Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition

International audienceDust grains of organic matter were the main reservoir of C and N in the forming Solar System and are thus considered to be an essential ingredient for the emergence of life. However, the physical environment and the chemical mechanisms at the origin of these organic grains are still highly debated. In this study, we report high-precision triple oxygen isotope composition for insoluble organic matter isolated from three emblematic carbonaceous chondrites, Orgueil, Murchison, and Cold Bokkeveld. These results suggest that the O isotope composition of carbonaceous chondrite insoluble organic matter falls on a slope 1 correlation line in the triple oxygen isotope diagram. The lack of detectable mass-dependent O isotopic fractionation, indicated by the slope 1 line, suggests that the bulk of carbonaceous chondrite organics did not form on asteroi-dal parent bodies during low-temperature hydrothermal events. On the other hand, these O isotope data, together with the H and N isotope characteristics of insoluble organic matter, may indicate that parent bodies of different carbonaceous chondrite types largely accreted organics formed locally in the protosolar nebula, possibly by photochemical dissociation of C-rich precursors

Chlorine Isotopic Compositions of Apatite in Apollo 14 Rocks: Evidence for Widespread Vapor-Phase Metasomatism on the Lunar Nearside ~4 Billion Years Ago

Compared to most other planetary materials in the Solar System, some lunar rocks display high delta (sup 37) Cl signatures. Loss of Cl in a H<<Cl environment has been invoked to explain the heavy signatures observed in lunar samples, either during volcanic eruptions onto the lunar surface or during large scale degassing of the lunar magma ocean. To explore the conditions under which Cl isotope fractionation occurred in lunar basaltic melts, five Apollo 14 crystalline samples were selected (14053,19, 14072,13, 14073,9, 14310,171 along with basaltic clast 14321,1482) for in situ analysis of Cl isotopes using secondary ion mass spectrometry. Cl isotopes were measured within the mineral apatite, with delta (sup 37) Cl values ranging from +14.6 1.6 per mille to +40.0 2.9 per mille. These values expand the range previously reported for apatite in lunar rocks, and include some of the heaviest Cl isotope compositions measured in lunar samples to date. The data here do not display a trend between increasing rare earth elements contents and delta (sup 37) Cl values, reported in previous studies. Other processes that can explain the wide inter- and intra-sample variability of delta (sup 37) Cl values are explored. Magmatic degassing is suggested to have potentially played a role in fractionating Cl isotope in these samples. Degassing alone, however, could not create the wide variability in isotopic signatures. Our favored hypothesis, to explain small scale heterogeneity, is late-stage interaction with a volatile-rich gas phase, originating from devolatilization of lunar surface regolith rocks ~4 billion years ago. This period coincides with vapor-induced metasomastism recorded in other lunar samples collected at the Apollo 16 and 17 landing sites, pointing to the possibility of widespread volatile-induced metasomatism on the lunar nearside at that time, potentially attributed to the Imbrium formation event

Insights into the Martian Regolith from Martian Meteorite Northwest Africa 7034

Everything we know about sedimentary processes on Mars is gleaned from remote sensing observations. Here we report insights from meteorite Northwest Africa (NWA) 7034, which is a water-rich martian regolith breccia that hosts both igneous and sedimentary clasts. The sedimentary clasts in NWA 7034 are poorly-sorted clastic siltstones that we refer to as protobreccia clasts. These protobreccia clasts record aqueous alteration process that occurred prior to breccia formation. The aqueous alteration appears to have occurred at relatively low Eh, high pH conditions based on the co-precipitation of pyrite and magnetite, and the concomitant loss of SiO2 from the system. To determine the origin of the NWA 7034 breccia, we examined the textures and grain-shape characteristics of NWA 7034 clasts. The shapes of the clasts are consistent with rock fragmentation in the absence of transport. Coupled with the clast size distribution, we interpret the protolith of NWA 7034 to have been deposited by atmospheric rainout resulting from pyroclastic eruptions and/or asteroid impacts. Cross-cutting and inclusion relationships and U-Pb data from zircon, baddelleyite, and apatite indicate NWA 7034 lithification occurred at 1.4-1.5 Ga, during a short-lived hydrothermal event at 600-700 C that was texturally imprinted upon the submicron groundmass. The hydrothermal event caused Pb-loss from apatite and U-rich metamict zircons, and it caused partial transformation of pyrite to submicron mixtures of magnetite and maghemite, indicating the fluid had higher Eh than the fluid that caused pyrite-magnetite precipitation in the protobreccia clasts. NWA 7034 also hosts ancient 4.4 Ga crustal materials in the form of baddelleyites and zircons, providing up to a 2.9 Ga record of martian geologic history. This work demonstrates the incredible value of sedimentary basins as scientific targets for Mars sample return missions, but it also highlights the importance of targeting samples that have not been overprinted by metamorphic processes, which is the case for NWA 7034

