T241 Subduction Zone Magmatism from Source to Eruption, in the Lab and in the Field: In Honor of Christy Till, GSA Mineralogy, Geochemistry, Petrology, and Volcanology Division Early Career Awardee

Wednesday, 25 September 2019
8:00 AM - 12:00 PM
Room 131ABC, North Building

SUPERSATURATION NUCLEATION AND GROWTH OF PLAGIOCLASE IN RESPONSE TO ERUPTIVE DECOMPRESSION

Paper number: T241-7
Program: Technical Sessions
Time: 9:50 AM – 10:10 AM

Authors

Abstract

Magma is forced into disequilibrium during decompression and returns to equilibrium through nucleation of microlites and growth on preexisting crystals. The resulting textures can be used to infer the decompression pathway of explosive and effusive eruptions, especially when compared with high temperature experiments. We performed a series of experiments using starting material from the climactic 1991 Pinatubo eruption. We decompressed experiments using continuous rates ranging from 100 to 0.3 MPa h-1. Plagioclase microlite number densities range from 107 to 108 cm-3. Plagioclase microlite area increased systematically from ~20 μm2 to ~1000 μm2 with increasing experiment duration. Average nucleation and areal growth rates of plagioclase are highest in the fastest decompressions (~107.5 cm-3 h-1 and ~10 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 μm2 h-1, respectively). We used these results to develop Supersaturation Nucleation and Growth of Plagioclase (SNGPlag), a numerical model that predicts the evolution of plagioclase crystallization in a decompressing magma. SNGPlag uses the MELTS web service to determine equilibrium plagioclase mode as a function of pressure and temperature. At each time step the model evaluates the difference between the calculated crystallinity and equilibrium crystallinity for a given pressure and temperature to determine the degree of supersaturation, which then sets plagioclase nucleation and growth rates. Growth rates are used to grow existing crystals whereas nucleation rates add new microlites. SNGPlag is thus experimentally and thermodynamically calibrated and produces results that can be used as a tool to quantify textures in natural volcanic rocks. We used SNGPlag to reproduce textures observed from the 2008 Chaitén, 1902 Santa Maria, 1980-1986 Mt St Helens, and 1800 BP Ksudach eruptions. Textural similarity to natural samples, or lack thereof, isolates the complex interplay and relative importance of variables and changing conditions. SNGPlag commonly predicts decompression rates that are slower than those indicated by previous work, likely caused by reliance on stepped decompressions or for failing to account for the effects of preexisting crystals on microlite nucleation and growth.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-332274

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.