| dc.description.abstract | Some of the most famous diamond deposits are associated with Kimberly-type pyroclastic kimberlite facies (KPK) located within diatreme part of kimberlite pipes. The origin of this type of kimberlite is one of the most debatable topics in kimberlite geology. The two competing models suggest either 1) explosive pyroclastic eruption with subsequent welding of the pyroclasts or 2) in-situ magma fragmentation without the formation of a pyroclastic deposit. The latter model suggests that a reaction between silicate fragments of the country rock and carbonatitic magma exsolves CO2 causing the magma fragmentation and freezing. Furthermore, an extensive interaction between carbonate-rich kimberlite magma and silicate country rocks was recently proposed for Snap Lake kimberlite dyke. This experimental study tested the two hypotheses by examining the reaction of felsic, intermediate, and mafic xenoliths with four carbonate-rich kimberlite mixtures (AN-1 and LS-26) and carbonate compositions. The experiments are conducted in a box furnace and in a piston-cylinder apparatus at Dalhousie University at 800-1100°C and pressure of 0.1 MPa and ~500 MPa to explore the effect of temperature, CO2, and H2O on the textures and the sequence of the reaction minerals.
Experimental results show greater degree of reaction and assimilation of xenoliths in the absence of H2O than in H2O-bearing runs. Experiments conducted at 0.1 MPa showed greater degree of assimilation with the mafic and felsic xenoliths and the carbonate-rich mixture. In CaCO3-Na2CO3 mixture runs, excessive CO2 degassing ruptured the capsule due to over-pressure. H2O bearing runs showed minimal to no reactions of the intermediate and mafic rocks with the kimberlite mixture. Additionally, Mg-rich LS-26 runs showed formation of Mg- and Fe-oxides encompassed by impure carbonate matrix. The developed reaction mineral phases and their textures were compared to the textures of natural BK1 Orapa KPKs of Botswana, Snap Lake, and Gahcho Kue kimberlites. Diffusion and dissolution of CaO and Na2O components reflected in compositional gradients experienced by the xenoliths during experimental metasomatism. Perovskite was the most common and stable phase that crystallized in all runs conducted with AN-1 mixture. H2O appears to have repressed the degree to which CO2 would exsolve and react with the xenoliths.
Keywords: Kimberly-type pyroclastic kimberlites, diatreme zone, magmaclast, subvolcanic-fluidization model, metasomatism, perovskite, piston cylinder, diamonds.
Pages: 80
Supervisor: Yana Fedortchouk | en_US |
