Clinopyroxene/liquid trace element partitioning in natural trachyte–trachyphonolite systems: insights from Campi Flegrei (southern Italy)

Abstract

Trace element partition coefficients between clinopyroxenes and associated glassy matrix (Cpx/L D) have been determined for 13 REE, HFSE4+,5+, U, Th, Sr, Pb, Sc and V from combined LA-ICP-MS/EMP analyses in selected trachytes and trachyphonolites from Campi Flegrei. Composition of clinopyroxene and glass is pretty homogeneous in the trachyphonolites, pointing to an overall attainment of the equilibrium conditions. In trachytes, conversely, phases show some compositional heterogeneity (due to the presence of clinopyroxene xenocrysts) that requested a more careful petrographic and geochemical inspection of the samples to assess the equilibrium clinopyroxene composition. In the trachyte clinopyroxenes, REE are compatible from Nd to Lu (Cpx/L D up to 2.9), like Y, Ti, Sc and V. The Cpx/L D for Eu is lower than those of the adjacent REE, highlighting Eu2+ contribution. High D values are also shown by U, Th, Pb, Zr, Hf, Nb and Ta relatively to basaltic and andesitic systems, whereas the D Sr is roughly similar to that found for less evolved magmas. Trachyphonolites are characterized by an overall decrease of the Cpx/L D for highly-charged cations (with the exception of V), and by a slight increase of D Sr. REE are still compatible from Nd to Lu (Cpx/L D up to 2.1), like Ti, Y, Sc and V. This variation is also predicted for REE and Y by models based on the elastic strain theory, being consistent with the slightly lower polymerization degree estimated for the trachyphonolites. However, the observed Cpx/L D (REE,Y) are matched by the modelled ones only considering very low T (≤825°C), which are believed unlikely. This mismatch cannot be attributed to effects induced by the water-rich composition of the trachyte–trachyphonolite suite, since they would lower the observed Cpx/L D (REE,Y). Moreover, the anomalous inflections of measured Cpx/L D for HREE suggests some crystal-chemical control, such as the entrance of these elements in a site distinct from M2. It is concluded that the large Cpx/L D determined for trachytes and trachyphonolites are likely induced by hitherto unconstrained changes of the Z3+ activities related to the composition of melt and/or solid. All these considerations strongly highlight the importance of a direct characterization of trace element partitioning in natural samples from magmatic systems poorly characterized by experimental studies.