INERT GASES IN FIFTEEN IRON METEORITES
PALMA, RUSSELL LUMIR
Doctor of Philosophy
The inert gases helium, neon, argon, krypton, and xenon were measured mass-spectrometrically in the metal phases of fifteen iron meteorites and in graphite and troilite mineral separates from the Odessa iron meteorite. The meteorites analyzed were chosen because of their low cosmic ray exposure ages; most had known exposure ages of less than 200 million years. The hope was that the prominent spallation component seen in prior iron meteorite inert gas measurements would be reduced enough to allow detection of a trapped gas component, should one exist. Approximately three gram samples were fused in a thoroughly out-gassed alumina crucible. Surface contamination was removed from the samples by acid treatment and by preheating the samples for two days at 300(DEGREES)C. Blank measurements were made for correction purposes before and after each sample. All of the meteorite samples show evidence of having a cosmogenic inert gas component. Small variations attributable to different trace element abundances and cosmic ray shielding are seen. Large deviations from the predicted cosmogenic compositions correlate with short cosmic ray exposure ages and often indicate the presence of a trapped atmospheric-like inert gas component. Those samples showing anomalous behavior in one inert gas tend to show anomalous behavior for all the inert gases. The inert gas data from most samples are consistent with the helium, neon, and argon concentrations being largely of cosmogenic origin and the krypton and xenon resulting from a mix of solar wind, atmospheric, and cosmogenic compositions. However, there are several meteorites which are anomalous with respect to the above interpretation. Krypton and xenon measurements had only been made in the metal phase of five iron meteorites, so determining the composition of those two gases was an important result of this work. The light krypton isotopes ('78)Kr and ('80)Kr are enriched in many samples relative to a mixture of atmospheric-solar wind and cosmogenic krypton. Prior investigations of xenon in iron meteorites had assumed only the presence of cosmogenic and atmospheric xenon, but the data from this study suggest that many xenon components may be trapped in the metal phase. The Braunau inert gas data indicate strongly that this meteorite contains primordial trapped gas. Furthermore, the Braunau xenon composition includes the admixture of a xenon component not hitherto seen in any meteorite, stony or iron. This new component is marked by a low ('128)Xe/('132)Xe ratio. The inert gases in the mineral separates from the Odessa iron meteorite also show contributions from spallation, but have distinctive components not seen in the metal phase. There are enrichments at ('80)Kr, ('82)Kr, and ('83)Kr in the troilite phase which are consistent with neutron capture on ('79)Br, ('81)Br, and ('82)Se, respectively, while a ('129)Xe excess results from ('129)I decay. A trapped gas component is strikingly evident in the neon and argon data from the graphite phase of Odessa.