Statistical perspective on the petrologic utility of polyphase groundmass compositions inferred via defocused beam electron probe microanalysis

Geostandards and Geoanalytical Research
By: , and 

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Abstract

Polyphase groundmasses (micro-scale minerals with or without glass) are generated from silicate liquids during the cooling of natural lavas often alongside larger minerals formed long before eruption. Many researchers have posited that compositions gleaned from the analysis of groundmasses closely approximate the compositions of the melts they were derived from, and these have been used frequently to model pre-eruptive magma conditions. However, it is difficult to confidently identify and sample these groundmasses once they are formed. Using a sample of lava that exhibits a wide degree of textural variation (ranging from holocrystalline to hypohyaline) we show that compositions of groundmasses sampled using defocused electron beams are significantly different from glass compositions in terms of mean composition and covariance. Despite this, several groundmass compositions qualify as ‘in equilibrium’ with matrix/rim olivine. When processed using available thermometers, however, modelled equilibrium temperatures are significantly higher than those produced using glass data, on average. Because of this, we prescribe caution in using polyphase groundmass data generated using defocused beam analysis even as a rudimentary approach.

Publication type Article
Publication Subtype Journal Article
Title Statistical perspective on the petrologic utility of polyphase groundmass compositions inferred via defocused beam electron probe microanalysis
Series title Geostandards and Geoanalytical Research
DOI 10.1111/ggr.12546
Volume 48
Issue 2
Year Published 2024
Language English
Publisher Wiley
Contributing office(s) Volcano Science Center
Description 14 p.
First page 345
Last page 358
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