Title: Development of novel sediment tracers: validation and application of conventional and new provenance proxies
Researchers: Dr Aoife Blowick and Dr Shane Tyrrell
Research Purpose and the underlying need for this research
An ever increasing number of detrital mineral grains are used as indicators of provenance, particularly heavy minerals such as zircon, rutile and apatite. More recently, the utility of framework components such as feldspar are being assessed as sand tracking tools. However, a number of issues exist in the application of provenance tools including: (a) Inadequate validation and rigorous testing of new provenance proxies; to warrant its use, a provenance approach must provide new insight not provided by an existing provenance tool and must be rigorously tested to establish the robustness of the proxy in the sedimentary environment. (b) Variable resilence in the sedimentary environment; reliance on extremely robust mineral grains such as zircon, which can survive multiple sedimentary cycles, results in the inability to distinguish first-cycle sedimentary delivery from poly-cyclic detritus. In turn, this means that unless all possible sources of intermediary grains (i.e. second cycle or more) are assessed (which in most cases is impossible) then an accurate sediment supply chain cannot be produced. (c) Source fertility bias; the propensity of a source rock to supply a specific mineral grain type has a first-order control on the provenance proxies released to a basin, and must be considered for every provenance application. A source rock with a high concentration of a specific mineral may release more grains of that specific mineral per unit volume than another source rock with a lesser concentration. As a result, grains supplied from a source in which they are underrepresented may become heavily diluted by the larger volumes of the same grain type from a more fertile source, even though both sources may have been supplying equal amounts. Smaller populations, which may bear critical provenance implications, may therefore be overshadowed or lost. This research aims to advance sand tracking capabilities through the development and rigorous testing of integrated, multi-proxy provenance approaches in parallel with identifying and mitigating the fundamental issues associated with modern provenance approaches, most notably recent framework approaches such as Pb in K-feldspar.
The project involves two main stands:
Strand 1: Validation of Pb isotopic fingerprinting of Plagioclase Strand 1 involves the validation of the use of the Pb isotopic composition of plagioclase as a sand tracking tool. This research extends, refines and adapts analytical capabilities previously validated for Pb isotopic fingerprinting of K-feldspar for characterising lower Pb contents typically observed in plagioclase. Plagioclase will be a particularly useful addition to the provenance toolkit arsenal, as (1) plagioclase has a different stability in the sedimentary environment to K-feldspar and other commonly used heavy mineral phases, (2) it is often sourced from rocks of basic composition which lack more felsic derived minerals such as K-feldspar and zircon and thus do not “show up” as contributing components in the detrital record, and (3) it is an important detrital component of active margin basins. The work involves the set-up of analytical protocols, including the use of an ion counter collector array for small volume ablations (as well as the collection of trace and major elements using quadrapole mass spectrometry) and the assessment of robustness of the Pb isotopic composition by testing plagioclase compositions in modern active margin basins with unambiguous sourcing. Building on this validation, the application of plagioclase alongside K-feldspar and more conventional heavy mineral suites will be completed to test its utility as a tracer of unique sources.
Strand 2: Assessment and Refinement of current provenance approaches Strand 2 comprises a number of linked yet independent studies in order to evaluate some of the fundamental assumptions of currently used provenance approaches, in particular recently established approaches such as Pb in K-feldspar. Studies include: (a) Investigation of feldspar weathering: Laboratory experiments which mimic natural weathering conditions along significant time scales (12+ months) and varying fluid pH conditions are used to simulate the chemical weathering of feldspars, in order to assess the reliability of the Pb isotopic composition of K-feldspar and plagioclase, and assess these processes on grain morphology. These experiments will demonstrate any perturbations in Pb isotopic composition caused by acidic weathering, underpinning the viability of Pb isotopic fingerprinting as a sand tracking tool. Complimentary investigations into feldspar weathering in the sedimentary environment include analysis of feldspar grains (both plagioclase and K-feldspar) in soil pedons above feldspar rich bedrock in all major climate regimes. By sampling grains systematically with increasing depth, in-situ depth profiles of Pb isotopic compositions are constructed, demonstrating any perturbations in Pb signals. (b) Investigation of population recognition, type and applications: With increasing numbers of Pb in K-feldspar studies, characteristic trends in Pb isotopic data are identifiable but standardised protocols for the identification of statistically robust grain populations has yet to outlined, often hampered by the inability to effectively visualise three-dimensional data in two-dimensional space. Using both case studies and newly acquired datasets with variable sample numbers (and numbers of grains analysed per sample) standard steps for population recognition will be established, in parallel with statistical tests outlined below. Assessment of unique data trends, such as linear arrays, will be assessed to establish potential source discrimination relationships. (c) Statistical analysis of novel approaches: Building upon previous statistical validation of conventional petrographic and heavy mineral analyses, statistical analysis of Pb isotopic fingerprinting of K-feldspar and plagioclase, including Monte Carlo simulations, are used to establish statistically robust guidelines for grain numbers required for studies.
The main hypothesis of the project builds upon the use of Pb isotopes in K-feldspar as a sand tracking tool. Pb isotopic fingerprinting of detrital K-feldspar grains has been shown to be a powerful way to track the sources and pathways of sands. However, as outlined above the mineralogical fertility of source rocks directly controls the utility of each provenance marker. K-feldspar is commonly sourced from rocks of felsic compositions and thus cannot be used to trace sediment contributions from more mafic source rocks (including many types of volcanic rocks). Plagioclase provides an alternative yet complementary way to trace these underrepresented source rocks, as it is commonly sourced from more mafic rocks and like K-feldspar, is unlikely to survive multiple sedimentary cycles, making it an important first-cycle detrital component in active margin basins. As such, if it can be shown that (i) the Pb isotopic composition of detrital plagioclase grains matches that of its source rock, and (ii) the Pb isotopic composition of detrital plagioclase grains remains unchanged by weathering, erosion, transport, burial and diagenesis (i.e. through a sedimentary cycle), Pb isotopic fingerprinting of plagioclase can be used to fill the potentially significant “mafic” gaps in current sand tracking capabilities, making it a vital component of the provenance toolkit.
Objective(s) and aims: The overall aim of the “Sediment Tracking” targeted project is to move toward first-order quantitative modelling of sedimentary systems and their detrital products. This specific post-doctoral project aims to help achieve this through the development and rigorous testing of integrated, multi-proxy provenance approaches in parallel with identifying and quantifying the various factors which may modify sand during and after transport, and to deal with the types of fundamental provenance-related issues outlined below. This will allow a wider range of trace and major components of sands/sandstones to be tracked and should ultimately help bring about the desired output of any provenance study – its use as a real predictive tool for both reservoir sandstone distribution and quality. This project links with two other projects based at NUIG, namely TP4.1PhD1, which will attempt to identify and assess the impact of sediment modification processes which can occur prior to final deposition; and TP4.1PhD2, which will assess post-depositional, provenance-related controls on reservoir sandstone quality. Part of this work is therefore to help supervise and provide technical expertise to these PhD projects while driving the advancement of novel analytical approaches to provenance research. IN addition – this project integrates with research in the broad HC4.1 targeted project and with other TPs in the hydrocarbons spoke as well as workers in the platform (NCIG).