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

Abstract

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.

Aragonite-calcite-dolomite relationships in UHPM polycrystalline carbonate inclusions from the Kokchetav Massif, northern Kazakhstan

“…The samples in this study belong to the carbonate-bearing garnet-pyroxene rocks of the diamondiferous Unit 1 of the Kokchetav Massif (Dobretsov et al, 1995;Korsakov & Hermann, 2006;Ogasawara et al, 2000;Pechnikov & Kaminsky, 2008;Shatsky et al, 1995;Schertl et al, 2004). Some diamondiferous garnet-pyroxene rocks have been considered to be isolated bodies within pelitic gneisses Sobolev & Shatsky, 1990).…”

“…Inclusions of single dolomite crystals in garnets from UHP metamorphic rocks from the Kokchetav Massif, N. Kazakhstan frequently contain diamonds (Korsakov et al, 2005;Ogasawara et al, 2000;Perraki et al, 2009), although they bear no indicator of the existence of aragonite and magnesite. This observation implies that dolomite was stable over the entire pressure-temperature-time path of the diamond-bearing rocks of Kokchetav (Hermann, 2003a).…”

Raman mapping of coesite inclusions in garnet from the Kokchetav Massif (Northern Kazakhstan)

“…The peak metamorphic temperature is constrained at 950-1000 • C [17][18][19][20]. A minimum pressure of 4.3 GPa is given by the presence of diamond and the maximum pressure of about 6.0 GPa is defined by the stability of dolomite [21,22].…”

Discrimination of metamorphic diamond populations by Raman spectroscopy (Kokchetav, Kazakhstan)

“…1. Details on these lithologies, their mineral assemblages and their diamond properties were published elsewhere [17,21,[23][24][25]. Besides these intragranular diamonds, diamonds in Kokchetav rocks might occur intergranular [26].…”

“…Diamonds in the carbonate rocks of Kokchetav are included in garnet, clinopyroxene and zircon [18,20,24] and occur as skeletal crystals or as cuboids (Fig. 1h).…”