MARE VOLCANISM IN OCEANUS PROCELLAUM AND MARE IMBRIUM: TIMING AND CHARACTARISTICS OF THE LATEST MARE ERUPTION OF THE MOON. T. Morota1, S. Kato1, J. Haruyama2, M. Ohtake2, and T. Matsunaga3. 1Nagoya Univ., Furo-cho, Chikusa-ku, Nagoya 464-8601, JAPAN ([email protected]), 2ISAS/JAXA, 3NIES.

Unraveling the timing and duration of mare volcan- duration of >~2.5 Ga), and that a peak of volcanic ac- ism on the Moon is essential for understanding its tivity may have occurred at the end of lunar volcanism thermal evolution. Basalt samples returned by the (~2.0 Ga). Apollo and Luna missions and lunar basaltic meteor- The Oceanus Procellarum and Imbrium regions are ites indicate that mare volcanism was active at least partly characterized by high abundances of heat- between ~4.35 and 3.1 Ga [e.g., 1–3]. producing elements such as thorium, potassium, and Remote sensing studies have suggested that some uranium [12-15] that make up the “Procellarum mare basalts in Oceanus Procellarum and Mare Imbri- KREEP Terrane (PKT)” [16]. The longevity of mare um (Fig. 1) are significantly younger than returned volcanism in the PKT can be closely correlated with samples, although the ages and spatial distribution of high abundances of radioactive elements in the crust of the latest eruptions are poorly constrained. On the basis this region. On the basis of a simple thermal conduc- of crater morphology measurement, Boyce et al. [4] tion model, Wieczorek and Phillips [17] demonstrated and Boyce [5] derived a model age of ~2.5 Ga for cen- that heat-producing elements concentrated in the lower tral Oceanus Procellarum and southwestern parts in crust of the PKT could cause directly partial melting in Mare Imbrium, and a youngest age of ~1.7 Ga in the the underlying mantle immediately after magma ocean Aristarchus region. Young [6] made crater size- crystallization, and that the melting zone would have frequency measurements in northern Oceanus Procella- remained under the central region of the PKT over rum and suggested that mare volcanism in this region much of the lunar history. In fact, the young units oc- may have continued until 1.57–1.97 Ga. Hiesinger et al. cur around the Aristarchus plateau and the Kepler [7-9] performed crater size-frequency measurements in crater, which are located in the central region of the numerous nearside maria and in some farside maria by PKT (Fig. 1). Although these results cannot determine employing Lunar Orbiter images. Their results are whether partial melting could be directly generated as consistent with previous studies suggesting that the a result of the high surface abundance of heat- latest volcanic activity occurred in the Oceanus Procel- producing elements, it is likely that crustal heating due larum and Imbrium regions. According to their model to heat-producing elements delayed cooling of the par- ages, the youngest mare basalts are located around the tial-melting zone. Aristarchus Plateau and have estimated surface ages of ~1.2 Ga, expanding the duration of mare volcanism to References: [1] Taylor S.R. (1982) Planetary Sci- more than ~3.0 Ga. ence: A Lunar Perspective. LPI, Houston. [2] Nyquist Prior to the SELENE (Kaguya) mission, high reso- L.E., Shih C.-Y. (1992) GCA, 56, 2213–2234. [3] Te- lution images covered limited areas of Oceanus Procel- rada K. et al. (2007) Nature, 450, 849–853. [4] Boyce larum and Mare Imbrium. The Terrain Camera (TC) J.M., et al. (1974) GCA, 1, 11–23.. [5] Boyce J.M. aboard SELENE was a panchromatic push-broom im- (1976) Proc. Lunar Sci. Conf. 7th, 2717–2728. [6] ager with two optical heads that acquired stereo data Young R.A. (1977) 8th Proc. Sci. Conf., 3457–3473. for the entire surface of the Moon with an average res- [7] Hiesinger H. et al. (2000) JGR, 105, 29239–29275. olution of 10 m/pixel [10]. Its high spatial resolution is [8] Hiesinger H. et al. (2003) JGR, 108, of great advantage in deriving reliable age, especially doi:10.1029/2002JE001985. [9] Hiesinger H. et al. for young or small areas, from crater size-frequency (2010) JGR, 115, doi:10.1029/2009JE003380. [10] distribution (CSFD). Morota et al. [11] performed new Haruyama J. et al. (2008) EPS, 60, 243–256. [11] Mo- crater size-frequency measurements for young (≤3.0 rota T. et al., (2011) EPSL, 302, 255-266. [12] Law- Ga) mare units in Oceanus Procellarum, related maria rence D., et al. (1998) Science, 281, 1484–1489. [13] (Mare Nubium and Mare Insularum), and Mare Imbri- Haskin L.A., et al. (2000) JGR, 105, 20403–20416. um using TC image data to investigate the timing and [14] Yamashita N., et al. (2010) GRL, 37, charactaristics of the latest eruption in the regions. doi:10.1029/2010GL043061. [15] Kobayashi S., et al. Figure 2 shows histogram of the model ages of (2010) SSR, 154, 193–218. [16] Jolliff B.L., et al. mare basalts obtained by Hiesinger et al. [7, 8] and (2000) JGR, 105, 4197–4216. [17] Wieczorek M.A., revised by Morota et al. [11]. The model ages show Phillips R.J., (2000) JGR, 105, 20417–20430. that mare volcanism continued until ~1.5 Ga (a total

Figure 1. (a) Terrain Camera mosaic image of the Oceanus Procellarum and Mare Imbrium regions. Ar, Aristarchus; Co, Copernicus; Ke, Kepler; Li, Lichtenberg. (b) Map of geologic units defined by Hiesinger et al. [7, 8] superposed on a Clementine false color map (750/415 on red, 750/950 on green, and 415/750 on blue). (c) Sketch map showing unit numbers [7, 8]. Units highlighted in light blue were reinvestigated by Morota et al. [11] and crater size- frequency measurements were performed for the areas highlighted in pink. (d) Map of model ages of mare basalts determined in Morota et al. [11] and Hiesinger et al. [7, 8].

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0 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 Model Age [Ga] Figure 2. Histograms of the model ages of mare basalts in the PKT obtained by Hiesinger et al. [7, 8] and revised by Morota et al. [11].