The Tarda Meteorite: A Window into the Formation of D-type Asteroids

Dynamic models of solar system evolution suggest that D-type asteroids formed beyond Saturn's orbit and represent invaluable witnesses of the prevailing conditions in the outer solar system. Here, we report a comprehensive petrographic and isotopic characterization of the carbonaceous chondrite Tarda, a recent fall recovered in the Moroccan Sahara. We show that Tarda shares strong similarities with the D-type-derived chondrite Tagish Lake, implying that Tarda represents a rare sample of D-type asteroids. Both Tarda and Tagish Lake are characterized by the presence of rare O-16-rich chondrules and chondrule fragments, high C/H ratios, and enrichments in deuterium, N-15, and C-13. By combining our results with literature data on carbonaceous chondrites related to C-type asteroids, we show that the outer solar system 4.56 Gy ago was characterized by large-scale oxygen isotopic homogeneities in (i) the water-ice grains accreted by asteroids and (ii) the gas controlling the formation of FeO-poor chondrules. Conversely, the zone in which D-type asteroids accreted was significantly enriched in deuterium relative to the formation regions of C-type asteroids, features likely inherited from unprocessed, D-rich, molecular-cloud materials.