A 33S Anomaly in the allende meteorite

Abstract

The isotope ratios 33S/ 32S and 34S/ 32S have been measured in sulphur fractions extracted from samples of the meteorites Allende and Eagle Station by leaching at successively greater acid concentrations and higher temperatures. On a three isotope plot of δ 33 S vs δ 34 S most of the data lie on or close to the mass fractionation line. The last fraction of sulphur extracted from a bulk Allende sample lies off the line and has an approximately 1%. excess in the 33/ 32S ratio. Previous searches for anomalous abundance patterns of 32S, 33S, 34S and 36S have been reported by HULSTON and THODE (1965a,b), THODE and REES (1971), and REES and THODE (1972). No isotope abundance variations were found, in the meteorite and lunar samples studied, which could not be explained on the basis of either mass dependent isotope fractionation or, in the special case of iron meteorites, cosmic ray production of 33S and 36S. We report here preliminary results of a renewed search for isotopically anomalous sulphur in which we are concentrating on the Allende and Eagle Station meteorites, both of which contain anomalous oxygen (CLAYTON et al ., 1973, 1976). In a first attempt to distinguish between normal sulphur and any possible anomalous sulphur, we have leached both bulk samples and hand separated components of these meteorites with hydrochloric acid. CLAYTON and RAMADURAI (1977) suggested that the presence of isotopically anomalous sulphur would be evidence for the existence of presolar grains which are relics of nucleosynthesis in certain zones of supernova expansion. In particular they suggested that sulphides of titanium are good candidates for isotopic analysis. These are not expected to exist in conventional solar equilibrium condensation sequences, but might be abundant in condensates from silicon burning shells of supernovae. Our chemical procedures were already completed when CLAYTON and RAMADURAI'S suggestions came to our attention and it must be stressed that so far, in all cases but one we have examined only sulphur from sulphides which are decomposed by HC1. Thus we may not have sampled sulphides of the type suggested by CLAYTON and RAMADURAI. All samples of the Allende meteorite were ground finer than 50μm before acid extraction of sulphur. Samples of sulphur were extracted from the various phases of the meteorites by using successively stronger hydrochloric acid leaches, longer times and higher temperatures of reaction. Sulphur initially released as H 2S was successively converted to CdS, Ag 2S and SF 6, this latter compound being analysed mass spectrometrically (THODE and REES, 1971). Analyses of nine SF 6 samples prepared from Ag 2S originally derived from Canyon Diablo troilite were also performed in order to monitor fluorination and mass spectrometry precision and to establish the zero points ofthe isotope variation scales. The results are shown in Table 1. The sulphur contents of the various samples were determined gravimetrically as Ag 2S. The bulk and matrix samples are probably a few percent low because of mechanical losses. The percentages of sulphur in each fraction of a sample extracted during each leaching stage are given in the table. The total sulphur content in the bulk and matrix samples of the Allende meteorite i.e., the sum of the sulphur contents of the individual fractions, varies from 1.8 to 2.08%, the highest percentage being in the matrix. These values compare with about 2 to 2.1% obtained by CLARKE et al . (1970).