U-TH-PB ISOTOPIC SYSTEMATICS OF LUNAR NORITE-78235

Presently favored models predict large U-Pb fractionatations during die Moon's earliest stages of evolution. Thus far, the extent of those fractionations is poorly constrained Ancient lunar highland rocks may provide evidence of initial Pb isotopic compositions and U/Pb ratios of early lunar magma sources and possibly of a primary magma ocean. For these purposes, we chose to study 78235, a pristine, high-Mg noritic cumulate thought to be relict, ancient, deep-seated lunar crust. This rock however, shows extensive shock-melt deformation was originally considered too damaged for a success isotopic study, and contains nonpristine components including impact-melt glass with Fe-Co-Ni metal of probable meteoritic origin. A leaching procedure was conducted on polymineralic separates in order to assure the removal of secondary Pb components. The Pb from leached separates (residues) do not form a linear trend on the Pb-Pb diagram, indicating open-system behavior either from mixtures of Pb or postcrystallization disturbances. Scatter in the U-Th-Pb data on evolution diagrams contributes to nonprecise ages and initial Pb compositions for 78235. In spite of these difficulties, we can present calculated initial Pb compositions and corresponding U-238/Pb-204 (mu) values, assuming reasonably precise radiometric ages from the literature for norite 78236. For example, on a concordia diagram and using initial Pb values equal to the isotopic composition of Canyon Diablo troilite (CDT), the U-Pb isotopic data from three residues of 78235 define a discordia, yielding an upper-intercept age of 4.426 +/- 0.065 Ga and a lower-intercept age of 3.93 +/-0.21 Ga The upper-intercept age is very similar to a Sm-Nd internal isochron age from the literature and is interpreted as the primary crystallization age of the norite. The lower-intercept age apparently dates the shock-melt event and is indistinguishable from that defined by Tera et al.'s ( 1974) cataclysm array. With the use of initial Pb values similar to those of CDT, the source of the norite must have had a mu value sufficiently low (e.g., <50) over a maximum span of approximately 135 m.y. so as not to greatly alter these initial values. These results would support the contention that high-Mg suite rocks are coeval with the ferroan anorthosites, both being produced during the earliest stages of lunar evolution. However, at least one other radiometric age determination from the literature, a Sm-Nd internal isochron age of 4.34 +/- 0.04 Ga for norite 78236, is in conflict with the 4.43 Ga age. This younger age can also be calculated from our U-Pb data by using more radiogenic initial Pb values than those of CDT. For example, an initial composition of Pb-206/Pb-204 = 44.2 and Pb-207/Pb-204 = 74.8, corresponding to a source with mu = 508, assuming a single-stage Pb evolution from CDT Pb values and 4.56 Ga as the age of the Moon, yields an upper-intercept age of 4.34 +/- 0.07 Ga. Assuming this age for the norite, the U-Pb data indicate that high-Mg suite rocks were derived from sources with high-mu values (e.g., >300) and support the idea that these rocks postdate the ferroan anorthosites. Resolution of the discrepancy about the nature of the source of the norite must await determination of a more precise age, probably best approached using the U-Pb zircon or badellyite method.