Isotopic and Geochemical Records of Magmatic Processes Captured in Crystals from Rhyolites at Okataina Volcano

Abstract

The Okataina Volcanic Centre (OVC) is a dormant silicic caldera in the Taupo Volcanic Zone of North Island, New Zealand that has been active since ca. 0.6 Ma. Eruption episodes at the OVC occur on a millennial scale and range from rhyolitic caldera-forming and dome-building events to mafic fissures. To understand how mafic and crustal inputs into magma genesis have varied across the life of the volcano, isotopic fluctuations in silicic minerals from ten temporally, volumetrically, and spatially varied rhyolite units (two caldera-forming, one syn-caldera, four intra-caldera, and three extra-caldera) were investigated. This study examines textures and compositions of major mineral phases along with, for the first time at the OVC, their sub-crystal isotopic compositions. Sub-crystal isotopic investigations consist of ⁸⁷Sr/⁸⁶Sr ratios within plagioclase, a ubiquitous and early-crystallizing phase, and δ¹⁸O within quartz, an abundant later-crystallizing phase. Results suggest diverse origins and transport of plagioclase and quartz cores prior to assembly in the erupting magma. In contrast, the outermost rims of plagioclase and quartz suggest crystallization in more uniform melts. Despite geochemical variations, sub-crystal isotopic compositions are homogeneous in plagioclase (⁸⁷Sr/⁸⁶Sr = ~0.705-0.706) and quartz (δ¹⁸O = ~7.6‰) to within the resolution of this study (±0.0004 average 2σ and ± 0.12‰ average 2sd, respectively). Higher-precision whole crystal Sr and Pb isotopes in plagioclase and oxygen isotopes in quartz were undertaken as well. These analyses also demonstrate prevalent homogeneity across the magmas, although variations in Pb isotopes are noted in caldera-forming magmas. Isotopic mixing models using sub-crystal and whole crystal isotopic ratios of plagioclase and quartz imply likely origins from subduction-modified basaltic melts that experienced ≥20% contamination from regional Torlesse metasediments. However, variations in mineral and whole rock ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb suggest contributions from additional sources are likely (e.g., Waipapa metasediments, metaigneous lower-crust, and/or unknown lower crust). Findings from this study demonstrate fairly consistent radiogenic and stable isotopic homogeneity across the history of this volcanic system, indicating steady inputs from mantle and crustal sources over time. This contrasts with other silicic systems that often exhibit dramatic shifts in isotopic compositions in pre- or post- caldera-forming eruptions (e.g., Yellowstone, Toba).