Search

Chondrites are undifferentiated meteorites that can provide information on the compositions of materials in the early solar System, including the building blocks of the terrestrial planets. While most chondrites belong to well-defined groups based on their mineralogy and chemical composition, a minor fraction have unusual characteristics and are classified as ungrouped chondrites. These ungrouped chondrites reflect the diversity of chondritic materials in the early solar system; however, they are not as well studied as grouped meteorites and their origins are poorly understood. In this study, we present high-precision mass-independent Cr, Ca and Mg isotope data for 17 ungrouped chondrites. The ε54Cr and ε48Ca (ε expresses parts per ten thousand mass-independent isotope deviation) data for ungrouped chondrites also provide important constraints for assessing their relationships to the known chondrite groups, and the radiogenic Mg isotope ratios (μ26Mg*) can be used to track the early solar system history. We also present the first high-precision data for a Kakangari (KC) chondrite, an enstatite chondrite, and for four enstatite-rich meteorites. The ε54Cr and ε48Ca values for the KC are −0.44 ± 0.04 and −1.30 ± 0.25, respectively, and ε48Ca value for SAH 97096 (EH3) is −0.19 ± 0.22 that overlaps with that of those of Earth-Moon system and ordinary chondrites. All the carbonaceous chondrite-like (CC) ungrouped chondrites show positive ε54Cr and ε48Ca values, and all the non-carbonaceous chondrite-like (NC) ungrouped chondrites and KCs (also belong to the NC trend) show zero or negative ε54Cr and ε48Ca values. This observation confirms the CC-NC dichotomy for primitive solar system materials. LEW 87232 (KC) also shows the highest 55Mn/52Cr ratio and ε53Cr value amongst all the chondrites. There is a positive trend between 55Mn/52Cr ratios and ε53Cr values among all the chondrites that mostly reflects a mixing between multiple chondritic components. Previously it has been reported that there is a bulk 26Al-26Mg correlation line amongst chondrites. This correlation has been interpreted as being due to mixing of CAIs (high 27Al/24Mg ratios and μ26Mg* values) and other silicate material (e.g., chondrules and matrix). By providing additional 26Al-26Mg chondrite data, we show that there is no 26Al-26Mg correlation line for the chondrites, ruling out the two-endmember (i.e., CAIs and other silicates) mixing model.

This study has been funded through the Czech Science Foundation project 13-22351S. Paul Savage was supported by the Marie Curie IOF ‘Isovolc.’ Frederic Moynier is grateful to the European Research Council under the European Community's H2020 framework program/ERC grant agreement # 637503 (Pristine) and the Agence Nationale de la Recherche for a chaire d'Excellence Sorbonne Paris Cité (IDEX13C445), and for the UnivEarthS Labex program (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). Parts of this work were supported by IPGP multidisciplinary program PARI, and by Region Île-de-France SESAME Grant no. 12015908. Moldavites are tektites genetically related to the Ries impact structure, located in Central Europe, but the source materials and the processes related to the chemical fractionation of moldavites are not fully constrained. To further understand moldavite genesis, the Cu and Zn abundances and isotope compositions were measured in a suite of tektites from four different substrewn fields (South Bohemia, Moravia, Cheb Basin, Lusatia) and chemically diverse sediments from the surroundings of the Ries impact structure. Moldavites are slightly depleted in Zn (~10–20%) and distinctly depleted in Cu (>90%) relative to supposed sedimentary precursors. Moreover, the moldavites show a wide range in δ66Zn values between 1.7 and 3.7‰ (relative to JMC 3-0749 Lyon) and δ65Cu values between 1.6 and 12.5‰ (relative to NIST SRM 976) and are thus enriched in heavy isotopes relative to their possible parent sedimentary sources (δ66Zn = −0.07 to +0.64‰; δ65Cu = −0.4 to +0.7‰). In particular, the Cheb Basin moldavites show some of the highest δ65Cu values (up to 12.5‰) ever observed in natural samples. The relative magnitude of isotope fractionation for Cu and Zn seen here is opposite to oxygen-poor environments such as the Moon where Zn is significantly more isotopically fractionated than Cu. One possibility is that monovalent Cu diffuses faster than divalent Zn in the reduced melt and diffusion will not affect the extent of Zn isotope fractionation. These observations imply that the capability of forming a redox environment may aid in volatilizing some elements, accompanied by isotope fractionation, during the impact process. The greater extent of elemental depletion, coupled with isotope fractionation of more refractory Cu relative to Zn, may also hinge on the presence of carbonyl species of transition metals and electromagnetic charge, which could exist in the impact-induced high-velocity jet of vapor and melts. Postprint

