SPECTRAL REFLECTLICE STUDIES OF DARK-WED IMPACT CRATERS: IXPL1,ATIONS FOR THE JOMPOSITIONANDDISTRIBUTION OF ANCIENT LUNAR BASALTS. B. Ray Hawke, and J.F. Lell, Planetary Geosciences Division, Hawaii Inst. of Geophysics, University of Hawaii, Honolulu,HI 96822.

Introduction: It has recently been proposed that lunar dark-haloed impact craters have been formed by the excavation of low-albedo materials £ran beneath lighter surface deposits and that the distribution of these dark-haloed craters can be used to determine the location and extent of early volcanic deposits which were subsequently covered with varying thicknesses of highland ejecta (1.2). If so, this would imply that basaltic volcanism may have pre-dated the formation of the last major basins, that early farside volcanism may have been widespread, and that at least some lunar light plains may be early volcanic deposits which were covered by high- lands debris. Support for this hypothesis has been provided by recent studies of the orbital geochemistry data sets which demonstrated that mafic anomalies are commonly associated with light plains units which exhibit dark-haloed impact craters (3,4). Because of the key role that dark-haloed craters may play in the solution of several major lunar problems, we have undertaken a program to study these features using the techniques of reflectance spectroscopy. Wnile our preliminary results for Copernicus H and three dark-haloed impact craters (DHCs) in the Schickard-Schiller region suggested'that these craters had exca- vated basaltic material, it was clear that additional spectra with higher spatial andrspectral resolution were needed.

Method: These near-infrared spectra were recectly obtained at the ?launa Kea Observatory 2.2 m telescope using the Planetary Geosciences Division indium antimonide spectrometer. The telescope's usual ff10 secondary mirror was replaced with the f/35 oscillating secondary intend- ed for thermal infrared observations. By using this mirror in its non-oscillating mode, it was found possible to collect spectra using a 0.7-arc second aperture. Hence it was possible for the first time to obtain spectra for very small lunar features (<3 km, depending on position on the lunar surface). Extinction corrections were made using the technique described by McCord and (6). halyses of the pyroxene band positions and continuum slopes were made using the methods presented by McCord et al. (7). Results: 1. Copernicus region - In a previous remote sensing study of this region (5), we concluded that Copernicus H, a 4.6 km DHC on the ejecta blanket of Copernicus, had excavated major amounts of "blue", high-Ti basalt fsm beneath the Copernicus ejecta blanket. me pre- sence of such basalt in the region had been suggested by Head (8) and Pieters (9). New spectra were obtained for the interior and exterior deposits of Copernicus H, an unnamed DHC north of Copernicus, and numerous geologic units in the region. An analysis of the spectrum of the interior of Copernicus H fully confirms previous sug- gestions that fresh mare material was exposed. The spectrum is nearly identical to that obtain- ed for the bright bowl of Draper C in and other fresh mare craters (e.g. Reiner K). The results of a Gaussian band analysis of the Copernicus H interior spectrum indicated that the material is dominated by high-Ca clinopyroxene which is typical of mare basalt. The dark- halo's spectral characteristics can best be explainedby the presence of submature mare material in which the abundance of impact glass has not reached the saturation level, as it has in much older mare surfaces. We also obtained a spectrum for a small DHC located within the obscure feature Gay-Lussac N which is north of Copernicus. This spectrum is very similar to that of Copernicus H ejecta. Apparently, this crater also exposed mare material present beneath the highlands material in the Copernicus ejecta blanket. It should be pointed out that the spectra obtained for both DBCs on the Copernicus ejecta blanket are very different from those of the dark-haloed volcanic craters in , Atlas, J. Herschel, and as well as the spectra of such regional pyroclastic deposits as those on the Aristarchus Plateau and near Rima Bode. 2. Schickard-Schiller region - The spectra of three craters are critical to understanding the origin of DHCs in the region. Ndggerath F is a relatively young DHC. The spectrum of its bowl is almost identical to that obtained for a fresh crater in the mare deposit located in the northern portion of the Schickard crater floor. The Nijggerath bowl spectrum is very dis- tinct from that of Schickard X, a fresh highlands crater in the wall of Schickard. The Schickard X spectrum is sitnilar to those obtained for numerous other fresh highlands craters on the western limb. These spectra were treated to continuum removal and Gaussian function band fitting procedure. Both Naggerath F and the Schickard mare crater exposed high-Ca clinopyroxene indicative of mare basalt (10). Schickard X crater excavated material with low-Ca pyroxene, typical of highland rocks. The spectrum obtained for the interior of Inghirami W is almost identical to that of the Niggerath F bowl. Relatively fresh mare basalts are exposed on the interiors of both of these dark-haloed impact craters. Spectra obtained for the dark-halo of Naggerath F exhibit characteristics which are inter- mediate between those of fresh mare basalts and the mature mare surfaces of the ponds in the NW and SE portions of the floor of Schickard. This dark material is composed of mare basalt which

