Petrologic province maps of the lunar highlands derived from orbital geochemical data

Abstract

Orbital and sample geochemical data from the lunar highlands are combined in variation diagrams to determine (a) the coincidence between the two data sets and (b) the geologic validity of extrapolating observed diagrammatic relations to an image format. We examined three variation diagrams: Mg* (= 100 Mg/Mg + Fe molar) versus (Th/Ti)c (ratio normalized to chondrites), Al versus Mg*/(Th/Ti)c, and Fe versus (Th/Ti)c. For the orbital data within each diagram, we establish a petrologic classification based on the data's clustering or coincidence with sample compositions or both. The spatial distributions of each diagram's classes are shown as petrologic units on color‐coded classification maps. The Mg*‐(Th/Ti)c diagram, which is similar to the Mg*‐(Sm/Ti)c diagram that has been used in sample studies, is inferior to the Al‐Mg*/(Th/Ti)c diagram in discriminating KREEP from Mg‐suite rock types: the orbital geochemical spectrometers analyzed the regolith, in which Mg varies much less than Al. Although the Fe‐(Th/Ti)c diagram shows a diverse distribution of orbital data similar to that shown by the Al‐Mg*/(Th/Ti)c diagram, it is unable to discriminate adequately between KREEP and Mg‐suite rock types. The Fe‐(Th/Ti)c diagram is nonetheless very useful because of its greater areal coverage. Orbital and soil‐sample compositions of the four highland landing sites correspond in both the Al‐Mg*/(Th/Ti)c and the Fe‐(Th/Ti)c data. The diagrams show some units of virtually pure anorthosite and norite. Most units show the effects of mixing of pristine rock types in the regolith. One unit, which dominates the eastern limb and farside highlands, has a composition so mafic that, with the chosen end members, it requires a 30% mare‐basalt component. We interpret this to mean that the highlands experienced a significantly greater amount of mare volcanism before the end of heavy bombardment than has been previously suggested. The average global composition of the highlands under the Apollo groundtracks is that of “anorthositic gabbro,” an average that has significant implications for the role of near‐global plagioclase fractionation in lunar crustal genesis. Our analysis has shown that the simultaneous use of orbital and sample geochemical data in variation diagrams can (1) bridge the mechanical gap between orbital and sample studies by relating orbital data to sample petrology, (2) present the results of several elemental databases as one map for rapid geologic analysis, and (3) display quantitative mixing relations of different crustal materials in a spatial format. This technique will be valuable for petrologic interpretation of the global geochemical data to be received from the future Lunar Geosciences Orbiter mission.