Ca isotopes record rapid crystal growth in volcanic and subvolcanic systems

Abstract

Significance Timescale estimates for crystal growth in volcanic systems, which are generally much shorter than absolute crystal ages, are thought to be shortest in mafic (vs. silicic) systems. However, this result may be influenced by assumptions for growth rates and/or methodological biases. We show that Ca isotopes can be used to constrain crystal growth rates in volcanic systems and confirm that large kinetic isotope effects can arise during plagioclase crystallization in natural phenocrystic, orbicular, and experimental samples. Combined with crystal-size distribution analyses, our approach can constrain timescales for phenocryst formation across all major bulk magma compositions, without having to assume crystal growth rates. Our results yield comparable rejuvenation timescales for the silicic and mafic systems studied. Kinetic calcium isotope effects can be used as growth-rate proxies for volcanic and subvolcanic minerals. Here, we analyze Ca isotopic compositions in experimental and natural samples and confirm that large kinetic effects (>2‰) can occur during magmatic plagioclase crystallization. Experiments confirm theoretical predictions that disequilibrium isotope effects depend mainly on the rates for crystal growth relative to liquid phase Ca diffusivity (R/D). Plagioclase phenocrysts from the 1915 Mount Lassen rhyodacite eruption, the ∼650-y-old Deadman Creek Dome eruption, and several mafic subvolcanic orbicules and plagioclase comb layers from Northern California have disequilibrium Ca isotopic compositions that suggest rapid crystal growth rates (>1 cm/y to 15 cm/y). The Ca isotope results, combined with complementary crystal-size distribution analyses, suggest that magmatic rejuvenation (and eruption) events, as reflected in crystal growth times, can be as short as ∼10−3 y. Although mafic systems are predicted to have shorter magmatic rejuvenation periods, we find similarly short timescales in both mafic and silicic systems. These results are consistent with a growing body of evidence suggesting that dominantly crystalline volcanic magma reservoirs can be rapidly reactivated by the injection of fresh magma prior to eruption. By focusing on a common mineral such as plagioclase, this approach can be applied across all major magmatic compositions, suggesting that Ca isotopes can be used as a tool for investigating the dynamics and timing of volcanic eruptions.