Lunar and Planetary Science XXXIII (2002) 1284.pdf

LUNAR MARE VOLCANISM IN THE EASTERN NEARSIDE INFERRED FROM CLEMENTINE UVVIS DATA. S. Kodama and Y. Yamaguchi, Department of Earth and Planetary Sciences, Nagoya University, Nagoya, Japan, [email protected], [email protected].

Introduction: The lunar mare basalts cover about the central northern Serenitatis. Se5 is exposed in the 17% of the lunar surface, but occupy only about 1% of east and west margins. Ejecta and/or floors of Bessel the total volume of the lunar crust [1]. In spite of their crater and Deseilligny crater, both located in Se3, indi- small proportion, they play a very important roll for cate higher contents of TiO2 than Se3. These materials understanding lunar thermal evolution, because mare are probably mixtures of Se2 and Se3, and this fact basalts are the results of lunar volcanism which was suggests that Se2 is distributed in the whole mare and caused by thermal evolution of the lunar interior, and is covered by Se3 and Se4. Stratigraphy and chemistry their composition reflects distribution of their source of the mare basalts in this region suggest that the vol- region. Information about spatial and temporal changes canism migrated from south to north, and TiO2 content of mare volcanism allows us to constrain the composi- decreased with time. tional distribution of lunar interior and thermal evolu- Mare Tranquillitatis. Mare Tranquillitatis lies in tional history. Therefore, it is necessary to establish the center of the study area (center: 7N, 40E; diameter: local and regional stratigraphy of mare basalts. In this 800km), neighboring to Mare Serenitatis in northwest. study, we used Clementine UVVIS data, composed of The mare basalts were divided into four groups defined multi-spectral high spatial resolution images, and ana- as follows: Tr1 (TiO2: <2%), Tr2 (8%), Tr3 (10%), lyzed the stratigraphy of lunar mare basalts in the east- Tr4 (>12%). Tr1 is exposed in the southern and north- ern nearside maria (Serenitatis, Tranquillitatis, Crisium, eastern parts, assumed to be widely sheeted on this Fecunditatis, Nectaris). mare. Tr2 is exposed in the southwest and center of Mare Tranquillitatis. Tr3 covers the central eastern and Methods: Mosaic images of the mare areas were northwestern region. Tr4 is distributed in the small area prepared by using Clemenitne UVVIS data. FeO and of western mare region. Distribution of these units sug- gests that mare volcanism lasted longer in the west than TiO2 contents maps of each mare were produced by applying techniques developed by Lucey et al. [2,3] to the east. Higher-Ti basalts are generally younger than the mosaic images. Mare materials have high content lower-Ti basalts. This trend contradicts to Mare Sereni- of Fe and low albedo relative to highland materials, so tatis. that they form clear boundaries with highland materials. Mare Crisium. Mare Crisium is located in the False color images generated with R415/R750 (as- northeast of the study area (center: 17.5N, 58.5E), and signed to red), R950/R750 (green), and R750/R415 occupies an elliptical area (420km N-S by 560km E- (blue) emphasize spectral differences and allow us to W). The mare basalts were divided into three groups distinguish individual mare units. As mare basalts have defined as follows: Cr1 (TiO2: >5%), Cr2 (4%), Cr3 (<1%). Cr1 is exposed in the southeastern part of the wide range of TiO contents, they can be classified by 2 mare. Cr2 is distributed at the center of the mare, west TiO contents. Stratigraphic relationships were con- 2 to Cr1. Cr3 lies mainly in the north-west region, though structed by examining composition of impact crater it is exposed near the south margin as a small patch. floors and shape of unit boundaries. Relatively high content of TiO (4-6%) in ejecta 2 around Picard and Peirce craters in Cr3 suggests that Results and Discussion: Stratigraphy of mare ba- Cr1 is extended to the west and north as well, not only salts and characteristics of volcanism in each mare re- in the southeast. Mare volcanism in this region seems gion are summarized below. All units were arranged in to migrated from the south to the north. TiO2 contents age. of the mare units are generally low, and decrease with Mare Serenitatis. Mare Serenitatis is located in the time. north of the study area (center: 27N, 19E; diameter: Mare Fecunditatis. Mare Fecunditatis (center: 4S, 740km). The mare basalts of this region were divided 52E; diameter: 690km) is located in the southeast of into five groups according to TiO2 content, and strati- Mare Tranquillitatis, and has subcircular southward graphic relations: Se1 (TiO2: 10%), Se2 (6-8%), Se3 extension (diameter: 250km). The mare basalts were (2%), Se4 (<1%), and Se5 (3-5%). Se1 and Se2 are divided into four groups defined as follows: Fe1 (TiO2: distributed in the south margin of Mare Serenitatis. Se1 7%), Fe2 (9-10 %), Fe3 (3%), Fe4 (<1%). Fe1 is dis- is extended from the northwestern Mare Tranquillitatis, tributed near the north margin and is also exposed as where Se1 is distributed more extensively. Se2 is also patches in the southwestern area. Fe2 is exposed in the exposed in the northwestern region of the mare. Se3 is north. Fe3 widely occupies the north region, assumed widely distributed in the center of the mare. Se4 lies in

