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Crystal nucleation and growth produced by continuous decompression of Pinatubo magma

Abstract

High-temperature decompression experiments demonstrate that crystal textures preserve a record of the style and rate of magmatic ascent. To reinforce this link, we performed a suite of isothermal decompression experiments using starting material from the climactic 1991 Pinatubo eruption. We decompressed experiments from 220 MPa to final, quench pressures of 75 or 30 MPa using continuous decompression rates of 100, 30, 10, 3, 1, and 0.3 MPa h−1. Amphibole, clinopyroxene, and plagioclase crystallized during the experiments, with plagioclase microlites dominating the assemblage. Total microlite number densities range from 107.6±0.4 up to 108.2±0.2 cm−3, with plagioclase accounting for up to 65% of the total number. Plagioclase microlite area increased systematically from 19 ± 8 to 937 ± 487 µm2 with increasing experiment duration. Our textures provide time-integrated records of crystal kinetics. Average nucleation and areal growth rates of plagioclase are highest in the fastest decompressions (~ 107.5 cm−3 h−1 and 10.1 ± 4.1 µm2 h−1, respectively) and more than an order of magnitude lower in the slowest experiments (~ 105.5 cm−3 h−1 and 0.8 ± 0.2 µm2 h−1, respectively). Both nucleation and growth rates are highest at high degrees of disequilibrium. We find that peak supersaturation-dependent instantaneous rates are generally more than an order of magnitude faster than average rates. We use those instantaneous nucleation and growth rates to introduce an iterative model to evaluate the effects of different decompression rates, decompression paths (continuous, single-step or multistep), and the presence of phenocrysts on final crystallinity and microlite size distribution.

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Acknowledgements

We thank Smithsonian for sample NMNH 116563-1. Helpful reviews from Jenny Riker and an anonymous reviewer improved this manuscript.

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Correspondence to Kenneth S. Befus.

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Communicated by Mark S Ghiorso.

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Befus, K.S., Andrews, B.J. Crystal nucleation and growth produced by continuous decompression of Pinatubo magma. Contrib Mineral Petrol 173, 92 (2018). https://doi.org/10.1007/s00410-018-1519-5

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  • DOI: https://doi.org/10.1007/s00410-018-1519-5

Keywords

  • Experimental petrology
  • Disequilibrium
  • Crystallization
  • Decompression
  • Pinatubo
  • Microlite