The Geochemical Evolution of Clinopyroxene in the Roman Province: A Window on Decarbonation from Wall-Rocks to Magma

We present results from atmospheric pressure experiments conducted at 1140, 1160 and 1180 °C under the buffering conditions of air, MH and NNO. The starting materials were a shoshonite and a phonotephrite from the Roman Province. These compositions were doped with variable amounts of CaO and CaO+MgO whose stoichiometric proportions reproduced the assimilation by magmas of calcite and dolomite, respectively. Results underline that, during magma-carbonate interaction, the oxygen fugacity exerts a primary control on clinopyroxene composition. With increasing fO₂, the content of Tschermak molecules, i.e., CaAlAlSiO₆, CaFeAlSiO₆, and CaTiAlSiO₆, in clinopyroxene significantly increases at the expenses of hedenbergite and enstatite components. The compositional variation of clinopyroxene described by our experiments is compared with the chemical analyses of natural crystals found in skarns and lavas at the Roman Province. This comparison provides that the simple ingestion of carbonate by magmas cannot explain the geochemical evolution of clinopyroxene in the Roman Province. At the periphery of magma chamber, the decarbonation reaction proceeds with the highest efficiency. This causes highly oxidizing conditions that, in turn, control the geochemical evolution of clinopyroxenes in skarns. However, the oxidative capacity of CO₂ fluxing progressively decreases from the skarn shell to the interior of the magma chamber. Consequently, the chemistry of clinopyroxenes found in lavas reflects spatially variable redox conditions.