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

• Published in: arXiv.org 2018-12
• Main Authors: Mollo, Silvio, Blundy, Jonathan, Scarlato, Piergiorgio, De Cristofaro, Serena Pia, Vanni Tecchiato, Flavio Di Stefano, Vetere, Francesco, Holtz, Francois, Bachmann, Olivier
• Format: Article
• Language: English
• Subjects: Barometers; Calcium magnesium silicates; Composition; Dependence; Diopside; Field strength; Fractionation; Lattice strain; Magma; Modulus of elasticity; Moisture content; Olivine; Partitions; Plagioclase; Rare earth elements; Yttrium
• ISSN: 2331-8422

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 modelling 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/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/DHFSE values that are sensitive to the magmatic conditions at which clinopyroxenes nucleate and grow. In order to better explore magma dynamics using clinopyroxene, a new 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 three lattice strain parameters, i.e., the strain-free partition coefficient (D0), the site radius (r0), and the effective elastic modulus (E).