Lunar and Planetary Science XXXIII (2002) 1284.pdf

LUNAR MARE VOLCANISM IN THE EASTERN NEARSIDE: S. Kodama and Y. Yamaguchi

to overlie on Fe1 and Fe2. Fe4 covers a large part of The volcanism in these maria seems to have started in southern area of main mare and is extended to the Mare Tranquillitatis, and spread to the other neighbor- southern extension, where no other units are found. ing maria. The TiO2 contents of the mare basalts are These observations suggest that the mare volcanism high in Mare Tranquillitatis, and are getting lower as spread to the south, and the contents of TiO2 decreases being distant from Mare Tranquillitatis. Moreover, with time in Mare Fecunditatis. ages of the mare basalts tend to become younger from Mare Nectaris. Mare Nectaris (center: 16S, 34E; Mare Tranquillitatis to the other maria in the study area. diameter: 240km) lies the south of Mare Tranquillitatis. The general trend in chemical composition of mare Sinus Asperitatis, located north to Nectaris, connects basalt is high Ti for the older units, although it is not Nectaris with Tranquillitatis and is also filled with the case only in Mare Tranquillitatis. mare basalts. The mare basalts were divided into only We are currently analyzing the mare basalts in two groups defined as follows: Ne1 (TiO2: <1%), Ne2 Oceanus Procellarum to compare mare volcanism in (3-4%). Most of mare surface is covered by Ne2. Ne1 the western nearside to that in the eastern nearside. is exposed in the northeast margin and around small craters distributed in the mare. In Sinus Asperitatis, the References: [1] Head III, J.W. (1976), Rev. of mare basalts have similar TiO2 content to that of Ne2 Geophys. Space Phys., 14, 265-300. [2]Lucey et al. unit. These two units might be a single unit, however, (1995), Science, 268, 1150-1153. [3]Lucey et al. stratigraphic relationship between these two units is not (1998), JGR, 103, 3679-3699. [4]Wilhelms, D. E. clear due to interceptions by Theophilus crater, Madler (1987), The Geologic History of the Moon. Chapter 11. crater, and their crater ejecta. Though contents of TiO2 USGS Prof. Paper, 1348, 300pp. [5]Basaltic Volcan- increase with time in this region, they are generally low ism Study Project (1981) Basaltic Volcanism on the in all units. Terrestrial Planets, Pergamon Press, Chapter 1, 236- Fig. 1 summarizes the relationship of the stratigra- 265. [6]Heiken, G. et al. (1991) Lunar Sourcebook, phy and chemical composition of the mare basalts, and Cambridge Univ. Press, Chapter 6, 183-284. locations of the maria, based on radiometric ages of lunar samples returned by Apollo and Luna missions.

Fig.1 Summaries of relationships of the stratigraphy, chemical composition of mare basalts, and locations of the maria. Ages, TiO2 content and locations of landing sites used in this study were from the reference [4][5][6]. Note that the ages of the mare units without the calibrated ages can not be compared with units in the other maria.