^(244)Pu Abundance in Ordinary Chondrites

The cosmic abundance of ^(244)Pu is an important parameter in models of nucleosynthetic chronology and as a reference value in studies of solar system chronology. Experimental determination of this abundance is a long-standing problem which has still not been fully solved. Two different approaches have been used, both based on measuring ^(244)Pu via its fission product Xe, but in two different classes of meteoritic material. One class consists of samples relatively rich in Pu and relatively poor in Xe components, especially trapped Xe, which interfere with identification offission Xe. Such materials, e.g., achondrites, phosphates, refractory inclusions, are, however, chemically fractionated, and lacking a stable or long-lived isotope of Pu the ^(244)Pu measurements must be translated to cosmic abundance by assuming geochemical coherence with another element, typically Nd. By this approach the best estimate (1) of cosmic ^(244)Pu abundance, stated relative to co-produced (r-process) ^(238)U, is ^(244)Pu/^(238)U = 0.004. The other class of material is bulk chondrite, believed to be an unbiased sampling of non-volatile elements. We believe that this is the best approach to determining the cosmic abundance of ^(244)Pu, since assessing whether Pu is geochemically coherent with Nd or another element requires independent knowledge of the unfractionated abundance of ^(244)Pu. The whole-rock chondrite approach is experimentally difficult, however. For some time the best estimate of ^(244)Pu abundance by this approach was ^(244)Pu/^(238)U = 0.0l5, based on an analysis of the LL6 chondrite St. Severin (2). More extensive and sophisticated analyses of St. Severin (3, 4) led to an improved estimate ^(244)Pu/^(238)U = 0.007, sharply lower than the previous value but still substantially higher than the value based on differentiated samples. While it is important to determine ^(244)Pu abundances in whole-rock chondrites other than the single meteorite St. Severin, previously available data on other meteorites have not been usefully precise. We have determined fission Xe concentrations (Table) by stepwise heating analysis ofa group of(unirradiated) ordinary chondrites selected to facilitate identification of fission Xe. Isotope dilution measurements of U, Th, Nd and Ce on aliquots of the samples used for gas analysis are under way. These samples were prepared from alternating cut slabs; the remaining slab faces have been examined for general petrological characterization and for distribution of phosphates, the principal hosts of Pu. The data available so far indicate fission Xe concentrations and ^(244)Pu/^(238)U ratios consistent with those in St. Severin. The fission Xe calculations require assumption of trapped Xe composition and are sensitive to this assumption. The tabulated fission concentrations are minima in that they assume trapped ^(130)Xe/^(136)Xe equal to the highest observed value. The trapped ^(130)Xe/^(136)Xe ratios for all these meteorites are similar and, as previously noted for St. Severin (4), are significantly different (higher) from AVCC