Jean Carignan

Magnesium isotope heterogeneity of the isotopic standard SRM980 and new reference materials for magnesium-isotope-ratio measurements

Multicollector ICP-MS has been used for the precise measurement of variations in the isotopic composition of the isotopic standard of magnesium (SRM980) provided by the National Institute of Standards and Technology (Gaithersburg, MD, USA). The SRM980 consists of metal chips weighing between 1 and 50 mg and each unit delivered by the National Institute of Standards and Technology corresponds to a bottle containing about 0.3 g. Height units were analysed. Variations in sample 25Mg/24Mg, and 26Mg/24Mg ratios are expressed as δ25Mg and δ26Mg units, respectively, which are deviations in parts per 103 from the same ratio in a standard solution. The differences in δ25Mg and δ26Mg of the SRM980 are up to 4.20 and 8.19‰, respectively, while the long-term repeatability of δ25Mg and δ26Mg are 0.09 and 0.16‰, respectively, at 95% confidence. However, when plotted in a three-isotope diagram, all the data fall on a single mass fractionation line. Overall limits of error of the SRM980 reported here fall within the previously reported overall limits of error. The isotopic heterogeneity not only corresponds to differences among units but has been found at the chip-size level. This result, due to the precision of the MC-ICP-MS technique, makes the SRM980 inappropriate for the international isotopic standard of magnesium. The SRM980 can still be used to report the excess of 26Mg, which is defined by the deviation from the mass-dependent relationship between 25Mg/24Mg, and 26Mg/24Mg ratios. Two large batches (around 10 g of Mg in each) of pure Mg solutions (in 0.3 M HNO3) have been prepared and characterised. These 2 solutions (DSM3 and Cambridge 1) are suitable reference material because they are immune to heterogeneity. DSM3 and Cambridge 1 are isotopically different (by 1.3‰ per u) and are available upon request from the first author. In addition, DSM3 has an isotopic composition very similar to the Mg-isotopic composition of carbonaceous chondrites (Orgueil and Allende). Because of the lack of heterogeneity and the cosmochemical and geochemical significance of DSM3, we urge the use of DSM3 as the primary isotopic reference material to report Mg-isotopic variations.

Stability of the Upper Nile drainage network (Ethiopia) deduced from (U–Th)/He thermochronometry: implications for uplift and erosion of the Afar plume dome

Abstract One of the best places to investigate the role of a mantle plume in creating topography at the onset of continental breakup is the Ethiopian volcanic province since it is the youngest and best preserved case of a large igneous province dissected by a system of rifts. In the center of the volcanic province, the northwestern Ethiopian plateau which surrounds the Afar depression displays the highest topography and experienced more than 1 km of surface uplift. Because hydrology and physiography are genetically linked, understanding the long-term evolution of these upper basins has implications regarding the morphotectonic evolution of the plateau as well as the paleo-hydrological evolution of the whole Nile River. We report the results of a combined thermochronological and morphological study aimed at understanding the long-term stability of the upper Blue Nile drainage network. Apatite and titanite He ages have been determined for crystalline basement samples collected below the lava pile, in various key sites of the drainage network. Titanite He ages, which range from 213 to 520 Ma, are in good agreement with published K-feldspar Ar–Ar data and most likely reflect post-Pan-African cooling of the Ethiopian basement. Apatite He ages, which range from 45 to 107 Ma, display a trend of decreasing age with increasing crustal depth in the Blue Nile gorge, which is interpreted as partial resetting of pre-existing ages due to basement burial because of the thick pile of flood lavas erupted 30 Ma ago. Simulations of apatite He age partial resetting suggest that erosion initiated in the Blue Nile canyon as early as 25–29 Ma ago, whereas erosion would have started after 11 Ma along the present retreat scarp. The early onset of erosion in the Blue Nile canyon suggests that: (i) volcanic and uplift-related tectonic divides represent long-lived (20–30 Myr) pre-breakup divides, and (ii) the elevated plateau physiography, which controls most of the present-day Nile hydrology, has existed since the Oligocene. We propose that the plateau represents the preserved part of a large uplifted dome, related to Afar plume impingement, and/or to massive underplating triggered by Oligocene continental flood basalt differentiation. In this case, the topographic development of the western Afar margin, which is related to extension and drift of the Arabian plate, would be mainly the result of the collapse of the Afar area from an initially elevated region.

Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry

Abstract Multiple-collector inductively coupled plasma mass spectrometry has been used for the precise measurement of the isotopic composition of Se in geological samples. Se is chemically purified before analysis by using cotton impregnated with thioglycollic acid. This preconcentration step is required for the removal of matrix-interfering elements for hydride generation, such as transitional metals, and also for the quantitative separation of other hydride-forming elements, such as Ge, Sb, and As. The analyte is introduced in the plasma torch with a continuous-flow hydride generation system. Instrumental mass fractionation is corrected with a “standard-sample bracketing” approach. By use of this new technique, the minimum Se required per analysis is lowered to 10 ng, which is one order of magnitude less than the amount needed for the N-TIMS technique. The estimated external precision calculated for the 82 Se/ 76 Se isotope ratio is 0.25‰ (2σ), and the data are reported as delta notation (‰) relative to our internal standard (MERCK elemental standard solution). Measurements of Se isotopes are presented for samples of standard solutions and geological reference materials, such as silicate rocks, soils, and sediments. The Se isotopic composition of selected terrestrial and extraterrestrial materials are also presented. An overall Se isotope variation of 8‰ has been observed, suggesting that Se isotopes fractionate readily and are extremely useful tracers of natural processes.

Re–Os systematics of UB-N, a serpentinized peridotite reference material

The reference material (RM) UB-N is a typical representative of earths upper mantle. It is a serpentinized garnet and spinel-bearing peridotite (a metamorphosed lherzolite) from the Vosges mountains, France, that is well characterized for major and many trace elements. In order to test whether UB-N is a suitable Re–Os reference material, 32 digestions in three different laboratories (CRPG/CNRS, MPI (Mainz) and University of Leoben) with four different digestion techniques (low-temperature acid attack, Carius tube dissolution, high-pressure asher (HPA-S) acid attack and alkali fusion) were performed. The results show that the low-temperature acid attack is unsuitable for the study of the Re–Os systematics of UB-N. Surprisingly, the well-established Carius tube acid digestion technique also fails to completely digest all Os-bearing mineral phases. Only alkali fusion and HPA-S acid attack yield the highest Os concentrations. Though sample inhomogeneity has been recognized (approximately 6% RSD for 2-g sample aliquots), it is possible to determine a well-defined average Os concentration of 3.85±0.13 ng g−1 (95% confidence; 19 digestions, fusion and HPA-S only). Rhenium-bearing minerals are very homogeneously distributed and replicates within each laboratory yield highly reproducible results independent of the digestion technique. A value of 0.2095±0.0040 ng g−1 (95% confidence; n=24) is assigned to the Re concentration. The best estimate for the whole-rock 187Os/188Os is 0.1278±0.0002 (95% confidence; n=12). The UB-N reference material now has well-understood Re–Os systematics that are typical of fertile upper mantle rocks. Analysis of this standard is, thus, highly recommended for the validation of Re–Os analytical procedures.

Timing of East African Rift development in southern Ethiopia: Implication for mantle plume activity and evolution of topography

Accurate determination of rifting chronology and associated uplift is crucial to understanding the evolution of the East African Rift System (EARS) and for identifying the significance of mantle plumes during continental breakup. This investigation of rift-related cooling along a major fault scarp in southern Ethiopia, using (U-Th)/He thermochronometry, shows that rifting started not before 20 Ma. Therefore, there is an absence of significant rift activity synchronous with the earliest volcanics of the EARS, which are Eocene in age. In contrast, this initial magmatic episode, which preceded the main flood basalts and rifting events by 15–20 Ma, is attributed to convective instabilities above the rising Afar mantle plume. A detailed spatial and temporal quantification of uplift and denudation along this rift shoulder shows that rift development in southern Ethiopia has been continuous since initiation in the Miocene. This direct evidence of denudation is inconsistent with the hypothesis that massive Plio-Pleistocene rifting and associated uplift occurred in this part of the EARS and could have triggered recent aridification. To the contrary, our study rather supports a major contribution of plume-related doming for creation of topographical barriers in the Ethiopian province.

Pb and Sr isotopic compositions of snowpack from québec, canada: inferences on the sources and deposition budgets of atmospheric heavy metals

