An integrated P-T-H2O-lattice strain model to quantify the role of clinopyroxene fractionation on REE+Y and HFSE patterns of mafic alkaline magmas: Application to eruptions at Mt. Etna

Abstract

A correct description and quantification of the geochemical behaviour of REE+Y (rare earth elements and Y) and HFSE (high field strength elements) is a key requirement for modeling petrological and volcanological aspects of magma dynamics. In this context, mafic alkaline magmas (MAM) are characterized by the ubiquitous stability of clinopyroxene from mantle depths to shallow crustal levels. On one hand, clinopyroxene incorporates REE+Y and HFSE at concentration levels that are much higher than those measured for olivine, plagioclase, and magnetite. On the other hand, the composition of clinopyroxene is highly sensitive to variations in pressure, temperature, and melt-water content, according to exchange-equilibria between jadeite and melt, and between jadeite/Ca-Tschermak and diopside-hedenbergite. As a consequence, the dependence of the partition coefficient on the physicochemical state of the system results in a variety of DREE+Y and DHFSE values that are sensitive to the magmatic conditions at which clinopyroxenes nucleate and grow.In order to better explore magma dynamics using clinopyroxene chemical changes, an integrated P-T-H2O-lattice strain model specific to MAM compositions has been developed. The model combines a set of refined clinopyroxene-based barometric, thermometric and hygrometric equations with thermodynamically-derived expressions for the lattice strain parameters, i.e., the strain-free partition coefficient (D0), the site radius (r0), and the effective elastic modulus (E). Through this approach, it is found that the incorporation of REE+Y and HFSE into M2 and M1 octahedral sites of clinopyroxene is determined by a variety of physicochemical variables that may or may not change simultaneously during magma differentiation. The applicability of the P-T-H2O-lattice strain model to natural environments has been verified using clinopyroxene-melt pairs from a great number of volcanic eruptions at Mt. Etna volcano (Sicily, Italy). DREE+Y and DHFSE values recovered by the model have been used as input data to quantify fractional crystallization processes in natural MAM compositions. Results from calculation illustrate that the concentration of REE+Y and HFSE in the magma is primary controlled by the geochemical evolution of clinopyroxene in terms of major cation exchange-equilibria and trace cation lattice strain properties.