Ilmenite Oxybarometry and Its Application for the Forecasting the Diamondiferous and Barren Kimberlites
摘要
The P–T–fO2 formation conditions and calculated compositions of fluids are estimated using Ilm–(Ol) oxybarometry and thermobarometry of the ilmenite xenocrysts in diamond-bearing and barren kimberlites of the Zimny Bereg region of the East European diamondiferous province (EEP). Our studies show that optimal conditions for diamond growth correspond to mostly aqueous calculated fluids with the lowest content of free atomic carbon. The C‒H‒O fluid compositions calculated from picroilmenite for highly and moderately diamondiferous kimberlites of the Zimny Bereg region correspond to low carbon and oxygen atomic content in the fluid, whereas the compositions of poor and barren kimberlites of the EEP North correspond to the low hydrogen and high carbon and oxygen atomic content. In comparison with previous methods of estimation of redox formation conditions of Ilm–(Ol) xenocrysts, the new method allows the correlation of their compositions with diamond potential of rocks. The method was also tested on Ilm–(Ol) inclusions in Kasai diamonds. The calculations showed the correspondence of C‒H‒O compositions with optimal conditions of their preservation. The method can be used in a complex of forecasting and prospecting works, which are targeted to the identification of diamond potential of primary sources based on the interpretation of compositions of picroilmenite from kimberlites and terrigenous rocks.