Oxygen isotope geochemistry of pyroclastic clinopyroxene monitors carbonate contributions to Roman-type ultrapotassic magmas

The oxygen isotope geochemistry and chemical composition of clinopyroxene crystals from Alban Hills pyroclastic deposits constrain the petrological evolution of ultrapotassic Roman-type rocks. Volcanic eruptions in the 560–35 ka time interval produced thick pyroclastic deposits bearing clinopyroxene phenocrysts with recurrent chemical characteristics. Clinopyroxenes vary from Si–Mg-rich to Al–Fe-rich with no compositional break, indicating that they were derived from a continuous process of crystal fractionation. Based on the delta18O and trace element data no primitive samples were recovered: monomineralic clinopyroxene cumulates set the oxygen isotope composition of primary magmas in the range of uncontaminated mantle rocks (5.5‰), but their SigmaREE composition resulted from extensive crystal fractionation. Departing from these mantle-like delta18OCpx values the effects of crustal contamination of clinopyroxene O-isotope composition were identified and used to monitor chemical variations in the parental magma. delta18O values in Si–Mg-rich clinopyroxene are slightly higher than typical mantle values (5.9–6.2‰), and the low SigmaREE contents are representative of early stages of magmatic differentiation. Delta18O values as high as 8.2‰ are associated with Al–Fe3+-rich clinopyroxene showing high SigmaREE contents. These delta18O values are characteristic of crystals formed during the late magmatic stages of each main eruptive phase. Geochemical modelling of delta18O values vs. trace element contents indicates that these ultrapotassic magmas were derived from fractional crystallization plus assimilation of limited amounts of carbonate wall rocks starting from a primary melt, and from interaction with CO2 derived from country rocks during crystal fractionation.