Caractérisation et datation des circulations de fluides dans le Cisaillement Sud Armoricain (Massif Armoricain, France)

In this study, petro-geochemical, isotopic and geochronological data have been acquired in order to characterize and date fluid circulations within the South Armorican Shear Zone. These circulations have been dated by U-Pb on both zircon and monazite and by muscovite 40Ar/39Ar analyses, extracted from mylonites and syntectonic granites displaying evidence of hydrothermal alteration. At 315 Ma, the South Armorican Shear Zone localized the circulation of magmato-metamorphic fluids, responsible for the crystallization of giant quartz veins parallel to its main strike. Around 300 Ma, an influx of meteoric fluids is recorded in mylonites, as well as in some granitic samples and in quartz veins oblique to the South Armorican Shear Zone, interpreted as crustal-scale tension gashes. These results have also been used to study the behaviour of isotopic chronometers during fluid-rock interactions. Although zircons do not seem to be affected, fluid circulations induced recrystallization of monazite and muscovite and therefore disturbed and even reset the U-Pb and K-Ar chronometers. In the syntectonic Questembert massif for example, 40Ar/39Ar dates obtained on six samples cover a time span of 16 Ma linked to a pervasive infiltration of fluids. Such results therefore illustrate the need to associate geochronology with a detailed petro-geochemical study, from whole rock characterization to the textural and/or chemical study of the minerals selected for dating.Dans ce travail, des données pétro-géochimiques, isotopiques et géochronologiques ont été couplées afin de caractériser et dater les circulations de fluides dans le Cisaillement Sud Armoricain. Ces circulations ont été datées par les méthodes U-Pb sur zircon et monazite et 40Ar/39Ar sur muscovite, dans les mylonites et les granites syntectoniques présentant des traces d'altération hydrothermale. A 315 Ma, le Cisaillement Sud Armoricain localise la circulation de fluides magmato-métamorphiques, responsables de la cristallisation de veines de quartz géantes qui lui sont parallèles. Vers 300 Ma, un afflux de fluides météoriques est enregistré dans les mylonites, certains échantillons granitiques et quelques veines de quartz, obliques au Cisaillement Sud Armoricain, interprétées comme des fentes de tension d'échelle crustale. Les résultats obtenus ont permis d'étudier le comportement des chronomètres isotopiques au cours des interactions fluide-roche. Si le zircon apparaît comme relativement robuste, les circulations de fluides provoquent la recristallisation de la monazite et de la muscovite, et ainsi perturbent, voir remettent à zéro les chronomètres isotopiques U-Pb et KAr. Par exemple, dans le massif syntectonique de Questembert, les dates 40Ar/39Ar obtenues sur six échantillons s'étalent sur 16 Ma, du fait de l'infiltration pervasive de fluides. Ces résultats illustrent donc la nécessité d'associer la géochronologie à une étude pétrogéochimique détaillée, depuis la caractérisation des roches jusqu'à l'étude texturale des monazites ou la documentation des zonations chimiques des muscovites, par exemple

