Testing the chondrule-rich accretion model for planetary embryos using calcium isotopes

Understanding the composition of raw materials that formed the Earth is a crucial step towards understanding the formation of terrestrial planets and their bulk composition. Calcium is the fifth most abundant element in terrestrial planets and, therefore, is a key element with which to trace planetary composition. However, in order to use Ca isotopes as a tracer of Earth's accretion history, it is first necessary to understand the isotopic behavior of Ca during the earliest stages of planetary formation. Chondrites are some of the oldest materials of the Solar System, and the study of their isotopic composition enables understanding of how and in what conditions the Solar System formed. Here we present Ca isotope data for a suite of bulk chondrites as well as Allende (CV) chondrules. We show that most groups of carbonaceous chondrites (CV, CI, CR and CM) are significantly enriched in the lighter Ca isotopes ($\delta^{44/40}Ca$ = +0.1 to +0.93 permill) compared with bulk silicate Earth ($\delta^{44/40}Ca$ = +1.05 $\pm$ 0.04 permill, Huang et al., 2010) or Mars, while enstatite chondrites are indistinguishable from Earth in Ca isotope composition ($\delta^{44/40}Ca$ = +0.91 to +1.06 permill). Chondrules from Allende are enriched in the heavier isotopes of Ca compared to the bulk and the matrix of the meteorite ($\delta^{44/40}Ca$ = +1.00 to +1.21 permill). This implies that Earth and Mars have Ca isotope compositions that are distinct from most carbonaceous chondrites but that may be like chondrules. This Ca isotopic similarity between Earth, Mars, and chondrules is permissive of recent dynamical models of planetary formation that propose a chondrule-rich accretion model for planetary embryos.Comment: 39 pages, 5 figures, 2 tables 1 supplementary material (1 table

Late delivery of exotic chromium to the crust of Mars by water-rich carbonaceous asteroids.

The terrestrial planets endured a phase of bombardment following their accretion, but the nature of this late accreted material is debated, preventing a full understanding of the origin of inner solar system volatiles. We report the discovery of nucleosynthetic chromium isotope variability (μ54Cr) in Martian meteorites that represent mantle-derived magmas intruded in the Martian crust. The μ54Cr variability, ranging from -33.1 ± 5.4 to +6.8 ± 1.5 parts per million, correlates with magma chemistry such that samples having assimilated crustal material define a positive μ54Cr endmember. This compositional endmember represents the primordial crust modified by impacting outer solar system bodies of carbonaceous composition. Late delivery of this volatile-rich material to Mars provided an exotic water inventory corresponding to a global water layer >300 meters deep, in addition to the primordial water reservoir from mantle outgassing. This carbonaceous material may also have delivered a source of biologically relevant molecules to early Mars

Ca and Sr isotopic fractionation at high temperature : constraints on the formation and the evolution of Earth