Elemental concentrations of Al, Ba, Cd, Cu, Mg, Mn, Pb, Rb, Sr, and Zn, as well as Pb and Sr isotopic compositions were determined in samples of snowpack obtained along two main transects from the province of Quebec (Canada); one north-south (between 47°N and 55°N; 1994) and the other within the St. Lawrence Valley (1997). Median enrichment factors (relative to upper crustal abundances) for Cd, Cu, Mn, Pb and Zn for all samples range from ≈300 to ≈42,000 and are indicative of an anthropogenic origin. Pb isotope ratios for snow samples retrieved in 1994 are highly variable (206Pb/207Pb = 1.148 to 1.193) and are characterized by the most radiogenic Sr isotope values (87Sr/86Sr ≥ 0.710). In contrast, the Pb and Sr isotope results for 1997 snow samples collected along the St. Lawrence Valley (below latitude 47°N), yield the most radiogenic Pb isotope ratios (206Pb/207Pb = 1.180 to 1.190) and 87Sr/86Sr ratios between 0.708 and 0.710. The former indicate that the atmospheric pollution in this region of Quebec is dominated by a mixture of anthropogenic emissions from U.S. (206Pb/207Pb ≈ 1.20) and Canadian (206Pb/207Pb ≈ 1.15) sources. Pb isotope ratios (206Pb/207Pb = 1.160 to 1.180) for 1997 samples collected north of latitude 47°N indicate input of an additional anthropogenic component, possibly that of Eurasian pollution being transported over the high Arctic during the winter season. A comparison of the Pb isotope results between lichens and snow samples from identical sample locations indicate that these either overlap (along St. Lawrence Valley), or are significantly different (north–south transect). The latter discrepancy may be attributed to either: (1) different time scales for the integration of the atmospheric signal (months for snow vs. years for lichens); (2) recording of the atmospheric signal at substantially different altitudes; or (3) the presence of an important, local point source of atmospheric pollution. Annual depositional budgets have been estimated for Pb, Cd, Zn, Cu, and Mn, and average values (g km−2 yr−1) are 1500, 130, 196,000, 1900, and 6400, respectively. Compared to previous depositional fluxes (1993–1994) estimated from adjacent regions in North America, those reported here are slightly lower with the exception of Mn and Zn. The nondecrease in depositional fluxes of Mn may be attributed to combustion of Mn-bearing fossil fuels by automotive vehicles. The exact cause for the elevated annual depositional values for Zn, however, remains enigmatic.

Tracing sources of atmospheric pollution in Western Canada using the Pb isotopic composition and heavy metal abundances of epiphytic lichens

The Pb isotopic composition and trace metal concentrations of epiphytic lichens collected from tree branches within northwestern North America are reported here, with a latitudinal coverage extending from the Beaufort Sea (Arctic circle) to the Canada–USA border. Overall, the trace metal concentrations and Pb isotope compositions correlate with latitudinal position, since lichens retrieved north of latitude 60°N are characterized by low enrichment factors (EF) (mainly between 10 and 30) for heavy metals (i.e. Pb, Zn) and radiogenic 206Pb/207Pb isotope values (∼1.170–1.180). Samples collected further south are characterized by higher EF for heavy metals and much lower Pb isotopic compositions (i.e. 206Pb/207Pb⩽1.150). Lichens retrieved in the immediate vicinity of major urban centers (i.e. Calgary, Alberta and Victoria, British Columbia) record distinct Pb isotopic values compared to the regional signal measured in adjacent (remote) samples. The total variation defined by the Pb isotopic compositions of the lichens may be attributed to the mixing of atmospheric particulates and aerosols derived from at least four end-member components, three anthropogenic and one natural. The latter is the predominant signal recorded in lichens retrieved north of 60°N, and is similar in Pb isotopic composition to a natural component identified in aerosols collected during the autumn season of 1994 at Alert (Canadian High Arctic). In contrast, samples collected further south reflect in-part mixing between Canadian and USA anthropogenic sources of atmospheric Pb. The third (unradiogenic) anthropogenic end-member most probably represents atmospheric emissions originating from one of the worlds major Zn/Pb smelters located at Trail, British Columbia.

Isotope tracing of atmospheric mercury sources in an urban area of northeastern France.

Mercury (Hg) isotope composition was investigated in lichens over a territory of 900 km(2) in the northeast of France over a period of nine years (2001-2009). The studied area was divided into four geographical areas: a rural area, a suburban area, an urban area, and an industrial area. In addition, lichens were sampled directly at the bottom of chimneys, within the industrial area. While mercury concentrations in lichens did not correlate with the sampling area, mercury isotope compositions revealed both mass dependent and mass independent fractionation globally characteristic of each geographical area. Odd isotope deficits measured in lichens were smallest in samples close to industries, with Delta(199)Hg of -0.15 +/- 0.03 per thousand, where Hg is thought to originate mainly from direct anthropogenic inputs. Samples from the rural area displayed the largest anomalies with Delta(199)Hg of -0.50 +/- 0.03 per thousand. Samples from the two other areas had intermediate Delta(199)Hg values. Mercury isotopic anomalies in lichens were interpreted to result from mixing between the atmospheric reservoir and direct anthropogenic sources. Furthermore, the combination of mass-dependent and mass independent fractionation was used to characterize the different geographical areas and discriminate the end-members (industrial, urban, and local/regional atmospheric pool) involved in the mixing of mercury sources.

