68th Annual Meteoritical Society Meeting (2005)
ASSEMBLY OF THE DESCARTES TERRANE: ARGON AGES OF LUNAR BRECCIAS 67016 AND 67455. R. A. Duncan1 and M. D. Norman2,3. 1Oregon State University, Corvallis OR 97331 USA. firstname.lastname@example.org. 2Australian National University, Canberra 0200 AU 3Lunar and Planetary Institute, Houston TX 77058 USA Introduction: The upper crust of the Moon records an integrated history of impact events extending from soon after the main accretionary phase of planet formation to the present day. An accurate reading of this impact history is important for several reasons, including a better understanding of the significance of large impact events for crust formation and biologic evolution on Earth, establishing absolute timescales of geological events on other planets, and understanding large-scale planetary dynamics of the Solar System. For example, recent models have linked a late cataclysmic bombardment of the inner Solar System at ~4.0 Ga to migration of the outer planets , despite the continuing debate over whether such a cataclysm really occurred . In order to improve our understanding of the impact history and development of crustal terranes on the Moon, we measured 40 Ar-39Ar ages of clasts from two Apollo 16 feldspathic fragmental breccias (FFB’s; 67016, 67455). These breccias are composed of anorthositic and melt breccia clasts in a fine-grained clastic matrix. They represent the Descartes terrane, a regional unit of the central nearside highlands. Clasts in these breccias sampled ancient crustal lithologies, including ferroan noritic anorthosites with ages of 4.4-4.5 Ga , and magnesian impact melts that predate at least some of the nearside basins . Methods: This project was initiated by Graham Ryder who separated chips of anorthositic and melt breccia clasts for Ar isotopic analysis. 0.8 to 3.7 mg of material from 28 samples were encapsulated in evacuated quartz vials and irradiated for 300 hr in the OSU experimental TRIGA reactor. Neutron fluence was monitored with Mmhb-1 hornblende (513.5 Ma). Samples were heated by a continuous power CO2 laser in 20-35 temperature increments; ages calculated from the released gas at these heating steps formed “plateaus’ comprising from 30-100% of the total. Uncertainties (2σ) for individual ages are typically 20-40 Ma. Results: Four anorthositic clasts from 67016 yielded welldefined ages ranging from 3842 to 3875 Ma (average 3862+12 Ma, 1σ SD). There is little evidence for older components in the Ar spectra of these clasts. In contrast, melt breccia clasts from 67016 (n=8) have more diverse spectra, with apparent ages of 4.1 to 4.2 Ga and evidence for older material (to 4.5 Ga,) in the highT fractions. Release spectra for 67455 were not as well behaved. Plateau ages of 3801 to 4012 Ma for three anorthositic clasts and 3987 Ma for one melt breccia clast were obtained. Conclusions: The Ar data are consistent with assembly of these FFB’s at ~3.9 Ga, coincident with many lunar impact melts. An assembly age of ~3860 Ma based on the 67016 anorthositic clasts would be younger than the age of Serenitatis inferred from Apollo 17 poikilitic melt breccias [3893 Ma; 5], possibly implicating a role for Imbrium or local craters in the formation of the Descartes breccias, rather than emplacement as Nectaris ejecta . Older ages for melt breccia clasts may indicate incomplete degassing rather than the emplacement age of the breccias. References:  Gomes et al. 2005. Nature 435:466-469.  Norman 2005. Australian J. Earth Sci, in press.  Norman et al. 2003. Meteoritics Planet. Sci. 38:645-661.  Norman et al. 2005. 68th Meteoritical Soc. Conf., this volume.  Stöffler and Ryder 2001. Space Sci. Rev. 96:9-54.