Water Transport in the Solar Nebula: Implications for the Mixing of Oxygen

Introduction: The affinity of oxygen for both gaseous and solid phases makes it and its isotopes important tracers of redox conditions and isotopic reservoirs in the solar nebula. The oxygen fugacity (fO 2) recorded by chondritic meteorites ranges over several orders of magnitudes, from very high values needed to explain such minerals as ferrous olivine in ordinary and carbonaceous chondrites to very reducing conditions such as that needed to explain Si-rich metal in enstatite chondrites [1]. In addition, the chondrite classes fall into distinct regions on an oxygen three-isotope plot [2]. These observations imply that distinct chemical and isotopic environments existed in the solar nebula. Whether these environments varied with location, time, or both remains to be determined. Ciesla and Cuzzi [3] recently demonstrated that the transport of water ice in an evolving protoplanetary disk would lead to fluctuations in the fO 2 inside the snowline that varied with both time and location and argued that these fluctuations may be partly responsible for the variations in mineralogy observed in primitive chondrites. Transport of water ice may also have served as a way of enriching the inner disk with 16 O-poor material that was the product of CO self-shielding either in the molecular cloud from which the solar system formed [4] or in the outer nebula itself [5]. Here we apply the model of [3] to investigate how the oxygen isotopic reservoirs mix during the changes in fO 2 associated with water transport under these two different models. We also discuss the implications for the oxygen isotopic compositions of thermally processed cometary grains. CO self-shielding models: CO self-shielding has been suggested to have operated in three different astrophysical settings to produce the oxygen isotope anomalies observed in chondritic materials (i-iii). In all three cases it is assumed that the bulk oxygen isotopic composition of the protosolar molecular cloud, as well as the average composition of the initial (thermally unprocessed) silicates, are 17,18 O-poor (δ 17,18 O SMOW ~-50‰). (i) Clayton [6] suggested that self-shielding may have occurred near the inner edge of the solar nebula (at the X-point) where the UV photon flux was high due to the young Sun. However, Lyons and Young [7] argued that the high temperatures of this region would lead to the isotopic reequilibration of H 2 O and CO, and thus erase any signatures of the self-shielding effect. (ii) Yurimoto and Kuramoto [4] suggested that …