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Start date: 2020 × Subject: Isotope geochemistry × Subject: Basement geology ×

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    Exploring the source attribution of environmental Pb – development of a continental-scale Pb isotope regolith map of Australia
    Desem, Candan Ulgen ( 2021)
    The element Pb is a particularly useful tracer as it is both a toxin of environmental concern and is often associated with mineralisation. As a result, Pb concentration data are often collected as part of environmental contamination and mineral exploration studies. Pb isotopic analysis, however, offers an important advantage in discriminating between different sources of Pb. The Pb-isotopic signature of regolith is a function of contributions from bedrock geology (including mineralisation) and exogenous inputs such as windblown dust, groundwater and anthropogenic contamination (industry, mine wastes, paints, transport, agriculture, waste handling and residential inputs). Pb-isotopic analysis allows us to estimate the relative contributions from each of these components and therefore provides an important tracing tool in both mineral exploration and environmental contamination studies. The spatial distribution of Pb in the regolith profile is also a function of the source of Pb (exogenous versus endogenous), depth and grain-size providing further diagnostic tools. Finally, Pb isotopic signatures in the continental crust show large natural variations which are transferred to the regolith and thus provide bedrock signatures that can be easily distinguished. Naturally occurring Pb has four stable isotopes with the following crustal abundances: 204Pb (1.36%), 206Pb (25.42%), 207Pb (21.11%) and 208Pb (52.10%). 206Pb is the decay product of 238U, 207Pb is the decay product of 235U and 208Pb is the decay product of 232Th. In contrast 204Pb is non-radiogenic and is often used as a stable reference isotope relative to which the accumulation of the other radiogenic isotopes over time can be quantified. Importantly, in mineralised systems, once an ore rich in Pb (e.g., galena, PbS) forms, the isotopic composition of its contained Pb does not evolve any further, as it does not contain significant amounts of U or Th. The Pb signature is thus ‘frozen in’ to the ore and will reflect the time at which the Pb was incorporated. As a consequence of these geological processes most ore deposits, and the products derived from them (leaded petrol, paint etc) have distinct Pb isotopic signatures that enable ‘fingerprinting’ of anthropogenic Pb contamination in the environment. In order to determine whether Pb-isotope signatures in the regolith are anomalous, an understanding of background Pb-isotopic signatures is critical. The use of large Pb-isotope datasets, however, has until recently been limited due to constraints associated with the availability and accessibility of lower cost, high-precision and high-throughput analyses. The advent of instrumentation such as Sector-Field Inductively Coupled Plasma Mass Spectrometers (SC-SF-ICP-MS) allowing rapid Pb-isotope analysis, now enables us to revisit the utility of Pb isotopes in this regard. Exploring the use of this new technology in the Australian context is a direct aim of this research. To this end, a continental-scale Pb isotope regolith map of Australia has been produced as part of this study, using catchment outlet (~ floodplain sediment) samples from Geoscience Australia’s National Geochemical Survey of Australia (NGSA) dataset. The dataset contains samples from 1300 locations across Australia obtained at a sampling frequency of approximately 1 sample/5200 km2 and ultimately covering ca. 81% of the continent. The Pb-isotope map is based on the coarse grain-size fraction (less than 2mm) of the top outlet sediment (0 – 10 cm depth) samples. Leaching techniques have been employed to separate loosely bound Pb (e.g. anthropogenic contamination) from Pb structurally bound in minerals (i.e. from underlying geology, mineralisation, or their weathering products). An analytical framework has also been established to achieve ‘fit-for-purpose’ data quality, with the elimination of the Pb-separation chemistry step required by conventional Multicollector-Inductively Coupled Plasma-Mass Spectrometer (MC-ICP-MS) or Thermal Ionisation Mass Spectrometer (TIMS) analysis combined with the high-throughput required in order to process large sample sets (> 1500 samples in this case). A sequential leach protocol (‘P618’) originally developed at CSIRO (Carr et al., 2011) has been utilised in which an ammonium acetate leach is followed by an aqua-regia (HNO3-HCl) digest. Pb isotope signatures were acquired using a Nu Instruments Attom single-collector sector-field ICP-MS (SC-SF-ICP-MS) with data quality comparisons also made with MC-ICP-MS and Quadrupole-ICP-MS, in which this research has demonstrated that both precision and accuracy achieved on a SC-SF-ICP-MS is greater than that achievable on a Quad-ICP-MS. The aims of this program were to (i) compare Pb signatures released using the different leaches, (ii) examine whether soil Pb isotope signatures can identify bedrock geology and metallogenic provinces as well as (iii) investigate the presence and extent of anthropogenic contamination across the continent. The data, in particular across older terranes, reveal a dominant contribution of Pb to the regolith from the underlying geology with major crustal elements well defined. A variety of exogenous Pb inputs from point sources of pollution are also apparent. The continent-wide Pb isotope regolith map of Australia demonstrates that the top coarse aqua-regia digested fraction of regolith samples in Australia, are dominated by Pb from the catchment bedrock geology. The influence and prominence of bedrock geologic Pb signatures are more easily visible in older terranes, such as the Archaean Yilgarn Craton and Proterozoic North Australia Craton of Australia where radiogenic signatures have had more time to establish. Apparently meaningful Pb-Pb isochrons on these top coarse fraction regolith samples demonstrate not only the data quality obtained from the Attom SC-SF-ICP-MS but also that the dominant bedrock geologic Pb-isotope signature is carried to shallow depths within the regolith profile in these areas. Visibly distinct Pb-isotope patterns which correlate with the major crustal elements and geological regions of Australia, further support the idea that the Pb-isotope signatures at shallow depths across Australia are governed by large-scale Pb reservoirs. At the smaller scale, variation in grain-size and depth of Pb-isotope signatures provide information as to how the Pb-isotopic signature varies within the regolith profile. Throughout this study, variation in Pb isotopic signatures with depth and grain-size have been found to be more pronounced where there is a distinct exogenous Pb input. In areas where a significant exogenous Pb input has not been identified (by means of an anomalous Pb isotopic signature), it appears there is some level of homogenisation of Pb-isotopic signatures within the regolith profile. Finally, this analytical program has revealed areas of anomalous Pb isotopic signatures in the regolith which would be useful targets for future investigation as the source of these signatures has not yet been determined.