31 research outputs found

    Ultra-trace element characterization of the central Ottawa River basin using a rapid, flexible, and low-volume ICP-MS method

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    Ultra-trace (<1 ng g-1) rare earth elements and yttrium (REE+Y) and high field strength element (HFSE) geochemistry of freshwater can constrain element sources, aqueous processes in hydrologic catchments, and the signature of dissolved terrestrial fluxes to the oceans. This study details an adapted method capable of quantifying ≥38 elements (including all REE+Y, Nb, Ta, Zr, Hf, Mo, W, Th, U) with minimal sample preparation in natural water aliquots as low as ≤2 mL. The method precision and accuracy are demonstrated using measurement of the National Research Council – Conseil national de recherches Canada (NRC-CNRC) river water certified reference material (CRM) SLRS-6 sampled from the Ottawa River (OR). Data from SLRS CRM are compared to those of new, filtered (HREE-enriched REE+Y patterns, small natural positive Y and Gd anomalies, and negative Eu and Ce anomalies. These REE+Y features are coherent downstream in the OR apart from amplification of Eu and Ce anomalies during REE removal/dilution. The OR samples capture a downstream decrease in sparingly soluble HFSE (Th, Nb, Ta, Zr, Hf), presumably related to their colloid-particulate removal from the dissolved load, accompanied by crustal Zr/Hf (32.5 ± 5.1) and supercrustal Nb/Ta (25.1 ± 7.7) ratios. Subcrustal Th/U (0.17-0.96) and supercrustal Mo/W (12.0-74.5) ratios in all ORB waters indicate preferential release and aqueous solubility of U>Th and Mo>W, with the latter attributed primarily to preferential W adsorption on soil or upstream aquatic (oxy)(hydr)oxide surfaces

    Pervasively anoxic surface conditions at the onset of the Great Oxidation Event: new multi-proxy constraints from the Cooper Lake paleosol

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    Oceanic element inventories derived from marine sedimentary rocks place important constraints on oxidative continental weathering in deep time, but there remains a scarcity in complementary observations directly from continental sedimentary reservoirs. This study focuses on better defining continental weathering conditions near the Archean-Proterozoic boundary through the multi-proxy (major and ultra-trace element, Fe and Cr stable isotopes, μ-XRF elemental mapping, and detrital zircon U-Pb geochronology) investigation of the ca. 2.45 billion year old (giga annum, Ga) Cooper Lake paleosol (saprolith), developed on a sediment-hosted mafic dike within the Huronian Supergroup (Ontario, Canada). Throughout the variably altered Cooper Lake saprolith, ratios of immobile elements (Nb, Ta, Zr, Hf, Th, Al, Ti) are constant, indicating a uniform pre-alteration dike composition, lack of extreme pH weathering conditions, and no major influence from ligand-rich fluids during weathering or burial metasomatism/metamorphism. The loss of Mg, Fe, Na, Sr, and Li, a signature of albite and ferromagnesian silicate weathering, increases towards the top of the preserved profile (unconformity) and dike margins. Coupled bulk rock behaviour of Fe-Mg-Mn and co-localization of Fe- Mn in clay minerals (predominantly chlorite) indicates these elements were solubilized primarily in their divalent state without Fe/Mn-oxide formation. A lack of a Ce anomaly and immobility of Mo, V, and Cr further support pervasively anoxic weathering conditions. Subtle U enrichment is the only geochemical evidence, if primary, that could be consistent with oxidative element mobilization. The leaching of ferromagnesian silicates was accompanied by variable mobility and depletion of transition metals with a relative depletion order of Fe≈Mg≈Zn\u3eNi\u3eCo\u3eCu (Cu being significantly influenced by secondary sulfide formation). Mild enrichment of heavy Fe isotopes (δ56/54Fe from 0.169 to 0.492 ‰) correlating with Fe depletion in the saprolith indicates loss of isotopically light aqueous Fe(II). Minor REE+Y fractionation with increasing alteration intensity, including a decreasing Eu anomaly and Y/Ho ratio, is attributed to albite breakdown and preferential scavenging of HREE\u3eY by clay minerals, respectively. Younger metasomatism resulted in the addition of several elements (K, Rb, Cs, Be, Tl, Ba, Sn, In, W), partly or wholly obscuring their earlier paleo-weathering trends. The behavior of Cr at Cooper Lake can help test previous hypotheses of an enhanced, low pH-driven continental weathering flux of Cr(III) to marine reservoirs between ca. 2.48-2.32 Ga and the utility of the stable Cr isotope proxy of Mn-oxide induced Cr(III) oxidation. Synchrotron μ- XRF maps and invariant Cr/Nb ratios reveal complete immobility of Cr despite its distribution amongst both clay-rich groundmass and Fe-Ti oxides. Assuming a pH-dependent, continental source of Cr(III) to marine basins, the Cr immobility at Cooper Lake indicates either that signatures of acidic surface waters were localized to uppermost and typically unpreserved regolith horizons or were geographically restricted to acid-generating point sources. However, in given detrital pyrite preservation in fluvial sequences overlying the paleosol, we propose that the oxidative sulphide corrosion required to drive surface pH(δ53/52Cr: -0.321 ± 0.038 ‰, 2sd, n=34) that cannot be linked to Cr(III) oxidation and is instead interpreted to have a magmatic origin. The combined chemical signatures and continued preservation of detrital pyrite/uraninite indicate low atmospheric O2 during weathering at ca. 2.45 Ga preserved in the rift-related sedimentary rocks of the Lower Huronian. The aqueous flux from the reduced weathering of mafic rocks was characterized by a greater abundance of transition metals (Fe, Mn, Zn, Co, Ni) with isotopically light Fe(II), as well as higher Eu/Eu* and Y/Ho. In most models of Precambrian ocean element inventories, hydrothermal fluids are viewed as the main supplier of several metals (e.g., Fe, Zn), although the results herein suggest that a riverine metal supply may have been substantial and that using Eu-excess as a strict proxy for hydrothermal flux may be misleading in near-shore marine sedimentary environments

