Amphibole–melt trace element partitioning of fractionating calc-alkaline magmas in the lower crust: an experimental study

Abstract

Amphibole is one of the most important hydrous minerals of the middle and lower continental crust and plays a key role in the formation of intermediate to silica-rich magmas. This study reports a consistent set of amphibole trace element partition coefficients derived from fractional crystallization experiments at 0.7 GPa in a piston cylinder apparatus. Starting materials were doped with trace elements on the 20–40 ppm level and measured using laser ablation (LA)-ICP-MS. Amphibole is stable from 1010 to 730 °C and systematically changes its composition from pargasite to magnesiohornblende to cummingtonite, while coexisting liquids vary from andesite to dacite and rhyolite. Amphibole–liquid partition coefficients increase systematically with decreasing temperature and increasing SiO2 in the liquid. Potassium displays an inverse behavior and partitioning decreases with decreasing temperature. Rare earth element (REE) partition coefficients, assumed to occupy the M4 site within the amphibole structure, increase continuously up to one order of magnitude. The calculated lattice parameters, ideal cation radius (r 0) and Young’s modulus (E) remain nearly constant with decreasing temperature. The high-field strength elements Zr and Hf that occupy the M2 site of the amphibole structure reveal a fivefold increase in partition coefficients with decreasing temperature and constant lattice parameters r 0 and E. Partition coefficients correlate with edenite, tschermaks and cummingtonite exchange vectors indicating that the maximum partition coefficient (D 0) for an ideal cation radius increases with decreasing edenite component, while the latter decreases linearly with temperature. Regressing Amph/L D Ca against trace elements results in fair to excellent correlations (r 2 0.55–0.99) providing a predictive tool to implement the trace element partition coefficients in numerical geochemical modeling. Our data result in positive correlations between Amph/L D Nb/Ta and Amph/L D Ca, and Mg#. Spoon-shaped REE patterns and subchondritic Nb/Ta ratios in tonalitic to granodioritic plutonic rocks and andesitic to rhyolitic magmas directly constrained by measured trace element compositions of coexisting liquids are consistent with hornblende gabbro fractionation in the middle to lower crust. The systematic change of the measured trace element composition of fractionating calc-alkaline liquids indicates that hornblende gabbro formation in the middle to lower crust also exerts an important control on some commonly used trace element ratios such as Sr/Y, Sr/Ba or Nb/Zr.