U - Th - Pb AND Rb - Sr SYSTEMATICS OF ALLENDE FUN CAI CMS -

Introduction: Among the calcium-aluminum-rich inclusions (CAIs) there exists a small subgroup referred to as FUN CAIs (for Fractionation and Unidentified Nuclear effects [1]) that exhibit unusually large mass-dependent fractionations and mass-independent isotopic variations [1-3]. Understanding the origin, conditions of formation, genetic relationship of these inclusions with other CAIs and the time of formation has been hampered by the rarity of these objects: since the first discovery of two FUN CAIs 37 years ago [1], only about 20 such CAIs have been identified [3-4].. Variable and mostly low compared to “normal” CAIs Al/Al in FUN CAIs derived from their internal Al-Mg isochrons [4, 5] suggests that these CAIs formed either prior to, or simultaneously with, injection and homogenization of freshly synthesized Al into the protosolar nebula [6]. Alternatively, the FUN CAIs could have formed significantly later, or experienced widespread resetting of the Al-Mg isotopic system. A recent investigation of the Hf-W relative chronometer in a single FUN CAI pointed to its formation contemporaneously with other CAIs in the nebula, and additionally suggested heterogeneous distribution of Al [7]. The uncertainty in the formation age of FUN CAIs could be resolved by dating with the U-Pb isotopic chronometer, but such determinations have been precluded by very low concentrations of U [7, 8]. Furthermore, only a few of the identified FUN CAIs are large enough to be analyzed for multiple geochemical and isotopic systems, especially isotopic dating using macroscopic (more precise but destructive) methods. Here we report preliminary U-Th-Pb and RbSr data for a recently discovered FUN CAI (CMS-1) discovered in the Allende meteorite. Sample: Allende FUN CAI CMS-1 is an irregularly shaped compact Type A inclusion. Its primary mineralogy consists of Ti-Al-rich pyroxene, melilite, abundant inclusions of spinel [4, 9]. Previous studies have reported on its Ti, Si, Mg, O isotopic compositions [4,9-10] as well as REE microdistributions [11]. The bulk REE pattern (based on estimated modal abundances of phases) is relatively flat with a small negative Ce anomaly and is enriched ~20× relative to CI chondrites [11]. Methods: A ~5 mg piece was gently crushed in a mortar and pestle. Coarse (>100 μm, 2.33 mg) and fine (2.00 mg) fractions were separated for U-Pb isotope analyses. These fractions were sequentially leached in cold dilute HNO3 (W1), hot 6M HCl (W2) and hot dilute HF (W3), leaving the residue (R) that was dissolved in hot concentrated HF. The leachates and the residues were spiked with Pb-Pb-Th-U-U, and Pb, U and Th were separated and analyzed using procedures similar to [12, 13]. The bulks after U, Th separation were spiked and further analysed for Rb-Sr isotopes using the procedures of [14]. Results: Uranium concentrations were calculated by numeric recombination of the leachates and residues. The recombined U concentration in the coarse fraction is 1.2 ppb, while the fine fraction contains 1.8 ppb; these concentrations are 30-100 times lower than in “normal” CAIs. A large portion (69-76%) of the U is located within the easily soluble fraction, notably W1. All other fractions contain less than 0.2 ppb U. Since easily leachable U is likely to be partially transferred from the matrix and/or introduced by handling, 1.2 ppb is considered an upper limit for the U concentration in this CAI. The total (recombined) concentration of Th is 440620 ppb, consistent with the ~20x enrichment (relative to CI) in REE [11]. Th is distributed uniformly throughout all leaching steps. Assuming that the parent material of this CAI had a CI-like Th/U ratio (~4), its original U concentration is expected to be 100-150 ppb U, typical for “normal” CAIs. Lead isotope ratios associated with the U-Pb and U-Pb decay schemes (Fig.1) are rather un-