An ad-hoc correction of classical-trajectory scattering simulations fully taking into account the quantum-mechanical principle of indiscernibility, produces a mass-independent isotope fractionation as observed in ozone.Variations with respect to temperature are reproduced for two different potential surfaces. Coupling the mass-independent fractionation to mass-dependent corrections gives us the possibility to estimate the kinetic constant for ozone-stabilizing collisions with a third body, which in turn allows to access the pressure dependence of the isotope fractionation.Obtained results are in qualitative agreement to published experimental data.

During the world's first nuclear explosion, in 1945, glassy melts called "trinitites", mostly derived from the sands at the surface of the test site, formed and were deposited at or near the hypocenter. The processes of formation of this fallout remain unclear. Here, we show how the oxygen and silicon isotopic compositions of three trinitites allow to refine their formation scenario. The three samples are typical of trinitites, being composed of various crystalline phases and of glassy phases divided into three chemical groups (CaMgFe, alkali, silica) that are mixed in various proportions in the three samples. The three samples show a large range of oxygen and silicon isotopic variations (-10.9 f 0.6 G delta 30Si G 4.2 f 0.6 %o, and 2.3 f 0.4 G delta 18O G 24.2 f 0.5 %o). At variance with the Hiroshima fallout deposits, no oxygen mass-independent isotopic fractionation was found in the three trinitites. The chemical and isotopic compositions of the chemical groups reveal that they result from different processes: the silica phases are molten fragments of the site material, while the CaMgFe and alkali phases are produced by the mixing of condensates and molten site material. Models show that the observed silicon isotopic variations resulted from Rayleigh distillation during condensation of the gaseous species injected into the cloud, while the variability in composition of the site materials also played an important role for controlling the oxygen isotopic compositions. From these observations, a general scenario, beginning with the vaporization of the site surface, producing a depression, is proposed. The vaporized material condensed and grew by agglomeration with other condensates and liquid materials. These agglomerates rained on the surface and quenched, forming the trinitites. This scenario is different from the formation of the Hiroshima glasses but shows some similarities to tektite formation.

Contrary to all terrestrial rocks, planets and meteorites exhibit oxygen isotope variations decorrelated with the mass difference of their atomic nuclei. It has been proposed that, in the protosolar nebula (PSN), these variations could result from mass independent isotopic fractionation (MIF) either during specific chemical reactions similar to those responsible for the formation of ozone in the Earth's atmosphere or during ultraviolet (UV)-photolysis of carbon monoxide (CO) gas in the PSN. However, these potential chemical MIF reactions (Chem-MIFs) are not identified in conditions close to the PSN, and there is no experimental demonstration that large MIF signature can be transferred to solids forming in the PSN. Here, we show that MIFs, up to 60‰ depletion in 16O, are produced by high-temperature reactions in a plasma during the condensation of carbonaceous solids from a gas containing two of the most abundant PSN molecular species (H2O and CH4). This effect is attributed to the formation in the plasma of the activated complex H2O2* followed by its stabilization by reactions with CHx• radicals. Although it is premature to assert that this reaction represents the main process resulting in MIF of oxygen isotopes in the solar system, our result demonstrates the potential importance of a Chem-MIF effect in a PSN where plasma zones develop.

A new kind of Hiroshima nuclear fallout, the Hiroshima glasses, was discovered around the Hiroshima Bay. Here, the chemical compositions and the silicon and oxygen triple isotope compositions were analyzed to understand the formation process of these new fallouts. The chemical analysis shows four different families of glasses: the melilitic glasses, the anorthositic glasses, the soda-lime glasses, and the silica glass. The silicon isotopic compositions show wide variations in the glasses, with delta 30Si varying between-23.0 +/- 1.8 %o and-1.5 +/- 1.1 %o. The oxygen isotopic compositions indicate the presence of mass-independent fractionation on approximate to 38 % of the analyses, reaching a Delta 17O of-3.1 +/- 0.6 %o. The chemical and silicon isotopic compositions of the Hiroshima glasses show that these glasses were formed by condensation within the nuclear fireball. Our scenario for the Hiroshima glasses formation, tested by modeling (GGchem code), considers a rapid condensation (1.7-5.5 s) in the nuclear fireball (3200-1000 K) at atmospheric pressure with a gas resulting from a mixing between air, and vaporized water and city materials. Chemical reactions during the Hiroshima glasses condensation are the most probable source for the oxygen mass-independent fractionation. The formation of the Hiroshima glasses by condensation implies that they may be an analog to the first condensates in the solar system: Calcium-Aluminum-rich Inclusions (CAIs), which are found in chondrites. The Hiroshima glasses exhibit similarities with CAIs in their chemical and isotopic compositions (Si and O).

