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Original paper

Petrogenesis of mineral micro-inclusions in an uncommon carbonado

Sautter, Violaine; Lorand, Jean -Pierre; Cordier, Patrick; Rondeau, Benjamin; Leroux, Hugues; Ferraris, Cristiano; Pont, Sylvain

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European Journal of Mineralogy Volume 23 Number 5 (2011), p. 721 - 729
published: 10/1/2011
DOI: 10.1127/0935-1221/2011/0023-2154

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ArtNo.: ESP147052305003

Abstract

A unique Brazilian sample of a carbonado, displaying unusually large amount of diamond clasts merged within a finegrained diamond matrix, has been studied by Secondary Ion Mass Spectrometry (SIMS), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) on Focused Ion Beam (FIB)-extracted foils. We found for the first time in the diamond clasts (stage-1) micrometer to nanometer-sized inclusions of augite, ilmenite and phlogopite (all Fe-rich). Inclusions of metallic phases (Fe, Ti, Cr, Al and alloys of Fe-Cr, Al-Cr, Al-Fe Cr) described in worldwide carbonado occur in the studied sample exclusively within the fine-grained matrix (stage-2). The carbon isotopic composition of the diamond clasts and the fine-grained matrix falls within the range 27 % to 32 % , like worldwide carbonado. The iron-rich silicate-oxide assemblage isolated inside clasts points to an initial growth of that diamond from mafic-rock minerals under oxidizing conditions (f O2</sub≫ IW). On the other hand metallic phases within the fine-grained matrix indicate an oxygen fugacity drop of at least 15 log units. This change in redox conditions is coeval with a deformation event under shearing stress at upper-mantle depth. During this metamorphic event, stage-1 diamonds were broken giving rise to the stage-2 fine-grained matrix, and syngenetic oxide inclusions were reduced to their metallic elements. This unique sample sheds new light on early 1970s hypotheses that interpreted carbonado as a high-pressure product from prograde metamorphism of crustal mafic rocks.

Keywords

carbonado • micro-inclusions • diamond texture • carbon isotope • fib-tem • redox conditions • metamorphic diamond • plastic deformation