Diamond-bearing and diamond-free metacarbonate rocks from Kumdy-Kol in the Kokchetav Massif, northern Kazakhstan

Dolomite marble from the Kumdy-Kol area of the Kokchetav Massif contains abundant microdiamond, mainly in garnet and a few in diopside. The mineral assemblage at peak metamorphic condition consists of dolomite + diopside + garnet (+ aragonite) +/- diamond. Inclusions of very low MgCO3 calcite and almost pure calcite occur in diopside and are interpreted as aragonite and/or aragonite + dolomite. Single-phase Mg-calcite in diopside with a very high MgCO3 component (up to 21.7 mol%) was also found in diamond-free dolomitic marble, and is interpreted as a retrograde product from aragonite + dolomite to Mg-calcite. The dolomite stability constrains the maximum pressure (P) at < 7 GPa using previous experimental data, whereas the occurrence of diamond yields the minimum peak pressure-temperature (P-T) condition at 4.2 GPa and 980 degrees C at XCO2 = 0.1. The highest MgCO3 in Mg-calcite constrains the minimum P-T condition higher than 2.5 GPa and 800 degrees C for the exhumation stage. As these marbles were subjected to nearly identical P-T metamorphic conditions, the appearance of diamond in some carbonate rocks was explained by high XCO2. A low XCO2 condition refers to high oxidized conditions and diamond (and/or graphite) becomes unstable. Difference in XCO2 for marble from the same area suggests local heterogeneity of fluid compositions during ultrahigh-pressure metamorphism.