Jeffrey R. Chiarenzelli

Oxygen isotope geochemistry of zircon

The high-temperature and small sample size of an I.R. laser system has allowed the first detailed study of oxygen isotope ratios in zircon. Low-magnetism zircons that have grown during metamorphism in the Adirondack Mts., N.Y. preserve primary δ18O values and low-magnetism igneous zircons are likewise primary, showing no significant affect due to subsequent granulite facies metamorphism. The measured fractionation between zircon and garnet is Δ(Gt-Zrc) = 0.0 ± 0.2‰(1σ) for most low-magnetism zircons in meta-igneous rocks. The consistency of this value indicates equilibration at temperatures of 700–1100°C and little or no change in the equilibrium fractionation over this temperature range. In contrast, detrital low-magnetism zircons in quartzite preserve igneous compositions, up to 4‰ out of equilibrium with host quartz, in spite of granulite facies metamorphism. The oxygen isotope composition of zircon can be linked to UPb ages and can ‘see though’ metamorphism, providing a new tool for deciphering complex igneous, metamorphic and hydrothermal histories. Zircons separated by magnetic susceptibility show a consistent correlation. Low-magnetism zircons have the lowest uranium contents, the most concordant UPb isotopic compositions, and primary δ18O values. In contrast, high-magnetism zircons are up to 2‰ lower in δ18O than low-magnetism zircons from the same rock. The resetting of oxygen isotope ratios in high-magnetism zircons is caused by radiation damage which creates microfractures and enhances isotopic exchange. Zircons from the metamorphosed anorthosite-mangerite-charnockite-granite (AMCG) suite of the Adirondacks have previously been dated (1125–1157 Ma) and classified as igneous, metamorphic or disturbed based on their physical and UPb isotopic characteristics. Low-magnetism zircons from the AMCG suite have high, nearly constant values of δ18O that average 8.1 ± 0.4‰(1σ) for samples ranging from 39 to 75 wt% SiO2. Only olivine metagabbros have lower average values (6.4‰), consistent with the hypothesis that they represent nearly pristine samples of the anorthosites parent magma. Whole-rock values of δ18O are also high in the AMCG suite and increase with SiO2 content, as predicted for a process of assimilation and fractional crystallization. Taken together, these data suggest that the elevated values of oxygen isotope ratios result from partial melting and contamination involving metasediments in the deep crust, before the crystallization of zircon. More normal values elsewhere in the Grenville Province record deep-seated, pre-1150 Ma regional differences.

Precambrian clastic sedimentation systems

Abstract The unique and evolving nature of the Precambrian geological environment in many ways was responsible for significant differences between Precambrian clastic sedimentary deposits and their Phanerozoic-modern equivalents. Some form of plate tectonics, with rapid microplate collisions and concomitant volcanic activity, is inferred to have led to the formation of greenstone belts. Explosive volcanism promoted common gravity-flow deposits within terrestrial greenstone settings, with braided alluvial, wave/storm-related and tidal coastline sediments also being preserved. Late Archaean accretion of greenstone terranes led to emergence of proto-cratons, where cratonic and rift sedimentary assemblages developed, and these became widespread in the Proterozoic as cratonic plates stabilised. Carbonate deposition was restricted by the paucity of stable Archaean terranes. An Early Precambrian atmosphere characterised by greenhouse gases, including CO2, in conjunction with a faster rotation of the Earth and reduced albedo, provide a solution to the faint young Sun paradox. As emergent continental crust developed, volcanic additions of CO2 became balanced by withdrawal due to weathering and a developing Palaeoproterozoic microbial biomass. The reduction in CO2, and the photosynthetic production of O2, led to aerobic conditions probably being achieved by about 2 Ga. Oceanic growth was allied to atmospheric development, with approximately 90% of current ocean volume being reached by about 4 Ga. Warm Archaean and warm, moist Palaeoproterozoic palaeoclimates appear to have become more arid after about 2.3 Ga. The 2.4–2.3 Ga Huronian glaciation event was probably related to continental growth, supercontinent assembly and weathering-related CO2 reduction. Despite many analogous features among both Precambrian and younger sedimentary deposits, there appear to be major differences as well. Two pertinent examples are rare unequivocal aeolian deposits prior to about 1.8 Ga and an apparent scarcity of Precambrian foreshore deposits, particularly those related to barrier island systems. The significance of these differences is hard to evaluate, particularly with the reduced palaeoenvironmental resolution because of the absence of invertebrate and plant fossils within Precambrian successions. The latter factor also poses difficulties for the discrimination of Precambrian lacustrine and shallow marine deposits. The temporal distribution of aeolian deposits probably reflects a number of possible factors, including few exposed late Archaean–Palaeoproterozoic cratonic areas, extensive pre-vegetative fluvial systems, Precambrian supercontinents and a different atmosphere. Alternatively, the scarcity of aeolian deposits prior to 1.8 Ga may merely reflect non-recognition or non-preservation. Precambrian shallow marine environments may have been subjected to more uniform circulation systems than those interpreted from the Phanerozoic-modern rock record, and Precambrian shelves probably were broad with gentle seaward slopes, in contrast to the narrow, steep shelves mostly observed in present settings. Poorly confined Precambrian tidal channels formed sheet sandstones, easily confused with fluvial or offshore sand sheets. Epeiric seas were possibly more prevalent in the Precambrian, but active tectonism as proto-continents emerged and amalgamated to form early supercontinents, in conjunction with a lack of sufficient chronological data in the rock record, make it difficult to resolve the relative importance of eustatic and tectonic influences in forming epeiric embayments and seaways. Other differences in Precambrian palaeoenvironments are more easily reconstructed. Ancient delta plain channels were probably braided, and much thicker preserved delta successions in the Precambrian are compatible with the inferred more active tectonic conditions. Pre-vegetational alluvial channel systems were almost certainly braided as well. Common fluvial quartz arenites are ascribed to differences in weathering processes, which probably changed significantly through the Precambrian, or to sediment recycling. Although Precambrian glacigenic environments were probably the least different from younger equivalents, their genesis appears to reflect a complex interplay of factors unique to the Precambrian Earth. These include emergence and amalgamation of proto-continents, the early CO2-rich atmosphere, the development of stromatolitic carbonate platforms, early weathering, faster rotation of the Earth and the possible role of changes in the inclination of the Earths axis.

Multi-element and rare earth element composition of lichens, mosses, and vascular plants from the Central Barrenlands, Nunavut, Canada

Abstract Lichen (n=12) and moss (n=6) species from a remote region of northern Canada have remarkably similar multi-element patterns suggesting they are non-specific accumulators of metals under existing conditions. Within individual species the concentration of many metals analyzed range over an order of magnitude. Many elements have a positive correlation with multi-element (n=48) and REE (rare earth element) totals. Others, such as Cd, K, and Zn have relatively consistent concentrations across all lichen and moss species, and across all sampling sites, indicating different accumulation and/or retention processes. Lichens and mosses have REE concentrations 1–3 orders of magnitude less than those of the average upper continental crust (UCC) but yield identical patterns. The correlation of other poorly soluble elements and key elemental ratios in lichen and moss are also similar to UCC and modern river sediment values. Metals including Sc, V, Cr, Fe, Co, Ga, Y, Hf, W, Pb, Th, and U show strong positive correlations with REE in lichen and moss. Rare earth elements may be useful as reference elements in environmental studies because of transport in the particulate phase, lack of significant anthropogenic sources, coherent group geochemistry, generally robust concentrations, and upper crustal signatures. Further, the REE may be helpful in identifying particulate deposition related to anthropogenic activities and enrichment of other elements by biogenic processes. The multi-element compositions of vascular plants (leaves and twigs) are fundamentally different from those of lichen and moss, lack correlation with REE, and are extremely enriched for many elements (100–1000× average upper continental crust) relative to the REE; perhaps because of limited REE solubility and transport via root systems. Enrichment factors for most metals of environmental concern are low; Pb is elevated but may be an artifact of low concentrations in local bedrock. Trace metal concentrations in lichen and moss at Otter Lake are similar to those measured across the Northwest Territories over 25 a ago.

Retention of arsenic on hydrous ferric oxides generated by electrochemical peroxidation

Electrochemical peroxidation (ECP), an emerging remediation technology, with direct electric current applied to steel electrode and small addition of H2O2, was used to remove As(III) from contaminated aqueous solutions. Bench scale experiments were conducted to evaluate the sorption and distribution of arsenic between the soluble and solid state hydrous ferric oxides (HFO) formed as part of the ECP process. ECP was effective in removing arsenic from the aqueous solution, with >98% of the applied As(III) adsorbed on HFO. Removal was complete within 3 min of ECP treatment and apparently independent of the initial pH of the water (3.5-9.5). In the absence of H2O2 more As(III) was adsorbed by solid state iron at pH 9.5 than at 3.5 (2600 vs. 1750 microg l(-1)). Thus H2O2 was crucial to oxidize As(III) to As(V) which is more strongly retained by HFO. Removal of As was not significantly affected by the concentration of H2O2 or by current processing time. The optimal operating conditions were pH < 6.5, H2O2 concentration of 10 mg l(-1) and current process time not exceeding 3 min. X-ray diffraction (XRD), diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy and transmission electron microscopy (TEM) were applied to study the HFO deposits. The XRD data indicated the prevalence of poorly ordered Fe minerals in the suspended ECP solids with a dominance of 5 line ferrihydrite in the absence of H2O2. At pH 3.5 and with 100 mg H2O2 l(-1), akaganeite was formed, whereas an incipient hematitic phase, reflection at 0.39 nm, occurred at pH 6.5. DRIFT data indicate that both As(III) and As(V) were specifically adsorbed onto HFO at acid and neutral pH. TEM observations indicated the presence of spherical shape ferrihydrite and provided evidence for possible formation of subrounded hematite and acicular shape goethite.

Initiation of ∼2.45-2.1 Ga intracratonic basin sedimentation of the Hurwitz Group, Keewatin Hinterland, Northwest territories, Canada

Continental to shallow-marine rocks of the Hurwitz Group, exposed as outliers across the Hearne Province in northern Canada, were deposited between ∼2.45 and 2.11 Ga on a 3.3-2.6 Ga granite-greenstone basement. Initiation of the Hurwitz Basin was characterized by low relief and rates of subsidence that never progressed beyond the intensity of regional sagging. During stage 1, polymictic conglomerate (lower Noomut Formation) was deposited in palaeovalleys cut into Archaean basement, and subarkose to quartz arenite ± quartz pebble conglomerate (upper Noomut Formation) was deposited on a low-relief sand plain. Abundant first-cycle quartz-rich sandstones are attributed to intense humid climate weathering in low-relief source areas and at sites of temporary storage, as well as to repeated fluvial and eolian reworking. Interbedding of pyritic palaeoplacers and continental redbeds denotes fluctuating levels of atmospheric oxygen; disappearance of pyritic palaeoplacers upsection signifies ultimate changeover to an atmosphere with sustained free oxygen. Stage 2 (Padlei Formation) was marked by a climatic shift toward cold (glacial?) conditions as indicated by dropstones, greywacke (till?) pellets and possible ikaite pseudomorphs in lacustrine rhythmites. Stage 3 (Kinga Formation) represents a return to wet/warm conditions with fluvial (± eolian) deposition of subarkose and quartz arenite on a low-relief sand plain (Maguse Member) and deposition of supermature quartz arenite in a vast (100,000 km2 minimum), shallow-water lake or series of lakes (Whiterock Member). Local chert beds at the top of the Kinga Formation (Hawk Hill Member) formed by spring discharge due to regional, gravity-driven, convective groundwater flow. An intracratonic setting is implied by: (1) a symmetric, basin-centred fill; (2) absence of features indicating a shelf-slope break; (3) absence of rift rocks; (4) a prolonged slow-subsidence history; (5) likely correlation with other continental to shallow-marine deposits scattered across the Keewatin Hinterland; and (6) positioning 1000–2000 km away from closest coeval passive margin sequences. We speculate that Hurwitz Basin formed due to lithospheric stretching related to a large-scale mantle upwelling during the protracted breakup of an earliest Palaeoproterozoic supercontinent (‘Kenorland’).

Stratigraphy, sedimentology and physical volcanology of the Henik Group, central Ennadai-Rankin greenstone belt, Northwest Territories, Canada: late Archean paleogeography of the Hearne Province and tectonic implications

Abstract Extensive areas of the Hearne and Rae provinces are underlain by Archean metavolcanic and metasedimentary rocks. In the Rae Province, supracrustal sequences with prominent continental to shallow-water quartz arenites have been interpreted by others as rift and passive margin deposits. In the Hearne Province to the southeast, the Ennadai-Rankin greenstone belt is exposed in an area ca. 700 × 200 km, forming the second largest greenstone belt in Canada. It comprises mainly greenschist-facies-grade mafic and felsic volcanic, siliciclastic, chemogenic and volcaniclastic rocks. Stratigraphic, sedimentologic and physical volcanologic data based on 1:50,000-scale mapping in the central part of the belt indicate deposition in a subaqueous mafic volcanic plateau-slope-basin system that was well removed from a continental influence. Water depth was below storm wave base as indicated by preservation of delicate lamination in pelitic and iron-formation units, absence of features suggestive of subaerial exposure, oscillatory flow or tidal currents, and lack of interfingering rocks that could represent laterally adjacent shelf, coastal or continental environments. Reconnaissance-level regional geologic and geochronologic data indicate an overall pattern of supracrustal deposition in the Hearne Province consisting of a large mafic volcanic plain (± isolated felsic volcanic centres) with lenses and belts of deep-water sedimentary rocks. At least two speculative, yet testable, tectonic models are consistent with the inferred paleogeography. For both, the Rae Province is considered to be the western continental hinterland to the ensimatic Hearne Province. The Rae and Hearne provinces may record late Archean continental extension, sea floor spreading, oceanic volcanic arc growth, diachronous back-arc spreading and collapse, and arc-continent collision, with thrusting of oceanic rocks on to attenuated Rae crust. Alternatively, the Hearne Province may represent northwestward accretion of multiple volcanic arc-trench pairs.

Iron-mediated reactions of polychlorinated biphenyls in electrochemical peroxidation process (ECP).

A study was conducted to explore some of the basic processes of polychlorinated biphenyl (PCB) destruction by a new technology termed electrochemical peroxidation process (ECP). ECP represents an enhancement of the classic Fenton reaction (H2O2 + Fe2+) in which iron is electrochemically generated by steel electrodes. Focus was on the extent of adsorption of a mixture of Aroclor 1248 on steel electrodes in comparison to iron filings. Commercially available zero-valent iron filings rapidly adsorbed PCBs from an aqueous solution of Aroclor 1248. Within 4 h, all the PCBs were adsorbed at 1%, 5%, and 10% Fe0 (w/v) concentrations. Little difference in adsorption was found between acidic (2.3) and unamended solutions (pH 5.5), even though significant differences in iron oxidation state and Fe2+ concentrations were measured in solution. PCB adsorption also occurs on steel electrodes regardless of the pH or electric current applied (AC or DC), suggesting the combination of oxidizing (free radical-mediated reactions) and reducing (dechlorination reactions) iron-mediated degradation pathways may be possible. Extraction of the iron powder after 48 h of contact time yielded the progressive recovery of biphenyl with increasing Fe mass(from 0.4% to 3.5%) and changes of the PCB congener-specific pattern as a consequence of dechlorination. A variety of daughter congeners similar to those accumulated during anaerobic microbial dechlorination of Aroclor 1248 in contaminated sediments indicate preferential removal of meta- and para-chlorines.

Volatilization of polychlorinated biphenyls from sediment during drying at ambient conditions

Air drying sediment from a Federal Superfund Site for 24 hours at ambient conditions (T=20°C; RH=25%) resulted in PCB volatilization losses of 14–23%, with 80–90% of the total loss occurring within the first eight hours. A sample dried for 24 hours lost 21.3% of PCBs via volatilization and an additional 7.5% during a second 24 hour drying period after rewetting. PCB loss was positively correlated with water loss (R2 > .99). Sediment samples (∼0.25 grams) suspended in 100 milliliters of double deionized water that was allowed to evaporate over a 7 day period lost 74–76% of their initial PCB concentration. Lower orthochlorinated congeners volatilized preferentially. These data indicate PCB loss through volatilization during drying at ambient conditions is more significant than previously recognized, particularly for sediments that have undergone extensive anaerobic microbial PCB dechlorination.

Heavy metals in lichens, southern district of keewatin, Northwest Territories, Canada

A suite of 12 saxicolous lichen species were collected from a remote site in the south-central District of Keewatin, and analyzed for As, Cd, Cr, Cu, Ni, Pb, Sb, V, and Zn. Concentrations of these metals are comparable to results from elsewhere in the NWT obtained up to 28 years ago, and from remote sites in northern Quebec, Alaska, Greenland and northern Finland. Heavy metal concentrations of the lichens were compared to that of 7 immediate substrates. Substrate is not a significant source of metals to the lichens because: 1) correlation between lichen and substrate chemistry is generally poor; and 2) metal concentrations in lichen species growing on a substrate lacking measurable concentrations of trace metals are analytically indistinguishable from the same species growing on enriched substrates. Interspecies calibration ratios, calculated with respect to C. stellaris, decrease from crustose to foliose to fruiticose varieties, indicating that the crustose types are the most efficient accumulators and/or retainers of heavy metals.

Photodecomposition of PCBs absorbed on sediment and industrial waste: implications for photocatalytic treatment of contaminated solids

PCBs in contaminated sediment and soil have been photodegraded in slurry suspension with TiO2 and simulated solar illumination without pre-treatment or amendments. Degradation of 81% of the PCBs in St. Lawrence River sediment was achieved in a 24 h irradiation period. Replicate experiments using five times as much sediment and half the light intensity yielded similar results. The observed degradation rates of individual PCB congeners correspond with gas chromatograph retention time and chlorine number. Lesser and ortho chlorinated PCBs are preferentially photodecomposed. The solubility of individual congeners controls the observed rate of decomposition by regulation of the transfer of PCBs to the aqueous phase and thus effectively retards access to the catalyst (anatase). The rate and degree of photodegradation can be enhanced by the use of lower wavelength ultraviolet light which leads to hydroxylation of PCB congeners increasing solubility and accessibility to photocatalytic reactions. Photodecomposition of PCBs in a slurry composed of furnace ash, core sands, and aluminum-rich (>45% Al2O3) slag from an aluminum foundry was increased from 45 to 88% during a 24 h irradiation period by using low wattage UVC lamps instead of solar equivalent illumination.