USING ZIRCON-HOSTED MELT INCLUSIONS TO TRACK MAGMA CHAMBER EVOLUTION: A CASE STUDY OF THE 74 KA YOUNGEST TOBA TUFF, SUMATRA

The mineral zircon provides a powerful means to investigate the pre-eruptive evolution of silicic magma systems via geochronological and geochemical studies. Additional sources of information, largely underutilized to date, are mineral and melt inclusions hosted within zircon. Zircon-hosted melt inclusions (ZHMIs) are common in volcanic rocks and have the potential to provide a unique temporal record of pre-eruptive magmatic volatile abundances because the zircon host can be radiometrically dated using U-Pb and U-Th methods.

We present early results from a study of ZHMIs from the ~74 ka Youngest Toba Tuff (hereafter, Toba), Sumatra, Indonesia. Zircon separated from Toba pumice contain abundant melt inclusions ranging in size from 5-50 μm, occurring with either spherical-ovid or irregular-wormy forms; the latter are suggestive of entrapment of melt tubes following dissolution events. Cathodoluminescence (CL) imaging of Toba zircon show that the majority of both ovid and irregular melt inclusions occur within areas that appear to represent mineral growth following earlier dissolution events. These zones have negligible CL variability and crosscut oscillatory-zone zircon, suggesting that remineralizion and entrapment of “dissolution” melt inclusions occurred relatively quickly. Some inclusions, however, do occur within regions with oscillatory CL zonation, suggesting entrapment also occurs during primary growth.

Electron microprobe measurements of Toba ZHMIs show compositions matching the most evolved portion of Toba pumice (total glass compositional span of 68-78 wt% SiO₂). No significant compositional differences exist between the “primary” and “dissolution” type melt inclusions, which both have anhydrous SiO₂ contents of 76-78 wt%, K₂O between 4.5-6 wt%, and Cl between 0.11-0.14 wt%. These ranges are very similar to that observed by Chesner and Luhr (2010) in most Toba quartz-hosted melt inclusions, consistent with both quartz and zircon saturating late in the Toba magmatic evolution. The compositional agreement of ZHMIs with Toba quartz inclusions and pumice glass indicates that, despite their small sizes, ZHMIs are free of significant boundary layer or post-entrapment crystallization effects. Consequently, ZHMIs can be reliable recorders of melt composition at the time of entrapment.