The occurrence of Mt Barca flank eruption in the evolution of the NW periphery of Etna volcano (Italy)

Abstract

Geological surveys, tephrostratigraphic study, and ⁴⁰Ar/³⁹Ar age determinations have allowed us to chronologically constrain the geological evolution of the lower NW flank of Etna volcano and to reconstruct the eruptive style of the Mt Barca flank eruption. This peripheral sector of the Mt Etna edifice, corresponding to the upper Simeto valley, was invaded by the Ellittico volcano lava flows between 41 and 29 ka ago when the Mt Barca eruption occurred. The vent of this flank eruption is located at about 15 km away from the summit craters, close to the town of Bronte. The Mt Barca eruption was characterized by a vigorous explosive activity that produced pyroclastic deposits dispersed eastward and minor effusive activity with the emission of a 1.1-km-long lava flow. Explosive activity was characterized by a phreatomagmatic phase followed by a magmatic one. The geological setting of this peripheral sector of the volcano favors the interaction between the rising magma and the shallow groundwater hosted in the volcanic pile resting on the impermeable sedimentary basement. This process produced phreatomagmatic activity in the first phase of the eruption, forming a pyroclastic fall deposit made of high-density, poorly vesicular scoria lapilli and lithic clasts. Conversely, during the second phase, a typical strombolian fall deposit formed. In terms of hazard assessment, the possible occurrence of this type of highly explosive flank eruption, at lower elevation in the densely inhabited areas, increases the volcanic risk in the Etnean region and widens the already known hazard scenario.

Appendix

⁴⁰Ar/³⁹Ar methodological approach

To chronologically constrain the geological evolution of the studied area, we selected two lava flow samples. The first one was collected from E1 lava flow along the left bank of the Simeto River (BT sample in Fig. 2). This lava flow is the oldest unit recognized in the studied area and was attributed to the earlier Na-alkaline volcanism (Ancient Alkaline Centers unit of Romano 1982) in the geological map of Romano et al. (1979). The second sample (MC sample in Fig. 2) belonging to the E5 lava flow was collected with the aim of defining the age of the Mt Barca eruption.

After removal of weathered portions, samples were crushed to obtain a grain-size fraction between 250 and 500 µm. The phenocryst fractions plagioclase, olivine, and pyroxenes were removed by density separation using heavy liquid at 27 and 29 × 10² kg/m³. A final clean separate was obtained after acid leaching with dilute HNO₃ and HF followed by hand picking for any obvious phenocryst intergrowths left in the sample. Samples were wrapped in Al foil and loaded into a quartz tube together with standards (DRA sanidine of 25.26 ± 0.05 Ma) for irradiation (duration: 1 h) with fast neutrons in the Cd-lined RODEO facility of the EU-JRC Petten HFR reactor (Netherlands). After irradiation, ca. 50% of the sample (ca. 200 mg) was wrapped in Al foil and loaded into a 21-position sample carousel in a double vacuum high temperature furnace. Gas extraction, purification, and mass spectrometric measurement follow the procedures described in Schneider et al. (2007). In summary, gas was extracted in steps of increasing furnace temperature between ca. 700°C and 1,200°C. The furnace segment of the extraction system is fitted with a cold trap that is used to trap any volatile components coming off the sample during heating. After cooling of the furnace tube, the gas is expanded into the purification line where it is exposed to two or three stages of cleaning by exposure to activated Fe–V–Zr and Zr–Al alloys at temperatures of 250°C and 450°C, respectively. Finally, the purified argon gas is admitted into the mass spectrometer for isotopic analysis. In this system, we use a Hiden HAL IV RC PIC-RGA 101 instrument fitted with a dual filament open electron bombardment source and a dual Faraday channeltron collector. The channeltron collector is operated in pulse-counting mode. Peak shapes are predictable with flat tops of better then ca. 0.4 m/e (mass over charge), and full separation of peaks (peak valleys in the parts per million range). Typically, beam intensities are measured at seven positions on the peak top from m/e −0.1 to m/e +0.2 for masses 36, 39, and 40, and single positions for masses 37 and 38. One baseline is measured at m/e 35.45. Intensity data are averaged per peak, and intensity information with the respective standard deviations are regressed in ArArCalc2.40 (Koppers 2002 and http://www.earthref.org/tools/ararcalc.htm). Beam intensities are corrected for hot and cold line blanks, intensity-dependent mass discrimination, and radioactive decay during the time interval between irradiation and analysis. Subsequently, we follow a standard intensity de-convolution scheme to correct for nucliogenic ³⁶Ar, ³⁹Ar, and ⁴⁰Ar using production rates measured from pure K and Ca silicate glass. Cold blanks were in the range 45, 5, 11, 20, and 14,000 cps at m/e: 36, 37, 38, 39, and 40, respectively, whereas the hot blanks varied from indistinguishable from the cold blank at 700°C to 55, 5, 17, 20, and 16,000 cps at 1,000°C and 130, 12, 27, 16, and 34,000 cps at 1,200°C.