Jean-Alix Barrat

Tissint Martian Meteorite: A Fresh Look at the Interior, Surface, and Atmosphere of Mars

A New Rock from Mars On 18 July 2011 a meteorite originating from Mars fell on the moroccan desert. Chennaoui Aoudjehane et al. (p. 785, published online 11 October) show that this meteorite was ejected from the surface of Mars 700,000 years ago and contains components derived from the interior, surface, and atmosphere of the red planet. Previous to this fall, only four other martian meteorites have been collected after being witnessed falling to Earth. All the other martian meteorites that are represented in collections around the world, have been found long after their arrival on Earth, and thus have suffered from exposure to the terrestrial environment. A meteorite that fell in Morocco in July 2011 provides a sample to study processes that operated on Mars 700,000 years ago. Tissint (Morocco) is the fifth martian meteorite collected after it was witnessed falling to Earth. Our integrated mineralogical, petrological, and geochemical study shows that it is a depleted picritic shergottite similar to EETA79001A. Highly magnesian olivine and abundant glass containing martian atmosphere are present in Tissint. Refractory trace element, sulfur, and fluorine data for the matrix and glass veins in the meteorite indicate the presence of a martian surface component. Thus, the influence of in situ martian weathering can be unambiguously distinguished from terrestrial contamination in this meteorite. Martian weathering features in Tissint are compatible with the results of spacecraft observations of Mars. Tissint has a cosmic-ray exposure age of 0.7 ± 0.3 million years, consistent with those of many other shergottites, notably EETA79001, suggesting that they were ejected from Mars during the same event.

Silicon isotopes in angrites and volatile loss in planetesimals

Significance Understanding volatile elements in the early solar system is a key step toward understanding the processes of planetary formation and the composition of Earth, but the origin of volatiles on Earth is not well understood. In this article, we present measurements of silicon isotope ratios in angrites, a class of meteorites dating from the first few million years after condensation of solids from the solar nebula. We show that the silicon isotope composition of angrites is consistent with a depletion of volatile elements on the angrite parent body through impact events during accretion. Such volatile-poor planetesimals may represent the material that formed the Earth. Inner solar system bodies, including the Earth, Moon, and asteroids, are depleted in volatile elements relative to chondrites. Hypotheses for this volatile element depletion include incomplete condensation from the solar nebula and volatile loss during energetic impacts. These processes are expected to each produce characteristic stable isotope signatures. However, processes of planetary differentiation may also modify the isotopic composition of geochemical reservoirs. Angrites are rare meteorites that crystallized only a few million years after calcium–aluminum-rich inclusions and exhibit extreme depletions in volatile elements relative to chondrites, making them ideal samples with which to study volatile element depletion in the early solar system. Here we present high-precision Si isotope data that show angrites are enriched in the heavy isotopes of Si relative to chondritic meteorites by 50–100 ppm/amu. Silicon is sufficiently volatile such that it may be isotopically fractionated during incomplete condensation or evaporative mass loss, but theoretical calculations and experimental results also predict isotope fractionation under specific conditions of metal–silicate differentiation. We show that the Si isotope composition of angrites cannot be explained by any plausible core formation scenario, but rather reflects isotope fractionation during impact-induced evaporation. Our results indicate planetesimals initially formed from volatile-rich material and were subsequently depleted in volatile elements during accretion.

Posteucritic magmatism on Vesta: Evidence from the petrology and thermal history of diogenites

We report on the petrology and the thermal histories of 13 diogenites in order to constrain the formation processes of the Vestan crust. We classify diogenites into unequilibrated and equilibrated diogenites in a scheme similar to that for basaltic eucrites. Pyroxenes in unequilibrated diogenites are chemically zoned, indicating that they crystallized rapidly from melts and escaped from global crustal metamorphism. The presence of unequilibrated diogenites casts doubt on the fact that all the diogenites formed at depth in the parent body, as commonly thought. Some diogenites probably crystallized in shallow intrusions or were extruded on the surface. These facts strengthen the geochemical evidence that diogenites and eucrites are not directly cogenetic and suggest that at least some diogenites have intruded the early formed eucritic crust. Thus, diogenites are certainly not the products of the crystallization of the magma ocean that triggered the differentiation of Vesta but are more likely cumulates associated with a later stage of magmatism. Furthermore, the intrusion of diogenites could have significantly thickened the early formed crust, making it difficult to excavate deep‐seated olivine mantle by moderate impact events.