    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

    Oxidative mafic rock weathering through geological time

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    THESIS 10939Oxidative chemical weathering requires the presence of free oxygen in the Earth\u27s atmosphere. Reconstructing the terrestrial oxidation history in the ancient rock record has helped to detect the intrinsic links between atmospheric composition and biological evolution of oxygenic photosynthesis. This thesis contributes to this theme via the study of oxidative chemical weathering at different stages of Earth\u27s history and at different weathering intensities. It presents detailed geochemical investigations of three profiles developed on mafic rock. Laterite (Bidar) and saprolite (Chhindwara) profiles hosted on Deccan Traps basalt (India) capture weathering in the Phanerozoic, while the ca. 1.85 Ga Flin Flon paleosol (Canada) hosted on greenstone captures weathering after the Great Oxidation Event (GOE, ca. 2.4-2.3 Ga)

    An estimate of 1.9 Ga mantle depletion using the high-field-strength elements and Nd-Pb isotopes of ocean floor basalts, Flin Flon Belt, Canada

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    The 1.9 Ga Flin Flon Belt is one of the best preserved Paleoproterozoic mafic volcanic provinces, hosting chemically and geologically distinguishable ocean floor and arc basalts. The ocean floor basalts, previously interpreted to have predominantly erupted in a back-arc basin, apparently tapped N-MORB-, E-MORB-, and OIB-like mantle sources. These ocean floor basalts are revisited in this study with a new major element, high-precision trace element, and isotopic (Nd and Pb) data set (similar to 60 samples). The main impetus behind the study was to combine trace element, particularly Nb/Th, and isotopic compositions to better approximate the contemporaneous depleted mantle, so as to establish more precisely the degree of depletion in the Paleoproterozoic. Through systematic examination of the basalt chemistry, samples influenced by subduction zone processes (low Nb/Th and Ta/W) were isolated first. The Ta/W ratio of the ocean floor basalts is higher than the primitive mantle value and provides strong evidence that the depletion imprint due to the preferential extraction of W in subduction zones was well established by the Paleoproterozoic. Next, a group of unexpectedly high Nb/Ta and high Nb/Th basalts was identified, whose chemical trends (e.g., anti-correlated Nb/Th and LOI) are best explained by post-depositional metamorphic dehydration, resulting in mobility of Th > Ta > Nb. The chemistries of the remaining, least subduction and dehydration modified, samples provide evidence of mixing between long-term (ca. 450 Ma), isotopically distinct mantle reservoirs (Delta + 3 epsilon(Nd) units) with relatively consistent Nb/Th ratios of 12.5-13.5. The average Nb/Th ratio of 13.0 +/- 0.9 for these ocean floor basalts is more precise, and lower, than previous estimates for the 1.9 Ga depleted mantle (similar to 14.4), which included samples that had suffered preferential Th > Nb loss. The magnitude of mantle depletion required by the new Nb/Th ratio of 13 still justifies significant net crustal growth from the Neoarchean (Nb/Th c. 11.1) to the Paleoproterozoic. When the Archean and Paleoproterozoic Th-Nb evidence is combined with insight from Pb and Nd isotopes, it appears that the mass of the depleted portion of the mantle increased between 2.6 and 2.0 Ga. Namely, the data are best explained with a model in which less depleted (lower?) mantle material was mixed into the quite strongly depleted post-Archean upper asthenosphere. This could possibly indicate the onset and establishment of whole mantle convection. (C) 2011 Elsevier B.V. All rights reserved

