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Mass Fractionation Laws, Mass-Independent Effects, and Isotopic Anomalies Nicolas Dauphas and Edwin A

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Mass Fractionation Laws, Mass-Independent Effects, and Isotopic Anomalies Nicolas Dauphas and Edwin A EA44CH26-Dauphas ARI 10 June 2016 9:41 ANNUAL REVIEWS Further Click here to view this article's online features: • Download figures as PPT slides • Navigate linked references • Download citations Mass Fractionation Laws, • Explore related articles • Search keywords Mass-Independent Effects, and Isotopic Anomalies Nicolas Dauphas1,∗ and Edwin A. Schauble2 1Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, Illinois 60637; email: [email protected] 2Department of Earth and Space Sciences, University of California, Los Angeles, California 90095 Annu. Rev. Earth Planet. Sci. 2016. 44:709–83 Keywords First published online as a Review in Advance on isotopes, fractionation, laws, NFS, nuclear, anomalies, nucleosynthesis, May 18, 2016 meteorites, planets The Annual Review of Earth and Planetary Sciences is online at earth.annualreviews.org Abstract This article’s doi: Isotopic variations usually follow mass-dependent fractionation, meaning 10.1146/annurev-earth-060115-012157 that the relative variations in isotopic ratios scale with the difference in Copyright c 2016 by Annual Reviews. mass of the isotopes involved (e.g., δ17O ≈ 0.5 × δ18O). In detail, how- All rights reserved ever, the mass dependence of isotopic variations is not always the same, ∗ Corresponding author and different natural processes can define distinct slopes in three-isotope diagrams. These variations are subtle, but improvements in analytical capa- Access provided by University of Chicago Libraries on 07/19/16. For personal use only. Annu. Rev. Earth Planet. Sci. 2016.44:709-783. Downloaded from www.annualreviews.org bilities now allow precise measurement of these effects and make it possi- ble to draw inferences about the natural processes that caused them (e.g., reaction kinetics versus equilibrium isotope exchange). Some elements, in some sample types, do not conform to the regularities of mass-dependent fractionation. Oxygen and sulfur display a rich phenomenology of mass- independent fractionation, documented in the laboratory, in the rock record, and in the modern atmosphere. Oxygen in meteorites shows isotopic vari- ations that follow a slope-one line (δ17O ≈ δ18O) whose origin may be as- sociated with CO photodissociation. Sulfur mass-independent fractionation in ancient sediments provides the tightest constraint on the oxygen par- tial pressure in the Archean and the timing of Earth’s surface oxygenation. 709 EA44CH26-Dauphas ARI 10 June 2016 9:41 Heavier elements also show departures from mass fractionation that can be ascribed to exotic effects associated with chemical reactions such as magnetic effects (e.g., Hg) or the nuclear field shift effect (e.g., U or Tl). Some isotopic variations in meteorites and their constituents cannot be related to the terrestrial composition by any known process, including radiogenic, nucleogenic, and cosmogenic effects. Those variations have a nucleosynthetic origin, reflecting the fact that the products of stellar nucleosynthesis were not fully homogenized when the Solar System formed. Those anomalies are found at all scales, from nanometer-sized presolar grains to bulk terrestrial planets. They can be used to learn about stellar nucleosynthesis, mixing in the solar nebula, and genetic relationships between planetary bodies (e.g., the origin of the Moon). They can also con- found interpretations based on dating techniques (e.g., 146Sm-142Nd) when they are misidentified as isotopic variations of radiogenic origin. To summarize, there is a world to explore outside of mass-dependent fractionation whose impact is promised to expand as analytical capabilities to measure ever-subtler isotopic anomalies on ever-smaller samples continue to improve. 1. INTRODUCTION In natural systems, isotopic variations can arise from a variety of processes such as phase equi- librium, unidirectional reactions, radioactive decay, and nuclear transmutations induced by par- ticle irradiation. The study of these variations is the basis of isotope geochemistry, spanning an ever-expanding range of applications, from biochemistry to nuclear astrophysics. Most isotopic variations in Solar System materials follow clear sets of rules. Phase equilibrium and unidirectional reactions are most often associated with mass-dependent variations, meaning that the degree of isotopic fractionation scales with the difference in mass of the isotopes involved. Radioactive decay produces variations in the abundances of the daughter isotopes that scale with parent-to-daughter ratios. Nuclear transmutations by particle irradiation, coming either from space (cosmogenic) or from surrounding rocks