Petrogenesis of mineral micro-inclusions in an uncommon carbonado

Research areas:
Year:
2011
Authors:
Journal:
EUROPEAN JOURNAL OF MINERALOGY
Volume:
23
Number:
5
Pages:
721-729
Month:
October
ISSN:
0935-1221
BibTex:
Abstract:
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.