Benchmarking the simulation of Cr isotope fractionation

A benchmark problem set consisting of four problem levels was developed for the simulation of Cr isotope fractionation in 1D and 2D domains. The benchmark is based on a recent field study where Cr(VI) reduction and accompanying Cr isotope fractionation occurs abiotically by an aqueous reaction with dissolved Fe 2+ (Wanner et al., 2012., Appl. Geochem., 27, 644–662). The problem set includes simulation of the major processes affecting the Cr isotopic composition such as the dissolution of various Cr(VI) bearing minerals, fractionation during abiotic aqueous Cr(VI) reduction, and non-fractionating precipitation of Cr(III) as sparingly soluble Cr-hydroxide. Accuracy of the presented solutions was ensured by running the problems with four well-established reactive transport modeling codes: TOUGHREACT, MIN3P, CRUNCHFLOW, and FLOTRAN. Results were also compared with an analytical Rayleigh-type fractionation model. An additional constraint on the correctness of the results was obtained by comparing output from the problem levels simulating Cr isotope fractionation with the corresponding ones only simulating bulk concentrations. For all problem levels, model to model comparisons showed excellent agreement, suggesting that for the tested geochemical processes any code is capable of accurately simulating the fate of individual Cr isotopes.

TIMS measurements of full range of natural Ca isotopes with internally consistent fractionation correction

This study presents static measurements of the Ca isotopic composition of standard reference materials SRM 915 a/b on a Triton Plus™ thermal ionization mass spectrometer with a specially developed Faraday cup array allowing simultaneous measurement of 40Ca and 48Ca. The total amount of Ca in all analyses was kept < 1 µg. With this setup the measurement uncertainties were 0.06 ‰ for 40Ca/44Ca and 0.12 ‰ for 48Ca/40Ca. Measuring all isotopes simultaneously better allows to test the internal consistency of different Ca isotope abundances reported in the literature. The exponential law was observed to correct incompletely instrumental mass fractionation. An improved fractionation correction based on the exponential law is proposed. It changes the 40Ca/44Ca ratio of SRM 915a (corrected relative to 42Ca/44Ca = 0.31221; 48Ca/44Ca = 0.08871) from 47.1635 ± 0.0028 to 47.1649 ± 0.0047. The measurements of SRM 915b were performed with different analytical conditions (runs were prolonged till complete filament load depletion). Even if the 40Ca/44Ca ratio of SRM 915b, when corrected with the simple exponential law, appears different (47.1532 ± 0.0038) from that of SRM 915a, it becomes coincident (47.1613 ± 0.0028) when corrected with a second-order refinement. This supports the use of the improved exponential law to obtain internally consistent Ca isotope ratio for natural samples.

Constraints on barium isotope fractionation during aragonite precipitation by corals

We present a barium (Ba) isotope fractionation study of marine biogenic carbonates (aragonitic corals). The major aim is to provide first constraints on the Ba isotope fractionation between modern surface sea water and coral skele- ton. Mediterranean surface sea water was found to be enriched in the heavy Ba isotopes compared to previously reported values for marine open ocean authi- genic and terrestrial minerals. In aquarium experiments with a continuous sup- ply of Mediterranean surface water, the Ba isotopic composition of the bulk sample originating from cultured, aragonitic scleractinian corals (d137/134Ba between +0.16 +/- 0.12permil and +0.41 +/-0.12permil) were isotopically identical or lighter than that of the ambient Mediterranean surface sea water (d137/134Ba = +0.42 +/- 0.07permil, 2SD), which corresponds to an empirical maximum value of Ba isotope fractionation of D137/134Bacoral-seawater = -0.26 +/- 0.14permil at 25°C. This maximum Ba isotope fractionation is close and identical in direction to previous results from inorganic precipitation experiments with aragonite- structured pure BaCO3 (witherite). The variability in measured Ba concentrations of the cultured corals is at odds with a uniform distribution coefficient, DBa/Ca, thus indicating stronger vital effects on isotope than element discrimination. This observation supports the hypothesis that the Ba isotopic compositions of these corals do not result from simple equilibrium between the skeleton and the bulk sea water. Complementary coral samples from natural settings (tropical shallow-water corals from the Bahamas and Florida and cold- water corals from the Norwegian continental shelf) show an even wider range in d137/134Ba values (+0.14 +/- 0.08permil and +0.77 +/- 0.11permil), most probably due to additional spatial and/or temporal sea water heterogeneity, as indicated by recent publications.

Cometary Isotopic Measurements

Isotopic ratios in comets provide keys for the understanding of the origin of cometary material, and the physical and chemical conditions in the early Solar Nebula. We review here measurements acquired on the D/H, N-14/N-15, O-16/O-18, C-12/C-13, and S-32/S-34 ratios in cometary grains and gases, and discuss their cosmogonic implications. The review includes analyses of potential cometary material available in collections on Earth, recent measurements achieved with the Herschel Space Observatory, large optical telescopes, and Rosetta, as well as recent results obtained from models of chemical-dynamical deuterium fractionation in the early solar nebula. Prospects for future measurements are presented.

Experimental determination of barium isotope fractionation during diffusion and adsorption processes at low temperatures

Variations in barium (Ba) stable isotope abundances measured in low and high temperature environments have recently received increasing attention. The actual processes controlling Ba isotope fractionation, however, remain mostly elusive. In this study, we present the first experimental approach to quantify the contribution of diffusion and adsorption on mass- dependent Ba isotope fractionation during transport of aqueous Ba2+ ions through a porous medium. Experiments have been carried out in which a BaCl2 solution of known isotopic composition diffused through u-shaped glass tubes filled with silica hydrogel at 10 C and 25 C for up to 201 days. The diffused Ba was highly fractionated by up to -2.15‰ in d137/134Ba, despite the low relative difference in atomic mass. The time-dependent isotope fractionation can be successfully reproduced by a diffusive transport model accounting for mass-dependent differences in the effective diffusivities of the Ba isotope species (D137Ba/D134Ba = (m134/m137)^b). Values of b extracted from the transport model were in the range of 0.010–0.011. Independently conducted batch experiments revealed that adsorption of Ba onto the surface of silica hydrogel favoured the heavier Ba isotopes (a = 1.00015 ± 0.00008). The contribution of adsorption on the overall isotope fractionation in the diffusion experiments, however, was found to be small. Our results contribute to the understanding of Ba isotope fractionation pro- cesses, which is crucial for interpreting natural isotope variations and the assessment of Ba isotope ratios as geochemical proxies.