Reconstructing the Evolution of the Submarine Havre Eruption, 2012: Insights from Vesicularity, Water Concentrations and Microtextural Analysis of Rhyolitic Glass

Tuesday, 31 January 2017
Marina/Gretel (Hobart Function and Conference Centre)
Samuel J Mitchell1, Bruce F Houghton1, Rebecca Carey2 and Iona M McIntosh3, (1)University of Hawaii at Manoa, Honolulu, HI, United States, (2)University of Tasmania, Earth Sciences, Hobart, TAS, Australia, (3)JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan
Abstract:
The 2012 eruption of Havre provides an exciting opportunity to study, in detail, a large volume, deep-submarine, silicic eruption. With few previously documented analogues, reconstructing the eruptive history of this event will advance our understanding of the effects of hydrostatic pressure on seafloor eruptions. We establish unit relationships, physical and chemical, between four pumiceous stratigraphic units produced during the event, as an essential requisite to understanding the submarine eruptive mechanisms. These units consist of: an extensive giant pumice deposit, an ash-lapilli-block unit proximally centred around the main dome and vent, a thin layer of ash and fine lapilli that covers the caldera and a more localised, thin unit of tubular pumice located away from the main eruptive vent. We present here macro- and micro-textural data and volatile analysis of pumice collected from known deposit localities via ROV in 2015.

Clast density/vesicularity distributions and total water (H2OT) contents in rhyolitic glass were determined for each of the units. Statistical analysis of clast density distributions reveals subtle differences between units. H2OT concentrations were measured through Fourier-transform infrared spectroscopy and contrasted with micro-Raman spectroscopy H2OT data. Microtextural images were collected using an electron microprobe to compare vesicle and microlite populations within banding and between clast populations and units.

Differences in primary H2OT content, density distributions, microtextures and microlite populations suggest that all four units were produced by multiple fragmentation processes and eruptive mechanisms during the Havre eruption. Specifically, we identify two distinct density distributions within the ash-lapilli-block unit implying two different emplacement mechanisms of this proposed density current deposit. The thinner tubular pumice unit shows geochemical similarities with an adjacent lava flow suggesting quench fragmentation producing this unique pumiceous deposit. Finally, a range of: densities, H2OT concentrations and microtextural-banding between individual giant pumice exteriors suggest at a more complex eruptive mechanism for giant pumice formation than the other units.

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