Trachyandesitic volcanism in the early Solar System

Significance Volcanism is a fundamental geological process on planets and was substantial during crustal growth on planetary bodies in the early Solar System, as witnessed by ubiquitous rocks of basaltic composition, e.g., on Earth, Moon, Mars, and asteroids. Besides basaltic volcanism, trachyandesite lavas are generated on Earth. The first occurrence of a trachyandesite lava in the meteorite collections demonstrates that trachyandesitic, alkali-, and silica-rich volcanism takes place not only on Earth today but already occurred on a small planetesimal ∼4.56 billion years ago. It sets new constraints on mechanisms and styles of early Solar System volcanism. Volcanism is a substantial process during crustal growth on planetary bodies and well documented to have occurred in the early Solar System from the recognition of numerous basaltic meteorites. Considering the ureilite parent body (UPB), the compositions of magmas that formed a potential UPB crust and were complementary to the ultramafic ureilite mantle rocks are poorly constrained. Among the Almahata Sitta meteorites, a unique trachyandesite lava (with an oxygen isotope composition identical to that of common ureilites) documents the presence of volatile- and SiO2-rich magmas on the UPB. The magma was extracted at low degrees of disequilibrium partial melting of the UPB mantle. This trachyandesite extends the range of known ancient volcanic, crust-forming rocks and documents that volcanic rocks, similar in composition to trachyandesites on Earth, also formed on small planetary bodies ∼4.56 billion years ago. It also extends the volcanic activity on the UPB by ∼1 million years (Ma) and thus constrains the time of disruption of the body to later than 6.5 Ma after the formation of Ca–Al-rich inclusions.

Si isotope homogeneity of the solar nebula

The presence or absence of variations in the mass-independent abundances of Si isotopes in bulk meteorites provides important clues concerning the evolution of the early solar system. No Si isotopic anomalies have been found within the level of analytical precision of 15 ppm in 29Si/28Si across a wide range of inner solar system materials, including terrestrial basalts, chondrites, and achondrites. A possible exception is the angrites, which may exhibit small excesses of 29Si. However, the general absence of anomalies suggests that primitive meteorites and differentiated planetesimals formed in a reservoir that was isotopically homogenous with respect to Si. Furthermore, the lack of resolvable anomalies in the calcium-aluminum-rich inclusion measured here suggests that any nucleosynthetic anomalies in Si isotopes were erased through mixing in the solar nebula prior to the formation of refractory solids. The homogeneity exhibited by Si isotopes may have implications for the distribution of Mg isotopes in the solar nebula. Based on supernova nucleosynthetic yield calculations, the expected magnitude of heavy-isotope overabundance is larger for Si than for Mg, suggesting that any potential Mg heterogeneity, if present, exists below the 15 ppm level.

Northwest Africa 5958: A weakly altered CM‐related ungrouped chondrite, not a CI3

Northwest Africa (NWA) 5958 is a carbonaceous chondrite found in Morocco in 2009. Preliminary chemical and isotopic data leading to its initial classification as C3.0 ungrouped have prompted us to conduct a multitechnique study of this meteorite and present a general description here. The petrography and chemistry of NWA 5958 is most similar to a CM chondrite, with a low degree of aqueous alteration, apparently under oxidizing conditions, and evidence of a second, limited alteration episode manifested by alteration fronts. The oxygen isotopic composition, with Delta ` O-17 = -4.3 parts per thousand, is more O-16-rich than all CM chondrites, indicating, along with other compositional arguments, a separate parent body of origin. We suggest that NWA 5958 be reclassified as an ungrouped carbonaceous chondrite related to the CM group.

Oxygen isotopic evidence for accretion of Earth’s water before a high-energy Moon-forming giant impact

We show that the bulk of Earth’s water was delivered before the high-energy collision that led to the formation of the Moon. The Earth-Moon system likely formed as a result of a collision between two large planetary objects. Debate about their relative masses, the impact energy involved, and the extent of isotopic homogenization continues. We present the results of a high-precision oxygen isotope study of an extensive suite of lunar and terrestrial samples. We demonstrate that lunar rocks and terrestrial basalts show a 3 to 4 ppm (parts per million), statistically resolvable, difference in Δ17O. Taking aubrite meteorites as a candidate impactor material, we show that the giant impact scenario involved nearly complete mixing between the target and impactor. Alternatively, the degree of similarity between the Δ17O values of the impactor and the proto-Earth must have been significantly closer than that between Earth and aubrites. If the Earth-Moon system evolved from an initially highly vaporized and isotopically homogenized state, as indicated by recent dynamical models, then the terrestrial basalt-lunar oxygen isotope difference detected by our study may be a reflection of post–giant impact additions to Earth. On the basis of this assumption, our data indicate that post–giant impact additions to Earth could have contributed between 5 and 30% of Earth’s water, depending on global water estimates. Consequently, our data indicate that the bulk of Earth’s water was accreted before the giant impact and not later, as often proposed.