Antimony isotope variations in natural systems and implications for their use as geochemical tracers

Abstract

Multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has been used for the precise measurement of Sb isotopic composition in geological samples, as well as Sb(III) and Sb(V) species in aqueous samples. Sb is chemically purified prior to analysis by using cation-exchange resin and cotton impregnated with thioglycollic acid (TCF). Purification through cation-exchange resin is required for the removal of matrix interfering elements such as transitional metals, whereas TCF is required for the separation of other hydride-forming elements such as Ge and As. The analyte is introduced in the plasma torch using a continuous flow hydride generation system. Instrumental mass fractionation is corrected with a “standard-sample bracketing” approach. Using this technique, the minimum Sb required per analysis is as low as 10 ng for an estimated external precision calculated for the 123Sb/ 121Sb isotope ratio of 0.4 ε units (2 σ). Sb isotope fractionation experiments reported here indicate strong fractionation (9 ε units) during Sb(V) reduction to Sb(III). Seawater, mantle-derived rocks, various environmental samples, deep-sea sediments and hydrothermal sulfides from deep-sea vents have been analyzed for their Sb isotope composition. We define a continental and oceanic crust reservoir at 2±1 ε units. Seawater ε 123Sb values do not vary significantly with depth and yield a restricted range of 3.7±0.4 ε units. Sb deposited in hydrothermal environments has a significant range of Sb isotopic composition (up to 18 ε units). These variations may reflect not only contributions from different Sb-sources (such as seawater and volcanic rocks), but also kinetic fractionation occurring at low temperature in aqueous media through the reduction of seawater-derived Sb(V) in more reducing environment. Our results suggest that Sb isotopes can be extremely useful tracers of natural processes and may be useful as paleoredox tracers in oceanic systems.