THE GROWTH HISTORY OF QUARTZ CRYSTALS FROM THE YOUNGEST TOBA TUFF, SUMATRA, INDONESIA

At 74 ka, a shallow compositionally zoned magma body explosively erupted 2800 km³ of silicic magma to form the Youngest Toba Tuff (YTT) and caldera. Pumice blocks from the YTT range from 68-77% SiO₂, and contain large euhedral quartz crystals, up to 2 cm in diameter. Melt inclusions, indicative of rapid crystal growth, and dissolution re-entrants, associated with an increase in magmatic temperature or dissolved volatiles, are common in the quartz crystals. In order to evaluate the cooling histories of these crystals and their pre-eruptive dynamics, 11 crystals from pumice blocks spanning the YTT compositional range were serially sliced (up to 6 slices per crystal) and the distribution of their melt inclusions and re-entrants were mapped. Distribution maps combined with cathodoluminescence (CL) images and geochemical data indicate the crystals have undergone contrasting cooling histories. Most crystals display 2-3 high density zones of melt inclusions, one in their centers, and another in the interior. In crystals from high SiO₂ samples, the third zone typically occurs near the crystal rim. Crystal slices with CL data indicate that quartz from low SiO₂ magma displays 2 CL zones, consisting of a dark interior zone and a thin bright rim. In contrast, quartz from high SiO₂ samples typically have a dark central zone, a bright interior zone, and a wide dark rim zone. Most melt inclusions (72-100%) are found in the dark CL zones. Quartz from low SiO₂ magma also contained abundant and long re-entrants, while those from high SiO₂ samples were far less common and shorter. Limited geochemical data on melt inclusions within the same crystal indicate that they become slightly more evolved from core to rim in crystals from the high-SiO₂ magma, whereas those from the low SiO₂ become less evolved towards their rims. Collectively, this data strongly suggests two very different cooling paths for quartz crystals from the high and low SiO₂ magmas. All crystals had a similar early period of rapid crystallization, indicated by central inclusion zones and dark CL zones. Quartz in the high SiO₂ magma record conditions dominated by undercooling and may have remained near the magma chamber roof prior to eruption. However, those in the low SiO₂ magma were experiencing slow crystallization and even dissolution in hotter less evolved magma before they erupted.