La formation du Système Solaire et son évolution restent peu connues aujourd’hui malgré le grand nombre de missions d’exploration spatiale mises en place depuis la deuxième moitié du 20ème siècle. Pour obtenir des informations sur les premières phases de la formation de la Terre et l’évolution du manteau terrestre, les météorites et les roches ignées sont de bons objets d’étude: les premières sont des témoins des conditions de la formation du Système Solaire et sont formées avant les planètes, les secondes nous renseignent sur la composition du manteau terrestre. Ces travaux de thèse se consacrent à l’analyse isotopique du calcium (Ca), du strontium (Sr) et du rubidium (Rb) d’une grande variété de roches terrestres et extra-terrestres en utilisant le MC-ICP- MS. Les isotopes stables du Ca et Sr permettent de tracer des processus physico-chimiques liés à la formation des roches. Le système Rb-Sr permet quant à lui de dater des évènements liés à l’histoire de ces roches. Les chondres, composants majoritaires des chondrites, ont été étudiés par analyse isotopique du Ca pour tes- ter et confirmer un modèle récent d’accrétion planétaire, le pebble accretion model. D’autres phénomènes peuvent affecter les corps qui peuplent notre système solaire. Notamment les chondrites peuvent subir des épisodes de chauffage et donc subir un métamorphisme thermique. La chronologie Rb-Sr permet de dater ces évènements de chauffage et d’en révéler l’origine qu’on estime alors liée à des impacts dans la ceinture d’astéroïdes. Les échantillons terrestres, des komatiites, MORBs et OIBs sont analysés pour estimer la composition isotopique du manteau en Ca et Sr ainsi que son évolution. La composition des isotopes stables du Sr dans le manteau semble homogène dans le temps alors que les isotopes du Ca révèlent la préservation d’hétérogénéités datant du début de l’histoire de la Terre. Les carbonatites, roches magmatiques contenant au moins 50% de minéraux carbonatés, sont étudiées en isotopie du Ca dans l’optique de révéler leur origine. L’enrichissement en isotopes légers du Ca des carbonatites comparé à la valeur moyenne du manteau reflète une contribution de matériel recyclé dans leur source mantellique. Ces travaux de recherche visent à explorer les grandes applications du fractionnement isotopique du Ca et du Sr à haute température.The formation of the Solar System and its evolution remain poorly known despite the explosion of space exploration in the mid 20th century. Meteorites and terrestrial igneous rocks are particularly useful objects of study for gaining insights into the formation and evolution of the Earth: the former existed before the planets and the latter reflect the composition of the terrestrial mantle. For this thesis, we performed Ca, Sr and Rb isotopic analyses using MC-ICP-MS technique on a variety of terrestrial and extra-terrestrial rocks. The fractionation of Ca and Sr stable isotopes allows for tracing processes and source effects and Rb-Sr system enables us to date primordial events. Chondrules, a major component of chondrites, are analysed for Ca isotopic composition to test and confirm the pebble accretion model for the formation of the Earth. The timing of the heating event of thermally metamorphosed carbonaceous chondrites is estimated using Rb-Sr chronology and reveals the process of the event as impacts in the asteroid belt. From Earth, komatiites, OIBs and MORBs samples are analysed to estimate the Ca and Sr isotopic composition and evolution of the mantle. The stable isotopic composition of Sr in the mantle is homogenous through the evolution of the mantle while Ca isotopes reveal preservation of early heterogeneities. Carbonatites, rare igneous rocks containing 50 % of carbonate minerals, are studied for Ca isotopic composition in order to indicate their origin. We suggest that the enrichment in lighter Ca isotopes of the carbonatites compared to the bulk silicate Earth’s value derives from a contribution of recycled components through subduction in their mantle source. This thesis explores the wide applications of Ca and Sr isotopic fractionation in high temperature geochemistry

Timing of thermal metamorphism in CM chondrites: Implications for Ryugu and Bennu future sample return

International audienceCarbonaceous chondrites are often considered potential contributors of water and other volatiles to terrestrial planets as most of them contain significant amounts of hydrous mineral phases. As such, carbonaceous chondrites are candidate building blocks for Earth, and elucidating their thermal histories is of direct importance for understanding the volatile element history of Earth and the terrestrial planets. A significant fraction of CM type carbonaceous chondrites are thermally metamorphosed or "heated" and have lost part of their water content. The origin and the timing of such heating events are still debated, as they could have occurred either in the first Myrs of the Solar System via short-lived radioactive heating, or later by impact induced heating and/or solar radiation. Since Rb is more volatile than Sr, and some heated CM chondrites are highly depleted in Rb, a dating system based on the radioactive decay of Rb-87 to( 87)Sr(lambda(87) Rb = 1.393 x 10(-11) yr(-1)) could be used to date the heating event relating to the fractionation of Rb and Sr. Here, we have leveraged the Rb-87/Sr-87 system to date the heating of five CM chondrites (PCA 02012, PCA 02010, PCA 91008, QUE 93005 and MIL 07675). We find that the heating events of all five meteorites occurred at least 3 Ga after the formation of the Solar System. Such timing excludes short-lived radioactive heating as the origin of thermal metamorphism in these meteorites, and relates such heating events to ages of collisional families of C-type asteroids

