Primitive clasts in the Dar al Gani 319 polymict ureilite: Precursors of the ureilites

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Primitive clasts in the polymict ureilite Dar al Gani (DaG) 319 include dark clasts, sulfide- or metal-rich clasts, and unusual chondritic fragments. The dark clasts consist mainly of phyllosilicates, sulfides and magnetite with or without fayalitic olivine. The sulfide-rich clasts consist of a silicate-rich matrix and heterogeneously distributed sulfide. The metal-rich clasts consist of a silicate-rich matrix with variable amounts of metal. The unusual chondritic fragments are chondrule and equilibrated chondrite fragments. Oxygen isotopic compositions of the silicate-rich matrices in the sulfide-rich or metal-rich clasts plot on the carbonaceous chondrite anhydrous mineral (CCAM) mixing line between Allende matrix and a dark clast in the Nilpena polymict ureilite. Their oxygen isotopic compositions are similar to those of the monomict ureilites. Considering its chondritic composition and oxygen isotopic composition, the silicate-rich matrix of the sulfide-rich clasts is the best candidate for the ureilite precursors. However, the matrix has an Mg/(Mg+Fe) ratio (mg ratio) of 0.56 and is too ferroan to produce the monomict ureilites with mg ratios of 0.74-0.95. Therefore, it may have experienced various degrees of reduction to produce precursors with the mg ratios, needed to form the monomict ureilites as residues during fractional melting. Oxygen isotopic compositions of the unusual chondritic fragments plot near the ordinary chondrites on a 3-isotope diagram, suggesting that they have no direct genetic relationship to the monomict ureilites. They were projectiles that collided with the ureilite parent body (UPB).
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Primitive clasts in the Dar al Gani 319 polymict ureilite: Precursors of the ureilites

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