The importance of considering temperature- and composition-dependent partition coefficients in differentiation models

Quantifying the parameters that govern magma differentiation is key to understanding the evolution of igneous systems . We present an approach combining thermodynamic modeling of magma differentiation using Rhyolite-MELTS with chemical composition, temperature, pressure, and oxygen fugacity ( X - T - P - f O 2 )-dependent partition coefficients ( D ) for rare earth elements. We apply this approach to a MORB system containing olivine, plagioclase, and clinopyroxene as the main mineral assemblage and compare the results to more classic approaches typically considering fixed D values. Our model highlights that the effects of temperature and composition on D values can compound or counteract each other depending on the mineral and element considered. Our results emphasize the value of using thermodynamic models alongside T - X -dependent D values to properly model the evolution of igneous systems. We show that this is of particular interest for modeling the evolution of rare earth element contents in minerals during differentiation. Using our results and corresponding thermodynamic constraints, we can provide D values for any mineral composition crystallized from this MORB system; based on chemical maps, we use our results to derive unprecedented maps of D values, highlighting strong partitioning variability at the thin-section scale. Therefore, using fixed D values can introduce significant mineral-melt partitioning errors compared to using T-X -dependent values, potentially leading to serious interpretation biases. • Partition coefficients (D) are dependent on P, T, fO 2 , and composition (X). • We show the effect of using variable D values, along LLD, on differentiation models. • We combine thermodynamic models with T-X-dependent D models. • We present the first maps of partition coefficients at the thin section scale.