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 within both apatite samples span from 0.007 to 0.56 wt% S (73 to 5622 ppm S).
The Samalas apatites imply a more complex S evolution than was initially predicted. The 536-811 BCE apatites indicate a degassing trend towards more positive δ34S values, contradicting the proposed hypothesis that degassing of S-bearing gases would deplete the melt of 34S. The outliers of 1257 CE apatites seem to imply the same degassing trend, but when we closely examined how S varies from core to rim in the apatites with the most distinct zoning patterns, there are three general processes suggested to have influenced S in the 1257 CE Samalas magma. 1) Grains with noticeable variations in S concentrations yet little change in δ34S values might be capturing sulfide crystallization. 2) Grains with both S concentrations and δ34S values decreasing from core to rim indicate degassing towards less positive δ34S values, as was initially hypothesized. 3) Two individual grains might be capturing magmatic recharge, as implied by their complex, light-dark-light-dark pattern.
To define how the S records in these apatites compare to those captured by the melt, we will be comparing our S data with those of Ding et al. (in prep), who collected these data for the melt inclusions and matrix glasses. We will also compare these data to a degassing model created by Ding et al. (in prep) to identify the S6+/ΣS and SO2/H2S ratios required to produce the trends observed in the apatites, melt inclusions, and matrix glasses. This study is the first to combine volatile records from apatite crystals and melt products. The methodology highlighted in this study and others from this multi-institutional effort will provide a novel perspective of how S evolves in volcanic systems, particularly those capable of climatically significant eruptions, and inform volcanic hazard assessments on Lombok and neighboring Indonesian islands.
Crayton J. Yapp
Robert T. Gregory
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Jackson, Yasmin, "Sulfur Behavior in the 1257 CE Samalas Magma (Lombok, Indonesia) as Revealed by Volcanic Apatite" (2022). Earth Sciences Theses and Dissertations. 28.