Tracking Magma-Crust-Fluid Interactions at High Temporal Resolution: Oxygen Isotopes in Young Silicic Magmas of the Taup Volcanic Zone

Oxygen isotopes are useful for tracing interactions between magmas, crustal rocks and surface-derived waters. We use them here to consider links between voluminous silicic magmatism and large-scale hydrothermal circulation in New Zealand's central Taup Volcanic Zone (TVZ). We present >350 measurements of plagioclase, quartz, hornblende and groundmass glass delta O-18 values from 40 eruptions from three discrete magmatic systems: Okataina and Taupo calderas, and the smaller Northeast Dome system. For each mineral, mean delta O-18 values vary by similar to 1 parts per thousand (delta O-18(plag) = +6.7-7.8 parts per thousand, delta O-18(qtz) = +7.7-+8.7 parts per thousand, delta O-18(hbl) = +5.4-+6.4 parts per thousand, delta O-18(glass) = +7.1-+8.0 parts per thousand), and inter-mineral fractionations mostly reflect high-temperature equilibria. Outliers (e.g., similar to+6 parts per thousand or >+10 parts per thousand plagioclase) represent contaminants incorporated on short-enough timescales to preserve disequilibrium (similar to 10(2) yrs for plagioclase). Melt delta O-18 values calculated from phenocrysts are similar to+7.3-+8.0 parts per thousand. Where multiple magmas were involved in the same eruption their delta O-18(melt) values are indistinguishable, implying that their parental mushes were isotopically well-mixed. However, small (<= 0.5 parts per thousand) but consistent delta O-18(melt) value gradients occur over millennial timescales at kataina and Taup, with short-term similar to 0.4-0.5 parts per thousand decreases in delta O-18(melt) values over successive post-caldera eruptions correlating with increases in Sr-87/Sr-86. These changes reflect tens of percent assimilation of a mixture of hydrothermally altered silicic plutonic material and higher-Sr-87/Sr-86 greywacke. These examples represent the first evidence for assimilation of altered crust into TVZ magmas. The subtle and short-lived isotopic signals of these interactions are only recognized through the high temporal resolution of the TVZ eruptive record and complementary radiogenic isotope data. Similar interactions may have been obscured in other nominally high- or normal-delta O-18 magmatic systems. Plain Language Summary Heat coming from large sub-volcanic magma systems drives convection of surface-derived waters through the upper few kilometers of Earth's crust, forming hydrothermal systems. The nature and depth of the interface between hydrothermal systems and their underlying magmatic heat sources are often not well constrained. High temperature alteration of rocks by surface-derived waters lowers rock oxygen isotope (O-18/O-16) ratios, which can thus be used to track infiltration of water into the crust. We measured the O-18/O-16 ratios of minerals from the products of 40 young eruptions from the highly active Taup Volcanic Zone, New Zealand, to assess whether the erupted magmas had melted and incorporated rocks that were altered in this way. At both Taupo and Okataina volcanoes, we observe periods of subtle (but statistically significant) progressive lowering of magma O-18/O-16 ratios over successive eruptions, suggesting that their magmatic systems at times overlapped and interacted with overlying hydrothermal systems. At both volcanoes, these reductions occurred as the magmatic system was rebuilt to shallow levels in the crust following very large (caldera-forming) eruptions. The subtle and short-lived signals of these interactions are only recognized here because of the unusually high eruption frequencies of Taupo and Okataina volcanoes.