Thorium abundances on the lunar surface

Abstract

Abstract. Measurements of absolute thorium abundances on the lunar surface are presented using both the high-and low-altitude data taken with the Lunar Prospector Gamma-Ray Spectrometer. An analysis of the uncertainties shows that the measured uncertainties are < 0.5 gg/g and are close to the theoretical limit of Poisson statistics. Currently, the overall systematic uncertainties are likely dominated by variations in background counts. The relative systematic uncertainties are thought to be no larger than 30% for the high-altitude data and 15% for the low-altitude data. A comparison of high-and low-altitude data show that most regions having thorium abundances > 7 gg/g are likely small area regions _< (150 km) 2.Using lunar topographic data, we have shown that the thorium abundances in the lunar highlands and portions of South Pole-Aitken (SPA) Basin are larger for lower elevations. We have also studied a number of regions with anomalously high thorium abundances such as the northwestern region of SPA Basin, the crater Arago in western Tranquillitatis, and the Compton/Belkovich region in the northeastern highlands. The Compton/Belkovich region appears to be enriched with evolved rocks such as alkali anorthosite and currently represents the only such extended region on the Moon that has been identified. In contrast, Tycho crater has very low thorium abundances which suggests that KREEP was not assimilated at depth in this portion of the Moon.

A Rb–Sr and Sm–Nd isotope geochronology and trace element study of lunar meteorite LaPaz Icefield 02205

“…Global spectroscopic coverage of the Moon obtained from orbit by the Clementine (1994) and Lunar Prospector (1998/99) missions allowed the sample data to be placed in the context of regional terranes . In particular, the global thorium concentration data collected by the Lunar Prospector spacecraft revealed a region on the Moon that is enriched in thorium and is now identified as the Procellarum KREEP Terrane Korotev, 2004;Lawrence et al, 1998Lawrence et al, , 1999Lawrence et al, , 2000Lawrence et al, , 2003. The KREEP reservoir (K -potassium, REE -rare earth elements, P -phosphorus, plus other incompatible elements like thorium) is commonly interpreted as a residual liquid resulting from the crystallization of a lunar magma ocean (LMO) that formed soon after accretion of the Moon (e.g., Warren and Wasson, 1979;Warren, 1985Warren, , 1988Jolliff, 1998).…”

Petrography, mineralogy, and geochemistry of lunar meteorite Sayh al Uhaymir 300

“…Recent Clementine and Lunar Prospector missions have revealed that the radioactive and geochemically incompatible elements, such as K and Th, are largely concentrated in the NW quadrant of the nearside, coincident with the Procellarum terrane (Lawrence et al 1998(Lawrence et al , 2000. On the basis of the remote sensing data provided by these missions, Jolliff et al (2000) recognized three distinct geochemical and petrologic terranes on the lunar surface: Procellarum KREEP Terrane (PKT), Feldspathic Highlands Terrane (FHT), and South Pole-Aitken Terrane (SPAT).…”

Global distribution of lunar composition: New results from Lunar Prospector

“…In contrast, the far side of the Moon is composed primarily of feldspathic material that contains only small areas of basaltic cover [e.g., Wilhelms, 1987, and references therein]. The surface distribution of radioactive elements is also skewed to the front side of the Moon [Metzger et al, 1973;Lawrence et al, 2000]. An additional lunar asymmetry is that the center of mass of the Moon is offset toward the Earth from its center of figure (CM-CF) by $2 km [Kaula et al, 1972;Bills and Ferrari, 1977;Smith et al, 1997].…”