  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).
