Induced Fission Track Measurements of Carbonaceous Chondrite Th/U Ratios and Th/U Microdistributions in Allende Inclusions

Citation

Stapanian, Maritza Irene (1981) Induced Fission Track Measurements of Carbonaceous Chondrite Th/U Ratios and Th/U Microdistributions in Allende Inclusions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/64g4-2a56. https://resolver.caltech.edu/CaltechETD:etd-10282005-135027

Abstract

A double irradiation fission track radiography technique has been developed to measure Th/U ratios in carbonaceous chondrites and map Th/U microdistributions in Ca-Al-rich inclusions. (Th + ²³⁸U) fission is induced by high dose 35-40 MeV proton irradiations. These irradiations are coupled with reactor ²³⁵U thermal neutron fission measurements to obtain the corresponding Th/U ratios.

The average solar system Th/U ratio is important in the theoretical modelling of the time scales for heavy element r-process nucleosynthesis. Earlier measurements (Morgan and Lovering, 1967, 1968) indicated CC Th/U ratios ranging from 2-6. This is in sharp contrast to ordinary chondrite, terrestrial, and lunar sample measurements which are tightly constrained to present day values of 3.8±0.5. An objective of this study was to check the 2-6 spread in CC ratios. The fission track technique, while not a high precision technique, can give individual meteorite measurements to within 12-20%. This is adequate to verify the existence of highly fractionated (relative to terrestrial) CC Th/U ratios. The results of our analyses of six bulk samples mainly type C2, but also including the Ivuna type C1 chondrite, show within the errors of the measurements that bulk CC Th/U ratios lie within the normal 3.8±0.5 range.

The real strength of the fission track technique lies in the ability to map Th/U microdistributions in-situ. Our technique has the sensitivity to make U measurements within 10% counting statistics errors on 100µ grains with 20 ppb. Th+U measurements of similar precision can be made on 100µ grains with 1 ppm Th+U. We have focussed our Th/U mapping experiments on the Allende meteorite- in particular, the calcium-aluminum-rich inclusions (CAI). The chemical and mineralogical composition of these CAI conform to model predictions for the earliest forming nebular condensates. Because of the refractory nature of Th and U, location and identification of Th,U-rich carrier phases can test solar nebula condensation models. Our major results are: (1) The high concentrations of Th and U in the rims of two Type A coarse-grained CAI attests to the importance of rims in understanding Th,U condensation and perhaps other refractory trace elements as well. (2) Analysis of a compact Type A inclusion shows that incorporation of rim material into total inclusion values is necessary to obtain Th and particularly U enrichments over Cl levels on par with the uniform enrichment of other refractory elements (Grossman et al., 1977). (3) The highly fractionated Th/U ratios observed in Type A perovskite (~20), and the general tendency for our CAI bulk measurements which show fractionated Th/U ratios to give high ratios suggests support of the Boynton (1978) proposal of higher U volatility compared to Th under the conditions of the early condensing nebula. Alternatively, our Type A inclusions may be atypical, having formed from a reservoir (gas?) of high Th/U. One mechanism for preferential depletion in the early stages of condensation could be alloying of U with Pt metals as discussed by Jones and Burnett (1980), although there is no strong evidence to support this specific mechanism. (4) The Th/U fractionations observed in Type A CAI suggest the strong likelihood for ²⁴⁴Pu/²³⁸U fractionations as well. Such inclusions would probably not be appropriate for determining the solar system Pu/U or Pu/Th ratios.

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