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Magma ascent within days confirmed by systematic diffusion modelling on xenocrystic olivine across the Auckland volcanic field, New Zealand

Jie Wu 1,2, Shane Cronin2, Marco Brenna1, Ingrid Ukstins2, Annaleise Klein3

  • Affiliations: 1Department of Geology, University of Otago, Dunedin, New Zealand; 2School of Environment, University of Auckland, Auckland, New Zealand; 3Australian Synchrotron, ANSTO, Clayton, Australia. 

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 267

  • Programme No: 1.3.14

  • Theme 1 > Session 3

Abstract

We selected seven basaltic volcanoes across the Auckland volcanic field (AVF) for a systematic evaluation of timescales of magma storage and ascent using diffusion modelling of major and trace elements and water within xenocrystic olivine of mantle origin. A total of 73 high-resolution profiles were measured on 71 olivine xenocrysts by electron microprobe. Profile components from a crystal interior to edge, include (1) a xenocrystic flat plateau with constant Fo# [100 × molar Mg/(Mg + Fe)] between 90 and 94, (2) a first magmatic overgrowth with Fo#80--83, and occasionally (3) a second reverse magmatic overgrowth with higher Fo# compared to the first overgrowth, reflecting recharge of more mafic magma. The boundaries between these components have smoothed transitional/diffusive Fo# areas. The very outer edge of the crystals is commonly characterised by a final thin (<10 µm) diffusive rim with a rapid decrease in Fo# (<80). The results show that total diffusion timescales within days are common to all examined volcanoes across AVF. Water profiles measured by FTIR show diffusion timescales within hours, representing rapid magma ascent after water-degassing starts at a few km depth. Our results reconfirm the conclusion of complex magma storage and fast magma ascent from a published study on the Pupuke maar alone, but also suggest shorter rise times from magma source regions, (with a mode at 2 days and 80% of examples <16 days) as well as complex conditions within deep magma storage over various timescales beneath the AVF.