Distribution of REE between clinopyroxene and basaltic melt along a mantle adiabat: effects of major element composition, water, and temperature

The distribution of rare earth elements (REE) between clinopyroxene (cpx) and basaltic melt is important in deciphering the processes of mantle melting. REE and Y partition coefficients from a given cpx-melt partitioning experiment can be quantitatively described by the lattice strain model. We analyzed published REE and Y partitioning data between cpx and basaltic melts using the nonlinear regression method and parameterized key partitioning parameters in the lattice strain model (D ₀, r ₀ and E) as functions of pressure, temperature, and compositions of cpx and melt. D ₀ is found to positively correlate with Al in tetrahedral site (Al ^T ) and Mg in the M2 site (Mg^M2) of cpx and negatively correlate with temperature and water content in the melt. r ₀ is negatively correlated with Al in M1 site (Al^M1) and Mg^M2 in cpx. And E is positively correlated with r ₀. During adiabatic melting of spinel lherzolite, temperature, Al ^T , and Mg^M2 in cpx all decrease systematically as a function of pressure or degree of melting. The competing effects between temperature and cpx composition result in very small variations in REE partition coefficients along a mantle adiabat. A higher potential temperature (1,400°C) gives rise to REE partition coefficients slightly lower than those at a lower potential temperature (1,300°C) because the temperature effect overwhelms the compositional effect. A set of constant REE partition coefficients therefore may be used to accurately model REE fractionation during partial melting of spinel lherzolite along a mantle adiabat. As cpx has low Al and Mg abundances at high temperature during melting in the garnet stability field, REE are more incompatible in cpx. Heavy REE depletion in the melt may imply deep melting of a hydrous garnet lherzolite. Water-dependent cpx partition coefficients need to be considered for modeling low-degree hydrous melting.