A formation model for carbonado diamond from impact-induced subduction to metamorphic resorption

• Carbon isotope values of carbonado diamonds indicate mixing with mantle fluids. • Plastic and brittle deformation features occur within the interior of the grains. • Nanoanatase encrustation predates the resorption-produced glassy diamond surface. • Impact-induced subduction of Archean organic material is hypothesized. • Carbonados show evidence for several events of fluid-diamond interactions. Carbonados are polycrystalline diamond aggregates exclusively of Archean age. Their localized occurrence in alluvial sediments of Brazil and Central Africa, as well as their unique characteristics and geochemical signatures including the glassy surface and the numerous millimeter-sized pores partially filled with secondary minerals have led to a wide range of proposals for their origin ranging from extraterrestrial impact to mantle formation. Here, we report on analyses of an extensive set of carbonados from Chapada Diamantina (Brazil). These diamond aggregates have carbon isotope compositions (–28.7 to –21.8 ‰) within the typical carbonado range. One exception (–15.6 ‰) demonstrates that intermediate compositions extending toward the mantle range are also present. Electron microscopy and micro-computed tomography analyses reveal plastic and brittle deformation within the diamond grains, but without shock-related structural features. The outer, glassy surfaces show no evidence of textural change, which rules out an impact origin and points to resorption. Nanocrystalline anatase (tetragonal TiO 2 with space group: I4 1 /amd) that encrust the euhedral diamond crystals implies that TiO 2 crystallization preceded resorption. Here, we propose a formation model for carbonado diamond aggregates in accordance with literature data and new observations: (1) impact-induced subduction in the Archaean, (2) rapid transport of sedimentary organic matter to the deep mantle and crystallization of porous diamond, (3) break-up of the diamond layer by ascending magma, (4) interaction with REE- and alkali-rich fluids/melts, (5) low-T anatase precipitation, (6) erosion and deposition in a conglomerate followed by metamorphic diamond resorption. Carbonados may thus represent a record of Archaean subduction of organic matter, offering unique insights into early Earth geodynamics.