Carbonatitic to hydrous-silicic growth medium of diamonds from Internatsionalnaya kimberlite pipe (Yakutia)

Abstract

Diamonds are pristine time capsules from Earth's mantle. Microinclusions in natural diamonds represent a bulk sample of high-density fluids/melts (HDFs) from which they are crystallized (e.g. Navon et al., 1988), and provide a unique opportunity to characterize diamond-forming HDFs and to understand their origin and evolution within the mantle. Here we report the compositions of micro-inclusions in 44 fibrous diamonds from the Internatsionalnaya kimberlite pipes located in the central field of the Yakutian diamond province. The compositions of HDFs from Internatsionalnaya diamonds fall on the carbonatitic to hydrous-silicic join. This compositional range is reported here for the first time in Yakutian diamonds. We also address the question of the genetic relationship between these fluids and their host kimberlite magma. The amount of major elements in the subsurface microinclusions has been determined using EDS. All analyses are normalized to 100% on a carbon -free basis (with excess oxygen for chlorine). Major- and trace-element compositions of the bulk microinclusion populations have been quantitatively analyzed by LA-ICP-MS. The abundance of carbonates, water and silicates in the diamonds were determined by FTIR. Additionally Raman spectroscopy has been used for phase identification in individual microinclusions. The nitrogen defect abundance and aggregation state of studied diamonds were calculated from FTIR spectra. The isotopic composition of carbon of some diamonds was measured using a continuous flow isotopic ratio mass spectrometer attached to a high temperature elemental analyzer. The diamonds studied here are opaque due to abundant microinclusions and look grey in the rough. The proportion of such diamonds in the whole diamond population from Internatsionalnaya is high (up to 5% of examined collections). Most of them have irregular roughly cubic shapes decorated by repetition of small octahedral faces (Fig.1 a). Some crystals have typical resorption features, such as the rounding of edges and etch-pits. The samples were polished into plates with a thickness of 100-200 µm. The polished plates are colorless. Most display an inner zone of high inclusion density and an outer inclusion-free rim (Fig. 1 b). Central cubic diamonds reveal fibrous internal texture defined by trains of microinclusions. The outer transparent parts of many diamonds are not fibrous: CL patterns show layered octahedral growth (Fig. 1 c, d). Fig. 1 Diamonds from Internatsionalnaya kimberlite pipe: (a) SEM image of diamond morphology; (b) polished plate with abundant microinclusions in the central part; (c) CL image of polished plates with zonal structure; (d) detail of (c) . The bulk nitrogen concentration in the studied diamonds ranges between 35 and 1438 ppm. The typical range for 31 diamonds is between 200 and 850 ppm and averages 450 ppm. The 20 studied diamonds are of type IaA and 21 are of type IaAB with IaB/IaA ratio from 4 to 30 %. The carbon isotope composition has been analyzed for 12 diamonds. The total range of δ