Journal Article

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

Wilson, Colin J. N., Chambefort, Isabelle, Barker, Simon J., Faure, Kevin, Elms, Hannah C., Rooyakkers, Shane M., Charlier, Bruce L. A.

2023

Overview

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 Taupō Volcanic Zone (TVZ). We present >350 measurements of plagioclase, quartz, hornblende and groundmass glass δ18O values from 40 eruptions from three discrete magmatic systems: Ōkataina and Taupō calderas, and the smaller Northeast Dome system. For each mineral, mean δ18O values vary by ∼1‰ (δ18Oplag = +6.7–7.8‰, δ18Oqtz = +7.7–+8.7‰, δ18Ohbl = +5.4–+6.4‰, δ18Oglass = +7.1–+8.0‰), and inter‐mineral fractionations mostly reflect high‐temperature equilibria. Outliers (e.g., ∼+6‰ or >+10‰ plagioclase) represent contaminants incorporated on short‐enough timescales to preserve disequilibrium (∼102 yrs for plagioclase). Melt δ18O values calculated from phenocrysts are ∼+7.3–+8.0‰. Where multiple magmas were involved in the same eruption their δ18Omelt values are indistinguishable, implying that their parental mushes were isotopically well‐mixed. However, small (≤0.5‰) but consistent δ18Omelt value gradients occur over millennial timescales at Ōkataina and Taupō, with short‐term ∼0.4–0.5‰ decreases in δ18Omelt values over successive post‐caldera eruptions correlating with increases in 87Sr/86Sr. These changes reflect tens of percent assimilation of a mixture of hydrothermally altered silicic plutonic material and higher‐87Sr/86Sr 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‐δ18O 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 (18O/16O) ratios, which can thus be used to track infiltration of water into the crust. We measured the 18O/16O ratios of minerals from the products of 40 young eruptions from the highly active Taupō Volcanic Zone, New Zealand, to assess whether the erupted magmas had melted and incorporated rocks that were altered in this way. At both Taupō and Ōkataina volcanoes, we observe periods of subtle (but statistically significant) progressive lowering of magma 18O/16O 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 Taupō and Ōkataina volcanoes. Key Points Large silicic mushes in the Taupō Volcanic Zone are isotopically well‐mixed with respect to oxygen but show subtle temporal δ18O variations Temporal trends in melt δ18O values reflect transient interactions with altered and unaltered assimilants after caldera collapses Muted (sub‐permil) melt δ18O value variability reflects limited isotopic contrasts between magmas and country rocks in this setting

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: caldera / Calderas / Contaminants / Convection / Earth crust / Heat sources / High temperature / hydrothermal alteration / Hydrothermal flow / Isotopes / Lava / Magma / magma‐crust interaction / Minerals / Oxygen / Oxygen isotopes / Plagioclase / Ratios / Resolution / Rock / silicic volcanism / Strontium isotopes / Taupō Volcanic Zone / Temperature / Tracking / Volcanic eruptions / Volcanoes