continental basement beneath East and Central Java. Volcanoes to the east of Merapi, and the Progo-Muria fault zone, where the upper crust is thinner and increasingly oceanic in nature have lower 87Sr/86Sr and δ18O, and higher 143Nd/144Nd indicating a stronger influence of the mantle source . Our new data represent a systematic and high-resolution arc-wide sampling effort that allows us to distinguish the effects of the upper crust on the compositional spectrum of individual volcanic systems along the Sunda arc.
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