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An experimental assessment of plagioclase shape as a proxy for mush solidification timescales

Lindoo, A. 1,2, Humphreys, M.C.S.1, Medard, E.3, Gordon, C.4, Mangler, M.F.5, Hughes, R.R.1,6; Brooker, R.A.2, and Holness, M.4

  • Affiliations: 1Department of Earth Sciences, Durham University, Durham, UK; 2School of Earth Sciences, University of Bristol, Bristol, UK; 3Laboratoire Magmas et Volcans, Université Clermont Auvergne - CNRS - IRD, OPGC; 4Department of Earth Sciences, University of Cambridge, Cambridge, UK; 5School of Ocean and Earth Science, University of Southampton, UK; 6Department of Earth and Environmental Sciences, University of Manchester, Manchester, UK

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 196

  • Programme No: 3.2.19

  • Theme 3 > Session 2

Abstract

Crystal sizes and shapes offer insights into magmatic and volcanic timescales. While studies have linked plagioclase aspect ratios to crystallization time in igneous intrusions (e.g., Holness, 2014), experimental validation under slow cooling rates and high crystal fractions remains limited. A causative mechanism remains lacking -- specifically, the role of crystal impingement in shaping plagioclase morphology is poorly understood. We conducted high-temperature, high-pressure experiments on anhydrous basalt compositions to simulate slow cooling (ΔT/Δt = 0.8°C/hr) and high crystal fractions (φ = 0.65), conditions representative of igneous intrusions. We measured plagioclase crystal shapes from BSE images and processed the length/width data using ShapeCalc [2]. Additionally, EBSD enabled examination of crystallographic growth relationships, while reanalysis of mafic dyke samples (Holness 2014;2017) allowed comparison of experimental and natural timescales. Our results show that in crystal-poor experiments, plagioclase aspect ratio correlates with crystallisation time, consistent with natural samples. This relationship reflects anisotropic growth mechanisms across crystallographic axes. However, in crystal-rich experiments, impingement of the c- and a-axes reduces aspect ratios, deviating from predictions under uninhibited growth. Modelling anisotropic growth with limited c- and a-axis dimensions reproduces these shapes, highlighting the impact of crystal impingement. These findings have important implications for interpreting magmatic solidification timescales using empirical relationships. By quantifying how crystal impingement influences plagioclase aspect ratios, we refine our understanding of when crystal shape can reliably indicate crystallization time in igneous intrusions and lava flow interiors.