Continents are unique to Earth and played a role in coevolution of the atmosphere, hydrosphere, and biosphere. Debate exists, however, regarding continent formation and the onset of subduction-driven plate tectonics. We present Ca isotope and trace-element data from modern and ancient (4.0 to 2.8 Ga) granitoids and phase equilibrium models indicating that Ca isotope fractionations are dominantly controlled by geothermal gradients. The results require gradients of 500–750 °C/GPa, as found in modern (hot) subduction-zones and consistent with the operation of subduction throughout the Archaean. Two granitoids from the Nuvvuagittuq Supracrustal Belt, Canada, however, cannot be explained through magmatic processes. Their isotopic signatures were likely inherited from carbonate sediments. These samples (> 3.8 Ga) predate the oldest known carbonates preserved in the rock record and confirm that carbonate precipitation in Eoarchaean oceans provided an important sink for atmospheric CO2. Our results suggest that subduction-driven plate tectonic processes started prior to ~3.8 Ga., Phase equilibrium modelling combined with Ca isotope measurements in ancient granitoids demonstrates that subduction of oceanic crust occurred repeatedly throughout the Archaean and that carbonate sediments were present in early Eoarchaean oceans (>3.8 billion years).

Stable Ca isotopes are an increasingly useful tool for understanding the sources and processes leading to the formation of magmatic rocks, yet Ca isotope fractionation during genesis of silicic continental crust is still poorly understood. Here, we present Ca, Sr, and Nd isotope, as well as major- and trace-element whole-rock geochemical data for A-, I-, and S-type granites (n = 30) from Australia/Tasmania, Canada, and France (δ44CaBSE of -0.6‰ to +0.2‰) and compare them to phase-equilibrium models for partial-melting (pelite, greywacke, MORB, enriched Archean tholeiite) and crystallization (hydrous arc basalt, A-type granite) that incorporate novel ab-initio predictions for Ca isotope fractionation in epidote and K-feldspar. The ab-initio calculations predict that epidote has similar δ44Ca to anorthite and that K-feldspar is the isotopically lightest known silicate mineral at equilibrium (Δ44Cakspar-melt of -0.4‰ at 1000 K). Our phase-equilibrium model results suggest that δ44Ca variations in all three granite types can be fully explained through magmatic processes, without necessarily requiring addition of isotopically exotic components (e.g., carbonate sediments). Heavy Ca isotope enrichments in A-type granites from the Lachlan Fold Belt, however, require isotopic disequilibrium between plagioclase and melt, which we use to constrain average plagioclase growth rates in these systems. This also serves to illustrate that whole-rock Ca isotope measurements can be used to estimate crystal growth rates, even in the absence of analyzable phenocrysts. In general, low Ca diffusivities and strong isotopic diffusivity ratios (D44/D40) in low-H2O granitic magmas should lead to resolvable isotopic disequilibrium effects in plagioclase, even at relatively slow growth rates (e.g., > 0.03 cm/yr). Combining our data with those from previous studies, we demonstrate that average granitoids and upper continental crust (with newly estimated δ44CaBSE of -0.25 ± 0.02‰, 2SE) have resolvable low δ44Ca compared to basalts and oceanic crust. Given that pressure has a major influence on Ca isotope fractionation across all of our models, this implies that melts feeding upper crustal granitoids dominantly evolve in the lower crust (10-14 kbar, through either partial-melting or fractional crystallization). This observation also suggests that heavier Ca isotopes are preferentially recycled back into the mantle through subduction and/or lower-crustal delamination events, but this is unlikely to have had a significant influence on the δ44Ca evolution of the upper mantle through geologic time., Earth and Planetary Science Letters, 608, ISSN:0012-821X, ISSN:1385-013X

