ALBERT

Notizen: Abstract The P-T path of magma associated with the 1944 Vesuvius eruption has been outlined on the basis of probe mineralogy and the relationships between the crystallising phases. Equilibrium P-T values, obtained from the reactions: 1. CaMgSi2O6(liq) = CaMgSi2O6(cpx) 2. NaAlSi3O8 (liq) = NaAlSi3O8 (plag) 3. CaAl2Si2O8 (plag)=CaAl2SiO6(cpx)+SiO2(liq) have been established for three intracrustal crystallisation stages: I) 8.0 kbar and 1255 °C; II) 4.0 kbar and 1178 °C; III) 0.5 kbar and 1105 °C. The H2O content in the magma has been estimated from an experimental calibration of $$a_{^{CaMgSi_2 O_6 } }^{liq}$$ as a function of $$X_{H_2 O}^{liq}$$ at P tot = 2 kbar. The estimated water contents of the magma for the three stages, I) 0.7%; II) 0.9%; III) 1.1%, are consistent with the pattern of activity of the 1944 Vesuvius eruption and with the relationship between the lavas. The shallow depth of H2O-saturation of the magma, 0.24 kbar at 1100 °C, is consistent with the eruption sequence of lava flows followed by lava fountain activity.

Notizen: 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.