Cherts are pervasive in the Precambrian and their O and Si isotopic compositions have been used extensively for reconstructing seawater paleo-temperatures. However, these reconstructions are obscured by the fact that cherts exhibit, at a given age, a large isotopic (O and Si) heterogeneity at bulk and micrometer scales. This is understood as reflecting the fact that cherts are an assemblage of silica precursors having different origins, and that diagenesis and metamorphism have modified the isotopic compositions acquired during the last equilibration with seawater. To test the contributions of the different processes that control the Si isotopic composition of cherts, we developed high-precision triple Si isotope measurements by Multicollector-Inductively Coupled-Plasma Mass-Spectrometer (MC-ICP-MS). This approach allows to identify deviations from the equilibrium Si isotopic fractionation at the level of a few ppm. Triple Si isotope data combined with trace elements and O isotope data in Precambrian cherts demonstrate that three different sources of silica were involved in the formation of cherts, and that both equilibrium and kinetic isotopic fractionation took place for Si. Archean seawater is predicted to have a triple Si isotopic composition kinetically fractionated relative to bulk silicate Earth. This requires a large kinetic isotopic fractionation during hydrothermalism of the oceanic crust and appears consistent with warm oceans in the Precambrian. However more work is required on the triple isotope systematics of Si inputs and output to seawater by rivers and authigenic clays formation. Triple Si isotope variations can thus be used as a new tool to unravel the origin of cherts and of their O and Si isotopic compositions. The present evidence of kinetic isotopic fractionations for Si isotopes also implies that Si isotopes should be associated to triple O isotope for paleotemperature reconstructions.

We propose a numerical model allowing to calculate the relative variations of isotopic ratios involved in a mass-independent isotopic fractionation (MIF) effect. This model is derived from classical trajectory simulation performed to reproduce the reactions yielding the isotopomers of ozone. In the ozone simulation, we did not introduce quantum mechanical selection rules for trajectories or the potential surface, but we separated instead exchange and non-exchange collisions, in order to introduce the fundamental quantum mechanical requirement according to which, for indistinguishable isotopes, the two possible reaction channels (elastic scattering or particle exchange) have to be superposed. The MIF effect is related to the molecular symmetry of the complex by the result that a different fraction of isotopically asymmetric complexes is stabilized than for symmetric ones.The model is applied on the results obtained experimentally for Mg and Ti isotopes in plasma. In plasma, Mg and Ti radicals resulting from the molecular dissociation of chlorides react with their parent molecules. In presence of hydrocarbons, isotope exchange rates are greatly enhanced when the intermediate activated complexes are adsorbed at the surface of the carbonaceous grains growing in the plasma. If a chemical reaction with the grain stabilizes the complex faster than its dissociation, MIF effects are observed. In such a chemical situation, the isotopic fractionation greatly exceed the usual theoretical predictions. Several characteristics of the MIF isotopic patterns are reproduced by the model.

Significance Both the physical effect and the chemical conditions at the origin of the oxygen isotope variations in the solar system have been puzzling questions for 50 y. The data reported here bring the MIF effect (the mass-independent fractionation originally identified on ozone) back to the center of the debate. Similar to Ti isotopes, we observe that the MIF effect for O and Mg is triggered by redox reactions in plasmas. These observations reinforce the idea of a universal mechanism observable in photochemical reactions when molecular collisions involving indistinguishable isotopes yield a symmetrical complex stabilized as a chemical product.

Some of the oldest traces for planktonic lifestyle have been reported in ca. 3.4 billion years old silicified sediments from the Strelley Pool Formation in Western Australia. Observation of flange appendages suggests that Archean life motility was passive and driven by drifting of microorganisms in their surrounding environment. Until now, the oldest traces for active motility are ca. 2.1 billion years old. Whether or not active motility already existed during the Archean eon remains an open question. In this study, we report the discovery of new 3.4 billion years old microfossils exhibiting a tail-like structure isolated from the Strelley Pool Formation. Exhibiting Raman spectra typically observed in organic-walled microfossils from the Strelley Pool Formation, these microfossils exhibiting a tail-like structure are syngenetic with their host rock. Composed of carbon, nitrogen, and, for one specimen, phosphorus, some of these organic-walled microfossils also exhibit significant level of aliphatic and amide moieties supporting their biogenicity. In addition, these microfossils exhibit a tail-like appendage sharing similar morphological features with locomotory organelles in modern microorganisms such as archaella, flagella, and cilia. This suggests that this observed appendage likely provided them with movement capabilities. If correct, with the ability to move, these microorganisms were capable of escaping from harsh environments and/or colonizing new ecological niches as early as 3.4 billion years ago.

