doi:10.1016/j.gca.2005.05.017 Volcanism in Mare Fecunditatis and Mare Crisium: Ar-Ar age studies VERA A. FERNANDES* ,† and RAY BURGESS Department of Earth Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, UK (Received July 16, 2004; accepted in revised form May 16, 2005) Abstract—The laser 40 Ar/ 39 Ar dating technique has been applied to five Luna 16 basalt fragments and one impact glass, and nine Luna 24 basalt fragments and one breccia. The textures of these basalts are fine-grained ophitic and coarse-grained basalts. The samples contain high levels of solar and lunar atmospheric argon acquired during their residence on the lunar surface. These trapped argon components are predominantly released at low temperature steps and can be distinguished from radiogenic and cosmogenic released at intermediate and high temperature steps. The apparent ages obtained for Luna 16 samples span a narrow range of 3.29 to 3.38 Ga. A young age of 0.988 Ga was obtained for a basaltic impact glass indicating the age of an impact event in the vicinity of Luna 16 landing site. The ages obtained for Luna 24 samples suggest the existence of at least three periods of volcanism occurring over a protracted interval of between 3.45 and 2.52 Ga. The long period of volcanism suggested for the Mare Crisium was likely due to a combination of geophysical and geochemical features in the surrounding and underlying areas of the Crisium Basin. Attempts at dating three Luna 20 samples were inconclusive due to their high trapped argon contents. Copyright © 2005 Elsevier Ltd 1. INTRODUCTION Six American Apollo missions and three Soviet Luna mis- sions brought 380 kg of lunar samples for laboratory study. Materials from these missions, plus the subsequent lunar me- teorite findings, have allowed scientists to take the first steps to construct a detailed geological history and evolution of the Moon. The few older rocks dated 4.2 Ga are cumulates of anorthosite and Mg-suite rocks and most have experienced a complex history. An age range for the eruption of lunar mare basalts of 3.9 to 3.1 Ga has been determined by different isotopic methods (summarized by Dalrymple, 1991; Papike et al., 1998; Stöffler and Ryder, 2001). Photogeologic evidence suggests that there were periods of volcanism occurring prior to the formation of the major nearside lunar basins (Schultz and Spudis, 1983; Bell and Hawke, 1984; Hawke et al., 1990; Head and Wilson, 1992; Head et al., 1997), which may have occurred as early as 4.2 Ga. The ages of lava flows younger than 3.1 Ga were first suggested after photogeologic observations by Schaber (1973), Schultz and Spudis (1983), Hiesinger et al. (2000), and Hiesinger and Head (2003) and more recently determined by Ar-Ar dating of Luna 24 samples (Burgess and Turner, 1998) and basaltic lunar meteorites (Fernandes et al., 2003; Borg et al., 2004). The Luna 16 (1970) landing site is in the northeastern part of the Mare Fecunditatis in an area not affected by cratering ejecta (Vinogradov, 1971), but between the Langrenus and Taruntius craters (McCauley and Scott, 1972) of Eratosthenian and Co- pernican ages respectively. This mare is within Fecunditatis Basin, one of the most degraded multi-ringed basins on the Moon (McCauley and Scott, 1972). The robotic mission drilled to a depth of 35 cm and collected 100 g of material from the upper regolith layer. Most of the fragments obtained were basaltic (Vinogradov, 1971) with an aluminous (13 wt.% Al 2 O 3 ), low Ti (5 wt.% TiO 2 ) chemical composition (Ma and Schmitt, 1979). Until recently, the age of only three fragments had been determined; this number has now been increased to 13 by Cohen et al. (2001) and the present study. The dominant age found for mare fragments obtained at this site is 3.4 Ga (Huneke et al., 1972; Cadogan and Turner, 1977; Cohen et al., 2001). The Luna 20 mission (1972) was the second unmanned Soviet mission and the only one to collect a regolith core from the eastern highlands of the lunar nearside. The Luna 20 land- ing site is 120 km north of the Luna 16 landing site (Vino- gradov, 1973) located on the hillside of Crisium Basin beyond the rings flooded with mare lavas, and considered to be inside a buried portion of the main rim (Wilhelms, 1987). Apollonius C crater (Copernican) is located several kilometers from the Luna 20 site, and Vinogradov (1973) suggested that its ejecta might constitute much of the Luna 20 material. A total of 50 g of material was collected with the main component being anorthositic rocks (ANT), with minor components of basaltic rocks and impact melts (Swindle et al., 1991). The age range of 3.75 to 4.5 Ga (Cadogan and Turner, 1977; Podosek et al., 1973; Swindle et al., 1991) obtained for Luna 20 fragments indicate that these fragments are from old crust and also that this region experienced a significant input from impact events as evidenced by the re-setting of the Ar-Ar ages of these samples. Previously, the ages of samples from Luna 20 have been used to constrain the time of Crisium Basin formation (Cadogan and Turner, 1977), however Podosek et al. (1973) have suggested that some material from the Fecunditatis and Tranquilitatis Basins is also present. Luna 24 (1976) was the third unmanned Soviet mission and the last time samples were ever purposely obtained from the lunar surface. The Luna 24 landing site is located on the southeastern region of Mare Crisium on the inner mare surface. A core of lunar regolith was drilled with a mass of 170 g. The * Author to whom correspondence should be addressed (veraafernandes@yahoo.com). † Present address: Inst. Geofísico, Univ. Coimbra, Av. Dr. Dias da Silva, Coimbra, Portugal. Geochimica et Cosmochimica Acta, Vol. 69, No. 20, pp. 4919 – 4934, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 4919