Recent remobilisation of shallow-level intrusions on Montserrat revealed by hydrogen isotope composition of amphiboles

Ion probe measurements of hydrogen isotopes were made on amphiboles representing different stages of the ongoing eruption of the Soufrière Hills Volcano, Montserrat. The majority (80%) of the andesitic amphiboles show relative intra- and inter-crystal δD homogeneity, with a mean of −38±12‰, consistent with known primary magmatic values. The remainder (20%) of amphiboles show marked δD heterogeneity, with a mean of −6±30‰. The heterogeneous amphiboles have ∼100 μm rims with δD values similar to the homogeneous crystals, but core values which are significantly heavier than primary magmatic values. Early in the eruption 50% of amphiboles are isotopically heterogeneous but post spring 1996 all amphiboles are homogeneous. We interpret these ion probe data in terms of development of a shallow andesitic magma chamber by repeated emplacement of andesitic magma in the shallow crust, over at least the last 100 yr. We suggest such shallow intrusions, emplaced during the volcano–seismic crises preceding the ongoing eruption, solidified. Some regions interacted with hydrothermal fluids and isotopic exchange took place generating the heavy hydrogen isotope signatures in the heterogeneous amphiboles. On onset of the ongoing eruption, such old igneous material was remobilised, with intimate mixing between the new magma batch and the old intrusions indicated by the close proximity of homogeneous and heterogeneous crystals. The isotopically primary magmatic rims on the heterogeneous crystals indicate hydrogen isotope exchange over a period of a few weeks, consistent with timescales of magma ascent at the Soufrière Hills Volcano. Once the old igneous material was flushed out, by spring 1996, the eruption proceeded by extruding material dominated by the new batch of magma. This resulted in a marked increase in extrusion rate.