Re-melting of rhyolite crystal mush and priming for caldera-forming eruption revealed by trace element zoning patterns in phenocrysts of Okataina deposits

The Rotoiti (~120 km3) and Earthquake Flat (~10 km3) rhyolite eruptions occurred in close succession at the Okataina Volcanic Center, New Zealand at ~50 ka. While zircon geochronology points to episodes of crystallisation on the 100,000 yr timescale and separate thermal histories for the magmas, little was known about the rates and processes of the final melt production and eruption. Crystal zoning patterns in plagioclase and quartz reveal the thermal and compositional history of the magmatic system leading up to the eruption. Plagioclase displays considerable within-crystal zonation: An = 37-74, MgO = 40-227 ppm, TiO2 = 45-227 ppm, Sr = 416-910 ppm and Ba = 168-1164 ppm. Resorption horizons in the crystals are marked by sharp increases (10-30%) in Sr, MgO and An that reflect changes in melt composition. Quartz cathodoluminescence images also reveal zoning that is truncated by resorption, accompanied by abrupt increases in Ti concentration (30-80 ppm) that reflect temperature increases of ~50-110°C. Elemental diffusion across some resorption horizons is restricted to zones of <20 microns, suggesting that re-growth occurred in <2000 years prior to eruption. Such episodes are brief compared to the longevity of the crystal mush zone from which the magmas were extracted. Some melt inclusions in the phenocrysts are depleted in Sr and Ba relative to accompanying matrix glass, indicating disequilibrium. The melt inclusions also display a wide range in water contents that is consistent with volatile fluxing. The textural and compositional features observed within the quartz and plagioclase crystals are best explained by periodic mafic intrusions repeatedly melting parts of a crystal-rich zone and recharging the system with silicic melt. These periodic influxes of silicic melt were capable of feeding a caldera-forming scale eruption.