Iron, Thorium & Lithologic Diversity on the Moon

NLSI Lunar Science Conference (2008) 2130.pdf IRON, THORIUM, AND LITHOLOGIC DIVERSITY ON THE MOON. B. L. Jolliff, R. L. Korotev, and R. A. Zeigler, Department of Earth and Planetary Sciences and the McDonnell Center for the Space Sciences, Washington University, Saint Louis, MO, 63130. (blj@wustl.edu) The two elements iron and thorium have proven to be extremely useful for placing lunar rock types into a global geochemical context. These two elements have been well measured from orbit, especially the global half-degree data set from Lunar Prospector [1], and the dynamic range is substantial. From lunar samples, we know well many of the correlations between these two and other elements, thus knowledge of Fe and Th concentrations tells much about a given rock or soil. In particular, Fe correlates inversely with Al and to some extent Ca, and because two of the main rock types exposed at the Moon’s surface are anorthosite and basalt, Fe is a good indicator of the proportions of these two distinct components. Thorium is one of the incompatible trace elements and its concentration correlates well with others such as U, K, Ba, Zr, and the REE, i.e., the elements enriched in KREEP, which was concentrated in late-stage magmatic residua of the magma ocean. Thus Th concentration is a good indicator of the abundance of this chemical component. In this abstract, we explore some of the observed relationships of different classes of lunar samples using these two elements, and we relate these relationships to their global distribution. The variation of Fe and Th according to rock type is shown in Fig. 1. Mare basalts have high FeO, typically >15 wt.%, and plot on the right side of Fig. 1. Many rocks of the feldspathic highlands, including ferroan anorthosite, plot at low values of Th and for most, low FeO. KREEP-rich rocks, including mafic impact melt breccias and members of the igneous alkali suite (alkali anorthosite, felsite, monzogabbro) plot at high values of Th (some exceeding that of KREEP basalt) and low to intermediate FeO. The magnesian-suite igneous rocks include members with a range of FeO such as troctolite, norite, and gabbronorite, and have low to intermediate Th. Among mare basalts, Th concentrations range up to about 4 ppm, but most are less than 2 ppm. Most of the chemical variability in mare basalts is in Ti content, and there is no clear correlation between Th and Ti in the basalts. Among non-mare igneous lithologies, the ferroan anorthositic and magnesian suites differ significantly in their trace-element characteristics. Many of these are cumulate rocks and as such do not necessarily have a trapped melt component; however, studies of the trace-element chemistry of their silicate minerals indicates that the magnesian-suite rocks were derived from relatively Th-rich (KREEP-rich) magmas. Indeed, the magnesian-suite rocks and the alkali-suite rocks, which form a more-or-less continuous differentiation trend, appear to be petrogenetically related, although such relationships have not yet been observed in a lithologic context such as in outcrop or in a well-understood suite of igneous rocks as, for example, one might collect in a terrestrial layered mafic intrusive. On the other hand, magnesian granulitic rocks and magnesian anorthosite have very low Th concentrations and their relationship to the ferroan-anorthositic suite and the magnesiansuite igneous rocks is not understood. Another set of rocks that is not well understood and not well represented among the samples are those formed by the extremes of igneous differentiation on the Moon, including granite (felsite), which can have Th concentrations as high as 50 ppm, but low FeO, and monzogabbro (aka quartz monzodiorite), which is more mafic than granite and can have Th as high as 30 ppm or more. Whether these rock types occur as significant rock bodies or only as small segregations in otherwise less evolved rock types is not known. It appears that fractionation of KREEP basalt (e.g., Apollo 15) coupled with silicate-liquid immiscibility leads to these evolved rock types. Lunar Prospector showed strikingly that enrichment in Th is strongly confined to the region of the Procellarum KREEP Terrane (PKT) [3] on the western near side (Fig. 2) [2]. A much smaller Th signature is seen in the deeply exhumed materials in the interior of the South Pole-Aitken Basin (SPA). These Figure 1. Variation of FeO and Th among lunar samples. From Chapter 2, New Views of the Moon, Fig. 2-5 [2]. NLSI Lunar Science Conference (2008) 2130.pdf data call into question the existence of a substantial KREEP layer globally at the base of the crust. The meaning of the SPA Th anomaly is unclear. Is this the Th signature of typical lower crustal rocks and is it hosted by a ferroan or magnesian component? The lunar meteorites, which number about 60, are proving to be a fairly good statistical representation of the Moon’s surface composition. They include numerous feldspathic regolith breccias, mare basalts, mixed basalt-feldspathic breccias, and even a few KREEP-rich examples. The feldspathic regolith breccias provide a good representation of the vast feldspathic highlands away from the PKT, and the basaltic meteorites provide examples of previously unsampled basalt types. Figure 3 shows the FeO-Th plot with several key groups of materials and how they correspond to the major lunar geochemical terranes. On the basis of the global Th distribution and the known locations of the Apollo samples, we have suggested that the magnesian suite of lunar rocks is petrogenetically related to the Procellarum KREEP Terrane [4]. Mafic igneous rocks are exposed in the central peaks of many highland impact craters [5] but it is not known whether these are magnesian rocks or ferroansuite mafic rocks complementary to the ferroan anorthosites. The context of the magnesian granulites is not known. A key test will be to determine the rock types found in the SPA basin. Lunar Prospector Mg data suggest that these rocks could be relatively ferroan; however, the contribution of upper mantle rocks and mare basalts needs to be determined, and the lithology of the ancient crustal rocks determined directly. Figure 2. Global Distribution of Thorium Figure 3. FeO vs. Th, from figure 2.2(b) from Lucey et al., 2006, New Views of the Moon. Remotely sensed Fe and Th provide an indication of the geologic setting of the most petrologically evolved lunar rocks, i.e., granite (felsite) and monzogabbro. Materials with high Th occur in the vicinity of Aristarchus Crater, in the western Procellarum region. This crater excavated rocks from below the mare, and among these is a low-Fe component, possibly alkali anorthosite or granite. Mixing trends among the ejected material with mare basalt suggest also that monzogabbro was concentrated in the target region. We look forward to the renewal of lunar exploration and the possibility that by visiting locations such as SPA basin and the Aristarchus region, we will extend our knowledge of the geologic and petrologic context for lunar materials significantly. Acknowledgements. Funding for this work is through NASA grants NNG05GI38G (BLJ) and NNX07AI44G (RLK). References: [1] Lawrence et al. (1998) Science, 281, 1484-9. [2] New Views of the Moon (2006) RiM-G 60. [3] Jolliff et al. (2000) J. Geophys. Res., 105, 4197-4216. [4] Korotev (2000) J. Geophys. Res., 105, 4317-4345. [5] Tompkins & Pieters (1999) Meteorit. Planet. Sci., 34, 25-41.