HR: 0800h
AN: V41D-0826    [Abstracts]
TI: A Melt Inclusion Study of the Middle and Oldest Toba Tuffs, Sumatra, Indonesia: Initial Results
AU: * Chesner, C A
EM: cachesner@eiu.edu
AF: Eastern Illinois University, Department of Geology/Geography, Charleston, IL 61920, United States
AU: Ens, R
AF: Eastern Illinois University, Department of Geology/Geography, Charleston, IL 61920, United States
AB: The Toba Caldera Complex in northern Sumatra was the site of 4 caldera forming eruptions in the past 1.2 m.y. Tuffs erupted at 1.2, 0.840, 0.501, and 0.074 Ma are known as the Haranggoal Dacite Tuff (HDT), Oldest Toba Tuff (OTT), Middle Toba Tuff (MTT), and Youngest Toba Tuff (YTT) respectively. A previous study determined the pre- eruptive volatile contents of the 2800 km³ YTT using melt inclusions in quartz crystals. In an effort to characterize the evolution of the Toba magma system, we recently studied melt inclusions from the MTT and OTT. Quartz crystals from 3 samples that spanned the compositional ranges of each tuff were chosen for study. In the MTT, melt inclusions were rhyolitic (73-77 wt. % SiO₂) and overlapped with the compositional range of the bulk rock samples. Melt inclusions in the OTT were also rhyolitic (74-77 wt. % SiO₂) but were far more silicic than their bulk rock samples (69-73 wt. % SiO₂). In contrast to the MTT, the most silicic melt inclusions in the OTT were found in the least evolved rock samples. Water contents determined by FTIR in the MTT range from 3.0-5.5 wt. %, while those in the OTT are 2.0-5.5 wt. %. Melt inclusions with the lowest water contents are found in the least evolved rock samples from both units. Both MTT and OTT inclusions contained about 20-120 ppm CO₂. In the OTT, inclusions with the highest CO₂ contents had the least silicic glass composition, but were from the most evolved bulk rock samples. Cl contents of the melt inclusions range from 1200-1800 ppm in the MTT and 1000-2000 ppm in OTT. Considerable Cl degassing is suggested by matrix glass analyses with distinctly lower Cl contents. S contents are low (<20 ppm) and near detection limits in melt inclusions from both units. Comparing MTT and OTT melt inclusions with those of the YTT indicates overlapping and indistinguishable major element geochemistry. The least evolved bulk rock samples from both the OTT and YTT contain the most evolved melt inclusions, unlike the MTT. Water contents of the MTT and OTT have a wider range than the YTT (4.0-5.5 wt. %) and gradients are recorded in the OTT and MTT magma chambers, whereas none is evident in the YTT magma. Dissolved S contents were low in all 3 magmas prior to their eruption.
DE: 1036 Magma chamber processes (3618)
DE: 1043 Fluid and melt inclusion geochemistry
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting