Sulfur Behavior in the 1257 CE Samalas Magma (Lombok, Indonesia) as Revealed by Volcanic Apatite
Explosive volcanic eruptions can inject massive amounts of volatiles, including SO2 and H2S, into the stratosphere, inducing drastic tropospheric cooling via sulfate aerosols. Greenland and Antarctic ice cores record evidence of these substantial releases of SO2 and H2S, including the massive releas...
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SMU Scholar
2022
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| Online Access: | https://scholar.smu.edu/hum_sci_earthsciences_etds/28 https://scholar.smu.edu/cgi/viewcontent.cgi?article=1030&context=hum_sci_earthsciences_etds |
| Summary: | Explosive volcanic eruptions can inject massive amounts of volatiles, including SO2 and H2S, into the stratosphere, inducing drastic tropospheric cooling via sulfate aerosols. Greenland and Antarctic ice cores record evidence of these substantial releases of SO2 and H2S, including the massive release during the ultraplinian 1257 CE Samalas eruption. The 1257 CE Samalas eruption is estimated to have released ~158 Tg of SO2, making it the largest volcanogenic release of SO2 within the last 2000 years. Vidal et al. (2016) propose that most of this sulfur (S) was hosted in a vapor phase present in the magma reservoir before the eruption. This study is part of a multi-institutional effort to characterize the pre-eruptive S behavior within the 1257 CE Samalas magma and determine the mechanism of S accumulation. In this study, S behavior within the Samalas magma was tracked by comparing measuring S concentrations and isotope compositions (34S/32S) in the 1257 CE and 536-811 BCE apatite. For the Samalas magma, we hypothesized that the magma acts as a closed system with respect to S, and sulfide crystallization and the degassing of S-bearing gases are the primary processes influencing the melt’s S concentration and isotope composition. Additionally, we proposed that the δ34S value of the melt would decrease as degassing depletes the melt of the 34S isotope. δ34S values from the two apatite samples are comparable to those from other arc volcanoes, which usually have very positive δ34S values due to the subduction of seawater sulfate, and suggest a large isotopic fractionation of 8.7‰, which rules out sulfide crystallization as the only process affecting the isotopic composition of the Samalas magma. The 536-811 BCE apatite exhibits a somewhat smaller range of δ34S values (+9.1 to +15.9‰, mean δ34S = +11.6‰) than the 1257 CE apatite (+7.0 to 15.6‰, mean δ34S = +10.4‰). Cl contents in many of the apatites were homogenous enough to derive a relative sensitivity factor, which was used to calculate S concentrations. S contents ... |
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