Plagioclase preferred orientation provides insights into magma flow dynamics of a basaltic dyke from the Budj Bim Volcanic Complex, Australia

Abstract

To understand the style, longevity and hazards of volcanic fissure eruptions, we need to understand the magma flow dynamics governing their feeder dykes. Microstructural analysis through optical microscopy and electron backscatter diffraction (EBSD) can be used to observe crystal alignment, shape and size from dyke samples to investigate magmatic fluid flow and deformation processes during the ascent of magma. Plagioclase is a mineral ubiquitously found in volcanic rocks that forms throughout magma crystallisation at a wide range of temperatures (~1300-400°C), and is therefore a mineral commonly used to investigate magma flow dynamics in dykes and sills. In particular, microlites of plagioclase (<0.1 mm) are amongst the last crystals to form at shallow crustal depths so have the potential to record the near surface flow behaviour and deformation processes that can lead to eruption.Here we examine elongate plagioclase microlites from oriented samples of a basaltic dyke from the Budj Bim Volcanic Complex, the only known fissure-fed eruption in the Newer Volcanics Province, south east Australia. Extensive quarrying of the Little Mount scoria cone has provided excellent exposure of the internal architecture of its feeder dyke. Optical microscopy observations show the dyke is comprised of zoned olivine phenocrysts (20%, <0.5 mm diameter), pyroxenes (20%, <0.1 mm diameter) and a fine-grained plagioclase-rich groundmass with minor oxides (5%, <0.05 mm diameter). EBSD of the plagioclase microlites shows they share a strong shape preferred orientation (SPO) and crystallographic preferred orientation (CPO). Plagioclase microlite orientation can be used as an indicator of magma flow, as elongate crystals may reorient due to flow and/or pure shear within the ascending magma. Using these novel data, we propose a model for plagioclase orientation, and therefore potential magma flow dynamics in the Little Mount dyke. Understanding the physical and chemical processes governing historic fissure eruptions such as the Budj Bim Volcanic Complex enables us to inform hazard mitigations for future fissure eruptions worldwide.