Pb isotopic investigation of aircraft-sampled emissions from the Horne smelter (Rouyn, Québec): Implications for atmospheric pollution in northeastern North America

This study reports metal abundances and Pb isotope ratios of emissions from the Horne copper smelter, and ambient (ground level) atmosphere at Rouyn, Québec retrieved during 2-week field studies in the winter and summer seasons of 2000. Plume aerosols were sampled during horizontal and vertical passes using a DHC-6 Twin Otter aircraft, which typically tracked the plume up to 30 km from the stack. Samples of the ‘background’ atmospheric conditions were taken simultaneously from a ground site meteorological station located 2.5 km in the predominant upwind direction from the stack.Overall, metal concentrations (ng/m³ of As, Cd, Cu, Pb, Se, Zn) for the in-plume samples are elevated by one or more orders of magnitude over those measured in the ground site samples. The Pb isotope compositions for the in-plume samples are extremely variable (²⁰⁶Pb/²⁰⁷Pb range from ∼1.120 to ∼1.170) but define well-constrained linear arrays in conventional Pb-Pb isotope plots. Correlations between metal concentrations (e.g. As/Pb, Cu/Pb, Zn/Pb) and the Pb isotope data are indicative of binary mixing between two anthropogenic end members; most probably imported industrial Pb found in recycled materials, and nonradiogenic Pb derived from Archean sulfide ore deposits. Pb isotope data from the winter ground site (‘background’) samples indicate that the ground level atmosphere at Rouyn contains a mixture of Pb emitted from the Horne smelter and long-range anthropogenic Pb transported from Eurasian pollution sources. Compared to the Pb isotope composition of epiphytic lichens, snowpack, and precipitations collected in 1990 to 1999 from adjacent geographic regions, the Pb isotope results obtained here suggest that aerosols emitted from the Horne smelter are dispersed into northeastern North America. However, future studies dealing with point source apportionment for this region of the globe will be rendered difficult due to the processing at the Horne smelter of recycled material containing industrial Pb from various sources.     

Sm-Nd, Lu-Hf and Pb-Pb signatures of gneisses and granitoids from the La Grande belt: extent of late Archean crustal recycling in the northeastern Superior Province, Canada

Sm-Nd, Lu-Hf and Pb-Pb isotopic signatures were determined for gneisses and felsic plutons of the La Grande granite-greenstone and the Nemiscau metasedimentary belts of Canada. The northern part of the La Grande belt exposes gneisses of the Langelier complex formed between ca. 2.8 and 2.9 Ga. The gneisses yielded εNd_(T) values between −2 and +1, εHf_(T) between −1 and +3, initial ²⁰⁷Pb/²⁰⁴Pb ratios of ∼14.9, higher than the value of 14.6 derived from the Stacey-Kramers growth curve, and crustal extraction ages ≥3.0 Ga. Modeling shows that the syn- to late-tectonic intrusions of granitoid rocks record at least 20%, and up to 40%, recycling of this gneissic basement. The southern part of the La Grande belt, where no basement gneisses are exposed, records lower proportions (5-20%) of this crustal end-member. In both parts of the La Grande belt, the post-tectonic plutons record the largest proportion of recycled crust, likely related to southeastward thrusting events in the Superior craton, crustal thickening and concomitant partial melting.The La Grande plutons have isotopic signatures more radiogenic than those of similar intrusions that formed concurrently, to the south, in the Abitibi greenstone belt. This reflects an ancient crust environment for the former vs. an oceanic setting for the latter. Mantle reservoirs with a protracted history of incompatible element depletion were present beneath the whole eastern Superior Province, in the late Archean.     

The lead isotope geochemistry and geochronology of late-kinematic intrusives from the Abitibi greenstone belt,and the implications for late Archaean crustal evolution

