Rapid Reactivation of Rhyolite Magma at Tarawera Volcano: Insights From Quartz Cathodoluminescence, Chemistry and Melt Inclusions

Four rhyolite eruption episodes at Tarawera volcano, New Zealand have each simultaneously emitted several physiochemically-distinct magma batches (1-10 km3) from multiple vents, during the last 25 kyrs. These episodes were separated on a millennial timescale, and show evidence of basaltic magma injection acting as an eruption trigger. At the scale of pumice clasts (4 cm), the rhyolite magma batches are compositional distinct, but homogeneous in composition (Sr varies by 20 ppm; Zr by 18 ppm) and Fe-Ti oxide equilibrium temperature (40°C). Mingled pumice in some deposits contains bimodal crystal and glass compositional populations indicating isolated crystallisation histories and short-lived contact between the magmas. At the sub-cm scale, matrix glasses have trace element compositions (Sr, Ba, Rb) that vary by factors up to 2.5, indicating incomplete mixing of separate melts. Some quartz-hosted melt inclusions are depleted in compatible trace elements (Sr, Ti, Ba) compared to enclosing matrix glasses indicating silicic melts have percolated through an earlier formed, partly crystalline mass. The matrix glass is enriched in some elements (e.g., Ba) relative to intruded basalts indicating it is not the product of direct mixing with basalt melt. Instead, the re-melting of felsic crystals deeper in the crystal pile is inferred. Quartz crystals display a variety of cathodoluminescence zoning patterns and resorption boundaries consistent with multi-stage crystallisation. These patterns relate to Ti content and thus reflect changes in melt chemistry and/or temperature (50-100°C). Complex thermal histories are indicated by quartz cores with high or low Ti content occurring side-by-side; mid-crystal zoning peaks in Ti and/or Ti-rich rims. The abrupt changes in Ti profiles (90 ppm over 30 µm) are consistent with short thermal fluctuations (1 kyr). These features are consistent with a crystal mush zone with isolated pockets of melt that are periodically re-charged from the upward percolation of new silicic melts generated by basalt intrusion. The resulting fluctuations in temperature and volatiles induced quartz resorption and regrowth. An important implication is that such rhyolite volcanoes could be reactivated into life over short periods.