Timescales of magma ascent recorded by olivine zoning patterns from Mount Leura and Mount Noorat, Newer Volcanics Province, Australia

Didonna, Rosa1, Handley, Heather1, Cas, Ray2, Fidel, Costa3, Murphy, Timothy1

1Department of Earth and Environmental Sciences, Macquarie University, Sydney, NSW, Australia 2School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, Australia 3Earth Observatory of Singapore, Nanyang Technological University, Singapore

Intraplate continental basaltic volcanic provinces (ICBVPs) occur on all continents, but the timescales of magmatic processes that lead to eruption in such settings are poorly understood due to the temporal infrequency and lack of spatial pattern in eruptions. Therefore, unravelling the timescale of magma ascent is a critical aspect to advance our understanding of volcanic hazard and risk. Here we focus on the Newer Volcanics Province (NVP) of SE Australia is an active intraplate basaltic province that contains over 400 volcanic centres. Volcanic landforms include maars, tuff rings, scoria cones, lava fields along with more complex eruption centres and the rate of activity has varied in space and time. Despite a large number of studies on the bulk-rock geochemistry and physical volcanology of the deposits (e.g. stratigraphy, eruption styles), few constraints are available on the timescales of magma ascent in the province.

We have investigated the olivine crystals within entrained mantle xenoliths and as individual crystals within the groundmass of basaltic volcanic rocks using compositional X-ray maps, backscattered electron (BSE) images and electron microprobe analyses (EMPA). We focus on NVP samples from Mount Leura (Lehurra kang) and Mount Noorat (Knorart) to shed a light on the dynamic processes that lead to the eruption and the relative timescales of magma ascent. Olivine crystals in mantle xenoliths are mainly unzoned with Mg#90 (Mg# = 100×Mg/[Mg+Fe]) with few crystals normally zoned (<Mg#75 rim). Olivine grains in the groundmass are commonly up to 1 mm in size and are mainly skeletal, with significant variation in Mg#, CaO, MnO and NiO content from core to rim. Olivine grains in the Mount Noorat samples are largely normally zoned with crystal interiors characterised by >Mg#90 and rims by <Mg#75. Olivine crystals from Mount Leura contain complexly zoned olivines suggesting a more complex crystallisation and transport history (Mg#77-79 cores and rims up to Mg#85). We model the chemical zonation patterns in olivine crystals that reveal a short time of magmatic processes before the eruption. The insight of magma storage, ascent and the pathway to the surface at NVP is a crucial information in understanding the volcanic hazard and mitigation risk in the region for which too little consideration is still given at the present.


Biography

Research Fellow in the Dept of Earth and Environmental Sciences at Macquarie University. PhD from Macquarie University in April 2020.

Research interests in igneous petrology and volcanology. Focus on crystal records of magmatic processes and timescales in the lead-up to volcanic eruptions combining field observations and detailed microscopy analytical techniques.

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