Global petrologic variations on the Moon: A ternary‐diagram approach

Abstract

A ternary‐diagram approach for determination of global petrologic variations on the lunar surface is presented that incorporates valuable improvements in our previous method of using geochemical variation diagrams. This new approach uses a ternary diagram that is subdivided into equally spaced segments along each of its three sides and has a triangular area in its center. The segments are assigned distinct colors that cover the range of the visible spectrum. The apexes are assigned the three primary colors, and the center triangle, which represents equal proportions of the three apexes, is assigned the color gray. The apexes are assigned the average Fe and (Th/Ti)c (ratio normalized to chondrites) compositions of KREEP/Mg‐suite rocks, mare basalts, and ferroan anorthosites. The composition of each picture element (pixel) within these Apollo orbital gamma ray databases is used to determine its position within the ternary diagram. The color corresponding to this ternary position is then placed on a classification map at that pixel's position within the orbital databases. Error databases produced for the Fe and (Th/Ti)c data were used to exclude pixels having high errors. The resultant classification map shows spatial transitions among petrologic units, allows direct determination of the relative proportions of each end‐member composition in a pixel, and increases the geologic interpretability of these data over that of the individual elemental databases. The classification units correspond remarkably well to observed geologic units, when we consider the spatial resolution (100 km) of the gamma ray spectrometer. Our results are as follows: (1) The highlands contain large areas of relatively pure ferroan anorthosite. (2) The average composition of the upper lunar crust is represented by an “anorthositic gabbro” composition, which supports the “magma‐ocean” hypothesis for lunar crustal genesis. (3) KREEP/Mg‐suite rocks are a minor fraction of the upper lunar crust. (4) Within the farside highlands, areas of KREEP/Mg‐suite rocks coincide mostly with areas of crustal thinning, which are probably areas of KREEP basalt extrusions or localized excavations of Mg‐suite rocks or KREEP‐rich rocks. (5) Portions of the east limb and farside highlands have considerable amounts of a mafic, chondritic Th/Ti component (like mare basalt) whose occurrences coincide with mapped concentrations of light plains that display dark‐halo craters; the presence of this component supports the hypothesis that mare volcanism had occurred within this highland region before the end of final heavy bombardment.