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)