V53B-3134
Should I Stay or Should I Go? The Effects of Weathering on Siderophile and Chalcophile Element Mobility in Mantle-Derived Sulfides

Friday, 18 December 2015
Poster Hall (Moscone South)
Jason Harvey, University of Leeds, Leeds, LS2, United Kingdom, Jessica M Warren, Stanford University, Stanford, CA, United States, Munir Humayun, Florida State University, Tallahassee, FL, United States and Richard Walshaw, University of Leeds, Leeds, United Kingdom
Abstract:
Jason Harvey1,2, Jessica M Warren2, Munir Humayun3, Richard D Walshaw1
  1. School of Earth and Environment, University of Leeds, Leeds, UK
  2. Dept. of Earth and Environmental Sciences, Stanford University, Stanford CA, USA
  3. National High Magnetic Field Laboratory, Florida State University, Tallahassee FL, USA

Peridotite xenoliths possess lower abundances of S and Os than peridotite massifs. This has been attributed to the ingress of meteoric water, which oxidizes sulfide, removing sulfur as soluble sulfate, while volatilizing Os to its more mobile OsO4 form[1].

Selenium is considered to be less mobile than S under oxidizing conditions[2]. This assumption was challenged recently when peridotite xenoliths from Kilbourne Hole were analysed for bulk-rock PGE-S-Se-Te-Re abundances[3]. The sulfides they contained had clearly experienced varying degrees of oxidation to Fe-oxyhydroxides, but yielded chondritic bulk-rock Se/S, consistent with values found in massif peridotites. This requires that Se is mobile during supergene weathering, otherwise supra-chondritic Se/S would be generated.

Here, we present the results of a LA-ICP-MS investigation into the highly siderophile element (HSE) and chalcophile element abundances of ca. 20 BMS grains from one of the same Kilbourne Hole xenoliths of the former bulk-rock study[3]. Osmium abundance and Os/Ir (where not fractionated by high temperature processes involving sulfide) are well correlated with both Se and S across a wide range of oxidation. Other platinum group elements (Ir, Ru, Rh, Pt, Pd) show no obvious signs of mobility with decreasing S (or Se) abundance. The positive co-variation between S and Se for a wide range of S abundance adds credence to the theory that Se may also be mobile during the oxidative breakdown of peridotite- hosted sulfides. Using Se as a proxy for S abundance in peridotite xenoliths should therefore be used with care. Moreover, the attribution of chondritic Se/S in peridotites to bulk-earth compositions may not preclude the loss of both S and Se resulting from sulfide weathering.

Refs: [1]Lorand et al. (2003) Geochim. Cosmochim. Acta 67: 4137-4153. [2]Dreibus et al. (1995) Meteoritics 30: 439-445. [3]Harvey et al (2015) Geochim. Cosmochim. Acta 166: 210-233.

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