USING GEOCHEMISTRY AND MINERALOGY IN THE SOURCE-DETERMINATION OF SHALE-HOSTED MIOCENE CONCRETIONS

Studies on carbonate concretions have established the prevalent model for the structure and growth of carbonate concretions, while the internal structure and mode of growth have not been fully understood. In this research, we conducted a detailed mineralogical investigation using a Transmission Electron Microscopy (TEM) to seek an understanding of the growth of the carbonate minerals and determine the sources of materials in these concretions. The samples studied were collected from Miocene–age Mudstones (Astoria Formation) located along Young’s River a few miles south of Astoria, Oregon. The concretions enclose a nucleus that vary in size and shape, and typically have the shape of slender, elongate to rhombic prisms. The concretions have three distinct regions, an outer thin layer that we labelled ‘crust’, and inner-middle layer that we labelled ‘mantle’ and an internal prismatic layer labelled ‘core’. To understand mineral growth of the pseudomorphs and to constraint the processes involved, a detailed mineralogical investigation at a much smaller scale is required. The zonation of the concretions into a core, mantle and crust have been suggested to be as a result of a progressive growth with one layer added to the outer edge of a growing concretion. As the concentric rings continue to grow, the changing chemistry in the progressive zones (core, mantle and crust) could be a reflection of the changing pore-water chemistry during their precipitation. High resolution TEM allows the visualization at nono-scale of pathways resulting in primary crystals, and by looking at increasingly smaller scales, processes occurring in microenvironments can be unraveled. TEM-Backscattered electron (TEM-BSE) images and the corresponding TEM-Energy dispersive spectroscopy (TEM-EDS) analyses showed that there are several precipitates dominated by Fe, Mn, Cu, Co, Al, Si, K and S in the various zones of the outward growing concretions. Investigation of the pseudomorphs using TEM showed that although the materials are composed entirely of calcite, there is also the mineralization of secondary minerals that are rich in Fe, Cu, Si, Al and other heavy metals such as Mn and Co. The core is dominated by Fe and Cu rich secondary minerals, whereas, the mantle and crust are rich in secondary minerals dominated by Si, Cu, and Al.