Cherts are pervasive in Precambrian and have significance in reconstructing paleo-environmental conditions.Cherts are an assemblage of different forms of sedimentary silica processed through diagenesis.They exhibit, for a given age and at micrometer scale, a large range of isotopic (O and Si) compositions.The inferences made from these isotopic compositions on Precambrian seawater temperatures depend on the models assumed for the mode of formation of cherts.Here we develop high precision silicon isotope measurements by Multicollector -Inductively Coupled Plasma -Mass Spectrometry to reach 2 standard error of ±3-8 ppm (e.g., 2 se = 3 ppm for 43 measurements in BHVO-2) on deviations from the slope in triple Si isotope diagram.This allows to tentatively identify the mechanism of Si isotopic fractionation and whether silica in chert was formed through equilibrium or kinetic processes.Triple silicon isotope data are reported here for cherts from Bitter Springs formation (0.85 Ga), Jixian group (from 1.3 to 1.5 Ga), Warrawoona group (3.5 Ga) and Onverwacht group (3.5 Ga).8 samples from Jixian group show large variations of massdependent Si isotopic compositions (δ 30 Si from 0.17 to 2.93 ‰) in agreement with previous ion probe data, and define an apparent mass fractionation law with a slope of 0.5119 ± 0.0011, intermediate between equilibrium (0.5178) and kinetic (0.5092) processes.We developed a model that explains these data by the formation of Jixian cherts from a mixture of amorphous silica precipitated at equilibrium with seawater and quartz precipitated kinetically from hydrothermal fluid.Correlations between Si and O isotopic compositions and trace element contents support this interpretation.By using Si isotopic fractionations measured between quartz and hydrothermal fluid and between amorphous silica and water, we can reconstruct the δ 30 Si of seawater in Jixian basin to be ≈ +2.5‰.The Si isotopic composition of cherts from other localities have slopes that deviate from kinetic and equilibrium, indicating complex dissolution and recrystallization of silica in pore waters during diagenesis.These new data allow to scrutinize the origin of cherts and better decipher primary and secondary processes at the origin of O and Si isotopic variations.

Forty years ago, the synthesis of ozone has revealed mass-independent fractionation (MIF) of oxygen isotopes. Among the numerous published interpretations of this ozone effect, the present paper reports that a titanium MIF observed during chemical reactions in plasma can be accounted for by one of these theoretical treatments. A gaseous mixture of TiCl4/C5H12 carried in a glass tube by a continuous flow of N2 was submitted to a HF discharge. Carbonaceous grains, presumably resulting from homogeneous condensation, show radial variations in Ti concentration with isotopic variations ranging from −25 to +125% for all mTi/48Ti isotope ratios (m standing for masses 46, 47, 49 and 50). It is proposed that the reduction rates by carbon radicals, of the metastable molecules formed by the isotope exchange reactions between Ti and TiCl4 are different if they involve distinguishable or indistinguishable isotopes. A numerical simulation reproduces the experimental isotope patterns defined by the mTi/48Ti isotope ratios.

The biogenicity of most of the putative Archean organic-walled microfossils discussed in the literature remains debated. Here, we report morphological and geochemical characterizations of an assemblage of microfossils isolated from the 3.46 Gyr-old Strelley Pool Formation (Western Australia), thereby providing a new set of data on the oldest authentic microfossils on Earth. Isolated microstructures/microfossils were studied by combining scanning electron microscopy imaging and elemental analyses, Raman spectroscopy and nanoscale secondary ion mass spectrometry (NanoSIMS). We identified four morphological types, namely filaments, films, spheroids and lenses. A minority of spheroids, films and lenses exhibits a continuous wall cell surface and high N concentrations and substantial P contents (as suggested by P-31(-)/C-12(2)- ionic ratios), i.e. exceptionally high morphological and geochemical preservation levels. In addition to the detection of P within microfossil walls, P was also found within micrometric patches in lenses exhibiting a fusiform shape. These patches may be remains of polyphosphate granules possibly formed during sporulation, suggesting, in turn, that cellular organization appeared as soon as 3.46 Gyr ago.

