Lunar and Planetary Science XXIX 1782.pdf

GEOCHEMICAL STUDIES OF LUNAR CRYPTOMARE. T. A. Giguere1,2, B. Ray Hawke1, G. Jeffrey Taylor1, P. G. Lucey1, 1Hawai`i Inst. of Geophys. and Planetology, University of Hawai`i, 2525 Correa Rd., Honolulu, HI 96822. 2Intergraph Corporation, 2828 Pa`a St. Ste. 2150, Honolulu, HI 96819.

Introduction the Ip (Light Plains) unit in the region around Taruntius crater. Global Clementine FeO and TiO2 maps with a Cryptomaria are ancient mare basalts deposits that variety of resolutions (1-35 km) were also used to are hidden or obscured by superposed higher albedo investigate selected cryptomaria [14, 15, 16]. material [1, 2, 3]. As such, they represent a record of the earliest mare volcanism, and may be a significant Results and Discussion volumetric contribution to the lunar crust. Previous remote sensing and geologic studies have provided Five geochemically anomalous regions were evidence for the distribution of ancient mare volcanism. selected for detailed examination. Schultz and Spudis [4] studied the distribution of dark- Balmer Region. This area exhibits elevated FeO haloed impact craters in the lunar highlands, most of values on the order of 4 wt. % over the surrounding which appear to have excavated dark mare basalt from highlands. Circular areas of even greater FeO values beneath a higher albedo surface layer. They suggested (14-16 wt. %) indicate the existence of impact ex- that basaltic volcanism predated the last major basin- cavation of an underlying mafic material. Some of forming impacts and that early mare volcanism may these areas have been identified as dark halo craters. It have been widespread. Hawke and Bell [5, 6] used near-IR spectra to demonstrate that many dark-haloed has been proposed that this region was the site of impact craters excavated ancient mare units buried by ancient basaltic volcanism and that the resulting basin and crater ejecta. Studies of the Apollo orbital deposits were later covered by a thin higher albedo geochemical data sets [7, 8, 9, 10] have shown that surface layer enriched in highland debris contributed mafic geochemical anomalies on the east limb and by surrounding large craters [8]. The existence of dark- farside of the Moon are commonly associated with light haloed craters and elevated FeO values support this plains deposits that exhibit dark-haloed craters. The proposal. Many of the proposed dark halo crater areas ages of these plains units indicated that the extrusion of have elevated TiO2, with peaks in the 2-3 wt. % range. mare basalt was a major process well before 4.0 b.y. The entire Balmer region has average titanium values In recent years, both Earth-based and spacecraft remote ~1 wt. % higher than the surrounding highland areas. sensing data have been used to characterize selected Although these TiO2 values fall within the range of lunar cryptomaria [2, 3, 11, 12, 13]. Still, many issues some highland areas, they are anomalously high and remain unresolved. We have utilized maps of FeO and thus support the concept of the presence of some mare TiO abundances produced from Galileo and 2 material in the surfaces for which these TiO values Clementine multispectral images coupled with the 2 were determined. nearside geologic map in a GIS dataset to investigate the nature and origin of ancient buried mare basalts. Terrain North of Taruntius Crater. The FeO values The purposes of this study include the following: 1) to are elevated in the light plains units to the north and study the composition of surface units in and around northeast of Taruntius crater. The northeast area varies each cryptomare region, 2) to determine the from 8-12 wt. % FeO. The north area ranges from 11- composition of the buried mare unit, 3) to investigate 14 wt. % with points of higher values (14-16 wt. %). the processes responsible for cryptomare formation, and The highlands north of Taruntius have FeO values <8 4) to locate previously unidentified cryptomaria. wt. %. Titanium concentrations follow similar patterns for these units with the area to the northeast at 0-2 wt. Method %, and the area to the north has TiO2 values of 1-2 wt. % with points of higher values (2-3 wt. %) that Clementine UVVIS and Galileo SSI images were spatially correspond to the high FeO points. The utilized in this study. The techniques described by highland terrain to the north exhibits lower values in Lucey et al. [14, 15] and Blewett et al. [16] were the 0-1 wt. % range. Taruntius has contributed applied to calibrated Galileo SSI images in order to highland material to the region as evidenced by the produce FeO and TiO abundance maps for the lunar 2 low iron rays to the southeast on Mare Fecunditatis. nearside. These maps have a spatial resolution of 1-2 km and were the primary data sets used in this Studies of orbital geochemical data and spectral investigation. We utilized GIS masking techniques to mixing models have shown the light plains north and isolate the light plains units (Ip) on the 1971 Wilhelms northeast of Taruntius contain 44-46 % highland and McCauley [17] 1:5M lunar nearside geolologic material and 54-56 % mare basalt [8, 12]. A dark halo map. The It (terra material) unit was used in addtion to crater on the rim of Taruntius crater combined with the Lunar and Planetary Science XXIX 1782.pdf

GEOCHEMICAL STUDIES OF LUNAR CRYPTOMARE: T. A. Giguere et al.

