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Variation of plagioclase shape with size in intermediate magmas: a window into incipient plagioclase crystallisation

Mangler, Martin F.; Humphreys, Madeleine C.S.; Wadsowrth, Fabian B.; Iveson, Alexander A.; Higgins, Michael D.

Variation of plagioclase shape with size in intermediate magmas: a window into incipient plagioclase crystallisation Thumbnail


Authors

Michael D. Higgins



Abstract

Volcanic rocks commonly display complex textures acquired both in the magma reservoir and during ascent to the surface. While variations in mineral compositions, sizes and number densities are routinely analysed to reconstruct pre-eruptive magmatic histories, crystal shapes are often assumed to be constant, despite experimental evidence for the sensitivity of crystal habit to magmatic conditions. Here, we develop a new program (ShapeCalc) to calculate 3D shapes from 2D crystal intersection data and apply it to study variations of crystal shape with size for plagioclase microlites (l<100 μm) in intermediate volcanic rocks. The smallest crystals tend to exhibit prismatic 3D shapes, whereas larger crystals (l >5-10 μm) show progressively more tabular habits. Crystal growth modelling and experimental constraints indicate that this trend reflects shape evolution during plagioclase growth, with initial growth as prismatic rods and subsequent preferential overgrowth of the intermediate dimension to form tabular shapes. Because overgrowth of very small crystals can strongly affect the external morphology, plagioclase microlite shapes are dependent on the available growth volume per crystal, which decreases during decompression-driven crystallisation as crystal number density increases. Our proposed growth model suggests that the range of crystal shapes developed in a magma is controlled by the temporal evolution of undercooling and total crystal numbers, i.e., distinct cooling/decompression paths. For example, in cases of slow to moderate magma ascent rates and quasi-continuous nucleation, early-formed crystals grow larger and develop tabular shapes, whereas late-stage nucleation produces smaller, prismatic crystals. In contrast, rapid magma ascent may suppress nucleation entirely or, if stalled at shallow depth, may produce a single nucleation burst associated with tabular crystal shapes. Such variation in crystal shapes have diagnostic value and are also an important factor to consider when constructing CSDs and models involving magma rheology.

Citation

Mangler, M. F., Humphreys, M. C., Wadsowrth, F. B., Iveson, A. A., & Higgins, M. D. (2022). Variation of plagioclase shape with size in intermediate magmas: a window into incipient plagioclase crystallisation. Contributions to Mineralogy and Petrology, 177(6), Article 64. https://doi.org/10.1007/s00410-022-01922-9

Journal Article Type Article
Acceptance Date May 16, 2022
Online Publication Date Jun 22, 2022
Publication Date 2022-06
Deposit Date May 23, 2022
Publicly Available Date Jul 14, 2022
Journal Contributions to Mineralogy and Petrology
Print ISSN 0010-7999
Electronic ISSN 1432-0967
Publisher Springer
Peer Reviewed Peer Reviewed
Volume 177
Issue 6
Article Number 64
DOI https://doi.org/10.1007/s00410-022-01922-9
Public URL https://durham-repository.worktribe.com/output/1206523

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Publisher Licence URL
http://creativecommons.org/licenses/by/4.0/

Copyright Statement
This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.






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