The origin of refractory minerals in comet 81P/Wild 2

Refractory Ti-bearing minerals in the calcium-, aluminum-rich inclusion (CAI) Inti, recovered from the comet 81P/Wild 2 sample, were examined using analytical (scanning) transmission electron microscopy (STEM) methods including imaging, nano-diffraction, energy-dispersive spectroscopy (EDX) and electron energy loss spectroscopy (EELS). Inti fassaite (Ca(Mg, Ti, Al) (Si, Al)(2)O-6) was found to have a Ti3+/Ti4+ ratio of 2.0 +/- 0.2, consistent with fassaite in other solar system CAIs. The oxygen fugacity (log f(O2)) of formation estimated from this ratio, assuming equilibration among phases at 1509 K, is -19.4 +/- 1.3. This value is near the canonical solar nebula value (-18.1 +/- 0.3) and in close agreement with that reported for fassaite-bearing Allende CAIs (-19.8 +/- 0.9) by other researchers using the same assumptions. Nanocrystals of osbornite (Ti(V)N), 2-40 nm in diameter, are embedded as inclusions within gehlenite, spinel and diopside in Inti. Vanadium is heterogeneously distributed within some osbornite crystals. Compositions range from pure TiN to Ti0.36V0.64N. The possible presence of oxide and carbide in solid solution with the osbornite was evaluated. The osbornite may contain O, but C is not present at detectable levels. The presence of osbornite, likely a refractory early condensate, together with the other refractory minerals in Inti, indicates that the parent comet contains solids that condensed closer to the proto-sun than the distance at which the parent comet itself accreted. The estimated oxygen fugacity and the reported isotopic and chemical compositions are consistent with Inti originating in the inner solar system like other meteoritic CAIs. These results provide insight for evaluating the validity of models of radial mass transport dynamics in the early solar system. The oxidation environments inferred for the Inti mineral assemblage are inconsistent with an X-wind formation scenario. In contrast, radial mixing models that allow accretion of components from different heliocentric distances can satisfy the observations from the cometary CAI Inti. (C) 2009 Elsevier Ltd. All rights reserved.