High pressure trace element partitioning between clinopyroxene and alkali basaltic melts

We present new experimental data on major and trace element partition coefficients (D) between clinopyroxene and a K-basaltic melt from Procida Island (Campi Flegrei Volcanic District, south Italy). Time-series experiments were conducted at 0.8 GPa and 1080-1250 degrees C aiming to investigate the role of the crystallization kinetics on trace elements partitioning behaviour at a pressure relevant for deep magmatic reservoirs. Results indicate that large ion lithophile elements (LILE) are incompatible (e.g., D-Sr <= 0.15), light rare elements (LREE; e.g., D-La <= 0.20) are always more incompatible than heavy rare elements (HREE), which in some cases result to be compatible with clinopyroxene (e.g., Due D-Dy = 1.40); high field strength elements (HFSE) are generally incompatible (D-HFSE <= 0.8), while transition elements (TE) range from slightly incompatible (e.g., D-V = 0.6) to highly compatible (e.g., D-Cr = 63). The calculated D values for LILEs, REEs, HFSEs, and TEs tend to decrease with the increase of temperature and to increase with increasing tetrahedrally-coordinated aluminium content, in agreement with the previous studies. Moreover, we observed the influence of the growth rate on the partition coefficients, with the highest D-REE values calculated in the runs with the highest growth rate (similar to 10(-7) cm s(-1)), due to the less efficient rejection of incompatible elements during rapid crystal growth, that in this study is not linked to disequilibrium conditions, but to the presence of pre-existing nuclei. Additionally, the apparent increase in DREE values with time observed in some runs is not referable to a change in time but rather to the different degrees of polymerization, expressed as the ratios NBO/T of these melts, strictly related to a loss of Fe occurred during the experiments, and thus to a different melt viscosity. Finally, the application of the experimental clinopyroxene-melt partition coefficients highlights that the deepest step of the magmatic differentiation in the Campi Flegrei Volcanic District is represented by the fractionation of about 20-30% of a clinopyroxenitic mineral assemblage from a basaltic parental magma. (C) 2021 Elsevier Ltd. All rights reserved.