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Thermodynamic controls on redox-driven kinetic stable isotope fractionation

Journal Article · · Geochemical Perspectives Letters (Online)
 [1];  [1];  [1];  [2]
  1. Stanford Univ., CA (United States)
  2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
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Stable isotope fractionation arising from redox reactions has the potential to illuminate the oxygenation of Earth's interior, oceans, and atmosphere. However, reconstruction of past and present redox conditions from stable isotope signatures is complicated by variable fractionations associated with different reduction pathways. Here we demonstrate a linear relationship between redox-driven kinetic fractionation and the standard free energy of reaction for aqueous chromium(VI) reduction by iron(II) species. We also show that the intrinsic kinetic fractionation factor is log-linearly correlated with the rate constant of reaction, which is in turn a function of the free energy of reaction. The linear free energy relationship for kinetic fractionation describes both our experimental results and previous observations of chromium isotope fractionation and allows the magnitude of fractionation to be directly linked to environmental conditions such as pH and oxygen levels. By demonstrating that the magnitude of kinetic fractionation can be thermodynamically controlled, this study systematically explains the large variability in chromium(VI) isotope fractionation and provides a conceptual framework that is likely applicable to other isotope systems.

Research Organization:
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Grant/Contract Number:
AC02-05CH11231
OSTI ID:
1571985
Journal Information:
Geochemical Perspectives Letters (Online), Journal Name: Geochemical Perspectives Letters (Online) Vol. 10; ISSN 2410-3403
Publisher:
European Association of GeochemistryCopyright Statement
Country of Publication:
United States
Language:
English

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58 GEOSCIENCES