Zinc Isotopes Provide Clues to Volatile Loss During Moon Formation 7000 K 2000 K Lunar formation involved the impact of a large object, nominally Mars-sized, with the proto-Earth. This produced a hot debris ring around the Earth, leading to production of the Moon. The high temperatures might have caused mass-dependent fractionation of the isotopes of volatile elements. The next slide shows data for zinc isotopes compared to those of more refractory elements. Left: Simulation by Robin Canup of the giant impact. Scale is hard to read, but depicts temperatures ranging from about 2000 K to 7000 K. Canup, R. (2004) Simulations of a late lunar-forming impact. Icarus 168, 433-456. Right: Model of the protolunar disk formed after the giant impact. The molten layer would have been much thinner than depicted, perhaps 50 km thick if all magma, though it might have contained substantial vapor. The important point is that it may have been possible to lose volatile elements from it. The drawing is based on one shown in Pahlevan and Stevenson (2007). Pahlevan, Kaveh, and David J. Stevenson (2007) Equilibration in the Aftermath of the Lunar-Forming Giant Impact. Earth and Planetary Science Letters, v. 262, p. 438-449. Cartoon depicting the protolunar disk formed by a giant impact with the Earth, based on diagram by Pahlevan and Stevenson (2007) Simulation of lunar origin by giant impact, showing temperatures reached. Moon forms mostly from materials heated to < 4000 K (Movie prepared by Robin Canup, based on Canup, 2004.) Zinc isotopes and lunar formation

Zinc Isotopes Provide Clues to Volatile Loss During Moon Formation Element Condensation Temperature (K)   W 1789 Ti 1582 Cr 1296 K 1006 Zn 726 Zn: Paniello et al (2012) Cr: Lugmair and Shukolyukov (1998) Ti: Zhang et al. (2012) W: Touboul et al. (2007) Overview: Ratios of zinc isotopes indicate evaporation of zinc (and other volatiles) during formation of the Moon. Abstract: Isotopic analyses of the oxygen and the refractory elements chromium, tungsten, and titanium show that Earth and Moon have the same isotopic compositions for these elements. In contrast, Randal Paniello and Frédéric Moynier (Washington University in St. Louis) and James Day (Scripps Institution of Oceanography, La Jolla) report from high-precision analyses of the isotopes of the volatile element zinc that the Moon differs significantly from Earth in the abundances of zinc isotopes. The authors ascribe this difference to evaporation processes during formation of the Moon by a giant impact. The refractory elements probably did not vaporize significantly during lunar formation, leaving their isotopic compositions unscathed by this significant event in solar system history Image: Deviation in the ratios of 53Cr/52Cr, 50Ti47Ti, 182W/184W, and 66Zn/64Zn in the Moon compared to Earth, in parts per ten thousand (epsilon units). Only lunar data with elevated values are plotted here, thus ignoring three samples that have 66Zn less than the terrestrial value. The large difference between Earth and Moon suggests a process that efficiently fractionated the heavier zinc isotope from the lighter one, likely the energetic Moon-forming giant impact, but did not affect the isotopic compositions of the more refractory elements. Table: Nominal 50% condensation temperatures of refractory elements, plus the volatiles K and Zn. K is included because, although volatile, its 41K/39K does not appear to vary in any planetary samples. Why Zn and K behave so differently during lunar formation is not known. Calculations done by Lodders (2003). References: Zn: Paniello, Randal C., Day, James M. D., and Moynier, Frédéric (2012) Zinc isotopic evidence for the origin of the Moon. Nature, vol. 490, p. 376-379. Cr: Lugmair, G. W. and Shukolyukov, A. (1998) Early solar system timescales according to 53Mn-53Cr systematics. Geochimica et Cosmochimica Acta, vol. 62, pp. 2863-2886. Ti: Zhang, J., Dauphas, N., Davis, A. M., Leya, I., and Fedkin, A. (2012) The Proto-Earth as a Significant Source of Lunar Material, Nature Geoscience, v. 5(4), p. 251-255, doi:10.1038/ngeo1429. W: Touboul, M., Kleine, T., Bourdon, B., Palme, H., and Wieler, R. (2007) Late formatin and prolonged differentiation of the Moon inferred from W isotopes in lunar metals. Nature vol. 450, p. 1206-1209. and Lodders, K. (2003) Solar System Abundances and Condensation Temperatures of the Elements, The Astrophysical Journal, v. 591, p. 1220-1247. Refractory elements Cr, Ti, and W are strikingly similar in the Moon and Earth, but volatile Zn is significantly different. Its elevated 66Zn suggests fractionation during lunar formation by a giant impact. Zinc isotopes and lunar formation