Partitioning of rare earth elements, Y, Th, U, and Pb between pargasite, kaersutite, and basanite to trachyte melts: Implications for percolated and veined mantle

TLDR

Amphibole/liquid partition coefficients vary with amphibole and melt composition, reflecting crystal‑chemical mechanisms of trace element incorporation. The study aims to evaluate how amphibole crystallization during melt migration affects trace element distribution in the upper mantle via reactive porous flow and veined fractional crystallization. Partitioning data for REE, Y, Th, U, and Pb were measured in 25 calcic amphiboles at 1.4 GPa and 950–1075 °C, and these values were used to model trace‑element redistribution during melt migration. The data show that REE and actinides occupy the M4 cavity, Pb the A site, HREE become compatible in melts with >50 wt % SiO₂ while LREE remain incompatible, and both reactive porous flow and veined fractional crystallization produce residual liquids and solids with highly variable La_N/Yb_N ratios.

Abstract

A new set of partitioning data for rare earth elements (REE: La, Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb), Y, Th, U, and Pb has been obtained for 25 calcic amphiboles (pargasites and kaersutites) crystallized from alkali‐basaltic and basanitic bulk rock compositions at pressure P = 1.4 GPa, and temperature T between 950° and 1075°C. The variations of amphibole/liquid partition coefficients and of their ratios relevant to petrogenetic studies are discussed with reference to the major element composition of the amphiboles and of the coexisting melt, and to the crystal chemical mechanisms for trace element incorporation. Our results support the conclusions that REE and actinides are incorporated into the M4 cavity in calcic amphiboles and distributed between the two available sites within that cavity and that Pb is incorporated into the A site. In our sample population, REE patterns are systematically enriched in heavy REE (HREE), as expected from the presence of significant cummingtonite component. No significant fractionation is observed between Th and U. The major factor controlling the amount of trace element incorporation is the SiO 2 content of the melt. The major implication of this study is that HREE can become compatible in amphibole in systems with SiO 2 content greater than ∼50 wt %, whereas LREE always remain incompatible. We use the new D REE amph/l values to calculate the effects of amphibole crystallization during melt migration in the upper mantle by reactive porous flow as well as fractional crystallization of amphibole during melt migration in veined systems. We show that both processes will lead to residual liquids and solids with extremely variable La N /Yb N ratios.