Sulfur isotope systematics of basaltic lavas from Indonesia: implications for the sulfur cycle in subduction zones

We report sulfur isotope compositions of basaltic and basaltic andesite lavas from selected volcanoes in the Indonesian are system covering the spectrum from low-K tholeiitic to high-K calc-alkaline compositions. The results of 25 samples from seven volcanoes, which are associated with different subduction regimes, show a range in delta S-34 values of+2.0-+7.8 parts per thousand (VCDT) with an average of +4.7 +/- 1.4 parts per thousand (1 sigma). Averages and within-suite variations of two larger sets of samples from Batur and Soputan volcanoes (+4.2 +/- 1.3 parts per thousand with n = 9 and +5.7 +/- 1.4 parts per thousand with n = 7, respectively) are comparable to those of the entire sample set. Sulfur concentrations are low (mostly between 2 and 74 ppm, average = 19 ppm) and do not show correlations with sulfur isotope composition and whole-rock chemistry, or systematic changes with time in any of the lava suites. From model calculations we infer that basaltic magmas will undergo sulfur isotope fractionation during degassing, most commonly towards lower delta S-34 values, but that the extent is limited at P-T conditions and oxidation states of interest. Hence, delta S-34 signatures of basaltic lavas will generally be within a few permil from primary magmatic values, even in cases of extensive sulfur loss. Consequently, magmas in the Indonesian are system originate from mantle sources that are enriched in S-34 relative to MORB and OIB sources and are likely to have delta S-34 values of about +5-+7 parts per thousand. The enrichment in S-34 is considered to reflect addition of slab-derived material, presumably from sediments rather than altered oceanic crust, with fluids being the most likely transport medium. Absence of correlation between delta S-34 values of Indonesian basalts and chemical proxies for source components or processes at the slab-wedge interface suggests that sulfur isotopes are relatively insensitive to variations in subduction setting and dynamics. This is supported by the modest range in delta S-34 Of the Indonesian volcanoes studied despite significant variations in the nature and amount of subducted material, and by the similarity with average S-34 enrichments in other oceanic are systems. (C) 2001 Elsevier Science B.V. All rights reserved.