CRYSTALLIZATION HISTORY OF QUARTZ PHENOCRYSTS FROM THE YOUNGEST TOBA TUFF

The largest Quaternary volcanic eruption on Earth occurred in Sumatra, Indonesia 74,000 years ago forming the Toba caldera. The 2800 km³ Youngest Toba Tuff (YTT) represents eruption of a compositionally zoned silicic magma chamber. Melt inclusions in quartz crystals from the YTT have been studied to provide constraints on pre-eruptive volatile contents and magmatic evolution. Several features of Toba quartz are unusual including their extraordinary size (up to 2 cm), euhedral hexagonal shapes, highly shattered condition, and abundant melt inclusions. Numerous melt inclusions are typically present in each quartz phenocryst and consist mostly of hexagonal dipyramidal negative crystal inclusions, up to 400 μm in size that are isolated or arranged in trains along growth surfaces. Hourglass inclusions with openings to the crystal rim are also common and can exceed 1 mm in length. Dependent upon cooling history of the host pumice sample, sealed inclusions can be glassy to extensively devitrified, contain proportional sized shrinkage bubbles or none at all, and occasionally host daughter crystals. Many inclusions are intersected by cracks in their host quartz crystal and consequently have large random sized or multiple vapor bubbles. Inclusions larger than 200 μm commonly have burst, breaking the host quartz crystal into numerous fragments. The unusual nature of the quartz phenocrysts coupled with the geochemistry of sealed glassy inclusions and other indirect evidence, suggests that Toba quartz have had a long and complex crystallization history. Because we expect these crystals to have traversed various temperature and compositional gradients during an extended pre-eruptive residence period of 150,000 years or more, they are ideal candidates for cathodoluminescence (CL) imaging. Using the framework of CL imaging, a complete textural analysis of Toba quartz phenocrysts and their melt inclusions will be presented. Growth histories of these quartz crystals as indicated by CL zoning will provide valuable context for melt inclusion geochemistry and further constrain magmatic processes that occurred in the Toba magma chamber prior to eruption of the YTT.