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Abstract: Cadmium isotopes were fractionated by the liquid–liquid extraction technique with a crown ether, dicyclohexano-18-crown-6. The isotopic ratios of mCd/111Cd (m: 110, 112, 113, 114, and 116) were measured precisely by the multi-collector inductively coupled plasma spectrometry (MC-ICP-MS). When the isotope enrichment factors were calculated, the odd atomic mass isotopes (111Cd and 113Cd) showed excesses of enrichment comparing to the even atomic mass isotopes (110Cd, 112Cd, 114Cd and 116Cd). This odd–even staggering property originates from the nuclear field shift effect. The contribution of the nuclear field shift effect to the observed isotope enrichment factor was estimated to be 5 to 30%. [Copyright &y& Elsevier]

Abstract: Mass-independent isotope fractionations found in laboratory-scale chemical exchange experiments are reviewed. The classic theory of stable isotope fractionation in chemical exchange reactions has been established by Bigeleisen, Mayer, and Urey in 1947. In 1996, the conventional mass-dependent theory was expanded by Bigeleisen to include a mass-independent term named the nuclear field shift effect. The nuclear field shift is an isotope shift in orbital electrons, which results from the isotopic difference in nuclear size and shape. Since the revised theory was proposed, the mass-independent isotope fractionation of various elements, (e.g., Ti, Cr, Zn, Sr, Mo, Ru, Cd, Sn, Te, Ba, Nd, Sm, Gd, Yb, and U), found in chemical exchange systems has been successfully explained as the nuclear field shift effect. In this review article, from both theoretical and experimental viewpoints, origins of mass-independent isotope effects are discussed. [Copyright &y& Elsevier]

Abstract: Microscopic pyrites with low 34S/32S ratios in ~3.47-Gyr-old sedimentary barites from North Pole, Australia have been interpreted as evidence for microbial sulfate reduction and/or sulfur disproportionation in the early Archean. We show that these microscopic sulfides have similar to slightly less negative Δ33S and slightly more negative Δ36S values compared to the enclosing sulfate crystals. This finding is consistent with a primary mass-independent signature overprinted by biological sulfate reduction, as calibrated by previous experimental laboratory culture studies. However, it is inconsistent with an overprint by abiological sulfate reduction or sulfur disproportionation, as predicted by isotope exchange theory and laboratory culture studies. Thus, our multiple sulfur isotope measurements support the contention that sulfate-reducing microbes had evolved by ~3.47 billion years ago. [Copyright &y& Elsevier]

Chromium isotopes were fractionated by continuous extraction using dicyclohexano-18-crown-6 as an extractant. This system was designed for accumulating isotope effect into the aqueous phase. After a long period of operation (∼2 weeks), the Cr isotope ratios in the residual aqueous phase were precisely measured by using a multi-collector thermal ionization mass spectrometer with the total evaporation technique. The analytical results showed a clear mass-independent isotope fractionation. The mass-independent property was consistent with that obtained from a batch extraction. The isotopic mass balance was verified to be preserved in the continuous extraction. The Bigeleisen's 1996 theory gave the consistent explanation on the Cr isotope effect, in which the mass-independent isotope effect was identified to be the nuclear field shift effect. [ABSTRACT FROM AUTHOR]

Abstract: The isotope geochemistry of sulfur in the SNC (Shergottite, Nakhlite, Chassignite) meteorites may carry important implications for the origin and chemistry of sulfates and sulfides in the Martian surface and subsurface. We report measurements of the sulfur isotope composition for three sulfur-bearing components of the Nakhla meteorite. These results yield a negative Δ 33S signal of −1.25‰ and a near zero Δ 36S signal for total sulfate as well as a small nonzero Δ 33S of −0.09‰ for sulfide phases. The Δ 33S of total sulfate measured here is more negative than the Δ 33S reported previously for water soluble sulfate and points to sulfur isotopic heterogeneity in the sulfate of Nakhla. Current understanding of mass-independent isotope effects associated with sulfur oxidation reactions rules out hydrothermal oxidation mechanisms for producing nonzero Δ 33S and points to a significant role for photochemical effects (including hyperfine chemistry), likely located in the Martian atmosphere. The near zero Δ 36S signal allows for the possibility of hyperfine chemistry. Because of the relationship between SNC sulfate and the sulfate deposits seen on Mars, these results place constraints on the generation of sulfur(VI) at the Martian surface and provide a case for future direct measurements of Δ 33S (and Δ 36S) of Martian sulfate. [Copyright &y& Elsevier]

Abstract: Even in the absence of nucleosynthesis, mass-independent isotope effect due to nuclear field can lead to a number of isotope anomalies found in refractory inclusions of chondrites. Isotope anomalies observed for Ba, Ca, Sr, Ti, and Cr in FUN inclusions may simply be explained by the nuclear field effects of these elements. A whole class of isotopic heterogeneities may therefore reflect evaporation/condensation processes in the solar nebula rather than nucleosynthetic effects. We speculate that the current estimates of the r-process component of the normal solar material may be affected by non-mass dependent fractionation effects. [Copyright &y& Elsevier]