Parameterized Lattice Strain Models for REE Partitioning between Amphibole and Silicate Melt

Abstract:

The distribution of REE between amphibole and silicate melt is important for understanding a variety of igneous processes involving amphibole. In general, amphibole-melt REE partition coefficients (D_REE) depend on pressure (P), temperature (T), and compositions of amphibole and melt. A previous study parameterized the D_REE in amphibole-melt solely as a function of melt composition [1]. Here, we use published REE partitioning data between amphibole and basaltic melt, the lattice strain model [2], and non-linear least squares regression method to parameterize key partitioning parameters in the lattice strain model (D₀, r₀, and E) as a function of P, T, and both amphibole and melt compositions. We focus on experimental data obtained by LA-ICP-MS and ion probe, and experiments close to equilibrium. Amphiboles and coexisting melts from the 38 experiments that we compiled span a wide range of compositions with the Mg# of amphibole and melt ranging from 36 to 100 and 15 to 99, respectively. Two models, which give nearly identical results, are explored in this study. In the first model, D₀ is a function of T and amphibole composition: it negatively correlates with T and Mg^M1,2,3 content in amphibole, and positively correlates with Ti^M1,2,3 content in amphibole. In the second model, D₀ is solely a function of the melt composition: it negatively correlates with the mole fraction of Ca in the melt. Interestingly, r₀ and E are both constant and identical between the two models, suggesting D₀ in the two models are equivalent. The latter allows us to develop a new thermometer for amphibole-melt equilibria. As an independent test, we compared model-derived temperatures with those reported in the phase equilibrium experiments. The predicted temperatures are within ±41°C on average of the reported temperatures, adding confidence to our parameterizations of D₀. Our two parameterized lattice strain models can be used to model REE fractionation between amphibole and basaltic melts during igneous processes. Furthermore, our amphibole composition dependent model allows us to develop REE-in-amphibole-mineral thermobarometers in the future. [1] Tiepolo et al. (2007). RiMG 67(1), 417-452. [2] Blundy and Wood (1994). Nature 372, 452-454.