203 Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder

We encourage papers that challenge and give insights into the interplay between timescales, methodologies, and processes to build a cohesive understanding of the lifespan, evolution, storage, and eruption of magma, from plutonic to volcanic systems.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 216AB

Constraining Complex Chronologies at Aira Caldera using a Confluence of Crystal Clocks

Paper number: 203-2
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Abstract

Crystals record variable magmatic timescales, from mush crystallization to defrosting, melt extraction, eruption triggering, and syn-eruptive ascent. Multiple methods are needed to capture this complexity, with some converging and others representing distinct processes; this reduces the risk of emulating the proverbial blind men describing an elephant.

 

To reconstruct the magmatic chronology leading to the caldera-forming eruption (CFE) of Aira, Japan, we integrate textural and compositional data with multiple diffusion chronometers for cores and rims of quartz, plagioclase, and pyroxene. We focused on a unique set of four high-silica rhyolite eruptions at Aira (30–33.5 ka): the 400 km³ CFE and three smaller precursors (<2 km³). We used multivariate statistics to distinguish pumice populations within each eruptive horizon to avoid sampling bias and fully characterize magmas. Using two independent chronometers, Ti in quartz and Sr in plagioclase, plus petrographic evidence (textures and melt inclusion faceting), we show that all magmas, including the CFE, were stored in melt-rich, eruptible conditions (>~750°C) for <1–2.5 ka, implying rapid defrosting of a common mush. Higher Li in plagioclase cores, higher Fe-Ti oxide and zircon saturation temperatures, and a less-evolved rhyodacitic population within the CFE point to more extensive mush rejuvenation by hotter, fluid-rich mafic recharge to produce the significantly greater melt volume. But this is only the start!

 

Plagioclase and quartz textures suggest only minor P-T changes during the next ~800 years of crystallization. Ti diffusion in quartz indicates a final recharge and resorption event followed by limited growth (10–50 µm) within ~5 years of eruption (at 800°C). FTIR measurements of H₂O and CO₂ in unexposed quartz-hosted melt inclusions (MIs) show most have a minimum of 4–5 wt.% H₂O (80–140 MPa), consistent with rhyolite-MELTS equilibration pressures (50–170 MPa). Many individual crystals contain multiple MIs with variable H₂O concentrations, up to 7 wt.%, suggesting deeper storage and incomplete H equilibration during ascent. Preliminary 3D modelling of H loss from MIs suggests ascent in several hours to days, consistent with previous estimates using diffusion along melt reentrants. Li diffusion in plagioclase from the CFE Ignimbrite supports final decompression within 5–15 hours.

 

Our work demonstrates the need for multiple independent methods with multiple phases, alongside careful textural and compositional characterization, to begin confidently reconstructing comprehensive magma histories.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414880

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