Oxygen isotope partitioning during oxidation of pyrite by H 2 O 2 and its dependence on temperature Liliana Lefticariu a,c, * , Arndt Schimmelmann b , Lisa M. Pratt b,c , Edward M. Ripley b,c a Department of Geology, Southern Illinois University, Carbondale, IL, USA b Department of Geological Sciences, Indiana University, Bloomington, IN, USA c Indiana Princeton Tennessee Astrobiology Initiative (IPTAI), NASA Astrobiology Institute, Indiana University, Bloomington, IN, USA Received 23 February 2007; accepted in revised form 20 August 2007; available online 19 September 2007 Abstract A detailed experimental study was conducted to investigate mechanisms of pyrite oxidation by determining product yields and oxygen isotopic fractionation during reactions between powdered pyrite (FeS 2 ) with aqueous hydrogen peroxide (H 2 O 2 ). Sealed silica-tube experiments utilized aliquots of pyrite that were reacted with 0.2 M H 2 O 2 for 7 to 14 days at 4 to 150 °C. No volatile sulfur species were detected in any experiment. The only gaseous product recovered was elemental oxygen inferred to result from decomposition of H 2 O 2 . Aqueous sulfate (S aq ) was the only sulfur product recovered from solution. Solid hydrated ferric iron sulfates (i.e., water-soluble sulfate fraction, S ws ) were recovered from all experiments. Ferric oxide (hema- tite) was detected only in high temperature experiments. Reactants were selected with large differences in initial d 18 O values. The oxygen isotopic compositions of oxygen-bearing reactants and products were analyzed for each experiment. Subsequent isotopic mass-balances were used to identify sources of oxygen for reaction products and to implicate specific chemical reaction mechanisms. d 18 O of water did not show detectable change during any experiment. d 18 O of sulfate was similar for S aq and S ws and indicated that both H 2 O and H 2 O 2 were sources of oxygen in sulfate. Low-temperature experiments suggest that H 2 O-derived oxygen was incorporated into sulfate via Fe 3+ oxidation, whereas H 2 O 2 -derived oxygen was incorporated into sulfate via oxidation by hydroxyl radicals (HO Å ). These two competing mechanisms for oxygen incorporation into sulfate express comparable influences at 25 °C. With increasing reaction temperatures from 4 to 100 °C, it appears that accelerated thermal decomposition and diminished residence time of H 2 O 2 limit the oxygen transfer from H 2 O 2 into sulfate and enhance the relative importance of H 2 O-derived oxygen for incorporation into sulfate. Notably, at temperatures between 100 and 150 °C there is a reversal in the lower temperature trend resulting in dom- inance of H 2 O 2 -derived oxygen over H 2 O-derived oxygen. At such high temperatures, complete thermal decomposition of H 2 O 2 to water and molecular oxygen (O 2 ) occurs within minutes in mineral-blank experiments and suggests little possibility for direct oxidation of pyrite by H 2 O 2 above 100 °C. We hypothesize that a Fe–O 2 mechanism is responsible for oxygenating pyrite to sulfate using O 2 from the preceding thermal decomposition of H 2 O 2 . Ó 2007 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Oxidation of sulfide minerals in the presence of water and oxygen (O 2 ) near the surface of Earth frequently results in production of highly acidic, sulfate-rich solutions associ- ated with weathering of metalliferous black shales, coal beds, and metal-sulfide ores. During the past two decades numerous studies have concentrated on understanding the mechanism of pyrite oxidation (Lowson, 1982; McKibben and Barnes, 1986; Luther, 1987; Moses et al., 1987; Wil- liamson and Rimstidt, 1994; Evangelou, 1995; Rimstidt and Vaughan, 2003), due in part to the environmental 0016-7037/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.08.022 * Corresponding author. Fax: +1 618 453 7393. E-mail address: Lefticariu@geo.siu.edu (L. Lefticariu). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 71 (2007) 5072–5088