Donato, Paola

This work is an extension of previous research in which we successfully tested the effectiveness of chlorine (Cl) versus sodium (Na) diagrams to unambiguously identify the provenance of obsidian artifacts attributable to Italian outcrops widely exploited during prehistory, i.e., Lipari, Pantelleria, Palmarola, and Monte Arci. In this further work, we found that the ratio R = Cl/Na decreases with the age of emplacement (t) of the obsidian outcrops and can be well described by the inverse law: . Obsidian samples were analyzed both by electron probe micro-analyzer (EPMA) and by Instrumental Neutron Activation Analysis (INAA). Radiometric dates have been obtained both from the literature and through 40Ar/39Ar dating. Data processing confirmed the time evolution of the Cl/Na ratio through the same equation, indicating that it can be used as an empirical estimator of the obsidian formation age. A best-fit analysis of the collected data gives the relation in units of ka. To verify the validity of this relation for non-Italian obsidians, we applied it to Sierra de Las Navajas (State of Hidalgo, Mexico) obsidians, estimating an age of 1.75 Ma, in agreement with the upper limit of 1.8–2 Ma generally accepted for these rocks. We propose that the Cl/Na ratio changes with time because of the differential loss of chlorine and sodium as a consequence of the micro-fracturing of the obsidian glass after the emplacement. If future tests on different obsidians can confirm the validity of this approach, a rough estimate of the age of emplacement of the obsidian outcrops could be derived from their geochemical compositions.

The peninsular and insular Italy are punctuated by Quaternary volcanoes and their rocks constitute an important aliquot of the Italian Quaternary sedimentary successions. Also away from volcanoes itself, volcanic ash layers are a common and frequent feature of the Quaternary records, which provide us with potential relevant stratigraphic and chronological markers at service of a wide array of the Quaternary science issues. In this paper, a broad representation of the Italian volcanological community has joined to provide an updated comprehensive state of art of the Italian Quaternary volcanism. The eruptive history, style and dynamics and, in some cases, the hazard assessment of about thirty Quaternary volcanoes, from the northernmost Mt. Amiata, in Tuscany, to the southernmost Pantelleria, in Sicily Channel, are here reviewed in the light of the substantial improving of the conceptual models, methodological approaches and the overall knowledge made in the last decades in the volcanological field study. We hope that the prest review can represent an useful and agile document summarising the knowledege on the Italian volcanism at the service of the Quaternary community operating in central Mediterranean area.

Magmatism accompanies rifting along divergent plate boundaries, although its role before continental breakup remains poorly understood. For example, the magma-assisted Northern Main Ethiopian Rift (NMER) lacks current volcanism and clear tectono-magmatic relationships with its contiguous rift portions. Here we define its magmatic behaviour, identifying the most recent eruptive fissures (EF) whose aphyric basalts have a higher Ti content than those of older monogenetic scoria cones (MSC), which are porphyritic and plagioclase-dominated. Despite these differences, calculations highlight a similar parental melt for EF and MSC products, suggesting only a different evolutionary history after melt generation. While MSC magmas underwent a further step of storage at intermediate crustal levels, EF magmas rose directly from the base of the crust without contamination, even below older polygenetic volcanoes, suggesting rapid propagation of transcrustal dikes across solidified magma chambers. Whether this recent condition in the NMER is stable or transient, it indicates a transition from central polygenetic to linear fissure volcanism, indicative of increased tensile conditions and volcanism directly fed from the base of the crust, suggesting transition towards mature rifting.

An integrated petrological and geochemical study on plagioclase crystals erupted at Vulcano Island (Aeolian Islands, Southern Italy) over the past 1000 years allowed us to draw a detailed model of the internal structure of the volcanic system, in which modes of magma interaction and timescales of storage at crustal depth are shown. The integration of compositional, textural and temporal record preserved in plagioclase crystals provides evidence for an articulated plumbing system constituted by several reservoirs connected at the crust-mantle boundary. Here, a basaltic-shoshonitic magma resides and is thought to feed the shallow magma reservoir of both La Fossa and Vulcanello centers, finally triggering eruptions throughout injections from depth. Textural and micro-compositional data on plagioclase crystals suggest the presence of three main magma levels located between ca. 17 and 2 km of depth beneath La Fossa Cone, which were intermittently reactivated over the whole period of activity considered. Plagioclase textures and compositional zoning indicate that the shallow (b11 km bsl) portions of the La Fossa plumbing system were particularly active over the last 1000 years, as crystals record the ascent and continuous episodes of magma recharge and mixing that affect the shallower reservoirs. The first stage of activity at Vulcanello (i.e. Vulcanello I eruption) was fed by slightly differentiated melts that directly rose from the deep basaltic-shoshonitic reservoir, residing for a short period of time into the crust before the eruption. Indeed, diffusion modeling calculations on Sr zoning in plagioclase indicate shorter timescales of residence (b2 years) for crystals erupted at Vulcanello compared to those of the La Fossa Cone eruptions (ca. 2–10 years). If compared with the time span between eruptions occurred at La Fossa, our time estimations may suggest that magma feeding the activity at La Fossa resides most of the time in reservoirs located below the plagioclase nucleation depth (~11 km of depth), finally rising up only few years before the eruption onset. According to our model, magmatic eruptions at Vulcano Island are related to the ascent of deep basic (basaltic/shoshonitic) magmas that trigger a sort of “reaction chain” through subsequent episodes of recharge and mixing toward the upper magmatic reservoirs.