Shock Recovery and Heating Experiments on Baddeleyite: Implications for U-Pb Isotopic Systematics of Martian Meteorites

Introduction: Radiometric ages of Martian meteorites, shergottites are generally young (i.e., ~165-475 Ma), and are in the late Amazonian chronostratigraphic unit [1]. Bouvier et al. [2-4] reported ~4.1-4.3 Ga old Pb-Pb ages for shergottites, and suggested that young Rb-Sr, Sm-Nd, and Lu-Hf ages so far obtained were affected by alteration of phosphates, interaction with Martian subsurface fluids, or intense shock metamorphism. Baddeleyite (ZrO2) with apparently primary igneous morphology is an important phase in shergottites for U-Pb age determination. In order to investigate shock effects on U-Pb isotopic systematics of baddeleyite, we undertook shock recovery and heating experiments on baddeleyite. Experimental: We used coarse-grained baddeleyite from Phalaborwa for a starting material. The baddeleyite was mixed with a coarse-grained terrestrial basalt with a weight ratio of 1:2. Shock-recovery experiments were performed using a propellant gun at NIMS [5]. The run products were placed in a vertical gas-mixing furnace and heated for 1-3 h at 1000-1300oC under log fO2 of IW+2.5 at 105 Pa. Textures were observed by a scanning electron microprobe and Raman spectra of shocked baddeleyite were obtained. In situ U-Th-Pb isotopic analysis was carried out with the SHRIMP II at NIPR [6]. Results and Discussion: We observed Raman peak shifts of 2-4 cm-1 in the 34-57 GPa samples. Lead loss from baddeleyite was not observed for the experimentally shocked samples. In addition, the U-Pb and Pb-Pb ages of shocked and heated baddeleyites are indistinguishable from those of unshocked baddeleyite within errors except minor lead loss from the baddeleyite shocked at 57 GPa and heated for 1 h at 1300oC. Although duration of peak shock-pressure and grain size of baddeleyite are different from the nature of basaltic shergottites, our experimental results suggest that it is hard to completely reset U-Pb isotopic systematics of baddeleyite in Martian meteorite by shock events below ~60 GPa. Young 238U-206Pb ages of ~200 Ma were presented for baddeleyite in Zagami and NWA 856 without the evidence on phase transition [7, 8]. Our results of Raman spectra for experimentally shocked baddeleyites are consistent with those of Zagami and NWA 856. The shock pressure of 57 GPa and heating up to a partial-melting temperature (1300oC) do not severely disturb U-Pb systems of baddeleyite, suggesting that U-Pb systematics of baddeleyite in shergottites give crystallization age of Martian volcanic rocks. Refs: [1] Nyquist L.E. et al. (2001) Space Sci. Rev. 96, 105-164. [2] Bouvier A. et al. (2005) Earth. Planet. Sci. Lett. 240, 221-233. [3] Bouvier A. et al. (2008) Earth. Planet. Sci. Lett. 266, 105-124. [4] Bouvier A. et al. (2009) Earth. Planet. Sci. Lett. 280, 285-295. [5] Sekine T. et al. (1987) J. Material. Sci. 22, 3615-3619. [6] Misawa K. et al. (2005) Geochim. Cosmochim. Acta. 69, 5847-5861. [7] Herd C. D. K. et al. (2007) Lunar Planet. Sci. XXXVIII, #1664. [8] Misawa K. and Yamaguchi A. (2007) Meteoritics & Planet. Sci. 42, A108.