The influence of oxygen fugacity and chlorine on amphibole–liquid trace element partitioning at upper-mantle conditions

The recycling of volatile and incompatible elements through the entire Earth's history is one of the most important processes for the chemical evolution of the lithosphere. In this context, amphiboles are important hydrous minerals playing a key role in the chemical differentiation of the crust-mantle system. We carried out amphibole-liquid partitioning (Amph/LD) experiments starting from a hydrous alkali basalt with variable chlorine content and doped in 37 trace elements of geochemical interest. Experiments were conducted with an end-loaded piston cylinder apparatus at a fixed pressure of 1.4g GPa, temperature from 1015 to 1050g gC, and at two different oxygen fugacity conditions (fO2;FMQ ≈-2.6 (log fO2 [experiment]-log fO2 [FMQ buffer]) and FMQ ≈+ 1.7). These conditions approach those of the Earth's upper mantle chemically metasomatised by subduction-derived agents that may occur in several geological settings. All runs show Ti-pargasites and kaersutites, either as individual crystals or replacing former clinopyroxenes, in equilibrium with alkali-rich (4g wtg %-6g wtg %) and silica-intermediate (52g wtg %-59g wtg %) melts. A comprehensive trace element dataset (LLEs-light lithophile elements, LILEs-light ion lithophile elements, REEs-rare Earth elements, HFSEs-high field strength elements, actinides, transition metals, chalcophiles) has been determined in glass and amphibole with a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). Although the melt polymerization has a major influence on Amph/LDs, we provide evidence that changes in the fO2 of both the system and Cl content in amphibole (up to 0.88g wtg %) also affect the Amph/LDs of several trace elements and their ratios. In particular, at higher fO2 conditions lower Amph/LD values for U and Mo are observed likely in relation to the low capability of amphibole to incorporate the more oxidized forms of these cations. The Amph/LDTh/Amph/LDU ratio is positively correlated with the Cl content in amphibole, while the opposite behavior is shown by Amph/LD ratios of other elements with the same charge and different ionic radius (e.g., Gdg /g Yb, Nbg /g Ta, and Pbg /g Ba). Dimensional modification of the amphibole structure in relation to the incorporation of Cl in the O(3) site are at the origin of the observed variations. General implications of our results are discussed.