Anomalous Oxygen Isotopic Fractionation in Vacuum Ultraviolet Photodissociation of Carbon Monoxide and Test of Self-Shielding: Relevance for Meteorite Oxygen Isotopes

Oxygen is the predominant elemental constituent of rocky planets and asteroids. It is the third most abundant element in the solar system after hydrogen and helium. The isotopic compo-sition of oxygen in three-isotope space (δ18O vs. δ17O plot) exhibits large heterogeneity among different bodies formed from the same primordial gas and dust mixture, termed the solar nebula. It is not possible to fully understand the formation and evolution of our own planetary system, unless we resolve the source of the oxygen isotopes. At present, there are two models: self shielding photochemistry in the solar nebula, and symmetry driven gas-phase and/or surface chemistry in the solar nebula, which can also involve photochemistry, but isn't required. Photochemistry is a dominant process at the outer layers of the nebular disk and isotopically selective photodissociation (a process known as isotopic self-shielding) of carbon monoxide, the most abundant nebular oxygen bearing molecule, has been suggested as a source of isotopically anomalous oxygen in the solar reservoir [1-3]. However, these models have had no experimental verification of the relevant isotopic fractionation associated with VUV-CO photodissociation at the relevant wavelengths. Recently, we have performed a series of CO photodissociation experiment with a windowless flow chamber at the Advanced Light Source (LBNL) synchrotron at Berkeley. These experiments demonstrate an anomalously enriched atomic oxygen reservoir is generated through CO photo-dissociation, but, without requiring isotopic self shielding [4]. These results emphasize the importance of chemistry in the solar nebula as we have previously demonstrated through recent laboratory experiments [5]. It is clear that the mass- independent oxygen isotopic composition, as observed in some of the first condensed solids in the solar system (Calcium-Aluminum rich Inclusions- CAIs, Chondrules etc.), can be generated through symmetry driven gas-phase chemical reactions. We will present new oxygen isotope data of VUV photodissociation of CO and discuss the relevance of this data to address the anomalous oxygen isotopic compositions of meteorites. References: [1] Clayton R. N. (2002) Nature, 415, 860-861. [2] Yurimoto H. and Kuramoto K. (2004) Science, 305, 1763-1766. [3] Lyons J. R. and Young E. D. and (2005) Nature, 435, 317-320. [4] Chakraborty S., Ahmed M., Jackson, T. L. and Thiemens M.H. (2008) Science, 321, 1328-1331. [5] Kimura Y., Nuth J. A., Chakraborty S., and Thiemens M. H. (2007) Meteoritics and Planetary Science, 42, 1429-1439.