Sulphur Isotope Fractionation in Troilite Oxidation, Evaporation and Dissociation, and the ³⁴S/³²S Ratios in the Canyon Diablo Metallic Spheroids

Abstract

<p>Canyon Diablo metallic spheroids are roughly spherical in form, approximately 1 mm in diameter, consist mainly of nickel and iron and are found in conjunction with Canyon Diablo meteorite fragments in the vicinity of Barrlnger Meteorite Crater, Arlzona.</p> <p>Conventional chemical techniques and a gas source mass spectrometer, using SF₆ as the analysis gas, were used to measure the sulphur contents and ³⁴S/³²S ratios in spheroid samples of different sizes. Also, material was removed from various depths within the spheroids by means of ablation using a mixer/mill so that the radial dependence of the sulphur contents and ³⁴S/³²S ratios could be studied. This was the first time that the ³⁴S/³²S ratios in the Canyon Diablo metallic spheroids have been measured.</p> <p>It was found that the spheroids are enriched in ³⁴S compared to the Canyon Diablo meteorite fragments and that the enrichment is greater in the smaller spheroids than in the larger ones. Also, the layers nearest the surface are more enriched than the inner portions.</p> <p>In order to investigate the possible causes of this enrichment the sulphur isotope effects associated with the oxidation, dissociation and evaporation of the meteoritic mineral troilite, (FeS), were measured. The troilite samples were heated in a furnace at approximately 1200ºC in a dry-air atmosphere for oxidation and in a vacuum for dissociation and evaporation. In the oxidation experiment it was found that the residue has a σ³⁴S value relative to the starting material of approximately -0.33 ‰ apparently independent of the extent of reaction. Also, dissociation and evaporation occur simultaneously in a two-branch reaction with the σ³⁴S values of the residues strongly dependent on the extent of reaction in a manner similar to the Rayleigh distillation process. The sulphur isotope effects associated with evaporation and dissociation were found to be (+4.8 ± 0.3) ‰ and (+12.9 ± 0.3) ‰ respectively. Consequently, the σ³⁴S values (relative to the starting material) of the initial products are respectively about -4.8 ‰ and -12.9 ‰. This is the first time that the sulphur isotope effect associated with the high temperature oxidation of troilite has been measured. It is also the first time that the sulphur isotope etfect associated with either the vacuum dissociation or evaporation of troilite has been measured. This latter information has proven useful in the interpretation of the sulphur content and σ³⁴S data obtained from the analyses of lunar soils. The equations governing the relationships between the σ³⁴S values of the residue and products in two-branch first-order reactions are presented.</p> <p>Published hypothetical accounts of the events that the Canyon Diablo metallic spheroids having their present chemical composition are reviewed and discussed. The following new explanation is presented that is consistent with the isotope ratio data and the chemical compositional differences between the spheroids and the meteorite fragments. It is suggested that the Canyon Diablo meteoroid included a portion containing greater nickel and cobalt contents than do the fragments presently found at the crater site. This portion was shock-melted by the impact and splash-dispersed to form the spheroids. The ³⁴S enrichment occurred when the spheroids lost sulphur by dissociation/evaporation upon exposure to the near-vacuum of the meteoroid's train.</p>