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Hawke, B.R. and Bell, J.F.

is considerably more mature than that exposed on the crater interior. Fresher material would be expected to be exposed on the interior because of mass wasting and downslope movement on the steep inner slopes. Spectra were also obtained for two areas in the dark deposits surrounding F. The spectrum for the darkest portion of the deposit exhibited characteristics which suggested the presence of a mare component. Inghirami W excavates material from beneath the inner facies of the Hevelius Fm. (11) while both Schickard R and Nfiggerath F are located in Imbrian-aged light plains deposits (1,5, 12) which are interpreted to have been ercplaced as a consequence of the Orientale impact event (11). NUggerath F, Inghirami W, and Schickard R excavate mare basalts from beneath light plains and other Orientale-related deposits. These results can probably be extrapolated to other near- by DHCs, which are both abundant (N, 15) and widely distributed in the Schickard-Schiller region and are concentrated on Orientale-related deposits. Hence, it seems likely that pre- orientale mare volcanic activity not only occurred, but was extensive in the region. The ex- istance of flat expanses of pre-Orientale mare basalt may have facilitated the formation of light plains by debris surges formed by the impact of Orientale ejecta. Our previous geologic studies of this region indicated that post-Orientale volcanic activity was more prevalent in the region than has commonly been recognized (5,13). 3. Dark-haloed Impact craters in other lunar regions - We have collected spectra for DHCs located on other portions of the lunar surface in order to determine if these craters also ex- cavated basaltic material from beneath a highlands-rich surface deposit. One such crater (64"~, 20's) is located within the continuous ejecta deposit of Petavius crater (Imbrian-age) which is superposed on an early Imbrian or late Nectarian plains unit (INp) on the interior of Balmer basin (14,15). An analysis of a near-infrared reflectance spectrum obtained for this crater indicated that material with spectral characteristics very similar to those exhibited by nearside mare basalts was excavated. Therefore, it seems likely that the surface of the INp unit is underlain by mare-like basalt. Numerous DHCs are located within Ip and INp units on the interior of Balmer basin (1,4) and nafic geochemical anomalies are also associated with these plains units (1,3,4,15,16). The Balmer basin appears to have been the site of early lunar volcanic activity. Spectra were also obtained for DHCs north and northeast of Mare Serenitatis. Preliminary analyses of the spectra of DHCs located southeast of Poseidonus and south of Hercules suggest that mare material was exposed by these craters. Additional spectra of DHCs in the light plains units east of Mare Frigoris have been collected but analyses are not yet complete. The results will be presented at the conference.

Conclusions: 1. Copernicus H excavated mare basalt from beneath the Copernicus ejecta blanket. 2. Several dark-haloed impact craters exposed mare basalts which underlay Orientale- related deposits in the Schickard-Schiller region. 3. Pre-Orientale mare volcanic activity not only occurred but was extensive in the Schickard-Schiller region. 4. Early basaltic deposits appear to have been covered by varying thicknesses of high- lands material in other portions of the lunar nearside. 5. The existence of level expanses of basaltic material nay have facilitated the forn- ation of light plains units in certain areas.

References: 1) P. Schultz and P. Spudis (1979) PLPSC 10, 2899. 2) P. Schultz and P. Spudis (1983) in prep. 3) B. Hawke and P. Spudis (1980) =,467. 4) B. Hawke et al. (1983) submitted to JGR. 5) B. Hawke and J. Bell (1981) PLPSC 12B. 665. 6) T. McCord and R. Clark (1979) Pub. A.S.P. 91,-511. 7) T. McCord et al. (1981) JGR 86, 10833. 8) J. Head (1974) PLSC 5, 207.- 9) C. Pieters (1977) PLSC 8, 1037. 10) J. Adams (1974) JGR 79, 4829. 11) D. Scott et al. (1977) USGS Map 1-1034. 12) D. Wilhelms and J. McCauley (1971) USGS Map 1-703. 13) B. Hawke and J. Bell (1982) NASA-TFI, in press. 14) D. Wilhelms and F. El-Baz (1977) USGS Map 1-948. 15) T. Hawell and C. Andre (1981) PLPSC 12B, 715. 16) E. Haines et al. (1978) PLPSC 9, 2985.

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