Continued investigation of the light element geochemistry of Tagish Lake

We have determined the abundance and isotopic composition of two separate carbonate generations within Tagish Lake. Nitrogen isotope analysis of the chondrite shows it to be organic-rich, with d15N values higher than in CI and CM chondrites

The Fountain Hills meteorite: A new CBa chondrite from Arizona

We describe a new member of the CR chondrite clan and compare it to other members of this group

Magmatic carbon in Martian meteorites: attempts to constrain the carbon cycle on Mars

One of the current goals of Martian exploration is to find evidence for extinct (or even extant) life. Carbon (an essential ingredient of life on Earth) is known to occur on Mars as CO2 in the atmosphere and frozen in the polar caps; it is inferred to be present as carbonates in the Martian crust and soils. We are attempting to define and quantify the different carbon reservoirs on Mars, so that we can follow Mars' carbon cycle. This paper discusses a primordial magmatic component that could be the starting point of such a cycle. The nature, distribution and isotopic composition of carbon was measured in a suite of Martian meteorites, comprising Chassigny and 11 shergottites. Other Martian meteorites were not included, as they sample rocks that have been altered by fluids at Mars' surface. Our results, obtained by high-resolution stepped combustion and mass spectrometry, show that the magmatic component has a very variable abundance of 1–100 ppm, with [delta]13C~[minus sign]20±4‰. This value is close to magmatic carbon determined for Moon and for Vesta (the parent body of the HED basaltic meteorites), but very different from that of Earth

Spectroscopic study of impurities and associated defects in nanodiamonds from Efremovka (CV3) and Orgueil (CI) meteorites

The results of spectroscopic and structural studies of phase composition and of defects in nanodiamonds from Efremovka (CV3) and Orgueil (CI) chondrites indicate that nitrogen atomic environment in meteoritic nanodiamonds (MND) is similar to that observed in synthetic counterparts produced by detonation and by the Chemical Vapour Deposition (CVD)-process. Most of the nitrogen in MND appears to be confined to lattice imperfections, such as crystallite/twin boundaries and other extended defects, while the concentration of nitrogen in the MND lattice is low. It is suggested that the N-rich sub-population of MND grains may have been formed with high growth rates in environments rich in accessible N (i.e., N in atomic form or as weakly bonded compounds). For the first time the silicon-vacancy complex (the "silicon" defect) is observed in MND by photoluminescence spectroscopy.Comment: 33 pages, 5 figures, submitted to Geochimica et Cosmochimica Act

A numerical model of ion implantation into presolar grains

Pre-solar grains such as diamond and silicon carbide (found in primitive chondritic meteorites), contain isotopically anomalous noble gases. There are two ways that these could have been incorporated into the grains: direct assimilation during grain formation and growth, or subsequent addition after the grains had formed. Herein we investigate the latter possibililty and explore the role of ion implementation within the interstellar medium (i.e. into pre-existing grains distributed in free space). Numerical models are derived to address the issue of how ion implantation would affect the grain population

Interstellar diamond in the efremovka CV3 chondrite: pyrolysis of different size fractions of grains

Amounts and isotopic compositions of C, N, and noble gases were measured in the same gas portions extracted by step heating (over the temperature interval of 300–2000°C) of two different-size fractions of interstellar diamond from the Efremovka CV3 chondrite. The analysis of these data and their comparison with the data obtained for the same fractions during their oxidation and with results of the pyrolysis of whole-rock diamond samples from other chondrites led us to conclude that (1) the fine-grained diamond is more heat-resistant than the coarse-grained diamond, perhaps, because of an increase in the percentage of defect grains and the concentrations of defects in them with an increase in the grain size; (2) graphitization and oxidation of nanometer-sized diamond grains result in an uniform (for example, layer by layer) mechanism of destruction; (3) N, He, and, to a lesser degree, Ne release during pyrolysis proceeds predominantly by means of diffusion, whereas Ar and Xe are liberated mostly during grain destruction; (4) coarse-grained diamond is enriched in associations of two and more N atoms, which can be released at lower temperatures than those of single N atoms; and (5) an increase in the thermal-metamorphism grade of the parental material of chondrites leads to a more rapid destruction of coarse-grained diamond than its fine-grained analogue. For example, the material of the Indarch chondrite, which was affected by the most intense thermal metamorphism among all chondrites considered, contains mainly fine-grained interstellar diamond

