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0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System THORIUM CONCENTRATIONS : IMBRIUM and ADJACENT REGIONS

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0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System THORIUM CONCENTRATIONS : IMBRIUM and ADJACENT REGIONS THORIUM CONCENTRATIONS IN THE IMBRIUM AND ADJACENT REGIONS OF THE MOON. A1 bert E. Metzger, Eldon L. Haines*, Maria I. Etchegaray-Ramirez, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91103, and B. Ray Hawke, Hawaii Institute of Geophysics, University of Hawaii, Honolulu, HI 96822. The orbital gamma-ray spectrometer deconvolution technique restores some of the inherent spatial resolution and contrast lost because of the substan- tial field of view of the instrument. The technique has previously been applied to the observed Th distributions in the Aristarchus, Apennine, and Smythii regions of the Moon overflown by Apollo (1,2). Application has now been made to the Imbrium region using that portion of the data field extending from 10°W - 42OW, over which the data coverage lies between 18ON and 30°N. The area enclosed not only fills in the interval between the Aristarchus- and Apennine-centered regions previously reported but also provides overlap regions which serve as a test of consistency. It is characterized by basalt flows of various ages, depths, and spectral proper- ties, craters of Copernican and Eratosthenian age, and probable areas of pyroclastic mantling . The Aristarchus and Apennine regions contain two of the three areas of maximum radioactivity observed along the Apollo 15 and 16 data tracks. For both regions the undeconvolved values for the 2' x 2" pixels comprising the data base, range over a factor of 3-4 with maximum values in excess of 8.5 ppm. By comparison, the Imbrium field contains a contrast of only 1.5, the values being more uniformly high, but with an upper limit of about 6.5 ppm. The substantially lower contrast makes modeling more difficult and less unambiguous. The family of acceptable Imbrium models contains areas of enhanced radioactivity at or near the craters Timocharis, Lambert, Euler and in the general vicinity of Brayley, La Hire and Delisle. The boundaries shown in the Figure may undergo some modif ication with improvements in model ing. Timochari s, a Copernican-age crater, was a1 so seen in deconvolving the Apennine region: its Th enhancement was ascribed to an ejected sub-mare crustal composition dominated by KREEP basalts (2). The Th enhancement at Lambert in the Figure lacks a pixel in which some of the crater resides and which would be expected to be included if we are modeling the ejecta blanket. Euler, when modeled as a one pixel region corresponding to its ejecta blanket, shows an enhancement comparable to that around Lambert. Euler, like Lambert, is Eratosthenian in age. On the other hand, enhancements are not seen centered around the Eratosthenian craters Delisle and Diophantus, whose ejecta blankets are comparable in size to that of Lambert and Euler (3). The Th enhancement near Brayley occurs north, east, and west of the impact structure but not at the crater itself. The region is covered to various depths with mare basalt flows. These basalt deposits are less than 0.5 km in thickness in the east where they are near the buried projection sf the outer Imbrium ring but are more than 1 Ian in thickness farther west (4). * Current address : 106 A1 berta Lane, Eugene, OR 97404 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System THORIUM CONCENTRATIONS : IMBRIUM AND ADJACENT REGIONS Metzger, A.E. et al. Spectral reflectance data suggests that the surface flows in this region are composed of Imbrium blue basalts and redder, complex, mottled material (5). The quality of fit of the Imbrium field is sensitively dependent on the shape of the Brayley feature. Matching the shape found in modeling the Aristarchus region results in a poorer fit having concentrations 20% - 30% lower for all enhanced regions. The results for the La Hire region are particularly significant. There appears to be a clear correlation between the Th enhancement in this region and the youngest of the three Eratosthenian basalt flows (Phase 111) mapped by Schaber et al. (6). These relatively young Phase I11 flows are clearly defined in fiwzun Apollo photography and are distinct in several remote sensing data sets. The correlation of the La Hire region with the Phase I11 flows suggests that the latest magma extruded in this region was rich in Th. The Th concentrations obtained for the La Hire region from these models approach those found in KREEP basalt. The Aristarchus region has been reworked following discovery of a 2" longitudinal error in the deconvolved field assignments. In the process, a family of fits superior to the best published model (1) was obtained. Major changes were found in the Th concentration of the Aristarchus crater ejecta blanket, up by 50% to values comparable to those found at Archimedes in the Apennine region, the southern part of the Aristarchus Plateau was reduced to a level indistinguishable from the mare region south and east of the plateau, and the Brayley region increased by 20%. Concentrations in the remaining areas were not affected significantly. The revised value for the Aristarchus crater and ejecta blanket requires a high concentration of KREEP basalts, and possibly a component of still higher Th concentration, such as quartz monzodiorite or granitic material (7). As indicated above, consistency in the overlap between the Imbrium and Aristarchus regions requires models whose quality of fit is outside the family of best solutions. On the east side, in the overlapping region, Timocharis, Lambert and the mare average all agree well. The Th concentration within the unenhanced mare average undergoes a decrease in the Imbrium region relative to that surrounding the Apennines, though most of the Imbrium region is consistent with that found for Imbrium in the Apennine region. This work was supported under NASA contract NAS 7-100 at the Jet Propulsion Laboratory, California Institute of Techno1 ogy. References 1 . Haines E.L., Etchegaray-Ramirez M.1 . and Metzger A.E. (1 978) &. Lunar Sci . Conf. 9th, p. 2985-3013. 2. Metzger A.E., Haines E.L., Etchegaray-Ramirez M.I. and Hawke B.R. (1979) Proc. Lunar Sci. Conf. loth, p. 1701-1718. 3. Wilhelms D.E. and McCauley J.F. (1971) U.S. Geological Survey, Map 1-703. 4. De Hon R.A. (1979) Proc. Lunar Sci. Conf. loth, p. 2935-2955. 5. Pieters C.M. (1979) Proc. Lunar Sci. Conf. loth, p. 2825-2849 and Frontispiece, Plate 1. 6. Schaber G.G., Thompson T.W. and Zisk S .H. (1 975) The Moon 13, 395-423. 7. Ryder G. (1976) Earth Planet. Sci. Lett. -29, 255-268. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System THORIUM CONCENTRATIONS: IMBRIUM AND ADJACENT REGIONS Metzger, A.E. et a1 . Distribution of Th in the Imbrium Region after deconvolving the observed data field. Numbers within the regions are the range of Th in ppm for the best models. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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