Presolar grains known to be present in meteorites are thermally and chemically extremely stable minerals such as diamond, graphite, silicon carbide, corundum (aluminum oxide) and silicon nitride. Although discovered first and by far the most abundant (ca. 1 per mill by weight in the most primitive meteorites), the diamonds are the least understood, and their very identification as being presolar is based on the isotopic composition of trace elements they carry. Noble gases have played a special role among these trace elements, and it is primarily the unusual isotopic composition of xenon - ca. 100% enrichment of the lightest and heaviest isotopes - which suggests that they came from supernova explosions.
How, when, and where introduction of xenon and other trace elements occurred may provide crucial information on formation and early history of the diamond grains, and there are strong indirect arguments that introduction was by implantation of ions. To test the viability of the process, a simulation experiment was performed: a noble gas mixture consisting of helium, argon, krypton and xenon ions with an energy of 700 electronvolts was implanted into a layer of terrestrial nanodiamonds of similar size as the presolar nanodiamonds (extremely small,only a few nanometer; s. figure), and after irradiation the release of the implanted noble gases was studied. Surprisingly, release as a function of temperature was bimodal, with one peak in the 200-700°C range and another one above 1000°C. This situation - after a single implantation - at first glance is similar to the case of the meteoritic nanodiamonds, there is a complication, however. In the case of the "stellar" diamonds differences in isotopic composition demand that at least two different events must have been involved in the introduction of noble gases: isotopically unremarkable noble gases are released primarily at low temperature, gases of presumably supernova origin at higher temperature.
If indeed, ion implantation is the mechanism by which trace elements were introduced into stardust diamonds and if, as the simulation study suggests, ion implantation results in the gases being located in two different sites within the diamonds of different thermal stability, the following sequence of events seems required:
- formation of diamonds presumably by chemical vapor deposition;
- irradiation of the diamonds (or a subfraction of them) with supernova trace elements;
- loss of the less retentively held fraction of implanted supernova material;
- irradiation of the diamonds at some later time (or of a different subfraction at an unspecified time) with trace elements of commonplace isotopic composition, possibly in the interstellar medium or the early Solar System;
- no more exposure to elevated temperature for any significant length of time (e.g. no more than ca. 10,000 years at more than 100°C).
A second important information from the implantation study is that the more retentively sited gases are isotopically fractionated relative to the starting composition. How this may have affected the inferred abundance and isotopic composition of the supernova implants into the stardust diamonds and how important the resulting changes are for the inferred nuclear processes remains to be worked out in detail.
Presolar diamond grain observed by transmission electron microscopy. High resolution image - obtained by F. Banhart (then at Max Planck Institute for Metal Research, Stuttgart) - shows the crystallographic[III] planes (distance 0.206 nanometer) of a typical-sized grain.
Journal
Nature