DECOMPRESSION EXPERIMENTS AS AN INSIGHT INTO ASCENT RATES OF SILICIC MAGMAS

Abstract

An experimental study of H2O exsolution, bubble growth and microlite crystallisation during ascent (decompression) of silicic magmas in the volcanic conduit is presented. Isobaric and decompression experiments were performed on a rhyolitic melt at 860 C, NNO+1, H2O saturation, and pressures between 15 and 170 MPa. Two sets of decompression experiments were performed, with decompression rates varying between 0.001 and 960 MPa/min: (1) from 150 to 50 MPa (high-pressure decompression), and (2) from 50 to 15 MPa (low-pressure decompression). The experiments highlight incomplete H2O exsolution for decompression rates>100 MPa/min, incomplete bubble growth for decompression rates>0.1 MPa/min, crystal nucleation time lags, and incomplete chemical re-equilibration to final pressures. The observed crystallisation process, i.e. growth versus nucleation, depends on the decompression range. Indeed, decompression-induced crystallisation during high-pressure decompressions is dominated by growth of existing crystals, whereas during low-pressure decompressions crystal nucleation is the dominating process. This study provides a means to infer magma ascent rates in eruptions of silicic magmas through a combined petrologic and experimental approach.