Electrical conductivities from laboratory measurements and geophysical data are often at odds, since common crustal silicates are significantly more insulating than are crustal conductivities determined by geophysical models. It has been suggested that the electrical conductivity of these rocks is in part controlled by intergranular C. The nature of this C is poorly known. Electron optical, diffraction, and spectroscopic studies reveal heterogeneous materials, with chemical and structural variations at the nanometer scale. The origin of the intergranular C is also in dispute. The intergranular C contains a host of elements, most notably Cl. It is unknown whether any of these elements are associated with graphite intercalation compounds (GIC), separate discrete mineral inclusions, or as organic compounds. The proposed study will focus on the chemistry and mineralogy of grain-boundary C in mantle xenoliths from alkali basalts, kimberlites, and samples recovered from the German continental scientific drilling site (KTB). Particular attention will be directed towards determining the local chemical composition of the C, with the view of identifying the hosts of the associated elements. It is necessary to ascertain the presence of GICs in the intergranular C because of their significant physical and chemical properties. Understanding the structure-property relationships of intergranular C requires a knowledge of the microstructure, including a quantitative appreciation of chemical bonding and its spatial distribution. Structural information will be revealed by high resolution transmission electron microscopy (HRTEM). The proposal describes studies that rely on the high-energy resolution and brightness of a TEM equipped with a field-emission gun (FEG) electron source and the high sensitivity of the new Gatan 766 DigiPEELS spectrometer. This experimental setup provides nanometer-scale, trace element, analytical capabilities. Electron energy-loss spectroscopy (EELS) with a TEM is an established spectroscopic technique that provides qualitative and quantitative chemical information. EELS is a highly sensitive microanalytical tool, and offers an advantage over energy dispersive x-ray spectroscopy (EDXS) for analyzing light elements.
