Ca isotopes record rapid crystal growth in volcanic and subvolcanic systems

Michael A Antonelli (1, 2) , Tushar Mittal (2) , Anders Mccarthy (3) , Barbara Tripoli (2) , James M Watkins (4) , Donald J. Depaolo (2)

1 IPGP (UMR_7154) - Institut de Physique du Globe de Paris
2 EPS - Department of Earth and Planetary Science [UC Berkeley]
3 School of Earth Sciences [Bristol]
4 University of Oregon [Eugene]

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.

Mots clés: crystal growth; timescales; Ca isotopes; volcanic eruptions; magma recharge

Domaines: Volcanologie

Michael A Antonelli, Tushar Mittal, Anders Mccarthy, Barbara Tripoli, James M Watkins, et al.. Ca isotopes record rapid crystal growth in volcanic and subvolcanic systems. Proceedings of the National Academy of Sciences of the United States of America, 2019, 116 (41), pp.20315-20321. ⟨10.1073/pnas.1908921116⟩. ⟨insu-02558381⟩