Rachel Milligan

          Fe-Ti oxides, chromite and ilmenite, are common minerals in kimberlite diamond-bearing kimberlites. They are brought to the surface during the eruption of kimberlitic magmas that are derived from the upper mantle. Previous studies have shown that, similar to diamonds, partial dissolution and interaction of Fe-Ti oxides with the kimberlite magma results in complex reaction rims and dissolution patterns. The nature of this interaction reflects both the chemical composition of the magma and fluid phases present. The goal of this study is to investigate the reactions that occur between chromite and ilmenite grains and the kimberlite melt, their implications for diamond preservation. The possible connections between resorption features and chemical composition are also investigated.

Chromite and ilmenite grains from two kimberlites in the Orapa cluster, Botswana, with different geological features were examined. Kimberlite A is a small pipe, filled with coherent kimberlite facies. Kimberlite B is larger and has two lobes filled with two different types of coherent kimberlite facies; the pipe also contains massive volcaniclastic and resedimented volcaniclastic facies. 75 grains were selected for examination of dissolution features under Scanning Electron Microscope: 20 chromites and 21 ilmenites from Kimberlite A and 10 chromites and 24 ilmenites from Kimberlite B. After the grains were imaged, they were mounted and polished to investigate reaction textures, zoning and reaction phases using Back Scatter Electron imaging, X-ray mapping and Wavelength Dispersive Spectroscopic analysis methods. Most of the chromite samples displayed rounded ovoid morphologies with oriented, euhedral, octahedral nodules. Very few of the imaged ilmenite grains display dissolution features, and most were surrounded with reaction phases such as perovskite and titanite. The results of the WDS analysis, BSE images and X-ray maps show that ilmenites from Kimberlite A show visible diffusive zonation and trending compositions. The grains have Mg-enriched, Fe-depleted rims indicative of a reduced kimberlite melt, with some reaction products (mostly perovskite) on the grain surface. Kimberlite B ilmenite grains have restricted compositions and are not visibly zoned. However, WDS analyses show a trend towards titanium-magnetite (depletion in Ti) around the rims of the ilmenite grains, as well as decreases of MgO and Cr2O3. This trend is more indicative of an oxidizing environment. Kimberlite B ilmenites also have large volumes of reaction products on the surface of the grains, both perovskite and titanite. In Kimberlite A, based on the volume of reaction products, ilmenite was likely closer to the liquidus composition than in Kimberlite B. Based on comparisons with experimentally produced surface features, Kimberlite A had a free fluid H2O phase, while Kimberlite B had less H2O, in a dissolved phase. Both kimberlites had low proportions of CO2 in a dissolved state. In both kimberlites there does not seem to be a correlation between the nature of dissolution features and the composition of the grains. The resorption features seen in Fe-Ti oxides are likely influenced by some other condition within the kimberlite, such as pressure, or temperature. It was determined that Kimberlite A, the simple kimberlite with free-fluid H2O and a reducing redox state, has a higher potential for diamond preservation than Kimberlite B.

Keywords: kimberlite, chromite, ilmenite, resorption, surface features
Pages: 94
Supervisor: Yana Fedortchuk