Experimental constraints on the crystallization and evolution of primitive magmas from Erebus volcano, Antarctica
Abstract
We present the preliminary results of phase equilibria experiments on a basanite and a phonotephrite, which are used to understand the evolution of alkali magmas at Erebus volcano, Antarctica. Field observations of the Erebus lava lake indicate complex magmatic behavior at depth. In addition, the remarkably complete fractional crystallization sequence of lava types seen in outcrop and their corresponding melt inclusions give an unprecedented look into the interplay between degassing and crystallization in both deep and shallow regions within the magmatic plumbing system. We aim for the first time to experimentally constrain the pre-eruptive conditions of the crystallization and evolution of the Erebus magma suite, with particular emphasis on magma evolution from basanite to phonotephrite. Phase equilibria experiments have been performed between 1000-1150 °C and 200-400 MPa with a range of H2O/CO2 ratios from XH2Ofluid = 0-1 with two natural compositions, a basanite (SiO2 = 42.13 wt%; MgO = 8.73 wt%) and a phonotephrite (SiO2 = 47.19 wt%; MgO = 3.13 wt%), collected on Ross Island, Antarctica. For the conditions thus far explored, augite is the liquidus phase in the basanite, followed by Mg-rich olivine. In the phonotephrite, Fe-Ti oxides are always present with the addition of minor apatite and feldspar in some samples. Preliminary results indicate that fractionation of Erebus basanite to phonotephrite likely occurred at high temperature (>1100 °C), at pressures above 300 MPa, and with sufficient water to stabilize plagioclase.