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We report a temperature calibration for Δ 47 from bivalve carbonate that lies within error of theoretical predictions (Schauble et al., 2006; Guo et al., 2009). The temperature sensitivity of this calibration is lower than several different earlier calibrations determined using either inorganic calcite, corals, foraminiferans and coccoliths, or brachiopods and bivalved molluscs, but it agrees with more recent Δ 47 -temperature relationships determined from measurements of clumped-isotopes in mollusc and brachiopod shells. We demonstrate that mollusc shell temperature calibrations originating from different laboratories that have not been corrected for instrument backgrounds may differ by as much as ∼0.07‰ in Δ 47 over the ∼0 to 30 °C temperature range even where dΔ 47 /d T agree. Because recent calibrations for Δ 47 vs. T from several different laboratories agree for bivalved mollusc shells, yet differ from an early calibration for bivalved molluscs, we suggest it is unlikely that temperature-Δ 47 variability is attributable to phylum-specific vital effects, and instead conclude that differences in calibration slope between phyla and/or inorganic calcite are more easily explained by variability in measurements made in different laboratories. Discrepancies in both calibration slopes and/or intercepts indicate that Δ 47 values measured in natural materials may be more significantly influenced by instrument-specific effects, as well as effects from sample preparation and handling and purification of CO 2 than current techniques are able to correct for, and therefore, temperatures obtained by comparing measurements of Δ 47 to independently determined calcite calibrations may err by a far greater amount than acknowledged in previous studies. [ABSTRACT FROM AUTHOR]

Abstract: We modeled the Δ17O of atmospheric O2 using 27 ordinary differential equations comprising a box model composed of the stratosphere, troposphere, geosphere, hydrosphere and biosphere. Results show that 57% of the deficit in 17O in O2 relative to a reference water fractionation line is the result of kinetic isotope fractionation attending the Dole effect, 33% balances the positive Δ17O of O(1D) in the stratosphere, and 10% is from evapotranspiration. The predicted Δ′17O O2 relative to waters is −0.410‰ as measured at the δ18O of air. The value for Δ′17O O2 varies at fixed δ18O with the concentration of atmospheric CO2, gross primary production, and net primary production as well as with reaction rates in the stratosphere. Our model prediction is consistent with our measurements of the oxygen isotopic composition of air O2 compared with rocks if rocks define a fractionation line with an intercept in δ′17O=103ln(δ17O/103 +1) vs. δ′18O=103ln(δ18O/103 +1) space less than SMOW but more positive than some recent measurements imply. The predicted Δ17O is less negative than that obtained from recent measurements of O2 directly against SMOW. Underestimation of Δ′17O O2 can only be ameliorated if the integrated (bulk) Δ′17O for stratospheric CO2 is significantly greater than measurements currently allow. Our results underscore the need for high-precision comparisons of the 17O/16O and 18O/16O ratios of atmospheric O2, VSMOW, and rocks. [Copyright &y& Elsevier]

First results from a new UV laser ablation MC-ICPMS method for measuring Mg isotope ratios in situ in meteoritical materials show that there are mass-dependent variations in δ25Mg and δ26Mg up to 1.5 ‰ per amu in chondrules and 0.3‰ per amu in a CAI from the Allende meteorite. In both cases the mass-dependent fractionation is associated with alteration. Comparisons with laser ablation O isotope data indicate that incorporation of pre-existing grains of forsterite with distinct Mg and O isotopic compositions and post-formation alteration both contributed to the variability in Mg isotope ratios in the chondrules, resulting in a correlation between high δ25Mg and low Δ17O. The laser ablation analyses of the CAI show that high-precision determinations of both δ25Mg and δ26Mg can be used to discriminate features of the 26Al-26Mg isotope system that are relevant to chronology from those that result from element mobility. [Copyright &y& Elsevier]

Abstract: The range in 56Fe/54Fe isotopic compositions measured in naturally occurring iron-bearing species is greater than 5‰. Both theoretical modeling and experimental studies of equilibrium isotopic fractionation among iron-bearing species have shown that significant fractionations can be caused by differences in oxidation state (i.e., redox effects in the environment) as well as by bond partner and coordination number (i.e., nonredox effects due to speciation). To test the relative effects of redox vs. nonredox attributes on total Fe equilibrium isotopic fractionation, we measured changes, both experimentally and theoretically, in the isotopic composition of an Fe2+–Fe3+–Cl–H2O solution as the chlorinity was varied. We made use of the unique solubility of FeCl4 − in immiscible diethyl ether to create a separate spectator phase against which changes in the aqueous phase could be quantified. Our experiments showed a reduction in the redox isotopic fractionation between Fe2+- and Fe3+-bearing species from 3.4‰ at [Cl−]=1.5M to 2.4‰ at [Cl−]=5.0M, due to changes in speciation in the Fe–Cl solution. This experimental design was also used to demonstrate the attainment of isotopic equilibrium between the two phases, using a 54Fe spike. To better understand speciation effects on redox fractionation, we created four new sets of ab initio models of the ferrous chloride complexes used in the experiments. These were combined with corresponding ab initio models for the ferric chloride complexes from previous work. At 20°C, 1000 ln β (β = 56Fe/54Fe reduced partition function ratio relative to a dissociated Fe atom) values range from 6.39‰ to 5.42‰ for Fe(H2O)6 2+, 5.98‰ to 5.34‰ for FeCl(H2O)5 +, and 5.91‰ to 4.86‰ for FeCl2(H2O)4, depending on the model. The theoretical models predict ferric–ferrous fractionation about half as large (depending on model) as the experimental results. Our results show (1) oxidation state is likely to be the dominant factor controlling equilibrium Fe isotope fractionation in solution and (2) nonredox attributes (such as ligands present in the aqueous solution, speciation and relative abundances, and ionic strength of the solution) can also have significant effects. Changes in the isotopic composition of an Fe-bearing solution will influence the resultant Fe isotopic signature of any precipitates. [Copyright &y& Elsevier]

Abstract: Several lines of isotopic evidence – the most direct of which is from Hadean Jack Hills zircons – suggest a very early history of crust formation on Earth that began by about 4.5Ga. To constrain both the fate of the reservoir for this crust and the nature of crustal evolution in the sediment source region of the Jack Hills, Western Australia, during the early Archean, we report here initial 176Hf/177Hf ratios and δ18O systematics for <4Ga Jack Hills zircons. In contrast to the significant number of Hadean zircons which contain highly unradiogenic 176Hf/177Hf requiring a near-zero Lu/Hf reservoir to have separated from the Earth’s mantle by 4.5Ga, Jack Hills zircons younger than ca. 3.6Ga are more radiogenic than –13ε (CHUR) at 3.4Ga in contrast to projected values at 3.4Ga of –20ε for the unradiogenic Hadean reservoir indicating that some later juvenile addition to the crust is required to explain the more radiogenic younger zircons. The shift in the Lu–Hf systematics together with a narrow range of mostly mantle-like δ18O values among the <3.6Ga zircons (in contrast to the spread towards sedimentary δ18O among Hadean samples) suggests a period of transition between 3.6 and 4Ga in which the magmatic setting of zircon formation changed and the highly unradiogenic low Lu/Hf Hadean crust ceased to be available for intracrustal reworking. Constraining the nature of this transition provides important insights into the processes of crustal reworking and recycling of the Earth’s Hadean crust as well as early Archean crustal evolution. [Copyright &y& Elsevier]