Noble gases in the grain-size fractions of presolar diamond from the Boriskino CM2 meteorite

Presolar diamond recovered from the Boriskino CM2 meteorite was separated into size fractions by sedimentation in an ultracentrifuge. The average grain sizes of the fractions ranged from ~1.4 to ~5.3 nm. Noble gases were measured in each diamond fraction and in the bulk sample in the course of stepped pyrolysis up to 700 and subsequent oxidation between 600 and 900. The following main results were obtained: (a) fine diamond fractions are strongly different from coarse ones in the elemental composition of the P3 component of noble gases, which is most likely related to element fractionation during implantation into diamond grains; (b)in presolar diamond from a single meteorite, a wide range of the Xe-P3/Xe-HL ratio was detected for the first time, from 0.130.06 for the fine fraction to 3.95 0.37 for the coarse fraction, which is suggestive of different energies of ions of the P3 and HL noble gas components during their implantation into diamond grains; (c) the calculated upper limit of 136Xe/132Xe for isotopically anomalous xenon (0.71 0.04) in the fine fraction is identical to a value of 0.7 determined by Hass and Lewis (1994) for Xe-LH in diamond from ther- mally metamorphosed meteorites. Such a coincidence suggests that Xe-HL was formed as an individual com- ponent before its implantation into the grains of presolar diamond as a result of mixing of the products of the r- and p-processes of nucleosynthesis during a supernova explosion with xenon of the isotopically normal com- ponent of noble gases. The elemental composition of the latter component is significantly different from that of the P3 component, at least with respect to 36Ar/132Xe

Nitrogen and argon release profiles in Luna 16 and Luna 24 regolith samples: The effects of regolith reworking

Fines, microbreccias and agglutinates from the Luna 16 mature regolith 1635 and fines from the immature/submature Luna-24 regolith have been analysed for N and argon isotopes in order to understand the origin of isotopically distinct N released at different temperatures. All high-resolution runs reveal a similarity in the release of 36Ar, 40Ar and N over a wide temperature interval. The similarity in the 40Ar and 36Ar releases and the near coincidence in the 1635 agglutinates implies that the implanted species were redistributed and homogenised during regolith processing such that, regardless of the huge difference in ion implantation energy between solar 36Ar and non-solar 40Ar, their present distribution and their release temperatures are now essentially equal. A small amount of 40Ar released in the lower temperature steps with elevated 40Ar/36Ar is considered to be trapped after reworking. While such mixing and homogenisation may also be expected for N components of different origins, to date all known stepped runs regularly demonstrate a reproducible variation in 15N, suggesting no homogenisation. We consider regolith N to be a mixture of several components trapped at different times, and some nitrogen that was not involved in the reworking. Relatively heavy N released around 500 °C appears to be the most pure form of the component trapped after reworking, probably from accreted meteoritic matter. Middle temperature isotopically lighter N appears to be a mixture of solar and non-solar N largely homogenised, and therefore solar N can not be seen in its pure form. Bulk 15N as well as formally deconvoluted 15N thermal profiles imply that the non-solar N has a variable 15N value. Several non-solar N sources are considered with their input resulting in increasing regolith 15N with time. Because N from meteorites and IDPs appear to be dominant, a mechanism is required to reduce the C/N ratio typical of meteoritic matter to that approaching the low value observed in the lunar regolith. Preferential loss of methane appears to be a viable explanation, following generation either by proton sputtering or in reducing vapour plumes