Pb isotope abundances are reported for six late-kinematic granitoid intrusives from the Quebec sector of the Abitibi greenstone belt. Leaching experiments on K-feldspar separates reveal the presence of radiogenic Pb, attributed to in situ decay of U and Th. Pb-Pb mineral isochrons were constructed with the K-feldspar data plus results obtained on the total-rock, sphene, apatite and other mineral phases; five localities show no evidence of post-emplacement disturbance and yield ages ranging from 2616 ± 19 to 2718 ± 12 Ma. These ages, which are corroborated by U-Pb dating of small populations of sphene, imply that the orogenic events in the Abitibi belt were terminated 2700–2710 Ma ago, and followed by a period of granitization which lasted for 80 to 100 Ma.The initial Pb isotope composition of the magmas shows that their source regions were isotopically heterogeneous; the time integrated ${}^{238}\text{U}{}^{204}\text{Pb}$ values for the source regions vary from 7.62 to 7.92 and the K-feldspar data indicate that similar heterogeneities were present at the scale of a single intrusion. The range of isotopic composition spans the compositional domain of the mantle, defined by sulfides associated with komatiites and some galenas, and that of the continental crust, defined by sulfides associated with Abitibi iron-formations. Consequently, the granitoid magmas are interpreted as partial melts of a continental crust comprising juvenile, mantle-derived rocks and non-negligible amounts of earlier formed sialic material. The Pb isotope data for the latter are consistent with the presence in the area of 3.0 to 3.4 Ga old sialic crust. The episode of crustal anatexis occurred as a consequence to the orogenic events which resulted in burial of altered supracrustal rocks rich in water and heat-producing elements.     

Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago

New lead, strontium and helium isotopic data, together with trace element concentrations, have been determined for basalts from the Cape Verde archipelago (Central Atlantic). Isotopic and chemical variations are observed at the scale of the archipelago and lead to the definition of two distinct groupings, in keeping with earlier studies. The Northern Islands (Santo Antão, São Vicente, São Nicolau and Sal) present Pb isotopic compositions below the Northern Hemisphere Reference Line (NHRL) (cf. Hart, 1984), unradiogenic Sr and relatively primitive ⁴He/³He ratios. In contrast, the Southern Islands (Fogo and Santiago) display Pb isotopes above the NHRL, moderately radiogenic Sr and MORB-like helium signatures. We propose that the dichotomy between the Northern and Southern Islands results from the presence of three isotopically distinct components in the source of the Cape Verde basalts: (1) recycled ∼1.6-Ga oceanic crust (high ²⁰⁶Pb/²⁰⁴Pb, low ⁸⁷Sr/⁸⁶Sr and high ⁴He/³He); (2) lower mantle material (high ³He); and (3) subcontinental lithosphere (low ²⁰⁶Pb/²⁰⁴Pb, high ⁸⁷Sr/⁸⁶Sr and moderately radiogenic ⁴He/³He ratios). The signature of the Northern Islands reflects mixing between recycled oceanic crust and lower mantle, to which small proportions of entrained depleted material from the local upper mantle are added. Basalts from the Southern Islands, however, require the addition of an enriched component thought to be subcontinental lithospheric material instead of depleted mantle. The subcontinental lithosphere may stem from delamination and subsequent incorporation into the Cape Verde plume, or may be remnant from delamination just before the opening of the Central Atlantic. Basalts from São Nicolau reflect the interaction with an additional component, which is identified as oceanic crustal material.     

Pb isotopic composition of late Archaean granites and the extent of recycling early Archaean crust in the Slave Province, northwest Canada

Initial Pb isotopic compositions have been determined for potassium feldspar from ca. 2.58 to 2.62 Ga plutonic rocks in the southern and central Slave Province of northwestern Canada to evaluate the extent of recycling of ancient crust within the province. Large differences in initial Pb compositions were measured which correlate with geographical areas of the province. Plutons in the east-central part of the province have initial compositions only slightly more radiogenic than estimated mantle values (²⁰⁷Pb/²⁰⁴Pb 14.8–14.9), and were dominantly deruved from juvenile crustal sources. In contrast, plutons in the Point Lake and western Contwoyto Lake areas of the western Slave Province have radiogenic compositions (²⁰⁷Pb/²⁰⁴Pb 15.1–15.2), and indicate significant recycling of pre-3.5 Ga crust. The Pb data support previous interpretations, based on Nd isotopes, for a major isotopic boundary in the central part of the province. Granites from the southern part of the province, near Yellowknife, have intermediate compositions which indicate: (1) the age of the protolith to the granitoids in the Yellowknife area is younger than at Point Lake, but older than in the eastern Slave; or (2) the granitoids in the Yellowknife area contain a mixture of an older Point Lake-type component and younger crust. The absence of pre-3.2 Ga crust in the Yellowknife area and lack of evidence for pre-2.8 Ga inherited zircons in the Yellowknife granitoids favour the former possibility. Evidence for recycling of ancient crustal sources, such as the Acasta Gneiss, is limited to a relatively small area of the west-central part of the province, suggesting that Acasta aged, or derived, crust is not widespread in the province. The marked regionality of isotopic composition may reflect a basement in the western part of the province which is itself a collage of crust of different age, being younger (ca 3.2-2.9) in the south, relative to the Point Lake region (3.9-3.2 Ga).