Plagioclase, orthopyroxene, clinopyroxene, glass magma-meter and application to Mount Ruapehu, New Zealand and Paraná volcanic province, Brazil
Harmon, Lydia Jane
A new phase equilibria “magma-meter” determines a number of magmatic storage and crystallization conditions, including pressure, oxygen fugacity (fO2), state of fluid saturation, and maximum temperature for glass-bearing rocks containing the assemblage plagioclase+orthopyroxene+clinopyroxene (plag+opx+cpx). This newly developed magma-meter can better constrain crystallization conditions of shallow, glass-bearing andesites to dacites. The magma-meter utilizes rhyolite-MELTS to determine crystallization conditions utilizing the glass compositions coexisting with the plag+opx+cpx assemblage in natural samples. The magma-meter retrieves crystallization conditions for experiments from the literature. We applied the magma-meter to the plag+opx+cpx-bearing systems: Mt. Ruapehu, in the southern Taupo Volcanic Zone, New Zealand, and the Palmas unit of the Paraná volcanic province, Brazil. The samples from Mt. Ruapehu and Paraná were tested from ~5 MPa to ~400 MPa and from super-liquidus to ~90% crystalline (~1200 ºC to ~700 ºC). Mt. Ruapehu is an active, structurally well-understood volcano; it serves as a methodological testing ground for the magma-meter. Results show a distribution of crystallization pressures for different eruptions, with modes of ~110 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 distribution indicates that the magma batches were stored over a range of pressures. Mt. Ruapehu magmas are water saturated, with fO2 of ΔQFM ~+1. Paraná includes extinct, super-eruption deposits, with preliminary results suggesting shallow (~90 MPa) storage conditions.