Insights into mafic enclave crystallisation at
Soufriere Hills Volcano from groundmass crystal
shape-size relationships

Abstract

Crystal size and shape in igneous rocks reflects the magmatic conditions experienced during their nucleation and growth. Whilst crystal size distributions are widely used in textural studies, crystal shape variability and its petrogenetic significance is not as well understood. Recent work has demonstrated that plagioclase nucleates as a prism and only develops its typical tabular shape during subsequent overgrowth (Mangler et al., 2022; 2023). It is yet unclear whether other phases besides plagioclase display a systematic evolution of shape with size.

Here, we reconstruct groundmass crystal 3D shape-size relationships in mafic enclaves from Soufriere Hills Volcano (SHV), Montserrat. Mafic enclaves preserve evidence of magma mixing events, a potential eruption trigger at SHV. Cooling of mafic enclaves in-situ provides a simple groundmass crystallisation scenario in which to explore new concepts of how crystal shape evolves with size.

Plagioclase evolves from prismatic to tabular with increasing size. Orthopyroxene potentially evolves from equant to bladed, with statistical validation being inconclusive. Amphibole does not show a systematic change in shape with size. Crystal growth modelling can reproduce the plagioclase shape-size relationships by modelling the growth of an initial prism to a range of overgrowth shapes. Overgrowth shape has been demonstrated experimentally to vary with melt composition, from higher relative growth rates in crystals grown in basaltic melts to lower relative growth rates in crystals grown in silicic melts (Mangler et al., 2023). Here, changing plagioclase shape with size is interpreted to reflect changing melt chemistry as successive populations nucleate and grow. The first plagioclase nucleate in basaltic melt and overgrow to tabular shapes. The melt becomes increasingly silicic with cooling and crystallisation. Successive populations nucleate in increasingly silicic melt, overgrowing to progressively more prismatic shapes. This is reflected in the plagioclase shape-size distributions in the mafic enclave, demonstrating
the petrogenetic significance of crystal shape.