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Bi-directional flow of magma recorded by melt inclusion morphology

Chun Terence LEUNG1, Wei-Ran Li1

Abstract

Melt inclusions (MIs) in volcanic rocks are commonly used to probe magma composition. However, less information was extracted from MI morphology which can vary with crystal growth rate and pre-eruption conditions. We investigate this using a centimetre-sized anorthoclase megacrystal from Mount Erebus, Antarctica ([1]) and phonolitic MIs of varying morphology. Similar megacrystals from Erebus were suggested to grow at 950 ±25 °C and variable pressures and travel a few cycles between the magma reservoir and lava lake before eruption (e.g., [2]). We analyse the morphology (e.g., area, aspect ratio, roundness) of MIs (n = 687) across the megacrystal using ImageJ software. We find that aspect ratio is a key feature that links the MI morphology to the anorthoclase composition (e.g., orthoclase content). This can be explained by a strong pressure dependence of the growth rate and orthoclase content of Erebus anorthoclase ([2]). To investigate the number of convective cycles witnessed by the megacrystal, we divided it into elliptical zones with the same centre but different major-minor axes and applied a statistical test to assess the difference between zones. We find that the innermost zone is distinct from those at the crystal mantle but similar to zones closer to the rim. Combined with pressure estimates from the anorthoclase, these imply at least one convective cycle before the final ascent to the surface. Our findings show that MI morphology variability can be linked to changes in crystal growth rates and pre-eruption conditions. [1] Li et al. (2023). [2] Moussallam et al. (2015).