Title: The chemostratigraphy and petrology of Mississippian volcanogenic clay bands (tuffs) in the Irish Midlands and their relation to potential volcanic centers
Researchers: Hilde Koch, Prof. Balz Kamber
In this study, I will conduct a comparative analysis of the geochemical composition of different altered volcanic ashes (bentonites) to assess whether sufficient variation exists to identify individual bentonites on the basis of their geochemical ‘fingerprints’. The aim is to geochemically characterise the Carboniferous bentonites in the Irish Midlands and to establish the correct stratigraphic position of beds. As a result, it should be possible to produce a list of volcanic ash horizons that can be used for chemostratigraphic correlation, enhancing our understanding of the timing of faulting and mineralisation. In addition to this, an attempt will be made to link these bentonite layers to their potentially related volcanic centres. The applied method is high-precision trace element geochemistry. The altered volcanic ash layers occur within the ore-hosting carbonates of the Irish Midlands. Therefore, Sr isotopes and trace elements of hosting carbonates will also be analysed to examine whether they can be used as a secondary stratigraphic tool to test the results from the bentonite layers. Diagenetic overprint of carbonates is one of the most significant impediments to reading the geochemical and isotopic record of the seawater from which carbonate formed. To get pristine seawater signatures, much progress has been made in terms of minimising sample volume and using micro-drilling devices to target specific, original areas in a sample, such as from plaquettes adjacent to thin sections. However, drilling still extracts material from well below the thin section surface. Therefore, the use of extensive sequential leaching steps is proposed in order to minimise contributions from non-carbonate minerals. The other opportunity to improve the sampling, as well as the whole process, is to develop a technique where samples will be extracted by using the laser ablation unit as a milling device (this study). The main objectives of this project can be split into two work packages; The tuff and the carbonate work package. The main objective of the tuff work package is to use high-precision trace element geochemistry to develop a chemical stratigraphy for volcanogenic clay bands in the Irish Midlands and to test their genetic relationship, if any, to proposed volcanic centres. Special emphasis will be placed on the stratigraphic potential of clay bands in the wider Navan area with implications for timing of faulting and mineralisation, while the carbonate work package focuses on the development of a new laser based sampling technique to enhance the use of carbonates as stratigraphic markers. As both work packages centre around the theme of stratigraphy, the results from both these studies can complement one another.