1 1 THORIUM CONTENT OF MARE BASALTS OF THE WESTERN PROCELLARUM REGION. B. L. Jolliff , J. J. Gillis , D. J. Lawrence2, and S. Maurice3, 1Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA; 2Los Alamos National Laboratory, MS-D466, Los Alamos NM 87545, USA; 3Observatoire Midi Pyrénées, 14 avenue Edouard Belin, 31400 Toulouse, France. ([email protected]) Introduction: Thorium data from the Lunar Pros- common 2° grid for FeO-TiO2-Th correlations, and for pector gamma-ray spectrometer (LPGRS) reveal a Th- a subset of areas, Th is resampled to 0.5 degree resolu- rich province in the Procellarum-Imbrium region that tion. Clementine data are from the 1999 USGS release has been called the Procellarum KREEP Terrane (PKT) [12] and we used the method described by [10] to [1-3]. Although it was known from Apollo gamma-ray calculate FeO and TiO2 profiles. results that a Th-rich province occurs in this region, In this abstract, we focus on a region from 6°S to LPGRS data showed clearly the extent of Th enrich- 26°N latitude and from 23°W to 63°W longitude. This ment. An unanticipated result was that within the PKT, area includes the crater Kepler on the eastern side and most areas, not just those exposing Imbrium ejecta such extends westward across Imbrian and Eratosthenian as the Fra Mauro and Alpes Formations, have elevated flows, including the region around Marius and Reiner Th concentrations relative to nearly all other regions of (Fig. 1). Basalts in this region include some that have the Moon’s surface. Even extensive areas of mare ba- moderate to high TiO2 concentrations [13; Fig. 1b]. To salt appear to have elevated Th (i.e., >3 ppm). This is a investigate variability among flows and to highlight surprising result because most mare basalt samples correlations that may be used to understand mixing of from Apollo and Luna missions and basaltic lunar me- basalt and non-basalt components, we show an east- teorites, with but few exceptions, have Th concentra- west compositional profile located at 8°N (Fig. 2). tions < 2 ppm [4,5]. Finding extensive volcanic flows Results and Discussion: The images shown in Fig. enriched in Th (and possibly also U and K) would in- 1 reflect a diversity of mare basalt compositions across crease the significance of the few exceptions among the a region that includes basalts ranging from Imbrian to samples and support the possibility that enrichment in Eratosthenian [11], and including a mare plateau region heat-producing radioactive elements played a direct role near Marius. Several prominent craters are seen in the in the generation of melts and in the extensive and pro- images including Kepler and Aristarchus, brightly longed resurfacing of the Procellarum region. rayed Copernican craters. These craters have punched The claim of thorium enrichment indigenous to through basalt and excavated high-Th Imbrium ejecta mare basalts must, however, be made with caution. The deposits, which can be seen near Kepler where they are Apollo 12 site serves as an example of a generally ba- exposed as knobby Alpes Fm. Toward Copernicus to saltic area where Th-rich nonmare components elevate the east, among exposures mapped by [11] as pre- Th concentrations of soils well above those of the local Imbrian rugged terrain, Th also appears to be strongly basaltic rocks [6,7]. Compositions of the Apollo 12 enriched. soils are consistent with mixing of observed Th-rich We concern ourselves with the nonmare units as nonmare materials, similar in composition to Apollo 14 well as the basaltic formations because of the need to soil, with Th-poor Apollo 12 basalt types [1,7], but such understand the effects of mixing nonmare materials mixing produces intermediate FeO as well as Th con- with basalt in mare regoliths. The contamination of sur- centrations. However, in Western Procellarum and rounding plains by ejecta from Kepler, for instance, is other areas within the PKT, FeO contents are very high obvious and extensive. In the profile of Fig. 2, Kepler and the nearest exposures of nonmare materials are ejecta affect compositions as far west as 42° longitude. many kilometers distant. These observations, coupled Farther west, however, FeO concentrations level off at with recognition of a small group of relatively Th-rich over 