The historic magmatic-hydrothermal eruption of the Breccia di Commenda, Vulcano, Italy

The historic Breccia di Commenda (BC) explosive eruption of Vulcano (Aeolian Islands, Italy) opened with a phase that generated a gray fine ash layer dispersed to the northwest (phase 1). The eruption continued with a dilute pyroclastic density current (PDC) that was dispersed to the east, followed by the emplacement of radially distributed, topographically controlled PDC deposits (phase 2). The last phase of the eruption produced a sequence of accretionary lapilli and gray fine ash dispersed toward the southeast (phase 3). The most impressive feature of the BC is its high lithic/juvenile clast ratio and the yellow color of the deposits of phase 2. Lithic fragments are mainly hydrothermally altered rocks, in the silicic and advanced argillic facies. Juvenile fragments, ranging from 20 % to 40 % by volume, are mainly confined to the ash component of the deposits and comprise rhyolitic to trachyandesite, poorly to non-vesicular fragments. The fine ash fraction of the deposits is richer in S, Cu, Zn, Pb, and As than the BC juvenile lapilli and bombs, and also the juvenile components of other La Fossa units, suggesting that the BC formed in the presence of an anomalously high amount of S and metals. Sulfur and metals may have been carried as aerosols by chloride- and sulfate-bearing micro-crystals, derived from the condensation of magmatic gas in the eruptive cloud. The high content of hydrothermally altered lithic clasts in the deposits suggests that explosions involved the fluid-saturated hydrothermally altered rocks residing in the conduit zone. However, the presence of a juvenile component in the deposits supports the idea that this explosion may have been triggered by the ascent of new magma. We categorize this eruption as magmatic-hydrothermal to emphasize that in this type of phreatomagmatic eruption the external water was an active hydrothermal system. Rock magnetic temperatures of non-altered lava lithic fragments indicate a uniform deposit temperature for the PDC deposits of between 200 and 260 A degrees C, with a maximum at 280 A degrees C. These homogeneous, relatively low temperatures are consistent with the idea that the phase 2 explosions involved the expansion of abundant steam from the flashing of the hydrothermal system. In addition, recent paleomagnetic dating of the BC provides an age of between 1000 and 1200 AD, younger than that reported in the previously published data, suggesting that previous interpretations and the recent history of La Fossa and Mt. Pilato require re-evaluation.