An experimental study of the kinetics of decompression‐induced crystallization in silicic melt

Abstract

[1] Experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression. Pinatubo dacite was held at 780°C, 220 MPa, f O2 = NNO + 2, H 2 O-saturated conditions for an equilibration period, decompressed to final pressures, P f , ranging from 175 to 5 MPa, and then held for 0.3-931 hours. According to the plagioclase liquidus curve in P H2O -T space for the relevant melt composition, these decompressions impose effective undercoolings, ÁT eff , of 34-266°C. Growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 P f < 150 MPa (ÁT eff = 34-93°C), and equilibrium crystal modes are achieved in <168 hours. Microlite nucleation is the dominant transformation process for 10 < P f < 50 MPa (ÁT eff = 125-241°C), and chemical equilibrium is not attained by 168 hours under these conditions. Slow, steady decompressions typically produced normally zoned, euhedral, and planar-faceted feldspar crystals, although anhedral morphologies were produced at very low P f . Contrary to expectation, slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure. Although counterintuitive, these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth (controlled by ÁT eff and melt viscosity) experienced during each decompression path. Analysis of liquid to solid transformation kinetics using TTT-style diagrams shows that crystallization occurs most rapidly at $100 MPa by a crystal growth mechanism. The next most efficient crystallization conditions are at 25 MPa, in a crystal nucleationdominated regime.INDEX TERMS: 3630 Mineralogy and Petrology: Experimental mineralogy and petrology, 3640 Mineralogy and Petrology: Igneous petrology, 8434 Volcanology: Magma migration, 8439 Volcanology: Physics and chemistry of magma bodies;

Constraining pre-eruptive magma conditions and unrest timescales during the Monte Nuovo eruption (1538 ad; Campi Flegrei, Southern Italy): integrating textural and CSD results from experimental and natural trachy-phonolites

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