12 research outputs found

    Trace element and Pb isotope fingerprinting of atmospheric pollution sources: A case study from the east coast of Ireland

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    Unravelling inputs of multiple air pollution sources and reconstructing their historic contribution can be a difficult task. Here, new trace metal concentrations and Pb isotope data were combined for a radionuclide (210Pb-241Am) dated peat core from the Liffey Head bog (LHB) in eastern Ireland in order to reconstruct how different sources contributed to the atmospheric pollution over the past century. Highest enrichments in the heavy metals Pb, Cu, Ag, Sn, and Sb, together with a Pb isotope composition (206Pb/204Pb: 18.351 ± 0.013; 206Pb/207Pb: 1.174 ± 0.012) close to that of the Wicklow mineralisation demonstrates significant aerial influx of heavy metals from local mining and smelting activities during the 19th century until ca. 1940s. A dramatic compositional shift defined by elevated Co, Cr, Ni, Mo, Zn, and V enrichments and a sharp transition towards unradiogenic 206Pb values (206Pb/204Pb: 18.271 ± 0.013–17.678 ± 0.006; 206Pb/207Pb: 1.170 ± 0.012–1.135 ± 0.007) is documented from the 1940s until ca. 2000. These are attributed to the atmospheric impact of fossil fuels and especially leaded petrol, modelled to have contributed between 6 and 78% to the total Pb pollution at this site. The subsequent turn to a more radiogenic Pb isotope signature since 2000 in Ireland is clearly documented in the investigated archive (206Pb/204Pb: 17.930 ± 0.006; 206Pb/207Pb: 1.148 ± 0.007) and reflects the abolishment of leaded petrol. However, there remains a persisting and even increasing pollution in Ni, Mo, Cu, and especially Zn, collectively originating from countrywide use of fossil fuels(peat, coal, heating oil, and unleaded vehicle fuels) for domestic and industrial purposes. This illustrates the continued anthropogenic influence on important natural archives such as bogs in Ireland despite the phase-out of leaded petrol

    Zinc isotope systematics in subduction zones - insights from Tonga arc lavas

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    International audienceTransition metal stable isotope systematics of terrestrial igneous reservoirs may help to better understand the recycling behavior of these elements through Earth’s interior. However, isotopic variations of ‘heavy’ elements caused by magmatic processes are subtle and difficult to resolve. Within this scope we have improved our precision on MC-ICP-MS Zn isotope measurements to 1) is amongst the most depleted for worldwide arc settings. Additional analyses of samples from IODP site 595 provide an insight to the input to this subduction factory. Primitive lavas (basalts and basaltic andesites; MgO>5 %) display subtle, yet resolvable Zn isotopic variations between the central Tonga islands Tofua, Late, Kao and Ata, as well as differences to 66/64Zn values of MORB and the subduction input. In addition, Zn-isotopes and Zn/La are correlated with fluid indices such as Ba/Th, suggesting potential mobilization of Zn from the subducting plate and/or overlying sediments into the mantle wedge prior to melting. Yet, negative correlations observed between Zn-isotopes and Sm/La of the lavas point towards a link between Zn-isotopic composition and relative mantle depletion beneath individual volcanic edifices. Concomitant occurrence of these features complicates a straighforward deconvolution of the Zn-isotope variations seen in arc lavas and fluid-iduced transport cannot be simply ruled out. In this presentation we will discuss and expand the Zn-isotope fractionation problematic in subduction settings aiming to address the following: i) what are the main mechanisms triggering Zn-isotope fractionation and what is the result of their combined effects? ii) can we identify and possibly quantify the subduction component from the Zn-isotope composition of the arc lavas? iii) what are the long-term effects of Zn recycling into the mantle on the evolution of the terestrial Zn signal

    Zinc isotope fractionation at destructive plate margins and potential implications for the global recycling signature

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    International audienceOur understanding regarding the behaviour of the fluid mobile element Zn at destructive plate margins is limited. In particular the fractionation mechanisms and input-output mass-balance remains to be investigated due to implications for the spatio-temporal cycling of this vital and socio-economically relevant element. In this study, we investigate the Zn isotope systematics of subduction input provided by IODP samples from the SW Pacific in comparison to lavas from the central Tonga arc, addressed as a worldwide endmember in terms of pre-subduction mantle wedge depletion. With an improved analytical precision, we report subtle, yet resolvable Zn isotope variations between the central Tongan islands, with an overall statistically relevant variation of 0.05‰ (at ±0.014‰ 2SD). The signatures are all > 0.1‰ lighter than the subduction input at this site, suggesting a fractionation process during subduction. After careful extraction of the isotopic effect caused by mantle melting processes (using DMM δ66/64Zn JMC-Lyon provided by Sossi et al. (2018) and Wang et al. (2017) and melt extraction indices such as Sm/La, Zr/Nb, and Zn/La), a pronounced negative correlation is observed between the Zn isotopic composition of the lavas and key fluid indicators such as Ba/Th and Ce/Pb. Together with predictions from ab initio calculations and mixing models performed between Indian DMM and Rayleigh dehydration of the subducting slab, we attribute the remaining, subtle Zn isotope variations to additions by Cl-rich fluids to the individual mantle wedges. A maximum of 5% chlorine-fluid contribution is suggested for the magmatic source of Tofua, whereas smaller proportions are estimated for Kao, Late and Ata. Overall, this study sheds new light on Zn fractionation mechanisms in sediment-poor subduction zones. Implications for the long-term Zn recycling will be addressed in this presentation.References:Sossi, P.A., Nebel, O., O'Neill, H.S.C., Moynier, F., 2018. Zinc isotope composition of the Earth and its behaviour during planetary accretion. Chemical Geology 447, 73-84.Wang, Z.-Z., Liu, S.-A., Liu, J., Huang, J., Xiao, Y., Chu, Z.-Y., Zhao, X.-M., Tang, L., 2017. Zinc isotope fractionation during mantle melting and constraints on the composition of Earth's upper mantle. Geochimica et Cosmochimica Acta 198, 151- 167
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