Nanometric inclusions of carbonates in Kokchetav diamonds from Kazakhstan: A new constraint for the depth of metamorphic diamond crystallization

Previous studies have revealed that microdiamonds from the Kokchetav ultra-high pressure metamorphic terrane of Kazakhstan contain nanometric scale inclusions of Si-, Fe-, Ti-, and Cr-oxides. Because the structure of SiO2 inclusions was not confirmed to be coesite or stishovite due to their very small size, such diamonds formerly served only as an indicator of a minimum pressure, ca. 4 GPa. Geothermobarometry applied to Kokchetav diamond-bearing rocks yielded a wide range of conditions: T = 700 °C–1250 °C, and P = 4–9 GPa. Our paper presents transmission electron microscopy studies with focused ion beam assistance that indicate that diamonds from marbles contain inclusions of aragonite (CaCO3) and magnesite (MgCO3), and that aragonite and nitrogen-bearing nanometric particles are associated with dislocations reflecting diamond growth at relatively high-temperature conditions. We determined the boundary of dolomite stability using the reaction CaMg(CO3)2 (dolomite) = CaCO3 (aragonite) + MgCO3. This allowed us to utilize available experimental data to evaluate the pressure at which diamond was crystallized. Taking into consideration uncertainties existing between experimental data produced in different laboratories, we propose the pressure for Kokchetav diamond crystallization to be ∼ 6 to 9 GPa. This evaluation is based on the assumption that temperature was determined correctly as 980 °C (minimum) and 1250 °C (maximum) for diamond-grade dolomitic marbles. Our data provide strong evidence that the metasedimentary rocks of the Kokchetav massif containing diamonds were subducted to the depth of ∼ 190–280 km.