Oxidation States of Mantle Minerals

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9903735Buseck The mantle comprises the largest fraction, by mass and by volume, of the Earth. Geologic processes within the mantle profoundly influence major features (ocean basins, mountains) at Earth's surface. In spite of its importance, all knowledge of the mantle is from indirect geophysical measurements or from small, rare samples. Diamonds and associated minerals are among the most prominent of these samples and are major source of information about the mantle. An important variable is their oxidation state, but this has been difficult to determine. We plan to measure the oxidation state of iron-bearing mantle minerals such as garnets, pyroxenes, and spinels, many of which occur as tiny inclusions within diamonds, in order to determine their ferric/ferrous ratios, which we will then use to calculate the oxidation state of the mantle. We will use an electron-beam-based analytical technique called electron energy-loss spectroscopy (EELS) for the measurements. EELS is done with a transmission electron microscope, which means measurements can be made on extremely small samples or regions of samples, a major benefit when dealing with, for example, inclusions in diamonds. A major goal will be defining the limits of EELS at the nanometer scale by using a series of well-characterized mantle minerals and comparing the technique to results from other methods that utilize coarser crystals. The results will be more precise and accurate analyses plus new insights into the oxidation state of the mantle.