A CONCISE COMPILATION OF PETROLOGIC INFORMATION ON POSSIBLY PRISTINE NONMARE MOON ROCKS

To facilitate systematic study of the surviving compositionally pristi ne (endogenously igneous) rocks of the ancient lunar crust, a compilat ion has been generated of all likely samples, along with key informati on on the petrologic characteristics and chemistry of each sample. The compilation includes 260 samples. Besides information related to the likelihood of each sample being truly pristine (i.e., mainly its textu re and siderophile element abundances), information is compiled on min eral content, listing major phases present as well as basic informatio n on mineral compositions, on size (expressed as mass), and on whether a reasonably comprehensive chemical analysis has been published. The compilation also classifies the samples into seven categories of confi dence in the pristine composition of the samples, reflecting an estima tion of the relative likelihood that each arguably pristine sample is in fact pristine. For many purposes, it is crucial to avoid inclusion of polymict rocks in a data base. On petrologic diagrams such as a plo t of average Mg' [Mg' = 100 x molar Mg/(Mg + Fe)] in a low-Ca mafic si licate vs. average An content in plagioclase, rocks in the top three c ategories of the confidence in pristine character appear distinctly bi modal in composition, with roughly half belonging to a ferroan suite c haracterized by high An despite relatively low Mg'. When samples of lo w to moderate pristine character are included, the bimodality appears less distinct. Sample mass can also be important. With a data base res tricted to samples more massive than 1 g, there is a clear distinction in feldspar content and bulk density between ferroan and nonferroan ( Mg-suite) rocks, such that only the ferroan-suite rocks are likely to have formed as flotation cumulates. With a data base including smaller samples, the same basic pattern is seen, but only in a blurred form, as the two rock types show considerable overlap in their modal feldspa r contents.