Mineralogy and geochemistry of the Peterborough Member, Oxford Clay Formation, Jurassic, UK: element fractionation during mudrock sedimentation

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Abstract
X-ray diffraction, electron microprobe and SEM analyses show that the Peterborough Member of the Oxford Clay Formation is composed mainly of quartz and illite, with lesser amounts of chlorite, kaolinite, K-feldspar, plagioclase, very small amounts of anatase and apatite, and variable amounts of calcite and subordinate siderite. The geochemistry, including major and a range of trace elements are used, with the mineralogy, to demonstrate the existence of several distinct components which have contributed to variations within the Peterborough Member. These components are (1) quartz, zircon and possibly other heavy minerals and K-feldspars, (2) carbonates, (3) phosphatic material, (4) organic material and (5) clays. Geochemical comparisons are made with various average shales demonstrating abnormally low Ba, Cu, CO, Mn, Na and Mg in the Peterborough Member. It is suggested that the current mineral assemblage is mostly detrital material, with little diagenetic alteration. The demonstration of fractionation between hydrodynamically heavy and light grains, even in an apparently fine grained sediment such as the Oxford Clay, has disturbing consequences for geochemical studies of sediment source areas where small numbers of small samples are employed.