Sulfur isotope systematics of the 1982 El Chichón trachyandesite: an ion microprobe study

Sulfur isotopic compositions were determined by ion microprobe for 36 spots on anhydrite crystals in trachyandesitic pumices erupted from El Chichón Volcano in 1982. Individual anhydrite crystals are homogeneous in δ³⁴S, within the ±1‰ (2σ) uncertainty of the method, but crystal-to-crystal variations are large (+2.5 to +10.9‰). The mean δ³⁴S for anhydrite (+6.4 ± 2.1‰, 1σ) is significantly lower than earlier results for bulk anhydrite separates (+9.0 to +9.2‰). The difference between the mean δ³⁴S values in these two populations may reflect a grain-size effect, with heavier sulfur concentrated in smaller anhydrite crystals, few of which were analyzed by ion microprobe. Variations in δ³⁴S show no correlation with complex textures in anhydrite revealed by cathodoluminescence color. Ion-microprobe analyses of δ³⁴S were also obtained on six ovoid-shaped inclusions of pyrrhotite, chalcopyrite, and/or intermediate sulfide solid solution hosted by silicate or oxide crystals, interpreted to be magmatic (δ³⁴S = −0.1 to +2.7‰; mean +0.7‰), and on four irregularly shaped multiphase sulfide fragments in the matrix, interpreted as xenocrystic, which range widely in δ³⁴S (−3.7 to +5.5‰). We evaluate four different mixing scenarios involving (1) magmatic anhydrite and sedimentary sulfate, (2) magmatic anhydrite and hydrothermal anhydrite, and anhydrite and coexisting sulfide crystals precipitated in different domains of a common magma reservoir that were affected by (3) different degrees of degassing or (4) different degrees of crustal sulfur contamination. The model involving physical contamination of sedimentary sulfate is considered untenable. The other three models are considered to be viable, but none of them can explain all observations. The results of this study and other recent investigations prompt a re-evaluation of the sulfur budget for the 1982 El Chichón eruption. We estimate that 2.2 × 10¹³ g of S was emitted, and that 58 wt.% of the sulfur was present as anhydrite prior to eruption, with the remainder in a vapor phase, with H₂S/SO₂ ≈ 9. The bulk magmatic δ³⁴S value for the 1982 El Chichón trachyandesite is estimated as +4.1 to +5.8‰, typical of the relatively heavy sulfur isotopic compositions that characterize subduction-related magmas.