    The Flin Flon paleosol revisited

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    A ca. 1.85 Ga paleosol developed on seafloor-altered mafic rocks is preserved beneath continental sandstones and conglomerates of the Missi Group near Flin Flon, Manitoba. Previous work has focused on a locality where the paleosol is developed on pillowed greenstone (chlorite + albite + epidote + calcite + quartz + Fe-Ti oxides +/- mica). This area has been revisited and a new (similar to 5 m) profile was excavated and sampled at a very high density for geochemical (major element; similar to 60 samples) and mineralogical (petrography, X-ray diffraction) investigation. The high sampling density has revealed a relatively sharp immobile-element offset (e.g., Al/Ti) in the profile. This could reflect weathering across two separate parent rocks or preferential addition of allochthonous material into the top of the profile. Regardless, comparison with a wide suite of volcanic rocks in the stratigraphy suggests that the original concentration of Fe and probably also Ca and Mg in the greenstone on either side of the boundary was similar, permitting quantitative treatment of major-element trends. The profile shows pronounced upward loss of Ca, Mg, Mn2+, and Fe2+ and a corresponding increase in Al, Ti, and total Fe. Physical weathering features, such as corestones, are well preserved and show progressive alteration upwards in the profile in parallel with the chemical trends. Deep in the profile, Na and Ca are lower than the greenstone due to the weathering of albite and epidote, whereas Mg, Fe, and Mn were largely retained in chlorite and secondary dolomite. Upwards in the profile, the loss of chlorite and dolomite and the increase of illite - mixed-layer clays and white mica (probably 2:1 layer or mixed-layer clays originally) and hematite are accompanied by a transition from green to maroon colour. Above a well-defined depth in the profile, carbonate is virtually absent, and the paleosol is highly depleted in Ca. This transition in the paleosol coincides with the immobile-element offset. The significance of this coincidence remains to be explored. Reworking of regolith (e.g., rip-up clasts) occurred during deposition of the Missi sediments, and burial and subsequent metamorphism resulted in the upward enrichment of K and Na and the associated conversion of pedogenic clays to illite and micas (muscovite and paragonite). Crosscutting quartz and epidote veins and tourmaline formation near the unconformity contact implies minor post-burial fluid movement. Despite these complexities, there is consistent retention of Fe through formation of pedogenic Fe oxides, attested by the near-constant Sigma Fe/Al ratio in the profile. There is very little evidence for subsequent Fe reduction by hydrothermal or organic fluids. Instead, trends in the Fe speciation are controlled predominantly by the modal variation of Fe-Ti oxides in the greenstone and spheroidal weathering textures in the paleosol

