Quartz Crystallization in the Youngest Toba Tuff Magma Chamber and its Remnants: A Complex Lineage Uncovered by CL Zoning

Abstract:

The giant >2800 km³ Youngest Toba Tuff (YTT) and its post-caldera, crystal-rich rhyolitic lava domes share identical compositions (69-76 wt. % SiO₂), mineralogies, mineral textures, and zircon and allanite U-Th ages (Barbee et al., 2014). To further understand the genetic relation between the YTT and the domes, we conducted an extensive in-situ cathodoluminescence (CL) study on serial slices of their unusual ≤2 cm, euhedral quartz crystals. Several important magma processes are recorded by CL zoning of Toba quartz, including (1) episodes of undercooling and irregular crystal growth (skeletal and dendritic zones, melt entrapment, growth embayments), (2) crystal dissolution (rounded cores and truncated zones), (3) quartz clustering (synneusis, parallel growth or twinning), and (4) fluctuating magma conditions and dynamics (contrasts in CL intensity). The associated features (1–3) are defined by growth zone configurations, whereas distinct cores, mantles, rims, and oscillatory zones mainly represent differences in trace element concentrations (4). Nearly all magma compositions are dominated by quartz with continuous subtle core to rim oscillatory zoning, while mid- to lower-SiO₂ samples contain the largest proportion of crystals with bright interior and rim zones consistent with crystal settling into a high-T environment or injection of hotter magma. The diversity of near-rim zoning patterns and CL boundary sharpness in single samples indicates coalescence of quartz with dissimilar, late-stage histories; however, interior zoning patterns broadly correlate with low- and high-SiO₂ bulk magma compositions similarly for the YTT and lava domes. Although a second generation of quartz crystallization is apparent in the groundmass of the lavas, its occurrence is not visually evident on the majority of dome (or recycled-YTT) phenocrysts. Processes promoting cluster formation appear important for quartz growth at Toba and may be related to those responsible in plutonic systems (Beane and Wiebe, 2012). The complex but comparable quartz growth histories and composite crystal forms between the YTT and domes suggest they experienced similar conditions during quartz crystallization and may share a common ancestry.