Lunar and Planetary Science XXXV (2004)
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EVOLVED LITHOLOGIES AND THEIR INFERRED SOURCES IN THE NORTHWESTERN PROCELLARUM REGION OF THE MOON. Bradley L. Jolliff, Department of Earth and Planetary Sciences and the McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130 Introduction: Compositional remote sensing from the Th-enrichment seen in regions dominated by basalt, the Lunar Prospector mission reveals the Procellarum- the data also suggest that some of the basalts themselves Imbrium region of the Moon, also referred to as the are significantly enriched in Th relative to the main Procellarum KREEP Terrane [1-4], to be an area of sig- groups of high-Fe basalts in the Apollo and Luna collecnificant enrichment of heat-producing residua (i.e., Th- tions and lunar meteorites. One trend in particular, rich) of the early lunar differentiation. Previous esti- shown by the arrow in Fig. 1, suggests mixing between mates place as much as 60-70% of the whole-Moon Th-rich basalt and a nonmare component that is excontent of Th into the crust and as much as 35-40% of tremely high in Th content, i.e., even higher than typical the crustal Th content into the Procellarum KREEP Ter- mafic impact-melt breccias from Apollo 14. Samples: Taking the remotely sensed data at face rane [5], which occupies only ~10-15% of the volume of the crust. Although these estimates have significant value, we find clues to what the rock components might uncertainty, the correspondence of the enrichment of Th be that produce these trends. We have previously pre(and other heat producers U and K) in this region is sented evidence that FeO- and Th-rich basalts (e.g., 4-6 consistent with extended igneous activity, manifested at ppm) occur in the Western Procellarum region [10-13]; the surface by extensive basaltic volcanism and subdued however, except for some of the Apollo 11 high-K batopography [3,4]. Such activity may have extended also salts (3-4 ppm Th) from Mare Tranquillitatis, only small to a significant depth, probably including the upper fragments of Th-rich basaltic materials have been found mantle. In this abstract, we present evidence based on in the sample collection. These include basaltic glass Apollo samples for some of the most extensively frac- from Apollo 14 (19.7 % FeO, 5.4 ppm Th [14]) and tionated lunar rocks types, including a Th-rich mare basaltic impact glass from Apollo 15 (18.9 % FeO, 8.3 basalt from Apollo 12, and monzogabbro (also known ppm Th [15]). Recently, in a survey of basaltic fragas monzodiorite), granite, and alkali anorthosite from ments from the Apollo 12 regolith, we identified a crysApollo 12 and 14 samples. We relate these to likely talline basalt fragment with 19.7 % FeO and 6.9 ppm Th exposures and sources indicated by compositional re- (Fig. 2). This sample, 12032,366-18, is a crystalline mote sensing. Western Procellarum (20°W to 80°W, 10°S to 43°N) Remote sensing: Figure 1 shows a distri14 bution of FeO vs. Th concentrations measured by to about 25 ppm compositional remote sensing for a region of the at 10 % FeO 12 western Procellarum basin at 0.5 degree resolution. The data for FeO are derived from Clemen(0.5° resolution) 10 tine spectral reflectance (CSR) [6,7] and Th data are from the Lunar Prospector gamma-ray spec8 trometer (LPGRS) [8,9] The data include a large number of points at high FeO, representing re6 ~5 ppm golith developed on extensive western Procellarum basalt flows. Data points at low FeO and 4 low Th correspond to nonmare or highland regions west of Oceanus Procellarum. Data at high 2 Th and intermediate FeO correspond to several 0 specific locations such as the Aristarchus region 0 5 10 15 20 25 and exposures of Fra Mauro and Alpes Formations around the craters Kepler, Reinhold, and FeO (CSR) Figure 1 Lansberg. Data points along the lower arm of the distribution represent mixing of basalt (high FeO) with mare basalt fragment whose composition is unlike the nonmare feldspathic materials along the western bound- common Apollo 12 basalt groups and is likely to have ary of Oceanus Procellarum (cyan square). Data points been ejected by impact from a distant site. Among nonmare materials, especially from Apollo on the upper arm lie along a mixing line between the compositions of basalts and typical Fra Mauro materi- 12, 14, and 15, fragments of alkali anorthosite, granite, als, noted by the green and red squares, respectively. It and monzogabbro have been found. Monzogabbro and is clear from the trends that impact mixing of Th-rich granite are especially enriched in Th, and they are likely nonmare materials such as Imbrium-derived mafic im- related to alkali anorthosite on the basis of incompatible pact melt into basaltic regolith has produced much of element signatures. Figure 2 shows FeO vs. Th con-
