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Proc Natl Acad Sci U S A. 2019 Oct 8;116(41):20315-20321. doi: 10.1073/pnas.1908921116. Epub 2019 Sep 23.

Ca isotopes record rapid crystal growth in volcanic and subvolcanic systems.

Author information

1
Department of Earth and Planetary Science, University of California, Berkeley, CA 94720; mantonelli@berkeley.edu.
2
Department of Earth and Planetary Science, University of California, Berkeley, CA 94720.
3
School of Earth Sciences, University of Bristol, BS8-1RL Bristol, United Kingdom.
4
Department of Earth Sciences, University of Oregon, Eugene, OR 97403.

Abstract

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.

KEYWORDS:

Ca isotopes; crystal growth; magma recharge; timescales; volcanic eruptions

PMID:
31548431
PMCID:
PMC6789932
[Available on 2020-03-23]
DOI:
10.1073/pnas.1908921116

Conflict of interest statement

The authors declare no competing interest.

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