Brigitte Zanda

Origin and age of the earliest Martian crust from meteorite NWA 7533

The ancient cratered terrain of the southern highlands of Mars is thought to hold clues to the planet’s early differentiation, but until now no meteoritic regolith breccias have been recovered from Mars. Here we show that the meteorite Northwest Africa (NWA) 7533 (paired with meteorite NWA 7034) is a polymict breccia consisting of a fine-grained interclast matrix containing clasts of igneous-textured rocks and fine-grained clast-laden impact melt rocks. High abundances of meteoritic siderophiles (for example nickel and iridium) found throughout the rock reach a level in the fine-grained portions equivalent to 5 per cent CI chondritic input, which is comparable to the highest levels found in lunar breccias. Furthermore, analyses of three leucocratic monzonite clasts show a correlation between nickel, iridium and magnesium consistent with differentiation from impact melts. Compositionally, all the fine-grained material is alkalic basalt, chemically identical (except for sulphur, chlorine and zinc) to soils from Gusev crater. Thus, we propose that NWA 7533 is a Martian regolith breccia. It contains zircons for which we measured an age of 4,428 ± 25 million years, which were later disturbed 1,712 ± 85 million years ago. This evidence for early crustal differentiation implies that the Martian crust, and its volatile inventory, formed in about the first 100 million years of Martian history, coeval with earliest crust formation on the Moon and the Earth. In addition, incompatible element abundances in clast-laden impact melt rocks and interclast matrix provide a geochemical estimate of the average thickness of the Martian crust (50 kilometres) comparable to that estimated geophysically.

Origin and Metamorphic Redistribution of Silicon, Chromium, and Phosphorus in the Metal of Chondrites

Chromium, silicon, and phosphorus concentrations of 0.1 to 1 percent by weight are common in metal grains in the least metamorphosed ordinary and carbonaceous chondrites. These concentrations are fairly uniform within single chondrules (but different from chondrule to chondrule) and are inversely correlated with the fayalite concentrations of the chondrule olivines. This relation shows that these chromium, silicon, and phosphorus concentrations could not have been established by condensation or equilibration in the solar nebula but are the result of metal-silicate equilibration within chondrules. Two generations of inclusions made by the exsolution of those elements have been identified: One formed during chondrule cooling and the other formed during metamorphism. The distribution and composition of the latter in type 3 to type 5 chondrites are consistent with increasing metamorphism relative to type 2 and type 3.0 material.

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.

Hydrocarbon Materials of Likely Interstellar Origin from the Paris Meteorite

We have examined some grains from the Paris meteorite through infrared and Raman micro-spectroscopy in order to investigate their carbonaceous and mineralogical components. In the mid- as well as far-infrared regions, the raw and global spectra of Paris resemble those of CM meteorites. However, we have obtained rather peculiar infrared spectra for some aromatic-rich micron-sized fragments of Paris displaying a very good match between its organic signatures both in the 3.4 ?m and 6 ?m regions, and the ones observed from the diffuse interstellar medium infrared sources toward the Galactic center, suggesting that this meteorite may have indeed preserved some organic matter of interstellar origin.

Propagation of high-energy particles inside solid matter: cosmic-ray-induced spallation in iron meteorites

Abstract A model is presented to evaluate the variations with depth of high-energy particle fluxes as they propagate inside spherical solid bodies. This model can be used to calculate the production rates of spallogenic nuclides inside meteorites of various sizes and compositions. The exponential attenuation of the flux, the gradual energy loss due to the Coulombian drag and the production of secondary particles in the spallation reactions are all taken into account in a self consistent way. It is shown that, in the energy range considered (300 MeV to 30 GeV), the particle fluxes are fairly insensitive to the actual angular distribution of the emitted secondary particles, as long as the assumed distribution remains within realistic limits. The propagation equations resulting from these interactions are solved on a spherical mesh consisting in a set of lines of various directions inside the sphere. The computed neutron and proton fluxes are presented in the case of an iron sphere irradiated by galactic cosmic ray (GCR) protons: these results show the critical importance of secondary particles and especially neutrons as depth increases within the sphere. The fluxes are used to calculate the production rates of some cosmogenic nuclides inside iron meteorites. The use of such production rates to gain information on exposure geometry of these meteorites and on the history of the GCR fluxes is discussed through a comparison with experimental data for the iron meteorite Grant. It is shown that the distribution of the different estimated ages within a meteorite could be used to distinguish between a spectral variation and an intensity variation of the GCR flux.