Formation Mechanism of Iron-Rich Olivine: Experimental Constrains into Early Fluid-Assisted Hydration and Dehydration Processes on Asteroids

Iron-rich olivine is one of the major minerals in the matrices of unequilibrated ordinary (UOCs) and carbonaceous (CV, CK, CO) chondrites whose petrologic type is >3.1. There has been an extensive discussion in the literature as to the formation mechanism of these olivines; however, their origin is poorly understood. The formation of ferroan olivine during hydrothermal alteration has been demonstrated to be thermodynamically viable. The stability of ferroan olivine is highly dependent on several variables, including temperature, water/ rock (W/R) ratio, pressure, oxygen fugacity, and bulk rock composition. So far, hydrothermal alteration experiments have not been successful at forming FeO-rich olivines with the compositions and textures observed in the matrices of chondrites. Therefore, understanding the formation conditions of FeO-rich olivines remains a key problem to explain the effects of hydrothermal alteration on chondrite matrices

Influx of nitrogen-rich material from the outer Solar System indicated by iron nitride in Ryugu samples

Large amounts of nitrogen compounds, such as ammonium salts, may be stored in icy bodies and comets, but the transport of these nitrogen-bearing solids into the near-Earth region is not well understood. Here, we report the discovery of iron nitride on magnetite grains from the surface of the near-Earth C-type carbonaceous asteroid Ryugu, suggesting inorganic nitrogen fixation. Micrometeoroid impacts and solar wind irradiation may have caused the selective loss of volatile species from major iron-bearing minerals to form the metallic iron. Iron nitride is a product of nitridation of the iron metal by impacts of micrometeoroids that have higher nitrogen contents than the CI chondrites. The impactors are probably primitive materials with origins in the nitrogen-rich reservoirs in the outer Solar System. Our observation implies that the amount of nitrogen available for planetary formation and prebiotic reactions in the inner Solar System is greater than previously recognized

A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu

Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe3+ to Fe2+ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss

Bulk Oxygen Isotopic Composition of Antarctic Micrometeorites: Effect of Atmospheric Entry

International audienceThe bulk O isotopic compositions of Antarctic micrometeorites are broadly compatible with that of carbonaceous chondrites, but systematic heavy O isotopic enrichments due to atmospheric entry were observed in partially melted particles

Dolomites in hydrated fine-grained Antarctic micrometeorites: Effective tools for analyzing secondary processes

International audienceWe report detailed transmission electron microscope (TEM) observations of carbonates from one hydrated fine-grained Antarctic micrometeorite (H-FgMM). These carbonates show the occurrence of complex chemical variations and microstructures that provide important evidence regarding the formation and evolution of rarely analyzed H-FgMMs. The chemical variations were identified at both micrometer and nanometer scales, indicating that these carbonates formed under localized fluid conditions that suggest a variable chemical microenvironment. Individual carbonates grew from isolated reservoirs of fluid. Moreover, these carbonates contain manganese amounts almost twice as high as those measured in CM chondrites but similar to those identified in CI chondrites. Their particular compositions indicate reducing and progressively evolving conditions in the fluid from which these carbonates precipitated, probably due to water consumption during phyllosilicates formation. In addition to the compositional variability, microstructural features are pervasive in these carbonates, similar to those described in heavily shocked meteorites indicating that these carbonates were probably modified during shock processes after their formation. Since carbonates are highly susceptible to shock metamorphism, we suggest that it is essential to investigate their structure in detail before interpreting the isotopic measurements related to the time of their formation. Additionally, associated with carbonates, ubiquitous phosphates were identified in the micrometeorite analyzed. Future studies of these mineral associations will provide us further insight into the formation and evolution of asteroids, especially since they were both identified in the surface materials of Ryugu and Bennu

Oxygen Isotopes of EPICA -- Dome C Extraterrestrial Dust Layers: Constraints on the Nature of the Impactors

International audienceThe oxygen isotopic compositions of the two extraterrestrial dust layers recorded in EPICA-Dome C ice core suggest a production by two distinct impacts, one of them possibly being by a comet

Transmission Electron Microscopy of CONCORDIA UltraCarbonaceous Antarctic MicroMeteorites (UCAMMs): Mineralogical properties

We performed mineralogical and petrographic studies of three UltraCarbonaceous Antarctic Micrometeorites (UCAMMs) by analytical transmission electron microscopy (TEM). The UCAMMs were identified in the CONCORDIA micrometeorite collection (2002 and 2006) recovered from central Antarctic snow, and are of probable cometary origin. UCAMMs are dominated by disordered carbonaceous matter that extends over surfaces of up to ∼90% of the particle. Embedded in this carbonaceous matter, we observed small and complex assemblages of fine-grained mineral phases, isolated minerals, glassy phases that resemble Glass with Embedded Metal and Sulfides (GEMS) that were first found in Interplanetary Dust Particles (IDPs), and rounded objects containing both glass and crystalline materials. The mineral assemblages are chondritic in composition, within a factor of 2. Crystalline materials represent at least 25% of mineral phases. This value is much larger than the upper limit of crystallinity measured in the diffuse interstellar medium (<2.2 wt%). Crystalline phases are dominated by low-Ca, Mg-rich pyroxenes, Mg-rich olivine and low-Ni Fe-sulfides. Exotic phases such as Mn-, Zn-rich sulfide and perryite have also been found as accessory minerals. The variety of high temperature mineral phases observed in UCAMMs is similar to that reported in chondritic porous IDPs and 81P/Wild 2 samples. The close association of high temperature crystalline phases with the low temperature carbonaceous matter in UCAMMs supports the hypothesis of a large-scale radial mixing in the early solar nebula. This new type of carbon-rich micrometeorites containing crystalline material provides the opportunity to analyze in situ, without any chemical processing, the association of materials formed at both low and high temperatures in the protoplanetary disk. A better knowledge of these samples is also important to prepare for future cometary missions, like ROSETTA in 2014

Oxygen Isotopes of EPICA -- Dome C Extraterrestrial Dust Layers: Constraints on the Nature of the Impactors

International audienceThe oxygen isotopic compositions of the two extraterrestrial dust layers recorded in EPICA-Dome C ice core suggest a production by two distinct impacts, one of them possibly being by a comet

Oxygen Isotopes of EPICA -- Dome C Extraterrestrial Dust Layers: Constraints on the Nature of the Impactors

International audienceThe oxygen isotopic compositions of the two extraterrestrial dust layers recorded in EPICA-Dome C ice core suggest a production by two distinct impacts, one of them possibly being by a comet

Transmission Electron Microscopy of Ultracarbonaceous Antarctic Micrometeorites of Possible Cometary Origin

International audienceTEM observations of ultracarbonaceous Antarctic micrometeorites show intimate mixing of disordered carbon and fine-scale assemblages of minerals like Mg-rich olivine, pyroxenes and Fe-Ni sulphides and alloys