Analysis and experimental investigation of Apollo sample 12032,366-18, a chemically evolved basalt from the Moon

Sample 12032,366-18 is a 41.2 mg basaltic rock fragment collected during the Apollo 12 mission to the Moon. It is enriched in incompatible trace elements (e.g., 7 ppm Th), but does not have a bulk composition that would be considered a KREEP (enriched in potassium, rare earth elements, and phosphorous) basalt. The sample is of particular interest because it may be representative of some of the mare basalts within Oceanus Procellarum that are inferred to be Th-rich, based on remote sensing data. The major mineral assemblage of 12032,366-18 is pyroxene, plagioclase, olivine, and ilmenite, and the bulk composition has 4.2 wt% TiO2, 11.7 wt% Al2O3, and 0.25 wt% K2O. The sample contains regions of late-stage crystallized minerals and glass (collectively termed mesostasis), including K-feldspar, apatite, rare earth (RE) merrillite, ilmenite, troilite, silica, and relatively sodic plagioclase adjacent to ferroan pyroxene. The mesostasis also occurs in several areas that are highly enriched in silica and intergrown with K-feldspar and very fine-grained, high-mean-atomic-number phases. We explore the petrology of this sample, including the origin of the Si-K-rich mesostasis to assess whether the mesostasis had formed by silicate liquid immiscibility (SLI). We used experiments to determine if the bulk composition of 12032,366-18 is representative of a bulk liquid composition, how the residual liquid evolves, and to investigate the partitioning of elements between phases as the melt evolves. Experiments support that the mesostasis formed by SLI after crystallization of minerals closely matches the major-mineral assemblage of 12032,366-18. Experiments bracket the onset of SLI and merrillite saturation between 1024 and 1002 degrees C. Some high field strength elements, such as Zr and P, partition preferentially into the Fe-rich liquid. From the experiments, we infer that the bulk composition of 12032,366-18 represents the magma from which it crystallized. Based on the Th-rich and KREEP-bearing chemistry of this sample, along with experimental evidence showing that the sample is representative of a bulk liquid composition and not a cumulate, we conclude that basalt fragment 12032,366-18 was delivered to the Apollo 12 landing site as ejecta from a distant impact and could represent an Oceanus Procellarum basalt. Missions to Oceanus Procellarum, such as Chang'E 5, have the potential to confirm whether some of those basalts are indeed enriched in Th and other incompatible trace elements as indicated by remote sensing.