Gardner, MF,Troll, VR,Gamble, JA,Gertisser, R,Hart, GL,Ellam, RM,Harris, C,Wolff, JA

2013

January

Journal of Petrology

Crustal Differentiation Processes at Krakatau Volcano, Indonesia

Validated

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Anak Krakatau plagioclase crustal xenoliths assimilation and fractional crystallization isotope micro-analysis ENERGY-CONSTRAINED ASSIMILATION OXYGEN-ISOTOPE COMPOSITION MAGMA CHAMBER PROCESSES OPEN-SYSTEM PROCESSES FRACTIONAL CRYSTALLIZATION PLAGIOCLASE PHENOCRYSTS CANARY-ISLANDS TRACE-ELEMENT SUNDA STRAIT SUBDUCTION ZONE

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149

182

Anak Krakatau is a basaltic andesite cone that has grown following the famous caldera-forming 1883 eruption of Krakatau. It breached sea level in 1927 and since the 1950s has been growing at an average rate of similar to 8 cm a week. We present new major and trace element data combined with whole-rock delta O-18, Sr and Nd isotope data for 1883, 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. Contact-metamorphic minerals such as cordierite and tridymite in metasedimentary 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 the Anak Krakatau magmas have a genetic relationship with the 1883 eruption products. The geochemical impact of crustal contaminants on whole-rock compositions is apparently small, and we conclude that low levels of assimilation of a quartzo-feldspathic sediment are recorded in Anak Krakatau magmas. Plagioclase phenocrysts from the 2002 eruption exhibit disequilibrium textures and complex compositional zoning, however, and are also isotopically variable with a total range in Sr-87/Sr-86 of 0 center dot 7043-0 center dot 7048 as determined by in situ laser ablation inductively coupled plasma mass 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.

DOI 10.1093/petrology/egs066