Crustal differentiation processes at Krakatau Volcano, Indonesia

AnakKrakatau is a basaltic andesite cone that hasgrownfollowingthe famous caldera-forming 1883 eruption of Krakatau. It breached sea level in1927 and since the1950s has been growing at an average rate of ~8 cm a week. We present new major and trace element data combined withwhole-rock δ8O, SrandNd isotope data for1883, 1993 and 2002 Krakatau eruptive products and the surrounding crust. Bombs erupted from Anak Krakatau during 2002 contain frothy metasedimentary and plutonic xenoliths that show variable degrees of thermal metamorphism, plastic deformation and partial melting. Contactmetamorphicmineralssuch ascordieriteandtridymite inmetasedimentary xenoliths are consistent with high-temperature metamorphism and incorporation at mid- to upper-crustal depth. Energy-constrained assimilation and fractional crystallization modelling of whole-rock data suggests that theAnakKrakataumagmas have a genetic relationshipwith the1883 eruption products. Thegeochemical impact ofcrustal contaminants on whole-rock compositions is apparently small, and we conclude that low levels of assimilation of a quartzo-feldspathic sediment are recorded in AnakKrakataumagmas. Plagioclase phenocrysts from the 2002 eruption exhibit disequilibrium textures and complex compositional zoning, however, and are also isotopically variable with a total range in 87Sr/86Sr of 0.7043-0.7048 as determined by in situ laserablation inductively coupled plasmamass spectrometry. This suggests that although shallow crustal assimilation appears to have had a limited effect on whole-rock chemistry, a complex late-stage differentiation history is recorded within the magma's cargo of crystals and xenoliths. © The Author 2012. Published by Oxford University Press. All rights reserved.