Corrigendum to “Testing the chondrule-rich accretion model for planetary embryos using calcium isotopes” [Earth Planet. Sci. Lett. 469 (2017) 75–83]

International audienc

The stable strontium isotopic composition of ocean island basalts, mid-ocean ridge basalts, and komatiites

International audienceOcean island basalts Mid-ocean ridge basalts Komatiites Kilauea Iki Bulk silicate earth A B S T R A C T The radiogenic 87 Rb-87 Sr system has been widely applied to the study of geological and planetary processes. In contrast, the stable Sr isotopic composition of the bulk silicate Earth (BSE) and the effects of igneous differentiation on stable Sr isotopes are not well-established. Here we report the stable Sr isotope (88 Sr/ 86 Sr, reported as δ 88/86 Sr, in parts per mil relative to NIST SRM 987) compositions for ocean islands basalts (OIB), mid-ocean ridge basalts (MORB) and komatiites from a variety of locations. Stable Sr isotopes display limited fractionation in a OIB sample suite from the Kilauea Iki lava lake suggesting that igneous processes have limited effect on stable Sr isotope fractionation (± 0.12‰ over 20% MgO variation; 2sd). In addition, OIB (δ 88/ 86 Sr = 0.16-0.46‰; average 0.28 ± 0.17‰), MORB (δ 88/86 Sr = 0.27-0.34‰; average 0.31 ± 0.05‰) and komatiites (δ 88/86 Sr = 0.20-0.97‰; average 0.41 ± 0.16‰) from global localities exhibit broadly similar Sr stable isotopic compositions. Heavy stable Sr isotope compositions (δ 88/86 Sr > 0.5‰) in some Barberton Greenstone belt komatiites may reflect Archean seawater alteration or metamorphic processes and preferential removal of the lighter isotopes of Sr. To first order, the similarity among OIBs from three different ocean basins suggests homogeneity of stable Sr isotopes in the mantle. Earth's mantle stable Sr isotopic composition is established from the data on OIB, MORB and komatiites to be δ 88/86 Sr = 0.30 ± 0.02‰ (2sd). The BSE δ 88/86 Sr value is identical, within uncertainties, to the composition of carbonaceous chondrites (δ 88/ 86 Sr = 0.29 ± 0.06‰; 2sd) measured in this study

Calcium isotopic evidence for the mantle sources of carbonatites

International audienceThe origin of carbonatites-igneous rocks with more than 50% of carbonate minerals-and whether they originate from a primary mantle source or from recycling of surface materials are still debated. Calcium isotopes have the potential to resolve the origin of carbonatites, since marine carbonates are enriched in the lighter isotopes of Ca compared to the mantle. Here, we report the Ca isotopic compositions for 74 carbonatites and associated silicate rocks from continental and oceanic settings, spanning from 3 billion years ago to the present day, together with 0 and C isotopic ratios for 37 samples. Calcium-, Mg-, and Fe-rich carbonatites have isotopically lighter Ca than mantle-derived rocks such as basalts and fall within the range of isotopically light Ca from ancient marine carbonates. This signature reflects the composition of the source, which is isotopically light and is consistent with recycling of surface carbonate materials into the mantl

Late delivery of exotic chromium to the crust of Mars by water-rich carbonaceous asteroids

The terrestrial planets endured a phase of bombardment following their accretion, but the nature of this late accreted material is debated, preventing a full understanding of the origin of inner solar system volatiles. We report the discovery of nucleosynthetic chromium isotope variability (μ54Cr) in Martian meteorites that represent mantle-derived magmas intruded in the Martian crust. The μ54Cr variability, ranging from -33.1 ± 5.4 to +6.8 ± 1.5 parts per million, correlates with magma chemistry such that samples having assimilated crustal material define a positive μ54Cr endmember. This compositional endmember represents the primordial crust modified by impacting outer solar system bodies of carbonaceous composition. Late delivery of this volatile-rich material to Mars provided an exotic water inventory corresponding to a global water layer >300 meters deep, in addition to the primordial water reservoir from mantle outgassing. This carbonaceous material may also have delivered a source of biologically relevant molecules to early Mars.ISSN:2375-254