Abstract
Abstract. The rare Earth elements (REEs) are important geochemical tracers for geological processes such as high-grade metamorphism. Aqueous fluids are considered important carriers for the REEs in a variety of geological environments including settings associated with subduction zones. The capacity of a fluid to mobilize REEs strongly depends on its chemical composition and on the presence of suitable ligands such as fluoride and chloride. In this study, we present structural and thermodynamic properties of aqueous yttrium–chloride and yttrium–fluoride species at a temperature of 800 ∘C in a pressure range between 1.3 and 4.5 GPa derived from ab initio molecular dynamics simulations. The total yttrium coordination by H2O and halide ions changes from seven to eight within the pressure range. For the yttrium–chloride species, a maximum number of three chloride ligands was observed. The derived thermodynamic data show that aqueous yttrium–fluoride complexes are more stable than their yttrium–chloride counterparts in chloride- and fluoride-rich environments at conditions relevant to slab dehydration. Mixed Y(Cl,F) complexes are found to be unstable even on the molecular dynamics timescale. Furthermore, in contrast to field observations, thermodynamic modeling indicates that yttrium should be mobilized at rather low fluoride concentrations in high-grade metasomatic systems. These results suggest a rather low fluoride activity in the majority of subduction-zone fluids because yttrium is one of the least-mobile REEs. Additionally, the simulations indicate that yttrium drives the self-ionization of hydration water molecules as it was observed for other high-field-strength elements. This might be a general property for highly charged cations in aqueous solutions under high-temperature and high-pressure conditions.
Highlights
Subduction zones have been the most important sites for exchange of matter and energy between the Earth’s crust and mantle for billions of years until now (Tang et al, 2016)
ab initio molecular dynamics (AIMD) simulations were performed for the hydrated Y3+ and for 11 different yttrium–halogen complexes: five YCl3n−n, n = 1–5; three YF3n−n, n = 1, 2, 3; and three mixed Y−(Cl, F) complexes
The Y–O distances derived from extended X-ray absorption fine structure (EXAFS) spectra by Vala Ragnarsdottir et al (1998) in the range of 2.36–2.39 Å are in good agreement with the atomic distances from the presented simulations, while the conference abstract by Mayanovic et al (2002) does not comprise quantitative data
Summary
Subduction zones have been the most important sites for exchange of matter and energy between the Earth’s crust and mantle for billions of years until now (Tang et al, 2016). Magnetotelluric anomalies (e.g., Worzewski et al, 2011; McGary et al, 2014) suggest the occurrence of a high proportion of melts and water-rich fluids in the subducted slabs due to partial melting (Zheng et al, 2016) and the dehydration of water-bearing minerals such as serpentine (Ulmer and Trommsdorff, 1995) and amphibole (Schmidt and Poli, 1998). It is known that REE patterns of subducted rocks are affected by the chemical composition of the metamorphic fluid (e.g., John et al, 2008; Zhang et al, 2008) due to their chemical complexation with dissolved anions, e.g., F−, SO24−, CO23− and Cl− (Tsay et al, 2014, 2017; Alt et al, 1993; Scambelluri and Philippot, 2001; Newton and Manning, 2010)
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