Abstract

Carbonados are irregular polycrystalline aggregates of diamond that occur in placers and low-grade metamorphic rocks. They are characterized by fine grain size, light carbon isotope ratio, a wide variety of inclusions of crustal materials (silicates, phosphates, and oxides), and an absence of minerals characteristically associated with diamonds from the upper mantle. We propose that all these properties, and also the presence of lonsdaleite in one carbonado, can be explained by impact metamorphism of crustal rocks containing organic carbon or a graphitic derivative, including those of the Archean and Proterozoic eras.

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