A new model for the evolution of diamond-forming fluids: Evidence from microinclusion-bearing diamonds from Kankan, Guinea
Introduction
Mineral inclusions are common in monocrystalline diamonds and denote peridotites and eclogites as the two main diamond host rocks (Meyer, 1987, Harris, 1992, Sobolev et al., 1999, Laiginhas et al., this issue, Banas et al., this issue). The involvement of fluids in diamond formation is suggested by the association of diamonds with cracks, veins and alteration zones in eclogite xenoliths (Schulze et al., 1996, Keller et al., 1998, Taylor et al., 2000, Taylor and Anand, 2004), with metasomatic minerals (Meyer, 1987, Anand et al., 2004, Bulanova et al., 2004), and by the morphology and symmetric growth patterns of the diamonds themselves (Sunagawa, 1981, Sunagawa, 1984, Bulanova, 1995, Davies et al., 1999). Direct evidence in the form of microinclusions carrying high density fluids (HDFs) is common in fibrous diamonds. The study of these inclusions permits a glimpse back in time to the story of diamond formation.
The presence of fluid inclusions in diamonds was first noted by Chrenko et al. (1967), who detected absorption bands of water and carbonates in the infrared (IR) spectrum of the coat of a coated diamond. Navon et al. (1988) determined the major element composition of the inclusions and suggested that the trapped material represents the fluids from which the diamonds grew. Such microinclusions have been found in cubic fibrous diamonds, the fibrous coats of coated diamonds and in clouds within octahedral diamonds (see Kamiya and Lang (1965) for details of the fibrous structure). They contain secondary minerals and a residual low-density fluid (Lang and Walmsley, 1983, Guthrie et al., 1991, Klein-BenDavid et al., 2006). Infrared spectroscopy and electron probe micro-analyser (EPMA) analyses of thousands of microinclusions in more than 100 such diamonds by Navon and co-workers (Schrauder and Navon, 1994, Izraeli et al., 2001, Shiryaev et al., 2005, Klein-BenDavid et al., 2007, Klein-BenDavid et al., 2008), reveal that the microinclusions have trapped high-density fluids (HDFs). Mineral inclusions ranging in size from ∼ 20 μm to < 1 μm are occasionally found in association with the HDFs (Izraeli et al., 2004, Tomlinson et al., 2006, Weiss et al., 2008).
The composition of the HDFs varies along two arrays between three end-members: a carbonatitic end-member rich in Ca, Mg, Fe, K and carbonate; a hydrous-silicic end-member rich in Si, Al, K and water; and a hydrous-saline fluid rich in Cl, K, Na and water. In most suites of diamonds studied so far, the HDFs span a range of compositions along one of the arrays only, but in Koffiefontein (RSA) and Diavik mine (Canada), where most HDFs fall along the carbonatitic-saline array, some diamonds containing carbonatitic–silicic compositions are also found. Klein-BenDavid et al. (2007) suggested that the three end-members are genetically linked and that the silicic and saline HDFs evolved from carbonatitic ones by crystallization and liquid immiscibility. Safonov et al. (2007a) experimentally confirmed the existence of such immiscibility. However, they suggested that upon cooling, the two immiscible melts evolve towards a single carbonatitic composition.
The HDFs in a few Siberian diamonds are highly carbonatitic, with MgO > 17 wt.% and MgO/CaO > 1, greater than in all other examined suites of diamonds (Klein-BenDavid et al., this issue, Zedgenizov et al., 2007, Zedgenizov et al., this issue). This new type of HDF, designated high-Mg carbonatites, falls in the compositional gap between the previously recognised low-Mg carbonatitic end-member and the bulk composition of kimberlites.
The individual microinclusions in a single diamond span a range of compositions that reflects variation in the chemistry of the trapped fluids. However, in most cases, radial (core to rim) variation of the HDF composition are limited and appear random. In most such diamonds, the standard deviation (1σ) of the major oxides is less than 10–20% (and rarely up to 30%) and the diamonds are regarded as homogenous. To date, only two diamonds with a significant sharp radial change in composition have been described (Klein-BenDavid et al., 2004, Shiryaev et al., 2005). In both cases, variations in cathodoluminescence (CL) and carbon isotopic ratios were also observed, suggesting introduction of a fresh HDF with limited mixing.
