Chondrule fragments from Comet Wild2: Evidence for high temperature processing in the outer Solar System

Abstract

Terminal grains from C2063,1,154,1,0 (Track 154) and C2061,1,113,5 (Track 113) from the Stardust collection of Comet Wild2's coma have been studied by TEM and NanoSIMS. Terminal grain 2 of C2063,1,154,1,0 consists of an Al-rich diopside (En 97–99%, Al2O3 9–11wt%), pigeonite (En 85% Wo 15% with TiO2 and Al2O3 contents of 0.5 and 5.2wt%) and minor forsterite and enstatite. The mineral assemblage and Al-rich, Ti-poor composition of the grain are consistent with being a fragment of an Al-rich chondrule, similar to those present in carbonaceous chondrites. The oxygen isotopic composition of the C2063,1,154,1,0 grain was determined by NanoSIMS analyses and found to be δ17O −10.6±5.7‰, δ18O −7.5±2.5‰ and δ17O +1.4±4.3‰, δ18O −6.5±1.6‰ (1σ errors) for the two sections. These figures are distinct from CAIs and consistent with an origin as Al-rich chondrule fragments. Terminal grain 5 of C2061,1,113,5 consists of low Ca pyroxene En 86–87% Fs 10–11% Wo 3–4% and ≤2wt% Al2O3 and in one section 5–10% of a Na-rich silicate phase. This assemblage may be a fragment of a low-Ca pyroxene-bearing chondrule and mesostasis. The original chondrule diameter for the C2063,1,154,1,0 and C2061,1,113,5 samples, by analogy with carbonaceous chondrite chondrules, might have been in the range 0.2–1.0mm. If they were of that size, then the presence of large grains of high temperature material (e.g. ≥1500K for such refractory assemblages) could be explained through commonly invoked models of radial drift from inner to outer Solar System, but only if the chondrules were first fragmented to dust within the inner Solar System. An alternative scenario is that some chondrule formation was associated with high temperature processing and planetesimals in the outer Solar System.