by decay of radioactive nuclides (nucleogenic), only affect certain isotopes in specific settings. In some cases, isotopic variations are found that depart from those rules and are categorized as anomalies. Isotopic anomalies were discovered when Black & Pepin (1969) measured the isotopic composition of neon released during stepwise heating of carbonaceous chondrites. They identified a component extremely rich in 22Ne, labeled Ne-E (letters A to D were already taken), that could not be related to neon in air by any of the processes discussed above. Years later, it was shown that this component has a nucleosynthetic origin, meaning that it is hosted in presolar grains of silicon carbide (Ming & Anders 1988) and graphite (Amari et al. 1990). Developments in mass Access provided by University of Chicago Libraries on 07/19/16. For personal use only. Annu. Rev. Earth Planet. Sci. 2016.44:709-783. Downloaded from www.annualreviews.org spectrometry and accompanying improvements in precision of isotope ratio analyses revealed that isotopic anomalies were widespread in planetary materials (Clayton 1978, Huss & Lewis 1995, Birck 2004). The divide between mass-dependent fractionation and isotopic anomalies is not clear-cut. The reason is that mass-dependent fractionation is not uniquely defined and can be described using various laws that depend on the cause for the fractionation (Marechal´ et al. 1999, Young et al. 2002). Often, isotopic variations are small, so mass-dependent fractionation cannot be easily distinguished from bona fide isotopic anomalies. This contribution therefore starts (Section 2) with a discussion of the theory of mass-dependent fractionation and reviews emerging applications of mass fractionation laws in Earth and planetary sciences. Isotopic anomalies are departures from mass-dependent fractionation and can be divided into clearly distinct categories: Some were produced in the Solar System by nebular, planetary, or 710 Dauphas · Schauble EA44CH26-Dauphas ARI 10 June 2016 9:41 geological processes, whereas others were inherited from stellar nucleosynthesis. Oxygen and sul- fur present a very rich phenomenology of isotopic anomalies in meteorites, planetary atmospheres, sediments, and igneous rocks. Although there is a near consensus on the general causes of some of these anomalies, important disagreements remain. The uncertainties have not prevented their use as geochemical tracers, however, and these are reviewed in Section 3. This section cannot give justice to the considerable amount of work done on this topic, and the reader is referred to relevant reviews (Clayton 1993, 2003; Farquhar & Wing 2003; Thiemens 2006). The best-documented reactions that produce oxygen and sulfur isotopic anomalies involve gas phase chemistry. Heavy elements that do not form volatile compounds can also display mass- independent fractionation caused by at least two chemical processes (the field shift effect and a probable magnetic isotope effect). Of these processes, the field shift effect is relatively well understood. In heavy elements, the nucleus can influence the strength of the chemical bonds that are formed, which can induce a fractionation that is not strictly mass dependent and is often characterized by an odd-even modulation of the isotope variations (Bigeleisen 1996a, Fujii et al. 1989). This phenomenon and examples drawn from nature and experiments are presented in Section 4. Early work on noble gases revealed the presence of material extraneous to the Solar System that was characterized by very large isotopic anomalies relative to terrestrial composition (Black & Pepin 1969). The search for the carriers of these anomalies led to the identification of presolar grains, which formed in the outflows of stars that lived and died before the Solar System was formed (Bernatowicz et al. 1987, Lewis et al. 1987). By analyzing the compositions of those grains in a laboratory setting, one can study the compositions of stars with a precision level that is unachievable by remote astronomical observations (Section 5). Some anomalies, albeit of smaller magnitude, are manifested at larger scales, ranging from millimeter-sized grains to planets. In presolar grains, isotopic variations are very large, so precision in isotopic analyses is often less of an issue than spatial resolution and isobaric interferences. At the other end of the size spectrum, bulk planetary bodies display isotopic anomalies that are very subtle and call for highly precise and accurate measurements (Clayton et al. 1973, Dauphas et al. 2002b). Such planetary- scale anomalies provide a means to (a) study stellar nucleosynthesis of elements that cannot be easily measured in presolar grains using in situ techniques, (b) trace genetic relationships between planetary bodies, and (c) understand how dust was size sorted, vaporized, and condensed in the solar
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