Warren reported in 2005 that "new" data for lunar meteorites and a synthesis of literature data have significant implications for the interpretation of global thorium (Th) data and for the Moon’s bulk composition. As presently calibrated (Prettyman et al. 2002), the Lunar Prospector gamma-ray data imply that the average global surface Th = 1.58 µg/g. However, that calibration yields implausibly high concentrations for the three most Th-poor documented sampling sites, it extrapolates to a nonzero Lunar Prospector Th, ∼0.7 µg/g, at zero sample Th, and it results in a misfit toward too-high Th when compared with the global regolith Th spectrum as constrained using mainly lunaite regolith breccias.
Another problem is manifested by Th versus K systematics. Ground truth data plot consistently to the high-Th/K side of the Prospector data trend, offset by a factor of 1.2. A new calibration is proposed that represents a compromise between the Th levels indicated by ground truth constraints and the Prettyman et al. (2002) calibration. Conservatively assuming that the Th versus K issue is mostly a K problem, the average global surface Th is estimated to be ∼1.35 µg/g. The Moon’s remarkable global asymmetry in KREEP abundance is even more pronounced than previously supposed. The surface Th concentration ratio between the hemisphere antipodal to the Procellarum basin and the hemisphere centered on Procellarum is reduced to 0.24 in the new calibration. This extreme disparity is most simply interpreted as a consequence of Procellarum’s origin at a time when the Moon still contained at least a thin residual layer of a global magma ocean. Allowing for diminution of Th with depth, the extrapolated bulk crustal Th is ∼0.73 µg/g. Further extrapolation to bulk Moon Th yields ∼0.07 µg/g, which is nearly identical to the consensus estimate for Earth’s primitive mantle.
Assuming chondritic proportionality among refractory lithophile elements implies Al2O3 of approximately 3.8 wt%. The Moon’s bulk mantle mg ratio is only weakly constrained by seismic and mare-basaltic data. KREEP and mare-free lunaite regolith samples, other thoroughly polymict lunar meteorites, and a few KREEP-free Apollo highland samples manifest a remarkable anticorrelation on a plot of Al2O3 versus mg. This trend implies that an important component of the Moon is highly magnesian. The bulk Moon is inferred to have an Earth-like oxide mg ratio of ∼87–88 mol%. The close resemblance between the bulk Moon and Earth’s primitive mantle extends to moderately volatile elements, most clearly Mn.
Unless major proportions of Cr and V are sequestered into deep mantle spinel, remarkably Earth-like depletions (versus chondrites) are also inferred for bulk Moon Cr and V. The 25–30 lunar meteorites (lunaites) represent a valuable adjunct to the rocks acquired by the Apollo missions from a limited region of the Moon’s central near side (a polygon can be drawn around the six Apollo sites to encompass a mere 2.7% of the Moon’s surface). Lunaite source craters are probably distributed almost randomly across the Moon’s surface (Gladman 1996). In contrast, the clustering of the Apollo sites in the central near side notwithstanding, site-to-site variation among Apollo rocks is generally much greater between missions (the average separation from the nearest-neighbor site is 420 km) than between two ends of a single mission’s traverse range
(separations ≤11 km, half <2 km).
Besides lunaites, the limitations of the Apollo sampling of the Moon are mitigated by very limited (“fines” only) sampling from three Luna sites, but these are tightly clustered (the maximum separation is 444 km) in an east-central nearside area not far from the Apollo sampling region(entirely within the 0.09 lunar circumferences of Apollo 17). Orbital spectrometry has also furnished data for surface (regolith) composition (e.g., Metzger et al. 1977; Lawrence et al. 2000, 2003) that is global in coverage, but only for a few elements and with limited sensitivity and precision. These orbital data indicate a remarkable degree of 1000 km scale heterogeneity, especially in terms of abundance of thorium and other incompatible elements (Lawrence et al. 2003; Elphic et al. 2000b). The main focus of this paper is to employ old data and new data (Warren et al. Forthcoming) for lunaites, along with Apollo and Luna samples, as ground truth with which to test and, if necessary, improve the current calibration (Prettyman et al. 2002; Lawrence et al. 2003) of the Lunar Prospector gamma-ray spectrometry (LP-GRS) data for thorium.
The ultimate goal is to estimate the bulk composition of the whole Moon and its crust as constraints on the Moon’s evolution and especially of its origin. Thorium is a key element for several reasons. Thorium is a natural source for several large γ-ray peaks and thus is exceptionally well suited to the LP-GRS technique. Geochemically, Th is the exemplar of a large suite of refractory-incompatible elements, including all of the rare earth elements except Eu. On the Moon, these elements, along with a few volatile-incompatible elements (K, Rb, and Cs), are strongly correlated and are concentrated in the distinctive crustal component KREEP (Warren and Wasson 1979a). It is widely assumed that all the refractory-lithophile elements, including the major elements aluminum and calcium, are in chondritic or near-chondritic proportions with one another in bulk planets (e.g., Taylor 1982; Hart and Zindler 1986; Sanloup et al. 1999). Thus, by constraining the bulk Moon content of Th, we establish a foundation for constraining the overall composition of the Moon. The distribution of Th is also important because the KREEP-linked radioactive elements Th, U, and K generate internal heat and because Th is a tracer for petrologically “evolved” crust. Apart from Th, the lunaites also help to constrain the key parameter of the overall mg (=Mg/(Mg + Fe)) ratio of the lunar mantle.
See review of Potassium-Uranium systematics:
Campbell, M. D., 1974, “Potassium-Uranium Systematics: Geologic Implications of Moon-Earth-Meteorite Origins,” Rice University Department of Geology Special Paper, 74 p. The data were plotted by hand (in those days) on Log-Log paper. See: http://www.i2mconsultants.com/downloads/corp/CampbellK-USystem-1974C.pdf
Managed & Built by I2M Consulting, LLC | © 2010-
