Crystal chemical control of clinopyroxene-melt partitioning in the Di-Ab-An system: implications for elemental fractionations in the depleted mantle

Abstract

The partitioning of fifteen trace elements (Rb, Sr, Zr, Nb, 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 ∼Di 65An 35 and ∼Di 55Ab 45 and produce clinopyroxenes distinct in chemistry while melt compositions range from 49 wt% SiO 2 to 61 wt% SiO 2. 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 Al 20 3 ( IVAl) content of clinopyroxenes in the anorthite-bearing experiments. Changes in D Ta/D Nb also correlate with IVAl 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 IVAl, consistent with the role of IVAl in charge balancing HFSE. However, IVAl 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 lherzolite as clinopyroxene Al 2O 3 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 lherzolite having clinopyroxenes with ∼5 wt% Al 2O 3. The fractionations among HFSE (Nb/Ta, Hf/Zr) and between HFSE and REE observed in both arc 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 MORB and arc lavas are similar in composition with both having significant HFSE anomalies. However, MORB do not typically record HFSE anomalies because of the complementary contribution of HFSE from enriched mafic veins interspersed within the peridotite.