C, N, and Noble Gas Isotopes in Grain Size Separates of Presolar Diamonds from Efremovka

Abstract

Nanometer-size presolar diamonds from the Efremovka CV3 chondrite were physically separated into several grain size fractions by ultracentrifugation. The coarsest size fraction is the most enriched in carbon-12; the others have broadly similar carbon isotopic compositions. Measurement of noble gases shows that their concentration decreases with decreasing grain size. This effect is attributed to ion implantation. Such an episode could occur in the envelope of a supernova that produced the diamonds, or in interstellar space; in either case, ions with energies above a certain threshold pass completely through the smaller diamond grains without being captured. Concentrations of nitrogen show only minor variations with grain size, indicating a different mechanism of incorporation into the diamonds.

Ion Implantation into Presolar Grains: A Theoretical Model

“…It is thus obvious that the interpretation of data acquired from grains extracted from meteorites will be controlled by the ability to obtain tightly defined grain-size fractions. These variations are comparable with those observed for noble gases in Efremovka diamond grain size fractions (Verchovsky et al 1998) especially for Ne and, perhaps, for He. This means that the isotope effects associated with ion implantation of noble gases into presolar diamond should be taken into account in order to find the original noble gas isotopic compositions (i.e.…”

“…Quite similar isotope variations for noble gases are observed in presolar diamonds (Verchovsky et al 1998) and SiC (Lewis et al 1994) grain-size fractions, as well as for Ba (Zinner, Amari & Lewis 1991;Prombo, Podosek, Amari et al 1993) and Sr (Prombo, Podosek, Amari et al 1992) in SiC grains. Thus, through measurements of the grain size and isotopic compositions of samples representing a collection of grains irradiated in space, it is possible to identify different implanted components provided their implantation energies are not identical, and also to calculate the implantation energies.…”

“…For example, the concentrations of noble gases in SiC grain-size fractions are clearly related to their grain size (Lewis, Amari & Anders 1994). Recent analyses of grain-size fractions of presolar diamonds (Verchovsky, Fisenko, Semjonova et al 1998) also show a strong grain-size dependence of noble gas concentrations. The only way that these relationships can be explained is through ion implantation into previously noble gas free grains.…”

The Fountain Hills unique CB chondrite: Insights into thermal processes on the CB parent body

“…We analyzed the concentration and isotopic composition of N in the two fractions by means of static vacuum gas-source mass spectrometry in conjunction with high-resolution stepped pyrolysis and combustion. The N analysis technique is described in Verchovsky et al (1998).…”

NWA 1152 and Sahara 00182: New primitive carbonaceous chondrites with affinities to the CR and CV groups

“…Nitrogen isotope compositions were measured on the Finesse multi-collector, high-precision, static vacuum, mass spectrometer, designed and built at the Open University (Verchovsky et al 1997(Verchovsky et al , 1998. Nitrogen isotopes (δ 15 N) are measured relative to AIR and can be measured to a precision of 1.0‰, with a minimum N content of 1 ng.…”