Differentiation of peraluminous granite en route to the surface

National audienceLeucogranites display petrological and geochemical heterogeneities that can be related to either primary processes at the source, or to secondary processes such as differentiation. In the Armorican Massif, a Hercynian domain in western France, syn-tectonic leucogranites were emplaced along major shear zones. The Lizio and Questembert massifs are derived from a similar metasedimentary source and emplaced at distinct depths, ca. 3-4 kbar and 1-2 kbar, respectively. The Questembert granite is more differentiated than the Lizio one, as demonstrated by a lower amount of modal biotite, a higher SiO2 content and a more pronounced peraluminous character. The high-SiO2 Questembert rocks can derive from the low-SiO2 Lizio ones by a 15 wt.% fractionation of a kfs + pl + bt + zrn + mnz assemblage. This magmatic evolution is well recorded by the oxygen isotope evolution of whole rocks, as well as quartz and muscovite separates, demonstrating that these granites crystallized from almost crystal-free liquids. We propose that the differences between the Questembert and the Lizio granites are related to processes acting during magmas ascent through the crust. Questembert is more differentiated than Lizio because the crystal-melt mixture covered a longer distance during its vertical migration, allowing for more crystal-liquid segregation. Regional deformation likely enhanced this process through a filter-press type process. Associated shear heating can explain the shallower depth of emplacement for the Questembert pluton as it moved the brittle-ductile transition zone toward the surface (Leloup et al., 1999). The shallower emplacement of the Questembert melt led to the exsolution of a fluid phase, which further interacted with surrounding rocks, developing a pervasive deuteric alteration. Leloup, P.H., et al. (1999). Geophys. J. Int. 136, 19-40

Magma differentiation in dykes: from field evidence to numerical study

International audienceMechanisms active during magma ascent in dykes are not still well quantified. From field observations, it is apparent that most dykes actually contain a crystalline load. The presence of a crystalline load modifies the effective rheology of such a system and thus the flow behaviour. For example, increasing both the density and viscosity of each crystal, compared to the melt, will cause a reduction of the ascent velocity and will modify the shape of the velocity profile from "Poiseuille like" to "Bingham-like". One issue related to the introduction of a crystalline load concerns the possibility for crystals to be segregated from a viscous granitic melt phase during magma ascent. The implications of such a process on magmatic differentiation have not previously been considered, nor has such a process been previously investigated via numerical models. In this study, we examine the flow dynamics of a poly-crystal bearing granitic melt ascending in a dyke via numerical models. Results showed that the melt phase can be squeezed out from a crystal-rich magma when subjected to a given pressure gradient range and that crystals behaviour is strongly dependant on their size and density. This demonstrates that crystal-melt segregation in dykes during granitic magma ascent constitutes a viable mechanism for magmatic differentiation. In order to quantify such a mechanism, results have been compared to two well-characterized granites from the Armorican Massif (France). Geochemically, these two granites originate from the same magmatic source, but the shallowest one (Questembert granite) can be derived from the deepest (Lizio granite) by a larger amount of crystal segregation during magma ascent in dykes. In our numerical models, we therefore use as initial conditions, the volume fraction and geochemical properties of the "Lizio-type" magma and compute the vertical distance required to end up with the "Questembert-type" magma. The comparison of the results allowed us to estimates the parameters such as pressur

Conditions and extent of volatile loss from the Moon during formation of the Procellarum basin

International audienceRocks from the lunar interior are depleted in moderately volatile elements (MVEs) compared to terrestrial rocks. Most MVEs are also enriched in their heavier isotopes compared to those in terrestrial rocks. Such elemental depletion and heavy isotope enrichments have been attributed to liquid–vapor exchange and vapor loss from the protolunar disk, incomplete accretion of MVEs during condensation of the Moon, and degassing of MVEs during lunar magma ocean crystallization. New Monte Carlo simulation results suggest that the lunar MVE depletion is consistent with evaporative loss at 1,670 ± 129 K and an oxygen fugacity +2.3 ± 2.1 log units above the fayalite-magnetite-quartz buffer. Here, we propose that these chemical and isotopic features could have resulted from the formation of the putative Procellarum basin early in the Moon's history, during which nearside magma ocean melts would have been exposed at the surface, allowing equilibration with any primitive atmosphere together with MVE loss and isotopic fractionation