A melt inclusion study of the Toba Tuffs, Sumatra, Indonesia

The Toba Caldera in Northern Sumatra, Indonesia is the site of Earth's largest Quaternary volcanic eruption. This eruption, dated at 74 ka, produced the 2800 km3 Youngest Toba Tuff (YTT) and ash-fall. Quartz-bearing silicic pumices ranging from 68 to 77 wt.% SiO2 indicate that the YTT magma was zoned compositionally and mineralogically. Prior to the YTT eruption, two other silicic quartz-bearing tuffs known as the Middle Toba Tuff (MTT) and the Oldest Toba Tuff (OTT) were erupted from Toba at 0.501 Ma and 0.840 Ma respectively. Although the volatile contents of the Toba magmas are poorly constrained, aerosols generated by the YTT eruption are generally thought to have caused a global volcanic winter. An evaluation of the pre-eruptive dissolved gas contents of the YTT magma is fundamental to understanding the global effects of this eruption. We used melt inclusions in quartz crystals to determine the dissolved H2O, CO2, S, Cl, and F contents of the YTT, MTT, and OTT magmas. Quartz crystals selected from pumice blocks and welded tuffs that spanned the compositional ranges of the three units were chosen for study. Major and trace element analyses of melt inclusions were also conducted to characterize melt evolution and provide context to the volatile data. Melt inclusions from the YTT, MTT, and OTT are rhyolitic in composition (73–77 wt.% SiO2) and have overlapping, indistinguishable major element trends. Inclusions from the large eruptions (YTT and OTT) have similar geochemical relationships where the most evolved melt inclusions occur in the least silicic bulk rock samples and are more evolved than their matrix glasses. In contrast, the least evolved melt inclusions occur in the most silicic samples and either overlap with, or are less evolved than their matrix glasses. These patterns can be explained if quartz crystals in the least silicic magmas were inherited from more evolved melt, while those in the most silicic melt formed in-situ. Several lines of evidence suggest that crystal settling of early-formed quartz crystals in long-lived magma bodies was an important process in the YTT and OTT magmas. Trace element data from the YTT melt inclusions have wide ranges (Rb = 250–830 ppm, Ba = < 10–485 ppm) and mostly support these models; however, incompatible trace elements are enriched 2–3× over their matrix glass compositions in all samples. In the smaller MTT, melt inclusions overlapped with the compositional range of the bulk rock samples and are consistent with in-situ crystallization. FTIR analyses indicate that the YTT, MTT, and OTT melts contained about 4.0–5.5, 2.0–5.5, and 2.0–5.5 wt.% H2O respectively. CO2 contents in all units are generally < 100 ppm and Cl is < 2000 ppm. Water gradients occur in all 3 tuffs, Cl gradients are evident only in the YTT, and the least silicic samples from all units had no detectable CO2. Sulfur contents in melt inclusions are low in all samples (< 32 ppm) and overlap with matrix glass S contents. Applying broad degassing constraints, about 1014 g of H2SO4 aerosols were likely loaded into the atmosphere during the YTT eruption, 2 orders of magnitude less than previous petrologic estimates. Considerably more Cl and F, about 1015 g of each, may have been part of the YTT aerosol cloud.