V43B-3111
Under Pressure: New Geobarometer Developed to Determine Crystallization Conditions and Storage Depths of Intermediate Magmas

Thursday, 17 December 2015
Poster Hall (Moscone South)
Lydia Harmon, Vanderbilt University, Nashville, TN, United States, Guilherme A R Gualda, Vanderbilt University, Earth and Environmental Sciences, Nashville, TN, United States, Mark S Ghiorso, OFM Research, Redmond, CA, United States and James Cowlyn, University of Canterbury, Christchurch, New Zealand
Abstract:
A new phase equilibria geobarometer successfully determines pressures of magma storage in plagioclase+orthopyroxene+clinopyroxene (POC) bearing intermediate magmas. The geobarometer utilizes rhyolite-MELTS to determine crystallization conditions utilizing the glass compositions coexisting with the POC assemblage in natural samples.

POC stability is strongly affected by water content and oxygen fugacity (fO2). We tested fO2 values ranging from +1 to +4 delta-QFM, which spans the fO2 range of intermediate magmas. Water content was tested from water-saturated to 4 wt% below saturation.

The geobarometer was applied to two intermediate POC-bearing systems – Mount Ruapehu in the southern Taupo Volcanic Zone, New Zealand, and the Palmas unit of the Serra Geral Formation in the Paraná Volcanics, Brazil. Both systems were tested from 25 MPa to 400 MPa and from 700 ºC to 1200 ºC.

Ruapehu is an active, structurally well-understood volcano in the Taupo Volcanic Zone; it serves as a methodological testing ground for the geobarometer. POC-bearing pumice clasts show a bimodal distribution of crystallization pressures for different eruptions, with modes of ~90 MPa and 130 MPa, consistent with field interpretations of different eruptive styles based on juvenile clast textures and previous knowledge of the magma plumbing system. The bimodal distribution indicates that the magma batches were tapped at different pressures (and depths). Ruapehu magmas are water saturated and have fO2 of delta-QFM equal to ~+1. The model substantiates these conditions, as rhyolite-MELTS calculations with lower water contents and different fO2 values do not produce the observed POC mineral assemblage.

Preliminary results from the Paraná Volcanics suggest juvenile, fiamme-like blob structures equilibrated at ~50 MPa, were water saturated, and had an fO2 between delta-QFM +1 and +1.5. The geobarometer has potential to unravel crystallization conditions of shallow, glass-bearing andesites to dacites.

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