Oxygen Isotopes and Mineralogy of an Al-Rich Chondrule Fragment in Stardust Track 154: Implications for Comet Wild2 Formation

It has frequently been suggested that high temperature, melted material got into the cometary formation region that Wild2 has sampled through radial drift from the inner Solar System e.g. [1]. However, recent nebular disk models show that transport of material in the ~mm size range from the inner to outer Solar System is inefficient and so it may be difficult to explain the proportion of high T material in Wild 2 [2]. Here we use O-isotopes and TEM studies of a Comet Wild2 particle to identify what we suggest is a fragment of an Al-rich chondrule fragment and consider current models of radial drift in the light of our results. Terminal particle #2 from C2063,1,154,0,0 (Track 154) was prepared by microtome cutting at NASA-JSC. TEM analyses have previously been made on 3 grids at the UoL (C2063,1,154,1,14; C2063,1,154,1,15; C2063,1,154,1,17). The sample is mostly composed of Al-rich, Ti-poor diopside (average composition En50Wo50, ≤11 wt% Al2O3) and pigeonite, with minor enstatite and forsterite [3]. This mineralogy is consistent with being a fragment of an Al-rich chondrule. Oxygen isotope analyses have been performed on C2063,1,154,1,15. NanoSIMS 50L isotope imaging mode was used. A 2 pA probe was rastered over an 8 x 8 μm2 area, and contamination by aerogel of the isotopic signature removed. The mass resolution was >10,000 with 16,17,18O, 28Si and 24Mg16O measured on electron multipliers. δ17O and δ18O were normalised to SMOW from San Carlos olivine measurements. Reproducibility determined from analyses on comparable areas of San Carlos olivine was 0.9% for δ18O and 4% for δ17O. The Track #154 grain analysis was corrected for instrumental mass fractionation using the San Carlos olivine results. The isotopic composition of the Track 154 particle was determined as: δ18O = -8.6 ± 1.9% and δ17O = -11.2 ± 4.3% (1σ errors). This isotopic composition is consistent with an origin as a fragment of an Al-rich chondrule [4]. If we assume that the Al-rich chondrule was originally mm-sized then it poses the question of how it got to or remained within the outer Solar System. Possibly radial drift from the inner Solar System occurred very rapidly in order to stop fallback to small orbital radii. Alternatively, this sample raises the possibility that some high T, melted material formed within the outer Solar System. A process to account for this could be an origin within hot, dense regions which were disrupted and led to separation of the material that formed the giant planets and comets like Wild2 [5].