Title: NCIG Platform development
Researchers: Dr Eszter Badenszki, Prof. Stephen Daly
Laser ablation analytical methods such as Pb isotopes in K-feldspar, Hf isotopes in zircon, are in routine use at the National Centre for Isotope Geochemistry (NCIG) at UCD. They contribute to a steady flow of articles in major journals (e.g., Badenszki et al., 2018; Flowerdew et al., 2013; Gagnevin et al., 2017; Kenny et al., 2017; Zhang et al., 2014, 2016, 2017) and conference contributions (Badenszki et al. 2017; Daly et al. 2016a, 2016b, 2017). To optimise the scientific potential of the NCIG equipment, we intend to set up a laser ablation split-stream system (LASS). In a LASS, the laser is coupled to two instruments allowing two distinct types of geochemical analysis to be undertaken simultaneously on the same sample volume, hence increasing the spatial resolution. Hitherto, it was necessary to do this work sequentially with the risk that the first method used might consume so much material that a second type of analysis could not be carried out. The main scientific challenge of this project is assessing the involvement of the deep crust, using Irish deep crustal xenoliths, in the formation of the Irish Carboniferous-hosted Pb-Zn deposits. Although Ireland is a world-class Zn province, the source of the metals is still contradictory. Shallow (<10 km depth) hydrothermal circulation has been proposed by Wilkinson & Hitzman (2015). However, several lines of evidence (e.g., Davidheiser-Kroll et al., 2014; Elliott, 2015; Hnatyshin et al., 2015) point to mantle and lower crustal involvement. Another important scientific effort relates to the lab’s involvement in multi-proxy sedimentary provenance studies. These contribute to the iCRAG Energy Resources Challenge and are also undertaken in collaboration with Guangzhou Institute of Geochemistry in the investigation of sediment budgets in continental-scale river systems such as the Yangtze system (Zhang et al., 2014, 2016, 2017) in relation to plate dynamics and Plesitocene to Holocene climate change. Time-wise, a significant part of my work involves setting up and testing lab instrumentation to ensure optimum data quality for iCRAG and external lab users. Most lab users require ongoing training and most lab sessions require intervention to maintain data quality.
Isotopic and elemental analysis that incorporates laser ablation split-stream sample introduction method is relatively new (Kylander-Clark et al. 2013). Using this method makes possible to collect different geochemical data (e.g., U-Pb geochronology and Hf isotopic ratios) from a single analytical spot hence makes analytical sessions more efficient in terms of time, spatial resolution and amount of used sample. This technique is not yet widely used in the geochemical community and having this setup would significantly increase the reputation/visibility of the NCIG labs at UCD. The Irish Zinc-Lead orefield is uniquely favoured to evaluate the scale of hydrothermal circulation because of the presence there of deep lower crustal xenoliths at four widely separated localities. These were carried to the surface from ~22-28km (and deeper levels) by Carboniferous volcanic activity. They provide the only possible direct samples of the lower crust and are of appropriate age. Isotopic data from the xenoliths correspond closely to those measured in minerals occurring within the ores, e.g., calcite and galena. While Zn contents of the xenoliths permit them to be metal sources, their mineralogy and texture provide an enriched template and a plausible extraction mechanism. Analysis of biotite and garnet in the xenoliths show significant enrichment in Zn (and other relevant metals) as well as order of magnitude depletion of Zn during alteration, providing a metal-release mechanism and pointing to a hydrothermal fluid system operating at least to depths of ~ 25km, much deeper than previously suspected. In collaboration with Dr Zengjie Zhang (Guangzhou Institute of Geochemistry), we are investigating sediment sources and sediment transport in the Yangtze River from Pliocene to Holocene times. The origin and evolution of this continental-scale drainage system has been debated for over a century. Using geochemical proxies, especially the Pb isotopic composition of detrital K-feldspar, we are evaluating the sediment contributions of the major tributaries, assessing major episodes of river capture and drainage linkages, evaluating the timing of major erosional events (such as the cutting through of the Three Gorges) and placing constraints on the initiation of the modern east-ward flowing Yangtze system in relation to the tectonic uplift of Tibet and associated climatic effects.
1. To develop methods for laser-ablation analysis at the National Centre for Isotope Geochemistry at UCD, which contributes to the Geochemistry Enabling Methodology within iCRAG. Applications include Pb isotopic analysis in feldspar, Hf and U-Pb isotopic analyses in zircon, and trace element analysis of these and other, mainly silicate, minerals. These techniques contribute primarily to the Raw Materials and Energy Security challenges of iCRAG. 2. To develop a laser ablation split-stream system (LASS) for simultaneous elemental and isotope ratio analysis using a QICP-MS and MC-ICP-MS Neptune instrument coupled with an excimer laser ablation system. 3. Train and assist external users in the laser ablation methods (iCRAG PhD students from NUIG, iCRAG postdocs from NUIG and TCD as well as external visitors). Time estimates are as follows: instrument set-up (usually half a day per session), training (1 day/new visitor) and routine assistance to users (at least two hours a day if all goes well). 4. Carry out administrative work associated with the day to day running of the laboratory and the research program. 5. To investigating deep crustal xenoliths and to determine if the deep crust within the Iapetus Suture Zone can have acted as a metal source for the Carboniferous-hosted Zn-Pb orefield, which sits above it.