Paper No. 157-4
Presentation Time: 9:00 AM-1:00 PM
SULFUR BEHAVIOR IN THE 1257 CE SAMALAS MAGMA (LOMBOK, INDONESIA) AS REVEALED BY SULFUR ISOTOPE RATIOS IN APATITE
The 1257 CE Samalas eruption on Lombok, Indonesia, released ~158 Tg of SO2, resulting in the largest volcanic SO2 release within the last 2000 years1. Pre-eruptive degassing is interpreted to have contributed ~80% of this SO2 release. In this study, we evaluate this proposed mechanism of excess sulfur by tracking sulfur behavior throughout the evolution of the Samalas trachydacite magma. We present sulfur isotope (δ34S) compositions in apatite microphenocrysts and inclusions in plagioclase from the 2550 BP and 1257 CE Samalas pumices. Secondary ionization mass spectrometry analyses of these apatites yield a broad range of positive δ34S values (+8.9 to +16.1‰ in 2550 BP apatite; +8.4 to +15.9‰ in 1257 CE apatite), revealing the trachydacite melt to be 34S-rich. These δ34S values increase with lower 32S ion yields, suggesting that the melt became more 34S-rich with sulfur loss. Sulfur concentration measurements will permit a more quantitative interpretation of the relationship between sulfur in the melt and δ34S values. Since 34S preferentially partitions into oxidized sulfur species, 34S enrichment of the melt is consistent with the increase in S6+/∑S ratios observed in melt inclusions2. Sulfur degassing likely contributed to the ~7‰-isotopic fractionation from an interpreted initial δ34S value of ~8.5‰, as this process has been previously observed to induce sulfur isotope fractionations of several ‰, even at magmatic temperatures3. Preliminary isothermal decompression models made using the D-Compress program4 suggest degassing from an initially highly oxidized system (i.e., ≥NNO+2) could produce the observed 34S-enrichment. With this study, we illustrate the potential of sulfur isotope records in apatite to enrich our current understanding of the processes preceding massive volcanic SO2 stratospheric injections, which can cause years-long, devastating global cooling events.
1Vidal et al. (2016)
2Ding et al. (2020)
3e.g., Sakai (1968)
4Burgisser et al. (2015)