ERUPTIVE PROCESSES AND MAGMA RESIDENCE AT THE TAUPO VOLCANIC CENTER (NEW ZEALAND): INSIGHTS FROM RHYOLITE-MELTS GEOBAROMETRY, DIFFUSION CHRONOMETRY, AND CRYSTAL TEXTURES

Rhyolitic magmas dominate in the continental crust and have produced some of the most voluminous eruptions in Earth’s history; thus, studying them is key to mitigating future hazards and understanding the magmatic construction of the crust. Here, we combine crystal textures (crystal size distributions; CSD), crystal zoning (diffusion chronometry), and glass compositions (rhyolite-MELTS geobarometry) to investigate the storage, longevity, and eruption of the ~530 km³, 25.4 ka Oruanui supereruption and the ~35 km³, 1.72 ka Taupo eruption from the Taupo Volcanic Center (TVC) in the Taupo Volcanic Zone (TVZ), New Zealand. These systems evolved in broadly the same crustal context and are the largest eruptions from the TVC and the most recent caldera-forming ones in the TVZ. Yet, they differ greatly in volume and came from substantially different magmatic systems. As such, we address each system’s particularities and their places in the magmatic histories of the TVC and the greater TVZ.

Matrix and melt inclusion glass compositions suggest that the giant Oruanui magma was more evolved and resided at lower pressures (76.7-78.3 wt. % SiO₂; 50-250 MPa) than the large Taupo magma (74.5-75.9 wt. % SiO₂; 280-470 MPa). Microlite textures also differ: Oruanui CSDs show no microlites while Taupo CSDs show an abundance of tiny crystals; timescales from the latter are ≤1 a. Given the differences in storage depths and compositions we find, we hypothesize that this textural discrepancy reflects differences in the eruptive process each system experienced. We explore four possible causes and conclude that a combination of these factors may be needed: differences in ascent time and/or liquidus temperature (due to differences in storage depths) and differences in decompression rates and/or styles. Phenocryst segments of CSDs from both systems are parallel and suggest residence times of 10¹-10³ a; Ti-in-quartz diffusion chronometry in Oruanui samples gives similar times (10⁰-10² a). This highlights that the residence of the final melt-rich, eruptible magma bodies in the crust was strikingly short (decades to millennia) for both systems. These timescales are similar to those determined for other large-to-giant volume eruptions (e.g., Bishop Tuff) and suggest that such short residence times may be characteristic of these systems.