Th/U Microdistribution in Enstatite Meteorites

Abstract

We report here our results to date for the mineralogical distribution of Th and U in the enstatite chondrites. By investigating the behavior of Th and U under the reducing conditions for which the enstatite meteorites formed, we hope to evaluate the suitability of these objectives for ^(244)Pu(_(1/2) = 82 m.y .) chronology, as well as gain insight into the origin of the E-chondrites. Induced fission tracks from a double irradiation have been used to map the distribution of Th and U in polished sections. The preliminary results are shown in Table I. The values are the mean contents of oldhamite (CaS) grains, and the uncertainties are the standard deviations from the mean. In enstatite chondrites both Th and U are concentrated in oldhamite. For example, in a section of Daniel's Kuil at least 70% of the total Th and 90% of the U are found in oldhamite. In Khairpur, alabandite (MnS) was also found to contain U but at a lesser concentration (~20 ppb). The smaller oldhamite grain size in St. Mark's and Abee complicates their study; however, Th+U are also found to concentrate in oldhamite in these two meteorites but at a somewhat lower value than seen in the E-6's. The Th-U ratios in oldhamite determined so far appear relatively unfractionated from the typical value of ~4. The whole rock Th/U value forHvittis was reported to be 5.0 (Morgan and Lovering, 1968) which agrees well with the oldhamite result. However, the whole rock Th/U for Khairpur of 2.5 (Morgan and Lovering, 1968) does not agree with the oldhamite value. This discrepancy will be investigated in an upcoming irradiation; in this irradiation we also hope to complete the data set of Table 1, look at Th/U on a grain-by-brain basis and examine the U distribution in a few enstatite achondrites (aubrites). But with the results to date, a good case can be made for a study of ^(244)Pu in the E-6 chondrites. The majority of Th and U is concentrated with little apparent fractionation in a single mineral, and the particular chemistry of oldhamite should enable a selective Pu-Xe measurement to be made.

Files

Additional details