Trace element partitioning between plagioclase and melt: Investigation of dopant influence on partition behavior

We present results from an ion microprobe study of REE-doped and natural concentration plagioclase-basalt run products of Drake (1972) that carries on from our earlier study (Bindeman et al., 1998). The goals of this work are (1) to determine plagioclase/melt partition coefficients for all REE for four analyzed plagioclase compositions (An40–80); and (2) to determine whether doping with REE influences partition coefficients of other REE and other trace elements. In combination with our analyses of Sr-doped runs (Bindeman et al., 1998), the new data allow us to compare partition coefficients (Di’s) of trace elements at natural concentrations with those in REE-doped and Sr-doped runs. In these comparisons, runs have the same run temperature and compositions, but different doping element(s). We find that ln(DREE) decreases as a linear function of REE atomic number, in contrast to most plagioclase-melt partition experiments. The slopes of the ln(Di) vs. %An and RT ln(Di) vs. %An dependencies of REE and monovalent and divalent cations increase from smaller to larger ions of each valence group. DLa/DY and DLa/DLu increase by more than 5 times as plagioclase composition changes from An80 to An40. Within each valence group, slopes on ln(Di) vs. %An and RT ln(Di) vs. %An plots increase linearly with ionic radius, with the trivalent REE showing the steepest slope vs. ionic radius dependence. Application of the elastic modulus model of Blundy and Wood (1994) to our results yielded good results, confirming that elastic properties determine partition behavior. We also present Di’s for U. We find no detectable effects of trace element doping on Di’s of 30 trace elements of varying size and charge. This implies that coupling between different trace elements is not a significant process during partitioning in natural systems, even for the case of heterovalent substitutions (such as REE). However, DREE and DY in REE-doped runs are 30–100% higher than DREE in undoped runs and Sr-doped runs. Doping with thousands of ppm of three selected REE (it does not matter which three) affects Di’s of Y and all REE in these runs, including those at natural concentration levels. We suggest that substituting trace cations largely compete for sites of substitution with the mineral-forming cations Na and Ca. We speculate that lower DREE in REE-doped runs are the result of a change in the REE substitution mechanisms at doped concentrations of all REE which leads to a different value of the Henry’s law constant. Copyright © 2000 Elsevier Science Ltd