The crystal structures of 212 experimentally synthesized, igneous clinopyroxenes were modeled from electronprobe chemical data The coexisting melts span a wide range of petrologically relevant, dry and hydrous compositions, characterized by variable enrichment in silica and alkalis Experimental conditions pertain to Earth's crust and uppermost mantle (P = 0–24 kbar; garnet absent) and a variety of f
O2 values (from CCO-buffered to air-buffered) and mineral assemblages (Cpx ± Opx ± Pig ± Ol ± Plag ± Spl ± Mt ± Amp ± Ilm) Unit-cell volume (Vcell) versus M1-polyhedron volume (VM1) relations were investigated over a range of pressures and temperatures using data derived from structure modeling and corrected for thermal expansivity and compressibility The relationships between pressure and clinopyroxene structural parameters were found to be dependent on the nature of the coexisting melt To reduce compositional effects, only clinopyroxenes belonging to mildly alkaline (MA) and tholeiitic (TH) series were considered Pressure was modeled as a linear function of Vcell, VM1, and Mg/(Mg + Fe2+)Cpx ratio A calibration based on the whole data set (MA+ TH) reproduced the experimental pressures within 14 kbar at the 1-σ level The maximum residuals were 35 kbar and 39 kbar for MA- and TH-clinopyroxenes, respectively Better statistics were obtained by considering MA- and TH-clinopyroxenes separately A calibration based on the 69 MA-clinopyroxenes reproduced the experimental pressures within 11 kbar (1σ) and with a maximum residual of 27 kbar A calibration based on the 143 TH-clinopyroxenes reproduced the experimental pressures within 10 kbar (1σ) and with a maximum residual of 34 kbar When these geobarometers are applied to natural samples for which P is unknown, the correction for compressibility is necessarily made through a trial-and-error procedure This expedient propagates an additional error that increases the above uncertainties and residuals by a factor of about 2 Applications to natural, igneous rocks for which the pressures of crystallization could be constrained based on experimental, petrological or geological evidence yielded pressure estimates that reproduced the expected values to within ca 2 kbar Compared to the MA-formulation, the TH-formulation appears to be less robust to variations in magma composition When applied to high-pressure (>10 kbar) clinopyroxenes synthesized from very low Na (Na2O < 15%) melts, the latter geobarometer can underestimate P by as much as 6 kbar Calculation of P through the present geobarometers requires clinopyroxene major-element composition and an independent, accurate estimate of crystallization T Underestimating T by 20 °C propagates into a 1-kbar increase in calculated P The proposed geobarometers are incorporated in the CpxBar software program, which is designed to retrieve the pressure of crystallization from a clinopyroxene chemical analysis

Clinopyroxene geobarometry of magmatic rocks. Part 2. Structural geobarometers for basic to acid, tholeiitic and mildly alkaline magmatic systems

The size distributions of crystals of olivine, plagioclase and oxides of the 1991/93 eruption at Mt. Etna (Italy) are analyzed. The simultaneous collection of this information for different minerals gives precious insight into the cooling history of lavas. Three distinct episodes are detectable: a storage of the magma in a deep reservoir, characterized by nearly constant and low nucleation and growth rates (near to equilibrium); an ascent phase, with an ever increasing nucleation rate related to volatile exsolution; and finally a quenching phase. In addition to geochemical and geophysical evidence, the similarity of the crystal size distributions of the present eruption with those of previous ones of this century makes it possible to exclude that crystal size distributions of Etnean lavas are due to mixing of different populations. This strongly suggests that the main features of the volcano feeding system have not changed despite observed variations in the magma output rates.

Effects of magma storage and ascent on the kinetics of crystal growth

The monitoring of seismic activity in eastern Sicily (southern Italy) has been recently improved, in the framework of the Poseidon Project, to investigate both tectonics and volcanic processes of Mount Etna. This effort has produced a homogeneous and complete data set which we use to image the deep structure of the volcano and to define the space and time distribution of the recent seismicity, encompassing the 1995 eruption, diffuse eruptions between 1995 and 1997, a further magma intrusion started in 1997, and an increase of volcanic activity in July 1998. We inverted P wave arrival times from 307 selected local earthquakes to obtain a three-dimensional velocity model of the volcano, with a simultaneous inversion for hypocenters and velocity parameters. The new tomographic images permit us to define the structure beneath the volcano from the surface down to 18 km depth. The main structural feature revealed by our inversion is a high-velocity body located beneath the central craters whose lateral extent increases from ∼6 km between 18 and 9 km depth to ∼12 km between 9 and 3 km depth. Near the surface the fast anomaly branches in two separate high-Vp regions, which are located below the summit craters and the eastern flank (Valle del Bove), respectively. The high-velocity features are interpreted as high-density cumulates of solidified magma that intruded the shallow crust. We hypothesize that magma ascends the crust within the relatively small high-velocity conduit (below 9 km depth) and is stored at depth shallower than 9 km within the broad high-velocity region, as also suggested by petrological data. A sharp increase of seismicity in 1997, with earthquakes occurring at the border of the high-velocity body, suggests that Mount Etna sustained recent episodes of intrusions, which possibly herald future eruptions. No large low-velocity anomaly interpretable as a melted magma chamber is imaged in the upper 18 km of the crust, while a broad low-velocity anomaly in the uppermost mantle, revealed by regional seismic data, possibly indicates the magma source region at depth. Earthquake occurrence from the deep source to the shallow intrusive area helps to trace the magma migration and the feeding of the volcanic system.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JB900427

Recent seismicity and tomographic modeling of the Mount Etna plumbing system

Subduction-like fluids in the genesis of Mt. Etna magmas: evidence from boron isotopes and fluid mobile elements

Low Pressure Crystallization Paths of H2O-Saturated Basaltic-Hawaiitic Melts from Mt Etna: Implications for Open-System Degassing of Basaltic Volcanoes

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 (Pt), 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 Pt 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 Pt, 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 Pt 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 Pt 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 Pt 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.

Carlo Aurisicchio

Papers

The 1983 Mount Etna eruption: thermochemical and dynamical inferences