Using Zircon-Hosted Melt Inclusions to Track the Late Volatile Evolution of the 74 ka Youngest Toba Tuff, Sumatra

Abstract:

Melt inclusions provide important constraints on the behavior of magmatic volatiles; however, our understanding is hindered by a lack of temporal constraints on volatile evolution. Melt inclusions in zircon crystals have the unique potential to provide a temporal record of pre-eruptive magmatic volatile abundances through radiometric U-Pb or U-Th dating of zircon hosts.

We present work on zircon-hosted melt inclusions (ZHMIs) from the ~74 ka Youngest Toba Tuff (hereafter: Toba), Sumatra, Indonesia. Zircon separated from Toba pumice contain abundant melt inclusions of ovoid or irregular-wormy forms, which range in size from 5-50 μm. Cathodoluminescence (CL) imaging of Toba zircon show that many ovoid melt inclusions and all the irregular melt inclusions occur within areas of zircon growth following earlier dissolution events. These zones crosscut oscillatory-zoned zircon and have negligible CL variability, suggesting that remineralizion and entrapment of melt inclusions may have occurred relatively quickly. Some inclusions do occur within regions of oscillatory CL zonation, indicating melt entrapment also occurs during primary crystal growth.

Electron microprobe analyses show no significant compositional differences between ovoid and irregular-shaped inclusions. Toba ZHMIs are chemically similar to quartz-hosted melt inclusions (Chesner and Luhr, 2010) and to the most differentiated Toba pumice glass (Chesner, 1998). Zircon saturation calculations (Boehnke et al, 2013) from ZHMI chemistries indicate that zircon saturated at ~750-770°C, consistent with Fe-Ti geothermometry from Toba pumice (700-780°C; Chesner and Luhr, 2010). Chlorine in zircon inclusions range from 0.11-0.17 wt%, which are within the range measured in quartz-hosted inclusions (0.11-0.20 wt%). Non-degassed matrix glass has a similar maximum chlorine content of 0.15 wt%, while out-gassed samples have Cl as low as 0.08 wt% (Chesner and Luhr, 2010). Evidence indicates that zircon saturated late and trapped melt compositions approaching the endpoint of Toba magmatic evolution prior to eruption.

SIMS measurements of melt inclusion H, C, and S are planned for this fall. Additionally, U-Th age measurements of zircon surrounding the inclusions are planned to constrain timescales of the late chemical evolution of the Toba system.