The accretion of volatile elements on Earth continues to be debated despite intense research focusing on the topic. Geochemical and cosmochemical observables have been explained by a variety of scenarios ranging from (1) an accretion of the volatile elements during the main phases of Earth’s formation to (2) an accretion of volatile-poor material followed by a late accretion of volatile-rich material after core formation ceased, called the late veneer. The mantle signature of elements that are both volatile and siderophile record volatile-related processes as well as differentiation-related processes. Studying the behaviour of such elements at metal–silicate equilibrium at high pressure and high temperature allows to separate the effect of differentiation from volatile-related processes on their abundances, and hence to discriminate scenarios capable of explaining the observed abundances.In this thesis work, the aim is to determine one or a variety of accretion scenarios of the Earth explaining the abundances of moderately volatile and siderophile elements in the mantle. Experiments were performed in piston cylinder and multianvil presses from 2 to 20 GPa and 1700 to 2600 K in order to measure the elemental and isotopic behaviour of a selection of moderately siderophile and volatile elements. Studying the partitioning of Sn, Cd, Bi, Sb and Tl allowed to determine the effect of temperature, pressure, fO2, and S content of the metal on the affinity of these elements for metal relative to silicate. The partitioning of these elements is reconstructed across Earth’s accretion, at appropriate pressure and temperature conditions. The studied elements present enhanced siderophility, preventing reconciliation of the experimental data with observed mantle abundances. A scenario involving partial equilibration of the mantle with the core, as well as a late delivery of volatile elements in the last 10 to 20 % of Earth’s accretion, as a large impactor, is able to explain the abundances of the studied elements, in agreement with other studies based on distinct observables. Around 3 wt.% of S in the core is needed to explain the Sn abundance. A late veneer consisting of 0.5 % of Earth’s mass is also required to explain the Bi abundance in the mantle. The study of Sn isotopic fractionation between metal and silicate yields a significant factor of ~0.3 ‰ at 2 GPa and 2000 K (∆122/118Snmetal−silicate), suggesting that Sn isotopes could be fractionated at core formation conditions, enriching the mantle in light isotopes. Carbonaceous chondrites present an identical Sn isotopic composition to the silicate Earth, making them the most likely contributors to the source of volatile elements on Earth, according to this study. These results are consistent with the favoured accretion scenario suggested by the elemental fractionation part of this study. As elemental and isotopic fractionations of siderophile elements between metal and silicate are strongly dependent on the metal composition, the effect of the Ni content of the metal on Fe isotopic fractionation was tested. The experiments performed in this study (22 experiments, from 0 to 70 % Ni in the metal) do not allow to detect any effect of Ni on the Fe fractionation factor. Therefore, this study brings new insight regarding the timing, process and origin of the volatile delivery on Earth, as well as information on the effect of core composition on the behaviour of siderophile elements during Earth’s differentiation., L’accrétion des éléments volatils sur Terre continue d’être débattue malgré de nombreux travaux de recherche sur le sujet. Les observables géochimiques et cosmochimiques ont été expliquées par une large gamme de scénarios allant de (1) l’accrétion des éléments volatils durant les phases principales de l’accrétion ter- restre, jusqu’à (2) l’accrétion de matériaux pauvres en volatils, avec une addition tardive de matériel riche en éléments volatils après la fin de la différenciation, aussi appelée vernis tardif. Les signatures mantelliques des éléments volatils et sidérophiles sont des enregistrements des processus liés à la volatilité et des processus de différenciation. L’étude du comportement de ces éléments dans un équilibre métal-silicate à haute pression et haute température permet de séparer l’effet de la différenciation de l’effet des processus volatils sur les abondances de ces éléments, et donc de discriminer les scénarios capables d’expliquer les abondances observées. Dans ce travail de thèse, nous cherchons à déterminer le ou les scénarios d’accrétion de la Terre permettant d’expliquer les abondances en éléments volatils et sidérophiles observées dans le manteau. Des expériences menées dans des presses piston-cylindre et multi-enclume de 2 à 20 GPa et de 1700 à 2600 K ont permis d’observer les comportements élémentaires et isotopiques d’une sélection d’éléments volatils et sidérophiles. L’étude du coefficient de partage de Sn, Cd, Bi, Sb et Tl a permis de déterminer l’effet de la température, la pression, la fO2, et la concentration en S dans le métal sur l’affinité de ces éléments pour le métal relativement au silicate. Le coefficient de partage de ces éléments est reconstruit au cours de l’accrétion terrestre, aux conditions de pression et température appropriées. Différents scénarios d’accrétion sont testés dans le but de reproduire les mêmes abondances finales que celles observées dans le manteau. Les éléments étudiés présentent une sidérophilité élevée, qui ne permet pas d’expliquer les abondances élevées observées dans le manteau. Un scénario impliquant un équilibre partiel entre noyau et manteau ainsi qu’une arrivée des éléments volatils dans les 10-20 derniers pourcents des phases principales d’accrétion de la Terre, sous la forme d’un impacteur de grande taille, permettent d’expliquer les abondances de ces éléments, en accord avec d’autres études basées sur d’autres observables. Environ 3 wt.% de S dans le noyau sont également nécessaires afin d’expliquer l’abondance en Sn. Un vernis tardif représentant 0,5 % de la masse terrestre est également requis afin d’expliquer l’abondance en Bi dans le manteau. L’étude du fractionnement isotopique de l’Sn entre métal et silicate a permis de mesurer un facteur significatif de ~0.3 ‰ à 2 GPa et 2000 K (∆¹²²/¹¹⁸Snmetal−silicate), impliquant qu’un fractionnement isotopique de l’Sn pourrait avoir lieu aux conditions de la différenciation terrestre, ayant pour effet d’enrichir le manteau en isotopes légers. Les chondrites carbonées présentent une composition similaire à la Terre silicatée ce qui en fait les meilleures candidates comme source des volatils sur Terre, d’après les résultats de cette étude. Ces résultats sont compatibles avec le scénario d’accrétion favorisé au vu des données élémentaires. Les fractionnements élémentaires et isotopiques des éléments entre métal et silicate étant fortement dépendant de la composition du métal, l’effet de la teneur en Ni dans le métal sur le fractionnement isotopique du Fe est testé. Les expériences réalisées (22 expériences, de 0 à 70 % Ni) ne permettent pas de détecter un effet du Ni sur le fractionnement du Fe. Cette étude apporte donc de nouveaux éléments de réponse concernant le timing et le mécanisme d’accrétion, et l’origine des éléments volatils sur Terre...