The geochemical evolution of clinopyroxene in the Roman Province: A window on decarbonation from wall-rocks to magma

Abstract

We present results from magma–carbonate interaction experiments designed to shed light on the geochemical evolution of clinopyroxene in the Roman Province (central Italy). Atmospheric pressure experiments were performed at 1140, 1160 and 1180 °C under NNO,MHand air oxygen buffering conditions. The starting materials were a shoshonite and a phonotephrite doped with different amounts of CaO and CaO + MgO whose stoichiometric proportions reproduced the assimilation by magmas of calcite and dolomite, respectively. The results show that clinopyroxenes, spinels and residual glasses are ubiquitous phases in all run-products. Calcite-doped runs crystallize more clinopyroxene than dolomite-doped runs at the same conditions. This leads to the formation of strong desilicated CaO-rich melts showing compositions comparable to those of magmatic skarns. During magma–carbonate interaction, the content of Fe3+ in clinopyroxene increases with increasing fO2 promoting the substitution of Al for Si in tetrahedral site. Local charge imbalances are also compensated by the incorporation of highly charged cations, such as Ti, into the crystal lattice. According to this cation substitution, Al–Ca–Fe3+–Ti-rich clinopyroxenes of the skarn environment testify to continuous CO2 fluxes produced by the thermal decomposition of carbonate wall-rocks. Nevertheless, the oxidative capacity of CO2 progressively decreases from the skarn shells towards the interior of the magma chamber driving the crystallization of Si–Fe2+–Mg-rich clinopyroxenes.