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Experimental determination of the equilibrium Fe isotope fractionation between Fe-aq(2+) and FeSm (mackinawite) at 25 and 2 degrees C

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  • Romain Guilbaud
  • Ian B. Butler
  • Rob M. Ellam
  • David Rickard
  • Anthony Oldroyd

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    Rights statement: NOTICE: This is the author’s version of a work that was accepted for publication. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. A definitive version was subsequently published in Geochimica et Cosmochimica Acta (2011)

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Original languageEnglish
Pages (from-to)2721-2734
Number of pages14
JournalGeochimica et Cosmochimica Acta
Volume75
Issue number10
Early online date21 Feb 2011
DOIs
Publication statusPublished - 15 May 2011

Abstract

We report the first experimentally-determined metal isotope equilibrium fractionation factors for a metal sulphide at ambient temperatures and pressures. Mackinawite, referred here as FeSm (where the subscript m indicates mackinawite), can be a reactive component in diagenetic pyrite formation and the extent of equilibration between FeSm and dissolved Fe(II) has direct implications the delta Fe-56 signatures recorded in diagenetic pyrite. The measured equilibrium Fe isotope fractionation between Fe(II)(aq) and FeSm is Delta Fe-56(Fe(II)-FeS) = -0.52 +/- 0.16 parts per thousand at 2 degrees C and Delta Fe-56(Fe(II)-FeS) = -0.33 +/- 0.12 parts per thousand at 25 degrees C and pH 4. At the experimental pH the equilibrium fractionation factor between all dissolved Fe(II) species and FeSm (Delta Fe-56(Fe(II)-FeS)) equates to the fractionation factor between Fe-aq(2+) and FeSm (Delta Fe-56(Fe2+-FeS)). The measured fractionations are of the same order as other non-redox fractionations measured in low-temperature Fe-C-O systems. We show that at low temperature, the Fe(II)(aq)-FeSm system is slowly asymptotic to isotopic equilibrium and consequently, FeSm is likely to partially conserve kinetically derived isotopic signatures generated on precipitation. Combined with the range of published kinetic fractionations measured on FeSm precipitation, our data suggest that, subject to the degree of isotope exchange during equilibration, FeSm can display delta Fe-56 compositions encompassing a range of similar to 1.4 parts per thousand. (C) 2011 Elsevier Ltd. All rights reserved.

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