Noble gases in meteorites: evidence for presolar matter and superheavy elements

Abstract

Primitive meteorites contain a variety of trapped noble gas components, differing in isotopic composition, host phase, and release temperature. The major components can be plausibly derived from solar noble gases by mass-dependent fractionation, but some minor components require nuclear processes and may be of presolar origin. Two such presolar components are two varieties of ‘Ne-E ’ (nearly pure 22 Ne, probably from the decay of 2.6 a 22 Na), residing in micron-sized grains of spinel and a carbonaceous phase, apparently a carbyne. A third is s -process Xe and Kr, enriched in isotopes 128, 130, 132 and 82, 84, 86. All seem to be condensates from highly evolved stars, that found their way into the early Solar System and survived in primitive meteorites. A more controversial component is ‘CCFXe’, enriched up to twofold in the heavy Xe isotopes, and attributed either to fission of an extinct, superheavy element ( Z = 111 - 115) or to direct nucleosynthesis in a supernova. Its host phases, chromite and carbynes, show none of the other isotopic anomalies expected for a supernova origin, and several other lines of evidence likewise favour a local over an exotic origin. However, conclusive proof is still lacking. To establish the true nature of these components, their host phases must be isolated, a task that is made difficult by their low abundance (0.1-1 % (by mass)), small grain size (0.01-1 μm) and uncertain composition. The most fruitful approach has been a combination of chemical techniques (etching, selective dissolution) and conventional physical methods of mineral separation.