The History, Relevance, and Applications of the Periodic System in Geochemistry

Geochemistry is a discipline in the earth sciences concerned with understanding the chemistry of the Earth and what that chemistry tells us about the processes that control the formation and evolution of Earth materials and the planet itself. The periodic table and the periodic system, as developed by Mendeleev and others in the nineteenth century, are as important in geochemistry as in other areas of chemistry. In fact, systemisation of the myriad of observations that geochemists make is perhaps even more important in this branch of chemistry, given the huge variability in the nature of Earth materials – from the Fe-rich core, through the silicate-dominated mantle and crust, to the volatile-rich ocean and atmosphere. This systemisation started in the eighteenth century, when geochemistry did not yet exist as a separate pursuit in itself. Mineralogy, one of the disciplines that eventually became geochemistry, was central to the discovery of the elements, and nineteenth-century mineralogists played a key role in this endeavour. Early “geochemists” continued this systemisation effort into the twentieth century, particularly highlighted in the career of V.M. Goldschmidt. The focus of the modern discipline of geochemistry has moved well beyond classification, in order to invert the information held in the properties of elements across the periodic table and their distribution across Earth and planetary materials, to learn about the physicochemical processes that shaped the Earth and other planets, on all scales. We illustrate this approach with key examples, those rooted in the patterns inherent in the periodic law as well as those that exploit concepts that only became familiar after Mendeleev, such as stable and radiogenic isotopes

K-40-Ar-40 constraints on recycling continental crust into the mantle

Extraction of potassium into magmas and outgassing of argon during melting constrain the relative amounts of potassium in the crust with respect to those of argon in the atmosphere. No more than 30% of the modern mass of the continents was subducted back into the mantle during Earth's history. It is estimated that 50 to 70% of the subducted sediments are reincorporated into the deep continental crust. A consequence of the limited exchange between the continental crust and the upper mantle is that the chemistry of the upper mantle is driven by exchange of material with the deep mantle

Mass-independent isotope fractionation of molybdenum and ruthenium and the origin of isotopic anomalies in Murchison

Dauphas et al.'s model for the nucleosynthetic origin of Mo and Ru anomalies in meteorites leaves the case of Murchison (CM2) unexplained.We explore the possibility that such a discrepancy is due to mass-independent effects controlled by nuclear field shift with, in particular, ‘‘staggering'' between odd and even masses.We first demonstrate the existence of such mass-independent fractionation of Mo and Ru isotopes by chemical exchange of Mo and Ru between DC18C6 crown ether and aqueous solutions. Our results fit the nuclear field shift theory of Bigeleisen. We then review the correlation between the mean-square charge radius (which controls the nuclear field shift) and the isotopic anomalies found in an Allende CAI and in Murchison. AlthoughMo and Ru in the Allende CAI show a clear indication of nucleosynthetic components, the mass-independent anomalies observed in Murchison show a strong correlation with the nuclear charge distribution. We therefore argue that some isotopic anomalies observed in meteorites may be due to nuclear field shift rather than nucleosynthetic processes. Such effects are temperature dependent and may represent either genuine nebular processes or analytical artifacts. This new interpretation may help assess the existence of anomalies due to the extinct isotopes 97Tc and 98Tc

Isotope fractionation of iron(III) in chemical exchange reactions using solvent extraction with crown ether

International audienceThis work reports on the chemical isotope fractionation of Fe(III) by a solvent extraction method with a crown ether of dicyclohexano-18-crown-6. The 56Fe/54Fe and 57Fe/54Fe ratios were analyzed by multiple-collector inductively coupled plasma mass spectrometry. We determined the dependence of the isotope enrichment factors () on the strength of HCl. The relative deviation of the 56Fe/54Fe ratios relative to the unprocessed material (104 56) increases from -15.3 to -6.3 with [HCl] increasing from 1.6 to 3.5 mol/L. Likewise, 104 57 increases from -22.8 to -9.6 under the same conditions. The correlation between 56 and 57 is mass dependent within the errors. The observed fractionation was broken down into the effects of competing extraction reactions and of a reaction between Fe(III) species (FeCl2+ and FeCl3) in the aqueous phase. We found that the isotope fractionation between the Fe(III) species is mass dependent, which we confirmed by calculating the reduced partition function ratios

Zr isotope anomalies in chondrites and the presence of Nb-92 in the early solar system

The presence of Zr isotope anomalies in the early solar system is demonstrated with the identification of Zr-92 excesses and Zr-96. deficits in several chondrites and the CAI Allende inclusions. The isotopic composition of Zr in carbonaceous, enstatite, and ordinary chondrites, along with four SNC meteorites, was analyzed by plasma source mass spectrometry. Most chondrite samples show negative Zr-96 anomalies, which indicate the presence of a pre-solar nucleosynthetic component. Six of them also display a distinct negative Zr-92 anomaly, reaching down to -2.7 +/- 0.8 epsilon units for Forest Vale (H4). The CAI inclusions from Allende, which are among the oldest known igneous objects of the solar system and have the highest Zr/Nb ratios, also show negative epsilon Zr-92 Of -2.4 +/- 0.5. Although a substantial fraction of the Zr isotope variability may be due to pre-solar nucleosynthetic processes, part of the Zr-92 excess must result from the decay of the now extinct Nb-92. (C) 2000 Elsevier Science B.V. All rights reserved