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New textural evidence on the origin of carbonado diamond; an example of 3-D petrography using X-ray computed tomography

Richard A. Ketcham and Christian Koeberl
New textural evidence on the origin of carbonado diamond; an example of 3-D petrography using X-ray computed tomography
Geosphere (Boulder, CO) (August 2013) 9 (5): 1336-1347

Abstract

Three-dimensional textural observations of inclusion and porosity patterns in a 23-carat carbonado diamond using high-resolution X-ray computed tomography reveal new information bearing on the nature and origin of this enigmatic material. A prominent patinaed surface is texturally linked to a banding and grading of inclusions and pore space beneath, extending several millimeters into the specimen. In situ observation demonstrates that almost all inclusions are polymineralic and show replacement textures, corroborating previous work indicating that the pore network is fully three-dimensionally (3-D) connected, and that virtually all macro-inclusions are secondary. Large metal inclusions are only found immediately adjacent to the margin of the specimen, and are thus also likely to be secondary or even tertiary. However, we also report pseudomorphs after a phase forming pristinely euhedral rhombic dodecahedra, individually and in clusters from 0.3 to 1 mm in diameter; although we could find no evidence of this phase persisting, it nevertheless represents the first "true" macro-inclusion reported in carbonado, which almost certainly formed syngenetically with the diamond material. The pore system is essentially trimodal, consisting of single and clustered pseudomorphs, oblate pores 0.1-0.3 mm in length with a clear preferred orientation, and 20 mu m to <1 mu m pores that form the connected network. Our observations support recent work suggesting that carbonado crystallized from a carbon-supersaturated fluid and suggest that the second stage may correspond with the creation of the pore alignment fabric. We further postulate that, although the present-day macro-inclusions are certainly secondary, the bulk material that comprises them may not be, and may instead be broken-down remains of the original included phase(s). While further verification is needed, a model built around this hypothesis may provide the simplest explanation to many of the unusual features of carbonado.


EISSN: 1553-040X
Serial Title: Geosphere (Boulder, CO)
Serial Volume: 9
Serial Issue: 5
Title: New textural evidence on the origin of carbonado diamond; an example of 3-D petrography using X-ray computed tomography
Affiliation: University of Texas at Austin, Jackson School of Geosciences, Austin, TX, United States
Pages: 1336-1347
Published: 20130814
Text Language: English
Publisher: Geological Society of America, Boulder, CO, United States
References: 39
Accession Number: 2013-070795
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
N03°00'00" - N12°00'00", E14°00'00" - E27°30'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201342
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