The 1983 Mount Etna eruption: thermochemical and dynamical inferences

Abstract

Thermochemical calculations and laboratory phase equilibration experiments on lavas of the 131 day 1983 Mt. Etna flank eruption of 0.1 km³ were undertaken to investigate possible systematic variations in inferred melt-phenocryst equilibration conditions as a function of time. The 1983 Mt. Etna lavas are multiply saturated; plagioclase, clinopyroxene and olivine, the dominant phenocrysts, occur in the ratio 1:1/2:1/4. Melts (glasses) plot close to the plagioclase saturated olivine-clinopyroxene low pressure cotectic on a Walker-O'Hara diopside-forsterite-silica diagram suggesting equilibration of melt and phenocrysts in a high level magma reservoir. Total pressures, temperatures and dissolved H₂O concentrations were calculated using the isoactivity method of Carmichael and coworkers based on about 300 elelctron microprobe analyses of coexisting olivine, clinopyroxene and plagioclase phenocrysts, microphenocrysts and groundmass microlites for samples collected 6, 46 and 125 days after the start of the eruption. Total pressures (P _t), temperatures and H₂O contents based on representative olivine-clinopyroxene pairs are 140 MPa, 1105°C, 2.4 wt% H₂O; 255 MPA, 1112°C, 1.0 wt% H₂O and 85 MPa, 1096°C, 1.8 wt% H₂O respectively for the early (283), middle (I83) and late (L83) samples. Corresponding equilibration depths are in the range 3 to 10 kilometers. Plagioclase feldspar phenocrysts, while showing more evidence of disequilibrium, provide compatible estimates of P _t and T when analysis is restricted to the low anorthite mode of the plagioclase frequency-composition histograms: 133 MPa and 1115°C; 260 MPa and 1117°C and 103 MPa and 1104°C, repectively for 283, I83 and L83. The pre-eruptive (i.e., in situ) temperature-pressure gradient calculated from olivine-clinopyroxene equilibria is 10.6 K/kbar. This compares well with independent estimates of the temperature-pressure derivative of the (pseudo) invariant point composition (10 to 12 K/kbar) in both model (e.g., diopside-forsterite-anorthite, Presnall et al. 1978) and natural (e.g., Walker et al. 1979; Grove et al. 1982) systems. Apparently, magma within the Etna reservoir was in a quasiequilibrium state buffered by its multiply-saturated character immediately preceding eruption. The temporal variation of computed P _t, T and H₂O concentrations for melt-phenocryst equilibrium agrees well with predictions based on simulations of the withdrawal of magma from a body zoned with respect to dissolved H₂O provided the temporal record of magma discharge is taken into account. Discharge varied by a factor of about 100 during the sample collection interval. The intermediate P _t but high H₂O content inferred for sample 283 reflects the withdrawal of H₂O enriched magma during an early phase of high average discharge of about (3∼50 m³/s) before evaculation isochrons became quasistationary. The high P _t and relatively dry I83 magma reflects the deepening of the evacuation isochrons after 50 days of intermediate discharge with the development of quasi-stationary isochrons in time and space. Sample L83 from day 125 near the end of the eruption reflects the “shoaling” of evacuation isochrons (hence low P _t and relatively high H₂O content) associated with the observed low (0.5 m³/s) discharge. Our results show that thermochemical modeling efforts provide important opportunities for testing the predictions of magma with-drawal simulations.