EarthRef.org Reference Database (ERR) -- Lundstrom et al. 1998

The partitioning of fifteen trace elements (Rb, Sr, Zr, Nh, Ba, La, Ce, Nd, Sm, Gd, Yb, Hf, Ta, Pb, and Th) between clinopyroxene and synthetic melt has been studied in two compositions along an isotherm in the diopside-albite-anorthite ternary at 1 bar pressure. The two compositions correspond to similar to Di(65)An(35) and similar to Di(55)Ab(45) and produce clinopyroxenes distinct in chemistry while melt compositions range from 49 wt% SiO2 to 61 wt% SiO2. The partition coefficients of high field strength elements (HFSE) increase by factors of 2-8 in Di-An experiments relative to Di-Ab experiments while other elements show very little change (+/-20%) between compositions. The change in HFSE partitioning correlates with increases in tetrahedral Al2O3 (Al-IV) content of clinopyroxenes in the anorthite-bearing experiments. Changes in D-Ta/D-Nb also correlate with Al-IV based on a survey of previously published determinations. Tests of models of trace element substitution energetics produce values for Young's modulus (E) and optimum D (D-o) consistent with previous results for clinopyroxene for mono-, di-, and trivalent cations. The wide variations in partitioning behavior for tetra- and pentavalent cations are also consistent with these models because the high values for E make partition coefficients and relative HFSE partitioning sensitive to small changes in composition. The overall increase in HFSE partitioning and D-o for the DiAn composition is consistent with D-o increasing as a function of Al-IV, consistent with the role of Al-IV in charge balancing HFSE. However,Al-IV must also cause lattice changes that affect the ability of the clinopyroxene to discriminate between Nb and Ta. There are two important implications to the observed dependence of HFSE partition coefficients on clinopyroxene aluminum content. First, HFSE will be fractionated from their adjacent REE within ultramafic samples during melting of spinel Iherzolite as clinopyroxene Al2O3 content decreases. Second, fractionations between Nb and Ta and Zr and Hf observed in mantle-derived magmas are consistent with extraction of melt in equilibrium with spinel Iherzolite having clinopyroxenes with similar to 5 wt% Al2O3. The fractionations among HFSE (Nb/Ta, HE/Zr) and between HFSE and REE observed in both are magmas and upper mantle peridotites may simply reflect prior depletion by major melting events (F > 10%) which left clinopyroxene as a residual phase. We speculate that the peridotitic sources for MORE and are lavas are similar in composition with both having significant HFSE anomalies. However, MORE do not typically record HFSE anomalies because of the complementary contribution of HFSE from enriched mafic veins interspersed within the peridotite. Copyright (C) 1998 Elsevier Science Ltd.