Investigating the Origin of Th in Mare Basalts of the Western Procellarum Region

1909: 33 Lunar and Planetary Science Conference, March 2002, Houston, Texas. INVESTINGATING Th IN MARE BASALTS: E.L. Flor et al. and fifth subROIs represent mature mare surfaces of differing compositions. These last two subROIs (north and south Schiaparelli) are distinguished on the basis of TiO2 content. This discrimination could be easily recognized in a 3dimensional plot of FeO, TiO2, and Th from the Schiaparelli surface subROI. Not only were these two areas distinguishable in 3-dimensional composition space, but by transferring the pixels back to image-space, they formed two separate and coherent groups (Fig. 1), suggesting that these two subROIs are separate flows altogether. We also note that Th and TiO2 exhibit a positive correlation in the southern Schiaparelli subROI and exhibit little correlation in the northern Schiaparelli subROI. Conclusions: The high TiO2 and Th signature in the southern region of the Schiaparelli ROI must be inherent to basalts and not a post depositional contamination as a result of mixing with high-Th nonmare crater ejecta. Analysis of the compositions of embayed craters (e.g. Schiaparelli, and Seleucus) and post mare craters that have excavated material underlying the maria (e.g. Aristarchus) suggests that the “basement” material (the material underneath the Schiaparelli ROI) contains elevated Th but low TiO2 and FeO concentrations. The Schiaparelli ROI, however, exhibits high TiO2 and FeO values, which suggests very little contamination by these other sources of Th. In fact, the region with the highest Th content, in the southern Schiaparelli subROI, also contains the highest FeO and TiO2 contents (Fig 1c,d). Therefore, the southern Schiaparelli basalt flow is inherently high in TiO2 and Th. In comparison, the northern Schiaparelli subROI exhibits some Th enrichment due to contamination. An annulus of relatively high-Th, low-TiO2 compositions is found along the border between the northern Schiaparelli subROI and the Dorsa Burnett ROI. Horizontal and vertical impact mixing on the Schiaparelli side of the boundary has resulted in elevating the Th concentration and decreasing the TiO2 (the FeO composition of the two units is too similar for mixing to have lowered the FeO composition of the northern Schiaparelli subROI). Although a portion of the northern Schiaparelli subROI is contaminated with Th from the Dorsa Burnett ROI, the central portion of the flow does not appear to be contaminated and contains ~5 ppm Th. Our results show that Th concentrations in the southern Schiaparelli subROI do not result from contamination from Th-rich crater ejecta, but instead may reflect a high-Th region within the PKT that extends below the crust and likely into the mantle. As Th is naturally radioactive and heat producing, high-Th concentrations in the mantle source regions of basalts would explain why basalt production was so voluminous in the PKT (covering 20% of the near side), why the PKT contains the oldest [10] and some of youngest [1] mare basalts to date, and contributed to relaxation of topographic features within the PKT. References: [1] Schultz, P.H., and P.D. Spudis, Nature, 302, 233-236, 1983; [2] Haskin, L.A. et al., JGR, 105 (E8), 20,40320,416, 2000; [3] Soderblom, L.A, et al., PLSC 8th, 1191-1199, 1977; [4] Nozette, and The Clementine Team, Science, 266, 1835-1839, 1994; [5] Binder, A.B., Science, 281, 1475-1476, 1998; [6] Lucey, P.G., et al., JGR, 105 (E8), 20,297-20,305, 2000; [7] Gillis, J.J., and B.L. Jolliff, JGR, in press; [8] Lawrence, D.J., et al., JGR, 105 (E8), 20,307-20,331, 2000; [9] Gillis, J.J., et al., LPSC XXXI, #2058, 2000; [10] Nyquist, L.E., C.-Y. Shih, GCA, 56, 2213-2234. Acknowledgements: This work was made possible by Washington University and the NASA Missouri Space Grant Consor-