Abstract
Abstract. 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 1.4 GPa, temperature from 1015 to 1050 ∘C, and at two different oxygen fugacity conditions (fO2; ΔFMQ ≈-2.6 (log fO2 [experiment] − log fO2 [FMQ buffer]) and ΔFMQ ≈+ 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 (4 wt %–6 wt %) and silica-intermediate (52 wt %–59 wt %) 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 0.88 wt %) 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., Gd / Yb, Nb / Ta, and Pb / 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.
Highlights
The partitioning of trace elements between minerals and silicate melts is a key topic in our understanding of how elements redistribute during Earth’s evolution, from primordial times to present
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
In the last five decades many efforts have been dedicated to the determination of the mineral–liquid partition coefficient (Min/LD) for most of the trace element groups mainly in major mantle-derived minerals like olivine, pyroxenes, garnet, and spinel and more typically crustal phases like feldspar, amphiboles, and micas, as well as accessory phases such as allanite, Published by Copernicus Publications on behalf of the European mineralogical societies DMG, SEM, SIMP & SFMC
Summary
The partitioning of trace elements between minerals and silicate melts is a key topic in our understanding of how elements redistribute during Earth’s evolution, from primordial times to present. How the f O2 affects the Min/LDs of trace elements is more evident by considering minerals with complex and more versatile crystal structure, such as those of amphiboles and micas (Ferraris and Ivaldi, 2002; Oberti et al, 2007). This is mainly due to their greater ability to counterbalance charge imbalance imposed by the incorporation of trace elements with different oxidation states (e.g., V, Ni, Mo, U) and to accommodate the incorporation of cations with both small (e.g., [6]Be2+ = 0.45 Å) and large (e.g., [8]Ba2+ = 1.42 Å) ionic radii
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