    A combined Y/Ho, high field strength element (HFSE) and Nd isotope perspective on basalt weathering, Deccan Traps, India

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    High-precision high field strength element (HFSE: Zr, Hf, Nb, Ta, Th, U, W, Mo), Y/Ho, and Nd isotope chemostratigraphy of two contrasting Deccan Traps weathering profiles - an ancient, deeply weathered laterite, and a younger (Quaternary), more moderately weathered saprolite - are used to reconstruct different aspects of basalt weathering. Precision of the HFSE analyses is demonstrated through a report of the long-term concentrations and ratios determined in United States Geological Survey (USGS) and Geological Survey of Japan (GSJ) basalt rock standards (BHVO-1, BHVO-2, BIR-1, BCR-2, JB-2). The oxyanion-forming members (U, Mo, W) are the most mobile of the considered HFSE group. Extreme loss of W, far exceeding those of U and Mo during certain stages of basalt alteration, is first reported here. The ability to strongly fractionate Mo and W during weathering may contribute to solving the unresolved mass imbalance between the crustal and marine inventories of W. By contrast, Zr, Hf, Nb, and Ta preserve the ratios of the parent basalt in the profiles due to their limited mobility; these are therefore of great potential value in reconstructing basalt flow stratigraphy and correlating lava flows in weathered flood basalt provinces. Of the HFSE, Th is not a good choice as a conservative element because it is strongly susceptible to addition of aeolian dust; this is evidenced by strong excursions in Th/Nb that are correlatable with alkali element enrichment and less radiogenic Nd-143/Nd-144 ratios. The chemical fingerprints of dust were identified in a paleo-flow top of the saprolite profile, suggesting that dust accumulation occurred during periods of quiescence between basaltic eruptions. During protracted exposure and laterite development, the magnitude to which dust overprints the basalt chemistry increases substantially as evident from much less radiogenic Nd isotope ratios and higher Th/Nb ratios in the Bidar profile relative to the protolith basalt. Attempts at quantifying the magnitude of dust accumulation in the laterite based on Th enrichment indicate a mass fraction of greater than 0.5 when the dust is assumed to have the chemistry of average upper continental crust. Although mixing models between the basalt and assumed dust composition cannot unambiguously constrain the dust source, the Nd isotope mixture preserved in the laterite points to a relatively young crustal dust source (e.g., similar to loess in composition) rather than the Precambrian shield rocks in the vicinity of the Deccan Traps. The contrasting topologies of dust-derived Nd and dust-derived Th in the laterite appears to record both physical transport of dust (Th) through permeable laterite horizons as well as transport by chemical dissolution and precipitation (Nd) at an inferred paleo-water table and in deep saprolite zones. Yttrium and Ho fractionate substantially during all observed stages of weathering, with Y/Ho ratios ranging from 26.5 to 21.9 in the moderately weathered saprolite profile and from 30.2 to 14.7 in the laterite profile. The single strongly superchondritic Y/Ho ratio of 30.2 in the laterite is restricted to a sample at depth, and appears to fingerprint the deposition of REE derived from dissolution higher in the profile. Decrease in the Y/Ho ratio relative to the protolith basalt (24.4-24.7) in both profiles inversely correlates with chemical weathering indices, and suggests that Y/Ho ratios have significant potential as a silicate weathering proxy. Consequently, suspended vs. dissolved river loads may record the differing behaviour of these elements during weathering. (C) 2014 Elsevier B.V. All rights reserved

    Unusually heavy stable Mo isotope signatures of the Ottawa River: Causes and implications for global riverine Mo fluxes

