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Diamond‐bearing and diamond‐free metacarbonate rocks from Kumdy–Kol in the Kokchetav Massif, northern Kazakhstan
Author(s) -
Ogasawara Yoshihide,
Ohta Masahito,
Fukasawa Kyoka,
Katayama Ikuo,
Maruyama Shigenori
Publication year - 2000
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1046/j.1440-1738.2000.00285.x
Subject(s) - diopside , dolomite , calcite , geology , metamorphic rock , aragonite , massif , diamond , mineralogy , geochemistry , metamorphism , coesite , mineral , eclogite , materials science , metallurgy , paleontology , subduction , tectonics
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 MgCO 3 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 MgCO 3 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 co 2 = 0.1. The highest MgCO 3 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 co 2 . A low X co 2 condition refers to high oxidized conditions and diamond (and/or graphite) becomes unstable. Difference in X co 2 for marble from the same area suggests local heterogeneity of fluid compositions during ultrahigh‐pressure metamorphism.