Magma Storage Region Processes Inferred from Geochemistry of Fe-Ti Oxides in Andesitic Magma, Soufriere Hills Volcano, Montserrat, W.I.

Analyses of Fe–Ti oxides help constrain models of magma storage region processes for the Soufrière Hills Volcano, Montserrat (W.I.), and provide clear evidence of the nature of transient heating events in the magma storage region. To constrain timescales of magma heating and remobilization, the TiO2 zoning patterns in a time series of natural titanomagnetites were compared with those produced in controlled phase equilibrium experiments on the andesite. Most samples of andesite erupted from 1995 to 2002 contain titanomagnetite crystals with uniform core compositions (TiO2∼7·8 wt %). Many crystals are characterized by rimward increases in TiO2, interpreted to be Ti diffusion gradients caused by heating of the andesite by invading basaltic magma. Some andesites erupted during periods of the highest observed mass eruption rate, however, contain titanomagnetite with uniformly low TiO2 contents from core to rim. The observation that no Ti diffusion gradients, and no elevated core TiO2 contents, occur in the vast majority of titanomagnetite grains in magma batches that were erupted more than 2 years after the onset of the present eruption strongly suggests, first, that heating of the batches of andesite occurred just before eruption, and, second, that injection of basaltic magma has continued throughout the eruption. Heat, but little mass, may be transferred from the invading basalt to the andesite in the magma storage region by injection of dikes or formation of sills. Ponding of basaltic magma at the base of a pre-existing andesitic magma storage region is the simplest explanation consistent with observations. The Fe–Ti oxide data strongly suggest that an internal conduit within the andesitic magma storage region carries magma from the zone of heating to the overlying conduit, which carries the magma through the upper arc crust. In this model, the magma chamber is being emptied from the bottom, at the contact between pre-existing andesite and newly intruded basalt.