Atmospheric Pb isotopic composition and trace metal concentration as revealed by epiphytic lichens:: an investigation related to two altitudinal sections in Eastern France

Abstract During Fall 1996, epiphytic lichens were collected along altitudinal sections in two areas of France (the Vosges mountains in the North-East, and the Alps, in Haute-Savoie) in order to verify any geographic distribution of atmospheric metals on a small scale. These lichens have various Pb isotopic compositions (206Pb/207Pb=1.126–1.147) which are correlated with the altitude of sampling. Lichens sampled near valleys display isotopic ratios significantly less radiogenic than those sampled at several hundred to thousand meters of altitude. In the Vosges sections, Pb concentrations and isotopic compositions of lichens may be used to define three zones: (1) valley: Pb-rich and non-radiogenic ratios, (2) transition: low-Pb and intermediate isotopic compositions, (3) mountain: heterogeneous Pb concentrations but more radiogenic and homogeneous Pb isotopic composition. Other metals (Zn, Cu, Cd, As), when normalised one to another, are not fractionated between these zones and display homogeneous relative abundance along the altitudinal sections of both sites. Variation of 206Pb/207Pb ratios with altitude is interpreted in terms of mixing of at least two pollution sources: one being the petrol (leaded and/or unleaded) combustion, and the other being of industrial origin. The latter is characterised by a more radiogenic isotopic composition. The Pb isotopic composition of flue gas residues from different municipal solid waste combustors in the Rhine valley and in other areas of France would suggest that these plants might be an important source of industrial Pb in the atmosphere. If the average industrial Pb in France has a 206Pb/207Pb close to 1.15, between 60 and 80% of the total Pb in lichens from the Rhine valley would come from gasoline combustion, whereas 85–90% of the Pb would have an industrial origin in lichens from higher altitude in the Vosges mountains. Although lichens from the Alps were collected at higher altitude, the percentage of industrial Pb for these lichens would be slightly lower (65%). Major winds and convection winds in the different valleys must then play an important role in term of distribution of atmospheric Pb in function of altitude.

Pb and Sr isotopic evidence for sources of atmospheric heavy metals and their deposition budgets in northeastern North America

Concentrations of Al, Ba, Cd, Cu, Mg, Mn, Pb, Rb, Sr, and Zn, as well as Pb and Sr isotopic compositions were determined in samples of snowpack obtained during the 1998 winter season from northeastern North America. Median enrichment factors, relative to upper crustal abundances, for Cd, Cu, Mn, Pb, and Zn for all samples range from ca. 50 to 36,000 and are indicative of an anthropogenic origin. The Pb isotope ratios correlate with geographic location because snowpack from eastern Ontario and the northeastern USA are characterized by the most radiogenic 206Pb/207Pb ratios (ca. 1.18–1.19), and these decrease systematically in an easterly direction towards the Atlantic coast (ca. 1.16). The Pb isotope data for the 1998 samples of snowpack indicate that atmospheric pollution in this region of North America is dominated solely by a mixture of anthropogenic emissions from US (206Pb/207Pb ca. 1.20) and Canadian (206Pb/207Pb ca. 1.15) sources. This result contrasts with that obtained for 1997 snowpack from similar geographic regions, because the Pb isotope data for the latter suggest the involvement of an additional anthropogenic component, possibly that of Eurasian pollution transported over the high Arctic. This difference in the distribution pattern of atmospheric pollution over northeastern North America between the 1997 and 1998 winter seasons may be related to the El Nino phenomenon. Sr isotope data for all the 1998 snowpack samples define a large range in 87Sr/86Sr values but most are between 0.709 to 0.710, similar to the value of present-day seawater (ca. 0.7092). However, the majority of snowpack samples from eastern Ontario are characterized by lower 87Sr/86Sr ratios (<0.709), and these correlate negatively with their enrichment factors of Pb. Such trends suggest the presence of a relatively ‘unradiogenic’ source of anthropogenic Sr, possibly related to emissions from coal-fired power plants located in the western and midwestern regions of the USA. Annual depositional budgets estimated from 1998 snowpack indicate that those for Pb, Cd, and Cu are highest in eastern Ontario, and significantly lower in the remaining regions. The former reflect a “meteorological corridor” in northeastern North America, which coincides with heavily industrialized and densely populated areas. Depositional budgets of Zn are extremely high in all areas investigated and their exact source(s) remains enigmatic. These may reflect either an increase in atmospheric industrial emissions (since the 1980s), a lower residence time in the atmosphere, or particular chemical speciation that affects the deposition of Zn from the lower troposphere. Depositional budgets for Mn are also high and may be related to the combustion of Mn-bearing fossil fuels by automotive vehicles.