Th (LPGRS)
Lunar and Planetary Science XXXV (2004)
2032.pdf
70 centrations for Apollo 12 materials, highlighting these granite Mostly Apollo 12 samples rock types. Also present among these rock fragments 60 are mare basalts at high FeO, a few nonmare rocks with 50 FeO <8%, mafic impact-melt breccias with FeO ~912% and Th ~10-25 ppm, and regolith breccias with mafic melt monzogabbro 40 breccia Avg compositions mainly intermediate between impact-melt breccia and mare basalt. 30 Ap 15 impact Gls Integration: The key point of comparison between 20 basalt Figs. 1 and 2 is that the trend of compositions between Alkali 12032,366-18 Anorth. mare basalt and mafic impact-melt breccia in Fig. 2 is 10 similar to the lower trend on the right side of Fig. 1 0 (note different scales), and the dashed arrow between 0 5 10 15 20 25 basalt 12032,366-18 and monzogabbro in Fig. 2 correFeO (wt%) sponds to the arrow on Fig. 1. The lower arm of the Figure 2 trend extending to low FeO is largely absent from Fig. 2 because there is no ready source of feldspathic Figure 3 nonmare material near the Apollo 12 region. Here, we are concerned primarily with the trend of compositions that occurs from high FeO toward very high Th. Moderately Th-rich basalts have been inferred from remote sensing to occur in the western Procellarum region along a swath stretching from south of the equator northward to Sinus Roris, and including regions of basalt flows mapped as Imbrian to Eratosthenian [11-13]. The surface along this swath has very high FeO concentrations (see Fig. 3) and so is taken to be regolith that has very little nonmare material mixed in. Using selection criterion as shown in the inset, Fig. 3 shows the distribution of Th in a section of Western Oceanus Procellarum that has very high FeO concentrations. In Fig. 3, points lying along the highest FeO-Th trend (see inset) are marked red and are shown in the map projection to cluster mainly about the Aristarchus crater. As shown in the inset, points along the high FeO-Th trend extend to lower FeO and even higher Th concenAcknowledgements: Ryan Zeigler, Jeff Gillis, Erica trations, projecting to about 25 ppm at 10-12 wt% FeO, Flor, Randy Korotev, David Lawrence, and Larry Haskin and higher than other areas in the region [cf. 16]. These are thanked for their contributions. This work was supvalues require the presence of a component that has ported by NAG5-8905 and 10227 (blj) & NAG5-10485 (L. significantly greater Th concentration than the Th-rich Haskin). impact-melt breccias from Apollo 12 and 14 samples. References: [1] Jolliff et al. (2000) JGR, 105, 4197; [2] Of the known lunar rock types, only monzogabbro and Korotev (2000) JGR 105, 4317; [3] Wieczorek and Phillips granite have high enough Th concentrations. The val- (2000) JGR 105, 20,417; [4] Haskin et al. (2001) JGR 105, ues that lie along the high-FeO, high-Th trend require 20,403 [5] Jolliff and Gillis (2002), in Workshop: The Moon basalt to have mixed with such materials, but at 0.5° per Beyond 2002; [6] Lucey et al. (2000) JGR, 105, 20,297; [7] Gillis et al., GCA submitted; [8] Lawrence et al. (2002a) pixel resolution, they do not extend below about 17% LPSC33, #1970; [9] Lawrence et al. (2002b) in Workshop: FeO (Fig. 1). We know from high-resolution Clemen- The Moon Beyond 2002; [10] Jolliff et al. (2001) LPSC32, tine data, however, that some materials of low FeO con- #2144; [11] Gillis et al. (2002) LPSC33, #1934; [12] Flor et tent were excavated by Aristarchus [17-18]. On the ba- al. (2002) LPSC33, abstract #1909; [13] Flor et al. (2003) sis of geochemical, petrologic, and lithologic LPSC34, abstract #2086; [14] Jolliff et al. (1991) PLPSC 21, associations, we may infer that low-FeO components 193; [15] Delano et al. (1982) PLPSC 13th, in JGR 87, A159; excavated by Aristarchus include, in addition to mon- [16] Lawrence et al. (2003) JGR 108(E9), 10.1029/2003/ zogabbro, its petrologic associates K-feldspar-rich gran- JE002050; [17] Lucey et al. (1986) JGR 91, D344; [18] Hawke et al. (2003) JGR 108(E6), 10.1029/2002JE001890. ite and alkali anorthosite.
Th (ppm)