Trace element partitioning coefficients between terrestrial silicate melts and plagioclase feldspar: Improved and simplified parameters

Abstract

Nernst partition coefficient data are compiled and parameterized against commonly measured and petrologically significant Input Variables, allowing plagioclase/meltD values for 37 trace elements to be calculated. With few exceptions plagioclase/meltDs increase with decreasing temperature, anorthite-content of plagioclase (An), wt% melt MgO and FeO* (all iron as FeO), molar melt Mg/(Mg + Fetotal); and with increasing wt% melt SiO2 and Na2O + K2O. High field strength cations and rare earth elements (REE) with similar ionic radii have nearly fixed ratios of LnD (e.g. LnDLa/LnDCe). These can be linked as a chain to construct a nearest-neighbour plagioclase/meltDREE (NN-DREE) profile from a point of origin at DCe. Correlations between LnDCe and An content in the partitioning data were regressed to connect these variables algebraically. Analysis of these NN-DREE profiles with the Lattice Strain Model (LSM) requires a temperature value, which was pre-defined by fitting simplified temperature-An paths to experimental data binned for melt H2O content. In the LSM, r3+0 is the size of the site into which REE substitute. Three published r3+0 parameterizations were tested. The Dohmen and Blundy (2014) parameterization of r3+0 [Å] (r3+0 = 1.331 − 0.067An) yielded the best fits to the NN-DREE profiles when values of D3+0 and E3+M were optimized. D3+0 is the strain compensated partition coefficient and E3+M is Young’s Modulus of the M-site hosting REE. Starting from the preferred r3+0 parameterization, NN-DREE profiles at many An contents were fit with a LSM by varying D3+0 and E3+M. These multiple fits yielded a tight linear trend of increasing LnD3+0 with decreasing An content [LnD3+0 = (−2.8323An) – 0.0538] applicable to all three temperature-An functions. Previously proposed parameterizations of Young’s Modulus E3+M can reproduce DREE profiles of very calcic plagioclase (>An0.9), but yield profiles that are too steep to fit DREE profiles of more sodic compositions. The LSM fits for dry melts (H2O < 1%) yield a trend of linearly-decreasing LnE3+M with decreasing An content [LnE3+M = (1.2808An) + 3.4897]. The temperature-An functions for wet (1–5% H2O) and very wet (>5% H2O) melts yield similar LnE3+Mvs An trends, but at lower LnE3+M to compensate for lower temperatures. With these parameterizations of temperature, r3+0, D3+0 and E3+M, the NN-DREE Lattice Strain Model reproduces the increasingly flatter and higher DREE profiles of plagioclase crystallizing from basaltic to rhyolitic melts, with An-content as the sole Input Variable needed.