Distribution and concentration of microdiamond in UHP dolomite marble

Abundant microdiamond occurs in dolomite marble in the Kokchetav massif, northern Kazakhstan. The concentration of diamond in this rock is extremely high in some domains in some samples; whereas same domains lack diamond although their mineral assemblages of silicates and carbonates are same with each other. The distribution and concentration of diamond in dolomite marble may give some keys to understand the formation mechanism and conditions of microdiamond in carbonate rocks. The continuous banded sample of the Kokchetav marble indicates the local heterogeneity of XCO2 during UHP conditions; forsterite-bearing assemblages in dolomitic marble is stable under extremely low XCO2 condition (e.g., XCO2=0.01), but diopside-dolomite pair in dolomite marble is stable at a little higher XCO2 conditions (e.g., XCO2=0.1). Forsterite-bearing dolomitic marble lacks diamond and graphite and is an indicator of the infiltration of H2O-rich fluid Diamond occurs in dolomite marble whose mineral assemblage is dolomite-diopside-garnet (+ aragonite), and is mainly included in garnet and slightly in diopside. The shape of microdiamond can be divided into three types: (1) star-shaped diamond which consists of core and surrounding subhedral part (type-S), (2) rugged surface diamond which consists mainly of core with minor amount of surrounding parts (type-R), and (3) very small-grained transparent diamond without core (type-T). These morphological features with the results of our micro-Laue diffraction and laser-Raman studies suggest that microdiamond formed as core at the 1st stage and then surrounding subhedral part overgrew on the core at the 2nd stage to form _grugged surface_h diamond and _gstar-shaped_h diamond. Type-R is an embryo of type-S. Diamond at the 2nd stage probably formed from CO2-H2O-bearing multi-component fluid, and type-T may be evidence for the precipitation of diamond from fluid without core as an seed crystal. We estimated the concentration of diamond in the highest domain of dolomite marble by microscopic observation of 3,447 grains of microdiamond in one thin section. Considering the error in volume estimation of each diamond grain, the concentration of diamond for the highest domain is about 2,700 carat/ton. In this domain, the population of type-T is 15 % and this is much higher than 5% for average samples. The extremely high concentration of microdiamond, high population of type-T and the strong heterogeneity of diamond distribution indicate (1) close relations between microdiamond formation and fluid, and (2) the local heterogeneity of the fluid effects in UHP dolomite marble.