Open-system degassing of sulfur from Krakatau 1883 magma

We present the first sulfur and oxygen isotopic data for tephra from the catastrophic 1883 eruption of Krakatau. Sulfur isotopic ratios in unaltered Krakatau tephra erupted August 26–27, 1883 are markedly enriched in 34S relative to mantle sulfur. High δ 34S values of +6.3 to +16.4‰ can best be explained by open-system or multi-stage degassing of SO 2 from the oxidized rhyodacitic and gray dacitic magmas with 34S enrichment of SO 2− 4 remaining in the melt. Lower whole-rock δ 34S values of +2.6‰ and +4.0‰ in two oxidized gray dacitic samples indicate more primitive subarc mantle sulfur in the 1883 magma chamber. Initial δ 34S of the rhyodacitic magma was probably in the +1.5‰ to +4.0‰ range and similar to δ 34S values measured in arc volcanic rocks from the Mariana Arc.