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

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