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Home Page  Researchers Oded Navon
Oded NavonInstitute of Earth Sciences Nano-inclusions in diamonds: The formation of diamonds and kimberlites in natureMy nanoscience research focuses on the study of nano-size inclusions in natural diamonds. Such inclusions are found in diamonds of fibrous texture. Each diamond carries millions of inclusions that are ~100 nm in size. The inclusions trapped the fluids from which the diamond grew at depth of 150-220 km below the surface. Now the fluid cooled and crystallized a multi-phase assemblage of minerals. Analysis of the fluids provides invaluable data on the composition of melts deep in the Earth's mantle. In a paper published in 2011 we compare the trapped fluids to melts that erupt from such depth ? kimberlites. We argue that the similarity in the trace element composition argue for a common source at depth and provides clues on the composition of kimberlitic magmas at depth. A second paper examines the ascent of such melt and the third is more technical, about the IR spectrum of water in the residual fluid left after the growth of mineral phases trapped fluids. Last year we completed three projects: Elements in high-Mg carbonatitic melts in diamond and kimberlites The trace elements of high-Mg carbonatitic high-density fluids (HDFs) trapped in six fibrous diamonds from Siberia exhibit patterns that are highly similar to those of Group I kimberlites, but are slightly more fractionated. The patterns of both are similar to the average pattern of post-Archaean xenoliths from the sub-continental lithospheric mantle (SCLM). The Siberian high-Mg carbonatitic HDFs are highly enriched in incompatible elements and have compositions comparable to those of high-Mg HDFs from Kankan, Guinea. However, in detail the latter show depletion of K, Rb, Cs, Nb and Ta and enrichment in Ba, Th, U and LREE relative to the Siberian HDFs. These differences correspond closely to those between the patterns of Group II and Group I kimberlites, respectively. Mixing, fractionation and melting were explored as possible scenarios to explain these similarities and to constrain the possible genetic relationships between HDFs, kimberlites and the SCLM. High-Mg HDFs and kimberlites can be produced by melting of a common source. The pattern of the calculated source for Siberian HDF and Group I kimberlites resembles that of average post-Archean, rather than Archean, SCLM. Batch melting of such a source can produce high-Mg HDFs at 0.5% partial melting and Group I kimberlites at ~2%. Kankan HDFs and Group II kimberlites can be produced by 0.1 and 0.8% melting of average Archaean SCLM that carries phlogopite +/- Fe-Ti oxides.
Figure 1: Melting an average Archaean xenoliths with composition that is similar to the average post-Archaean SCLM closely reproduce the pattern of Udachnaya high-Mg HDFs and Group I kimberlites. The close correspondence between the trace-element composition of surface kimberlites and HDFs that were trapped at depth indicates that kimberlitic melts do not change their incompatible trace element contents much on their way to the surface (except for a possible loss of alkalis). The new data on the HDFs suggest a close genetic relation between high-Mg carbonatitic HDFs and kimberlites and reveals the similarity of the trace element of both to that of the post-Archaean SCLM. This similarity may reflect the interaction of such melts with the lithospheric keel, its melting to produce HDF and/or kimberlites or melting of deeper sources that led to formation of HDFs and kimberlite and to widespread metasomatism of the SCLM. The ascent of kimberlitic magma The rate of propagation of dykes is controlled by the rate of the fracturing at the tip and by the flow rate of magma inside the dyke. When high energy is needed to fracture the host rock and magma viscosity is low, the rate of propagation is controlled by the rate of fracturing (fracture controlled regime).When the energy to fracture the host rock is low, propagation is controlled by the magma flow rate (magma-controlled regime). We study the transition between these regimes for the case of a constant magma vesicularity and constant mass of gas in the cap. In the fracture-controlled regime the propagation rate only weakly depends on the amount of the gas in the gas cap, whereas at the magma-controlled regime it is significantly enhanced with increase the mass of gas at the cap. The gas pressure in the cap opens the dyke in front of the magma and allows magma flow rates that are significantly higher than predicted by models that ignore the gas cap. The maximum propagation rate is obtained at the transition between the fracture- and magma-controlled regimes. If the gas mass in the gas cap is high enough, a gas pocket can separate from the magma as a distinct unconnected pocket and propagate as a gas-filled crack at a constant velocity. Pressure decreases during ascent leads to higher vesicularity and faster gas filtration through the magma and into a gas cap. Gradual increase of the mass of gas in the cap is important in accelerating the propagation rate of dykes. The IR spectrum of water trapped in diamonds Infrared (IR) spectra of 55 microinclusion-bearing diamonds, combined with the bulk major-element chemistry of the trapped high-density fluids (HDFs) and spectroscopic and mineralogical data on the daughter phases in the microinclusions, were used to investigate the various shapes of IR absorption of water in fibrous diamonds and to decipher their origin. The difference in shape is most prominent for the OH-stretching mode at ~3400 cm-1. Low-Mg carbonatitic to silicic compositions show similar spectrum to that of pure water. Their extra width and high 3600 cm-1/3400 cm-1 ratio is attributed to absorption by daughter mica phases. High-Mg carbonatitic HDFs are characterized by a much wider OH-stretching mode, which extends towards lower energies. We attribute this wide band to absorption by hydrated K-bearing carbonates. This is supported by: 1) Positive correlation between the K2O content in these HDF compositions and the width of the OH-stretching band. 2) Correlated reduction of the low-energy absorption of the OH-stretching band and of the absorbance of carbonates with increasing temperatures. In diamonds carrying saline HDFs, the width of the OH-stretching band is narrower than that of pure water. The similarity in width and shape to spectra of salt-solvated water and the little change in the band shape with increasing temperature suggest that most of the water in saline microinclusions might be solvated in the daughter salts. This allows the identification of saline HDFs in diamonds by IR spectroscopy even though it is insensitive to chlorides. The variation in the shape of the OH-stretching band of water and the quantification of the absorption of the various daughter mineral phases in the microinclusions allows semi-quantitative determination of the composition of fluids trapped in diamonds.
Figure 2: High-resolution SEM image of an open microinclusion in a fibrous diamond.
Figure 3: IR spectrum of the OH-stretching mode of the various HDFs compared with that of pure water. Specific research topics related to Nanoscience and Nanotechnology:
List of publications in Nanoscience and Nanotechnology (2011-2012)
Five most significant publications and number of citations:
Success storiesUnderstanding the formation of diamonds in natureCooperation with other universities in Israel:Within Hebrew University:
Research grants:
Students, postdocs and researchers:
Staff scientists: Omri Dvir |
