On the retention of primordial noble gases in the Pesyanoe meteorite

Abstract

A sample of the Pesyanoe meteorite has been examined by X-ray diffraction microtechnique, in an effort to find minerals of known gas-trapping tendency and thus explain the high helium content of the meteorite. Determinations of enstatite, feldspar, and troilite by earlier workers have been confirmed. The existence of many variant minor pyroxene phases, including clinopyroxene and rhombic pyroxene with stacking faults, shows that the history of the meteorite must have involved several complex heating and cooling cycles. In addition, there exists a variety of Sulfides, which includes not only both magnetic and non-magnetic troilites, daubréelite, and oldhamite, but also a new mineral, MgS, the composition of which has been checked by spectrographic analysis. All the sulfides present show a high degree of crystal imperfection. Vaterite (μ-CaCO 3) has been found, but it is not known whether it is of terrestrial or antecedent origin. The high concentration of dislocations in the sulfides indicates a comparable abundance of other lattice defects. It is suggested that lattice defects of various kinds are the most probable loci of the gas atoms. The gases apparently entered the sulfide crystals during their growth, either in a closed magma chamber or in a hot gas stream containing both noble gases and sulfur compounds. Some circumstantial evidence favours the latter alternative.