intermediate iron values suggest a volcanic origin for beneath the surface of the light plains unit. It appears the underlying mare surface with later contamination that this light plains unit was produced by the by variable amounts of highland material as a result of contamination of a mare deposit with highlands-rich Taruntius and other impacts [8]. ejecta from Hercules and Atlas craters. Schiller-Schickard Region. The area around the Southern Central Highlands. Most of the southern craters Schiller and Schickard near the extreme portion of the lunar central highlands exhibit FeO southwestern limb of the Moon's Earth-facing values that range between 5 and 9 wt. %. However, a hemisphere, contains numerous unusual features which small area with anomalously high FeO values has been have provoked controversy since the early days of identified near Maurolycus crater. The highest FeO lunar study. These include the crater Wargentin, values (13-15 wt. %) are centered on the dark-rayed whose floor is topographically higher than the crater Buch B (dia. = 6 km) which is located on the rim surrounding terrain; the large crater Schickard (dia. = and wall of Buch C. Lesser FeO enhancements are 227 km), whose floor exhibits distinct mare deposits as associated with the dark-rayed crater Maurolycus A well as a light plains unit; the elongated crater (dia. = 15 km) and Barocius M (dia. = 17 km) which Schiller; and dark-haloed impact craters (DHCs). may also excavate dark material. However, it should Previous studies [2, 5, 6, 11] have demonstrated that be noted that none of these craters is located on a light DHCs in the region have excavated ancient (>3.8 Ga) plains deposit. Perhaps mafic intrusions were mare basalts from beneath highland-bearing material excavated from depth by these dark-rayed craters. emplaced by the Orientale impact event. The Galileo FeO map shows that iron values vary References: from 11-15 wt. % in the Schiller-Schickard (SS) [1] Head, J. W. and Wilson, L. (1992) Geochim. et cryptomare. The surrounding highlands exhibit FeO Cosmochim. Acta 56, 2144-2175. [2] Antonenko, I. et al. values that range from 5-9 wt. %. The post-Orientale (1995) Earth, Moon and Planets 56, 141-172. [3] Head, J. W. mare ponds have FeO abundances of >15 wt. %. DHCs et al. (1993) J. Geophys. Res. 98 (E9), 17149-17181. [4] in the region exhibit higher FeO values than the Schultz, P. H. and Spudis, P. D. (1979) LPSC 10, 2899-2918. [5] Hawke, B. R. and Bell, J. F. (1981) LPSC 12B, 665-678. surrounding plains material. The cryptomare region [6] Bell, J. F. and Hawke, B. R. (1984) J. Geophys. Res. 89, and the post-Orientale mare ponds have slighly higher 6899-6910. [7] Hawke, B. R. and Spudis, P. D. (1980) Proc. TiO2 values (1-2 wt. %) than the surrounding Conf. Lunar Highlands Crust. 467-481. [8] Hawke, B. R. et highlands (<1 wt. %). These results support previous al.(1985) Earth, Moon, and Planets 32, 257-273. [9] Clark, suggestions that ancient mare basalts were excavated P. E. and Hawke, B. R.(1987) Earth, Moon, and Planets 38, from beneath plains units emplaced by the Orientale 97-112. [10] Clark, P. E. and Hawke, B. R.(1991) Earth, impact event. In addition, it appears that the light Moon, and Planets 53, 93-107. [11] Blewett, D. T. et al. plains deposits in the SS region contain a major (1995) J. Geophys. Res. 100 (E8), 16959-16977. [12] component of mare basalt. Blewett, D. T.et al. (1995) Geophys. Res. Let. 22, 3059- Northeast Nearside (NEN) Region. Dark-haloed 3062. [13] Hawke, B. R. et al. (1993) Geophys. Res. Let. impact craters occur on the extensive light plains 20, 419-420. [14] Lucey, P. G. et al. (1995) Science 268, deposits on the northeastern portion of the lunar 1150-1153. [15] Lucey, P. G. et al. (1998) J. Geophys. Res. nearside. Hence, ancient mare volcanism may have In Press. [16] Blewett, D. T. et al. (1997) J. Geophys. Res. occurred in at least some parts of the NEN region. 102 (E7), 16319-16325. [17] D. E. Wilhelms and J. F. Gartner D is a small (dia. = 8 km) with a partial dark McCauley (1971) U.S.G.S Map I-703. [18] Hawke, B. R. et al.(1993) LPSC 24, 617-618. [19] Grolier, M. J. (1974) halo which excavates material from beneath the surface U.S.G.S Map I-841. of a light plains unit in the interior of Gartner crater. Two near-IR spectra obtained for Gartner D exhibit characteristics which clearly indicate that mare basalt was exposed by this impact event [18]. The Galileo iron map shows enhanced FeO values associated with Gartner D and other nearby impact craters. A light plains unit was mapped in the area south of Hercules and Atlas craters by Grolier [19]. The spectrum for a dark-haloed crater south of Hercules indicates that mare basalt was excavated from beneath the highlands-rich ejecta blanket emplaced by the Hercules impact event [18]. Other impact craters in the vicinity excavated FeO-rich mare material from