U-Pb ISOTOPIC SYSTEMATICS OF EXPERIMENTALLY SHOCKED BADDELEYITE

Introduction: The age significance of isotope chro-nometers in shocked, but not totally molten, extrater-restrial materials is still a matter of debate [1–3]. Numerous isotopic data from different isotopic systems have been obtained for meteorites, but most meteorites with apparent shock ages do not show petrographic evidence of strong shock metamorphic overprint. Deutsh and Schärer [4] carried out comparative isotope investigation on experimentally shocked zircon. In that case, lead isotope fractionation was not observed in experimentally shocked samples despite equilibrium shock pressures up to 59 GPa. Baddeleyite (ZrO 2) is an important and useful mineral for U-Pb dating of lunar and Martian meteorites. ZrO 2 crystallizes in baddeleyite structure (monoclinic) under pressur of ~4 GPa and temperature of ~1000 °C [5], and transforms to a tetragonal and then to a cubic fluorite structure at high temperatures. Baddeleyite is stable at ~4 GPa in the phase diagram, and, by compression , monoclinic baddeleyite shows sequential transition to two orthorhombic phases up to 70 GPa [5]. We performed shock recovery experiments on baddeleyite at the shock pressures of 24, 34, and 47 GPa, to understand the shock effects on U-Pb isotopic systematics of baddeleyite. Here we report the results of SEM-CL observation, Raman spectroscopy, and U-Th-Pb isotope analysis of experimentally shocked baddeleyite. Samples and Techniques: The shock recovery experiments were performed using a single stage 30 mm-bore propellant gun at the National Institute for Materials Science, Japan [6]. We used coarse-grained baddeleyite (200–250 μm in size) from Phalaborwa, South Africa, (2059.8 Ma) for a starting material. The baddeleyite is mixed with a coarse-grained terrestrial basalt sample, S690-7a, from North Kona, Hawaii [7] with a weight ratio of 1:2. The mixture was encapsulated in a cylindrical container made of SUS304 stainless steel and pressed at 29 MPa. We performed three experiments under different shock pressure conditions at 24, 34, and 47 GPa. Porosity of the samples before the shock experiments was 26–30 %. Textures of the run products were examined by a scanning electron microscope with a cathode lumines-cence detector (SEM-CL; JEOL JSM-5900LV). We analyzed Raman spectra with a Micro Raman spectrometer (JASCO NRS-1000) operated with a focused