The oldest traces for planktonic lifestyle have been reported in ca. 3.4 billion years old silicified sediments from the Strelley Pool Formation in Western Australia. Observation of flange appendages suggests that Archean life motility was passive and driven by drifting of microorganisms in their surrounding environment. Until now, the oldest traces for active motility are ca. 2.1 billion years old. Whether or not active motility already existed during the Archean eon remains an open question. Here we report the discovery of new 3.4 billion years old tailed microfossils. These microfossils exhibit a lash-like appendage that likely provided them with movement capabilities. This suggests that these microfossils are the oldest remains of active motile life forms. With the ability to move in liquids and on organic and/or mineral surfaces, these microorganisms were capable of escaping from harsh environments and/or colonizing new ecological niches as early as 3.4 billion years ago. The existence of these deep-rooted Archean motile life forms offers a new picture of the Archean biodiversity, with unanticipated evolutionary innovative morphological complexities.

Insoluble organic materials (kerogens) isolated from ancient sedimentary rocks provide unique insights into the evolution of early life. However, establishing whether these kerogens are indeed syngenetic with the deposition of associated sedimentary host rocks, or contain contribution from episodes of secondary deposition, is not straightforward. Novel geochemical criterions are therefore required to test the syngenetic origin of Archean organic materials. On the one hand, the occurrence of mass-independent fractionation of sulphur isotopes (MIF-S) provides a tool to test the Archean origin of ancient sedimentary rocks. Determining the isotope composition of sulphur within kerogens whilst limiting the contribution from associated minerals (e.g., nano-pyrites) is however challenging. On the other hand, the Xe isotope composition of the Archean atmosphere has been shown to present enrichments in the light isotopes relative to its modern composition, together with a mono-isotopic deficit in Xe-129. Given that the isotopic composition of atmospheric Xe evolved through time by mass dependent fractionation (MDF) until similar to 2.5 to 2.0 Ga, the degree of MDF of Xe isotopes trapped in kerogens could provide a time stamp for the last chemical equilibration between organic matter and the atmosphere. However, the extent to which geological processes could affect the signature of Xe trapped in ancient kerogen remains unclear. In this contribution, we present new Ar, Kr and Xe isotopic data for four kerogens isolated from 3.4 to 1.8 Gy-old cherts and confirm that Xe isotopes from the Archean atmosphere can be retained within kerogens. However, new Xe-derived model ages are lower than expected from the ages of host rocks, indicating that initially trapped Xe components were at least partially lost and/or mixed together with some Xe carried out by younger generations of organic materials, therefore complicating the Xe-based dating method. Whilst non-null Delta S-33 values and Xe-129 deficits relative to modern atmosphere constitute reliable imprints from the Archean atmosphere, using Xe isotopes to provide information on the syngenetic origin of ancient organic matter appears to be a promising - but not unequivocal - tool that calls for further analytical development.

Isotopes of heavy elements are produced in various amounts by nuclear processes in stars1,2. Consequently, the presence of isotopic anomalies in the Solar System is considered to reflect the presence of presolar grains condensed in previous generations of stars3 and not a (proto-) Solar System process. However, for oxygen, the major rock-forming element, it has been shown that physico-chemical reactions applicable to the presolar cloud or the protoplanetary disk were a possible source of isotopic variations due to mass-independent isotopic fractionation (MIF)4,5. Here we show that MIF effects are not restricted to oxygen, but can also be produced for titanium. Titanium-rich grains experimentally condensed from a TiCl4(g)/C5H12(g) plasma exhibit MIF effects from −25% to +120% for all Ti isotopic ratios. These large Ti isotopic variations follow the model developed for oxygen MIF6 and mimic the Ti isotopic anomalies observed in some presolar grains. This effect is ascribed to the reactions between chemically indistinguishable isotopes6 and could contribute to the complexity of isotopic anomalies observed in Solar System materials1,7–14. The abundances of isotopes in presolar grains are presumed to reflect their nucleosynthetic generation in stars. However, here Robert et al. report experimentally detected evidence of mass-independent fractionation of titanium, implying that titanium isotopic abundances in presolar grains may not be as representative as once thought.

Significance The debate on the onset of plate tectonics in the Earth’s history has partially originated from the controversial criteria of using felsic crust to trace plate tectonics in the past. Here, we demonstrate how Ti isotope ratios can be used as a proxy for the affinity of felsic rocks to plume or island arc settings. Our study shows that, contrary to what was previously assumed, Ti isotopes cannot serve as a direct evidence for plate tectonics from 3.5 billion years ago, and must be combined with other information on SiO 2 contents of crustal rocks to be reliable.