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 °C at X co2 = 0.1. The highest MgCO3 in Mg–calcite constrains the minimum P–T condition higher than 2.5 GPa and 800 °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 X co2. A low X co2 condition refers to high oxidized conditions and diamond (and/or graphite) becomes unstable. Difference in X co2 for marble from the same area suggests local heterogeneity of fluid compositions during ultrahigh‐pressure metamorphism.