Carbon- and hydrogen-bearing fluids have been commonly associated with mantle metasomatism. In the case of diamond growth, sinusoidal rare earth element (REE) patterns of many peridotitic garnet inclusions (Harris et al., 2004, Promprated et al., 2004, Stachel et al., 2004) and rare occurrence of magnesite (Wang et al., 1996), phlogopite (Sobolev et al., 1997) and yimengite (Bulanova et al., 2004) inclusions indicate the involvement of fluids rich in incompatible elements. Indeed, the trapped HDFs contain water and carbonate and have elevated levels of K, Na, Rb, Cs, Ba, Zr, Hf, Ta, Th, U and LREE compared to primitive mantle and chondritic values (Akagi and Masuda, 1988, Schrauder et al., 1996, Rege et al., 2005, Tomlinson et al., 2005, Zedgenizov et al., 2007). Due to the high mobility and chemical reactivity of those fluids, they may act as an efficient agent for mantle metasomatism.
Akagi and Masuda (1988) and Schrauder et al. (1996) showed that the REE patterns of the HDFs are similar to those of kimberlites and proposed a genetic relation at depth. The requirement for an enriched source for kimberlite formation (Dalton and Presnall, 1998, Le Roex et al., 2003, Brey et al., 2008), together with petrologic and geochemical evidence connecting kimberlites and mantle metasomatism (Greenwood et al., 1999, Kamenetsky et al., 2004, Van Achterbergh et al., 2004), suggest close relations between metasomatic fluids, low-volume magmas (kimberlites, carbonatites) and some episodes of diamond formation.
Here we report major- and trace-element data for a suite of seven microinclusion-bearing diamonds from the Kankan district in Guinea. We recorded the radial chemical changes of the trapped HDFs and discuss their evolution in light of previously suggested models. Combining our data and recent results on diamonds from other sources, we propose a new model for the formation and evolution of the HDFs, as well as a relationship between their composition and the chemistry of garnet inclusions found in other Kankan diamonds (Stachel et al., 2000).
Section snippets
Samples
Seven diamonds from Kankan, Guinea (Table S1) were laser-cut and polished on both sides to create thin slabs that permit light transmittance. They were then cleaned ultrasonically in HF 60% and HNO3 69% for 2 h and washed with distilled water and ethanol before analysis.
Six of the diamonds are coated. Their octahedral cores are clear and their bright CL reveals concentric octahedral growth. The coats of three diamonds consist of a white uniform inner zone rich in inclusions, surrounded by a
FTIR
Analyses were made using a Bruker IRscope II microscope coupled to a Nicolet 740 FTIR spectrometer (Globar source, KBr beamsplitter, MCT detector, He–Ne laser). Spectra were taken in the range of 550–4000 cm− 1 with resolution of 4 cm− 1. Nitrogen concentration and aggregation states were determined using a computer program supplied by D. Fisher (DTC Research, Maidenhead) and the absorption coefficients of A centres (double substitution of carbon by two nitrogen atoms, IaA type spectrum), B centres
The diamonds
The octahedral cores of all diamonds carry 1000–2000 ppm nitrogen, mostly in A centres and 5–30% in B centres, corresponding to temperatures of 1100–1200 °C for residence time of 1000 to 10 My, respectively (Taylor et al., 1990, Leahy and Taylor, 1997). The coats carry only A centres (770–1100 ppm), but absorption peaks at 1344 and 1130 cm− 1 indicate 80–260 ppm nitrogen in C centres near the rims of diamonds ON-KAN-381, 382 and 383 (Figs. 2 and S1). All diamond spectra have the 3107 cm− 1 band
Diamond structure and nitrogen aggregation
The octahedral cores of the diamonds are clear and show fine layer-by-layer growth in CL images (Fig. 1b, d). All carry nitrogen in A and B centres, indicating long residence times in the mantle.
The fibrous coats of the high-Mg carbonatitic diamonds consist of a microinclusion-rich inner part and a clear outer part (Fig. 1a) suggesting transition from initial fast growth to slower rates during the formation of the rims (e.g. Sunagawa, 1981, Sunagawa, 1984). The uniform CL intensity and nitrogen
Conclusions
The nitrogen in Kankan diamonds resides in A and B centres in the cores, only A centres in the coats and A + C centres in their rims, indicating multiple growth events that were significantly separated in time. Mineral inclusions and nitrogen aggregation state indicate that cores grew at ∼ 5 GPa and 1100–1200 °C. The shift of quartz IR bands in the microinclusions indicates similar pressure during coat formation.
The contrasting trends of evolution revealed by the variation of HDF composition from
Acknowledgements
We thank Tamar Shalev, Suzy Elhlou and Norman Pearson for their help with the analyses. Research was funded by BSF grant #2004161 to Oded Navon and ARC Linkage and Discovery grants to S.Y. O'Reilly and W.L. Griffin. Analytical data were obtained using instrumentation funded by ARC LIEF, and DEST Systemic Infrastructure Grants and Macquarie University. Reference minerals were supplied by the Hebrew University Mineral collection. Y.W. thanks support of the Kameron foundation for his travel to
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