The solubility of rhenium in silicate melts: Implications for the geochemical properties of rhenium at high temperatures

Abstract

The solubility of rhenium (Re) in a haplobasaltic melt (anorthite-diopside eutectic composition) has been experimentally determined using the mechanically assisted equilibration technique at 1400°C as a function of oxygen fugacity (10⁻¹² < fO₂ ≤ 10⁻⁷ bar), imposed by CO-CO₂ gas mixtures. Samples were analysed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). This is a true microanalytical technique, which allows small-scale sample heterogeneity to be detected, while providing a limit of detection of 2 ppb Re. Time-resolved LA-ICP-MS spectra revealed the presence of suboptically sized micronuggets of Re in all samples, which, because they are present at the 0.5 to 10 ppm level, dominate the true solubilities of Re (<1 ppm at the conditions of the experiment) in bulk analyses of the samples. Nevertheless, the micronuggets could be filtered out from the time-resolved spectra to reveal accurate values of the true Re solubility. A number of time series of samples were taken at constant fO₂ to demonstrate that the solubilities converge to a constant value. In addition, solubilities were measured after increasing and decreasing the imposed fO₂. The results show that Re dissolves in the silicate melt as ReO₂ (Re⁴⁺) and ReO₃ (Re⁶⁺) species, with the latter predominating at typical terrestrial upper-mantle oxygen fugacities. The total solubility of Re is described by the following expression (fO₂ in bars): [Re/ppb] = 9.7(±1.9) × 10⁹ (fO₂) + 4.2 (±0.3) × 10¹⁴ (fO₂)^1.5Assuming an activity coefficient for Re in Fe-rich metal of 1, this gives a value of D_Re^met/sil of 5 × 10¹⁰ at log fO₂ = IW-2, appropriate for metal-silicate partitioning in an homogenously accreting Earth. Thus, Re is indeed very highly siderophile, and the mantle’s abundance cannot be explained by homogenous accretion.

Introduction

Rhenium (Re), together with the six platinum group elements (Ru, Rh, Pd, Ir, Os, and Pt) and Au, comprise a group of elements known as the highly siderophile elements (HSEs), the defining geochemical property of which is that they have metal-silicate distribution coefficients D_M^met/sil in excess of 10⁴. Their high metal-silicate distribution coefficients have resulted in their partitioning into the metal of the Earth’s core. This, together with the fact that the HSEs are heavy elements with intrinsically low solar abundances, means that their abundances in the Earth’s mantle are extremely low. In fact Re, with a bulk silicate Earth abundance of 0.2 ppb, is the rarest of all the naturally occurring elements, apart from the noble gases (e.g., O’Neill and Palme, 1999). Nevertheless, advances in geochemical analytical capabilities now enable the HSEs to be determined in a wide variety of igneous rocks. Interpretation of these analytical data requires some knowledge of the high-temperature geochemical properties of the HSEs, in particular quantification of their metal-silicate and sulfide-silicate distribution coefficients.

In addition, identification of the valence states of Re in the high-temperature geological environment is a useful first step towards understanding the geochemical behaviour of this element. The common valence states of Re are 4+, 6+, and 7+, but previous experimental work (e.g., O’Neill et al 1995, Righter and Drake 1997) has suggested that Re dissolves in silicate melts as species with unusually low valence states (1+ or 2+). These low valence states are so rare that they are effectively unknown in solid state chemistry, and consequently, there is no indication of even the most fundamental chemical properties of Re in such states. However, these earlier experimental studies used bulk analytical methods. Some recent parallel studies on other HSEs raise the possibility that these earlier studies may have been compromised by the “micronugget effect” O’Neill et al 1995, Ertel et al 1999, whereby the small amounts of Re chemically dissolved in the silicate melts may have been masked by larger amounts present as micronuggets. Micronuggets are submicroscopic particles, which seem to form something akin to a colloidal suspension. In this study we use a microanalytical technique, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), to see through the micronugget contamination and determine the true solubilities of Re in a haplobasaltic silicate melt (Anorthite-Diopside 1-atm eutectic composition). We demonstrate that the previous work was indeed in error from micronugget contamination. The valence states of dissolved Re in silicate melts are in fact the familiar 4+ and 6+. Micronuggets also probably affected previous experimental studies of Re metal-silicate partitioning, which are consequently biased to low values; the present results demonstrate unambiguously that Re is indeed an HSE with an enormous metal-silicate distribution coefficient at likely core-forming conditions.