Lawrence, and 7 co-authors, reported in 1999 that the Lunar Prospector (LP) Gamma-Ray Spectrometer (GRS) has been used to map the global abundance of thorium on the lunar surface. A global map of absolute thorium abundances on a 2 degree by 2 degree equal-area pixel scale is presented using new detector modeling results and low altitude LP data. Because thorium is relatively abundant in KREEP-rich material, these data provide fundamental information regarding the locations and importance of terranes that are rich in KREEP-bearing materials. [KREEP = potassium (K), rare earth elements (REE), and phosphorus (P)], which according to the most widely accepted model of lunar formation as of 1999 is thought to have formed between the lunar crust and mantle.
The authors concluded that while there are many complexities associated with how KREEP was formed and distributed, these new data provide insight about how KREEP was distributed on the lunar surface. The observation that the highest surface Th abundances are localized in and around Imbrium Basin is consistent with the assumption by Haskin [1998a] that there exists a single Th-rich province on the Moon. If the general picture of the Haskin model is assumed to be correct, the Imbrium impact spread large amounts of Th-rich material over the Moon, while other large impacts (e.g., SPA) distributed very little Th-rich material.
There also may have been periods of KREEP volcanism that produced Th-rich units such as the post-Imbrium Apennine Bench Formation, located particularly around the Apennine bench, Aristillus, around the massifs of the Alpes, and near Sinus Iridum [P. D. Spudis, pers. comm., 1999]. Following these periods of KREEP volcanism, mare basalt filled in much of the Imbrium basin with varying but lower amounts of Th-rich material. The additional high-Th material was excavated in various locations around Imbrium basin when impacts that produced the craters Aristarchus, Mairan, Kepler and Aristillus punched through the relatively thin mare basalt layer (<5 km) exposing underlying high-Th material. This picture is complicated by the fact that some craters such as Plato and Copernicus do not appear to have excavated Th-rich material. However, according to basin ejecta modeling described by Haskin [1998b], it is possible that ejecta from large basins such as Imbrium can be considerably inhomogeneous in spatial extent. So the lack of a high-Th signature at Plato and Copernicus may mean these regions happened to be places where the Imbrium ejecta were absent or had lower Th abundances.
Within SPA basin, there is a general Th enhancement of 1.5 to 3 ug/g which is consistent with this material being 50:50 mantle:lower crustal material. There is also a region of higher Th abundances in the northwestern portion of SPA basin. Because of the close association of this moderately Th-rich region in SPA with the Imbrium antipode, it is possible that some of the Th-rich material in SPA was deposited by the Imbrium impact as proposed by Haskin [1998a]. Finally, the authors have discovered a new, moderately high Th region in the northern highlands near the craters Compton and Belkovich that does not appear to be associated with any other abundance anomalies.
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