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    The accurate use of Mo isotope mass balance modelling of ancient oceans relies on the assumption that the δ98Mo of modern riverine inputs represents a reasonable estimate of the past. A growing number of studies of global rivers have demonstrated significant variation in δ98Mo from the bedrock sources of Mo. The Ottawa River, Canada, was previously identified as having an anomalously heavy Mo isotope composition, with a δ98Mo signature close to seawater (2.3‰), for a seemingly pristine natural river. To further explore this unusual signature, we collected and filtered 29 water samples from the Ottawa River, tributaries, and small lakes, and analysed them for Mo isotopes as well as major and trace elements. Here, we fully document heavy δ98Mo signatures throughout the Ottawa River and its absence in surrounding areas of the wider Ottawa River basin (ORB). Our results reveal a progressive upstream increase in both Mo concentration and δ98Mo signatures in the Ottawa River towards values even heavier than seawater, up to 3.13‰ – the heaviest Mo isotopic signature of river water measured to date. In contrast, the tributaries and lakes display far lighter and more consistent δ98Mo signatures within the range previously found in other rivers. Weathering of an isotopically heavy bedrock source, fractionation during weathering and retention of light isotopes in soils have all been proposed as sources of heavy δ98Mo in rivers; however, none of these mechanisms can satisfactorily explain our new observations. Colloidal and particulate processes that remove elements downstream, as inferred from some trace element proxies, also cannot explain the decreasing δ98Mo, since the preferential removal of light Mo isotopes is predicted from these processes. Similarly, the downstream trends show no apparent relationships with constructed dams or known potential industrial sources. Therefore, our findings from the Ottawa River are best explained as the dilution of a yet unidentified point source of heavy Mo upstream of sampling, or a significant permanent sink for light isotopes existing only in the upper reaches of the catchment. In both cases, anthropogenic contribution from a large mining district in the headwaters of the river must be considered and should be explored further. Fractionation of Mo in waste rock storage facilities have been previously identified and may provide an unnatural sink for isotopically light Mo through the Rayleigh-type fractionation of dissolved Mo on oxyhydr(oxide) mineral surfaces. The implied anthropogenic alteration of the natural Mo cycle highlights the significant and wide-reaching effects of unnatural point sources of Mo on the cumulative δ98Mo signatures of the catchment, and emphasises the necessity for detailed geochemical screening of anomalous river water isotope signatures before natural isotope compositions are inferred

    Deep-water seep-related carbonate mounds in a Mesoproterozoic alkaline lake, Borden Basin (Nunavut, Canada)

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    The Mesoproterozoic (1.1 Ga) Borden Basin contains extremely large, deep-water dolostone seep mounds (Ikpiarjuk Formation) whose distribution is controlled by faults. Four mounds were investigated along measured stratigraphic sections. Petrographic study revealed several depositional components, and a mixture of at least two distinct carbonate sources. Stable isotope data showed no significant methane contribution to the carbonate phases. Detritus-corrected REE+Y patterns, obtained using solution ICPMS, depict binary mixtures between basin-water-derived precipitates and seep-fluid-derived carbonate. The purest pelagic REE+Y signal is from mound tops, suggesting that mound accumulation ceased when the seep fluid waned. The REE+Y pattern of the pelagic precipitates resembles that of modern alkaline lake water. The shale-normalised pattern of the basin water is LREE-depleted, has a positive Ce anomaly and pronounced Y-excess, but lacks the La and Gd anomalies typical of seawater. The seep-fluid-related dolomite has flat shale-normalised REE+Y patterns, no Ce anomaly, and a negative Eu anomaly. This combination of characteristics points to circum-neutral (Ca and Mg-bearing?) fluids that interacted with the underlying basement before seeping into the lake bottom through faults. The chemostratigraphic patterns of the mounds result from the relative contribution of elements from the basin water vs. seep fluids. When combined with published geochemical data for coeval black shale surrounding the mounds, the new data suggest a lacustrine setting, surrounded by catchments with a preponderance of moderately to strongly weathered alkali basalt whose runoff drove the lake to alkalinity. Seep fluid was probably evaporatively concentrated basin water that acquired new geochemical characteristics both during evaporation and through water-rock interaction in the subsurface. The new understanding of this depositional stage of the Borden Basin highlights the importance of lacustrine deposits in the Mesoproterozoic, and presents an obvious impediment to using carbonate stable isotope or trace element geochemistry to reconstruct global atmosphere-hydrosphere conditions for this time for any units that cannot be demonstrated conclusively to be of marine origin. (C) 2015 Elsevier B.V. All rights reserved
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