New petrological constraints on the last eruptive phase of the Sabatini Volcanic District (central Italy) : Clues from mineralogy, geochemistry, and Sr-Nd isotopes

We report results from mineralogical, geochemical and isotopic analyses of the three youngest pyroclastic products (ca. 86ky) belonging to the Sabatini Volcanic District (Roman Province, central Italy). By means of thermometers, hygrometers and oxygen barometers, we have estimated that the crystallization temperature of magma progressively decreases over time (910-740°C), whereas the amount of water dissolved in the melt and fO2 progressively increases as compositions of magmas become more differentiated (4.5-6.4wt.% H2O and 0.4-2.6 δQFM buffer, respectively). Thermodynamic simulations of phase equilibria indicate that geochemical trends in mafic magmas (MgO>4wt.%) can be reproduced by abundant fractionation of olivine and clinopyroxene (~50wt.% crystallization), while the trends of more evolved magmas (MgO≤4wt.%) originated by fractional crystallization of plagioclase and sanidine (~45wt.% crystallization). The behavior of trace elements highlights that magmatic differentiation is controlled by polybaric differentiation that includes: (1) prolonged fractionation of mafic, anhydrous minerals from a primitive, H2O-poor magma at depth and (2) extraction of a more evolved, H2O-rich magma that crystallizes abundant felsic and subordinated hydrous minerals at shallow crustal levels. Assimilation and fractional crystallization modeling also reveal that magmas interacted with the carbonate rocks of the subvolcanic basement. The effect of carbonate assimilation accounts for both trace element and Sr-Nd isotopic variations in magmas, suggesting a maximum degree of carbonate assimilation of less than 5wt.%.