Abstract-In this paper we describe the recovery, handling and preliminary mineralogical investigation ofthe Tagish Lake meteorite. Tagish Lake is a type 2 carbonaceous chondrite which bears similarities to CIl and CM chondrite groups, but is distinct from both. Abundant phyllosilicates as well as chondrules (however sparse) and common olivine grains in the matrix preclude any other classification.The bulk density of Tagish Lake (1.67 glee) is far lower than CI or CM chondrites (2.2-2.3 and 2.6-2.9 glee, respectively), or any other meteorite for that matter. We have identified two lithologies: a dominant carbonate-poor lithology and a less-abundant carbonate-rich lithology. The meteorite is a breccia at all scales.We have noted similarities between Tagish Lake and some clasts within the enigmatic meteorite Kaidun; possibly there are genetic relationships here worth exploring. In the paper we describe a clast ofCMl material within Tagish Lake which is very similar to a major lithology in Kaidun.

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The canonical initial 26Al/27Al ratio of 4.5 x 10(-5) has been a fiducial marker for the beginning of the solar system. Laser ablation and whole-rock multiple-collector inductively coupled plasma-source mass spectrometry magnesium isotope analyses of calcium- and aluminum-rich inclusions (CAIs) from CV3 meteorites demonstrate that some CAIs had initial 26Al/27Al values at least 25% greater than canonical and that the canonical initial 26Al/27Al cannot mark the beginning of solar system formation. Using rates of Mg diffusion in minerals, we find that the canonical initial 26Al/27Al is instead the culmination of thousands of brief high-temperature events incurred by CAIs during a 10(5)-year residence time in the solar protoplanetary disk.

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available online at http://meteoritics.org 269 Abstract-We describe the petrologic and trace element characteristics of the Yamato 86029 (Y-86029) meteorite. Y-86029 is a breccia consisting of a variety of clasts, and abundant secondary minerals including coarse-and fine-grained phyllosilicates, Fe-Ni sulfides, carbonates, and magnetite. There are no chondrules, but a few anhydrous olivine-rich grains are present within a very fine-grained phyllosilicate-rich matrix.Analyses of 14 thermally mobile trace elements suggest that Y-86029 experienced moderate, open-system thermal metamorphism. Comparison with data for other heated carbonaceous chondrites suggests metamorphic temperatures of 500-600ºC for Y-86029. This is apparent petrographically, in partial dehydration of phyllosilicates to incompletely re-crystallized olivine. This transformation appears to proceed through 'intermediate' highly-disordered 'poorly crystalline' phases consisting of newly formed olivine and residual desiccated phyllosilicate and their mixtures. Periclase is also present as a possible heating product of Mg-rich carbonate precursors.Y-86029 shows unusual textures rarely encountered in carbonaceous chondrites. The periclase occurs as unusually large Fe-rich clasts (300-500 µm). Fine-grained carbonates with uniform texture are also present as small (10-15 µm in diameter), rounded to sub-rounded 'shells' of ankerite/siderite enclosing magnetite. These carbonates appear to have formed by low temperature aqueous alteration at specific thermal decomposition temperatures consistent with thermodynamic models of carbonate formation. The fine and uniform texture suggests crystallization from a fluid circulating in interconnected spaces throughout entire growth. One isolated aggregate in Y-86029 also consists of a mosaic of polycrystalline olivine aggregates and sulfide blebs typical of shock-induced melt recrystallization.Except for these unusual textures, the isotopic, petrologic and chemical characteristics of Y-86029 are quite similar to those of Y-82162, the only other heated CI-like chondrite known. They were probably derived from similar asteroids rather than one asteroid, and hence may not necessarily be paired.

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