ELSEVIER Earth and Planetary Science Letters 160 (1998) 709–722 Open-system degassing of sulfur from Krakatau 1883 magma Charles W. Mandeville a,Ł , Akira Sasaki b , Genji Saito b , Kevin Faure b , Robert King c , Erik Hauri d a Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192, USA b Geological Survey of Japan, 1-1-3 Higashi, Tsukuba, Ibaraki 305, Japan c Instituto de Geologia de Geologia Economica Aplicada, Universidad de Concepcion, Casilla 4107, Concepcion 3, Chile d Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road N.W., Washington, DC20015, USA Received 7 November 1997; revised version received 18 May 1998; accepted 18 May 1998 Abstract 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 34 S relative to mantle sulfur. High Ž 34 S values of C6:3 to C16:4‰ can best be explained by open-system or multi-stage degassing of SO 2 from the oxidized rhyodacitic and gray dacitic magmas with 34 S enrichment of SO 2 4 remaining in the melt. Lower whole-rock Ž 34 S values of C2:6‰ and C4:0‰ in two oxidized gray dacitic samples indicate more primitive subarc mantle sulfur in the 1883 magma chamber. Initial Ž 34 S of the rhyodacitic magma was probably in the C1:5‰ to C4:0‰ range and similar to Ž 34 S values measured in arc volcanic rocks from the Mariana Arc.  1998 Elsevier Science B.V. All rights reserved. Keywords: pyroclastics; sulfur; degassing; Krakatoa 1. Introduction Exsolution and expansion of dissolved gases (H 2 O, CO 2 , SO 2 , Cl) from magma is important not only for fragmentation during volcanic eruptions but in the case of SO 2 and Cl may also have impacts on climate [1]. A drop in mean annual Northern Hemi- sphere temperature of 0.3–0.4ºC has been attributed to the 1883 eruption of Krakatau [2]. The most re- cent petrologic estimate of sulfur degassed from 12.5 km 3 of magma during the 1883 eruption is 2:8 ð 10 12 g S [3]. This estimate is approximately an order of Ł Corresponding author. Tel.: C1 (212) 769-5380; Fax: C1 (212) 769-5339; E-mail: cmandy@amnh.org magnitude less than estimates derived from studies of acidic layers in Antarctic and Greenland ice cores [4,5], despite consideration of all erupted magma compositions [3]. Likely sources of excess sulfur as- sociated with this eruption may be from nonerupted andesitic magma in the zoned chamber, and vapor- ization of seawater by pyroclastic flows [3]. Assessment of the source of dissolved sulfur in 1883 magma requires evaluation of modifications to the initial sulfur isotopic composition during de- gassing. The magnitude and direction of an isotopic shift accompanying degassing depends largely on pressure, temperature, the amount and species of volatiles lost and whether the process is ‘open’, or ‘closed’ [6,7]. Degassing processes may be modelled 0012-821X/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S0012-821X(98)00122-8