Petrogenesis of mineral micro-inclusions in an uncommon carbonado

作者：Sautter Violaine^*; Lorand Jean Pierre; Cordier Patrick; Rondeau Benjamin; Leroux Hugues; Ferraris Cristiano; Pont Sylvain

来源：European Journal of Mineralogy, 2011, 23(5): 721-729.

DOI：10.1127/0935-1221/2011/0023-2154

• carbonado
• micro-inclusions
• diamond texture
• carbon isotope
• FIB-TEM
• redox conditions
• metamorphic diamond
• plastic deformation

A unique Brazilian sample of a carbonado, displaying unusually large amount of diamond clasts merged within a fine-grained 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 parts per thousand to -32 parts per thousand 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 (fO(2) > 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.

• 出版日期2011-10