20%, TiO2 is high (10-12%), and Th ranges from basaltic glasses from Apollo 14 and Apollo 15 samples ~5-6 ppm, decreasing to the west through an Eratosthe- lead us to suggest that Th enrichment might actually be nian mare unit. Continuing west, across an Imbrian ba- indigenous to some of the basalts, themselves [1,3]. salt surface near 60°W, TiO drops to 7% and Th, to 3 Here, we report results of a preliminary “high- 2 ppm. The younger flows have both higher TiO2 and resolution” study of some of the Western Procellarum higher Th concentrations, although the very highest basalts and correlations between Th, FeO, TiO2, and TiO2 basalts do not have the highest Th. In some of the geologic formations that support such a conclusion. basalt-dominated parts of the profile, but not every- Methods and Data: We use a combination of data where, small-scale variations in Th correlate inversely including LPGRS Th from the low altitude phase [8], with TiO , and to a lesser degree, with FeO (e.g., 48- FeO and TiO derived from Clementine spectral reflec- 2 2 60°W). In areas of nonmare exposures at 25–45°W, the tance (CSR) [9,10], Lunar Orbiter images, and geologi- inverse correlation of Th with FeO and TiO is strong. cal maps [11]. Compositional data are convolved to a 2 Abstract #2144: 32nd Lunar and Planetary Science Conference, March 12-16, Houston, Texas, 2001. TH CONTENT OF WESTERN PROCELLARUM BASALTS: B. Jolliff et al. Figure 2 Western Procellarum, 8°N Latitude FeO (wt.%) Kepler 20 TiO2 (wt.%) Th (ppm) mostly mostly 15 Eratos- Imbrian thenian Alpes Fm. basalts basalts 10 5 relative concentration 0 80 70 60 50 40 30 20 longitude (W) compositions to have ~21 wt.% FeO, then their Th con- centrations would range from ~2–5 ppm. The intersec- tion of the two divergent trends suggests a range for the Ti-rich Eratosthenian basalts of about 3–5 ppm Th. Figure 3 KREEP-rich 12 Imbrium ejecta 10 8 TiO2, CSR (wt.%) Th, LPGRS (ppm) 6 subsampled, 2x2° concentration 4 mare 2 KREEP-poor basalt feldspathic materials 0 6 8 10 12 14 16 18 20 22 FeO, CSR (wt.%) Implications: From this preliminary analysis, we conclude that basalts in the western Procellarum region, particularly the younger, Ti-rich Eratosthenian basalts are enriched in Th relative to most common, sampled basalt types from the Apollo sites, but similarly to Apollo 11 high-K basalts. Such enrichments appear to be distributed extensively and not just in a few odd lo- cations, supporting a link to the source region of the basalts and not just enrichment by crustal assimilation or lateral contamination from the ejecta of impacts into A plot of FeO vs. Th and TiO2 (Fig. 3) subsampled Th-rich, sub-mare material. If the enrichments were a from the region shown above illustrates the first order characteristic of the source region, they may have pro- mixing relationships. For Th, two trends of data points vided the heat needed to explain the prolonged and ex- extend toward lower FeO from the cluster at 19–21 tensive basaltic resurfacing in the Procellarum region. wt.%. The trend projecting to low Th represents the Acknowledgements: This work was supported by western edge of the PKT and mixing with Th-poor feld- NASA grants NAG5-4172 and NAG5-8905. References: [1] Jolliff et al. (2000) JGR 105, 4197; [2] Wiec- spathic highlands. The trend projecting toward high Th zorek and Phillips (2000) JGR. 105, 20,417; [3] Haskin et al. (2000) at intermediate FeO extrapolates toward Apollo 14-like JGR 105, 20,403; [4] Taylor et al. (1991) Chapter 6, Lunar Source- impact-melt breccia compositions; this material, proba- book, Cambridge; [5] Korotev (1998) JGR 103, 1691; [6] Korotev et bly Imbrium ejecta, lies beneath the mare and in a few al. (2000) LPSC31, #1363; [7] Jolliff et al. (2000) LPSC31, #1671; [8] Lawrence et al. (2000) JGR 105, 20307; [9] Lucey et al. (2000) places such as near Kepler and farther east, projects JGR 105, 20297; [10] Gillis et al. (2001), this vol.; [11] Wilhelms and above the mare surface. At the high FeO end of this McCauley (1971) USGS Map I-703; [12] Eliason et al. (1999) trend lie the most basalt-rich soils. If we take the basalt LPSC30, #1933; [13] Gillis and Jolliff (2001) LPSC32, this vol. Abstract #2144: 32nd Lunar and Planetary Science Conference, March 12-16, Houston, Texas, 2001. .