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Mass-independent fractionation of Hg isotopes by biological processes
A284
Goldschmidt Conference Abstracts 2006
Strontium isotopes in Samoan basaltic melt inclusions
Mass-independent fractionation of Hg isotopes by biological processes
M.G. JACKSON, S.R. HART, L. BALL
T.A. JACKSON1, D.M. WHITTLE2, M.S. EVANS3, D.C.G. MUIR1
Woods Hole Oceanographic Institution, USA (mjackson@whoi. edu) We measured 87Sr/86Sr ratios on 41 olivine-hosted melt inclusions from nine Samoan basalts (taken from 5 Samoan volcanoes) using laser ablation multi-collector (LA-MC) ICPMS. 87Sr/86Sr ratios are corrected for mass bias after eliminating major isobaric interferences from Rb and Kr. The external precision averages ±320 ppm (2r) for the 87Sr/86Sr ratios on natural Samoan basalt glass standards of a similar composition to the melt inclusions. Surprisingly, all of the Sr-isotope ratios measured by LA-MCICPMS on Samoan melt inclusions fall within the range recorded in whole-rocks using conventional methods. However, melt inclusions (grey diamonds and open squares) from two Samoan basalt bulk rock samples are extremely heterogeneous in 87Sr/86Sr (0.70459–0.70926). Melt inclusions from a third high 3He/4He Samoan basalt (open circles) are isotopically homogeneous, exhibit 87Sr/86Sr values from 0.70434 to 0.70469, and mark the unradiogenic extreme in the Samoan array. The isotopic variability (or lack thereof) is mirrored in Rb/Sr from the same inclusions (Fig. 1). Several melt inclusions yield 87Sr/86Sr ratios higher than their host rock, indicating that assimilation of oceanic crust and lithosphere is not the likely mechanism contributing to the isotopic variability in these melt inclusions. Additionally, none of the 41 melt inclusions analyzed exhibit 87Sr/86Sr ratios lower than the least radiogenic basalts in Samoa (87Sr/86Sr = 0.7044), providing an additional argument against assimilation of oceanic crust and lithosphere as the source of the isotopic diversity in the melt inclusions. Instead, the source sampled by the high 3He/4He Samoan basalt may be a more appropriate, common unradiogenic endmember that is the source of much of the isotopic heterogeneity in melt inclusions observed in other Samoan basalts.
Fig. 1. Sr-isotope and Rb/Sr ratios in Samoan basalts (light field) and melt inclusions (dark fields, representing melt inclusions from 9 basalt samples). Internal (error bars on data points) and external (bars on periphery) precision are 2r.
doi:10.1016/j.gca.2006.06.576
1
Water Science and Technology Directorate, Department of the Environment, Burlington, Ont., Canada L7R 4A6 (t.a.jackson @ec.gc.ca) 2 Department of Fisheries and Oceans, Burlington, Ont., Canada L7R 4A6 ([email protected]) 3 Water Science and Technology Directorate, Department of the Environment, Saskatoon, Sask., Canada S7N 3H5 (marlene. [email protected]) Variations in the stable isotope composition of mercury (Hg) in food web organisms (crustaceans, forage fish, and top predator fish) and sediment cores from three different lakes were measured by multicollector inductively coupled plasma mass spectrometry (performed by Activation Laboratories Ltd.). Total Hg and methylmercury (MeHg) analyses were done as well. Our purpose was to investigate possible biotic fractionation of Hg isotopes. The 198 Hg/202Hg, 199Hg/202Hg, 200Hg/202Hg, and 201Hg/202Hg ratios of Hg in standard solutions and sample digests (adjusted to concentrations of 2–5 ng mL 1 and analysed in the order standard– sample–standard following reduction of Hg(II)–Hg(0) with SnCl2) were used to calculate d198Hg, d199Hg, d200Hg, and d201Hg values. Evidence for mass-independent fractionation of Hg isotopes with odd mass numbers (199Hg and 201Hg) was found in the biotic assemblages from all three lakes, but only mass-dependent effects were observed in the sediments. Thus, in plots of 198Hg/202Hg against 200Hg/202Hg the organisms and sediments formed a single linear regression line, but plots of 199Hg/202Hg and 201Hg/202Hg against 200Hg/202Hg showed that the organisms were anomalously rich in 199Hg and 201Hg relative to sediments. Similarly, the d199Hg and d201Hg values of each biotic assemblage were higher than d198Hg and d200Hg and had a different pattern of variation. The deviations of the 199Hg/202Hg and 201Hg/202Hg ratios of the organisms, and the corresponding d199Hg and d201Hg values as well, increased with MeHg concentration, whereas d198Hg and d200Hg gave inverse correlations. Analysis of MeHg and inorganic Hg fractions of fish proved that MeHg was highly enriched in 199 Hg and 201Hg, as implied by the data for whole organisms. Moreover, isotopic signatures of organisms showed effects of habitat and diet (e.g., isotopic differences between planktonic and benthic crustaceans and fish that prey on them) and a distinctive anomaly found consistently in lake whitefish. We conclude that biological processes involved in the biogeochemical cycle of MeHg in aquatic ecosystems mediate mass-independent fractionation of Hg isotopes. This phenomenon is attributable to effects of nuclear spin and penetration of the inner electron shells of Hg by valence electrons of ligands owing to the high nuclear charge of Hg and inefficient shielding of it by d- and f-electrons. doi:10.1016/j.gca.2006.06.577
Goldschmidt Conference Abstracts 2006
Strontium isotopes in Samoan basaltic melt inclusions
Mass-independent fractionation of Hg isotopes by biological processes
M.G. JACKSON, S.R. HART, L. BALL
T.A. JACKSON1, D.M. WHITTLE2, M.S. EVANS3, D.C.G. MUIR1
Woods Hole Oceanographic Institution, USA (mjackson@whoi. edu) We measured 87Sr/86Sr ratios on 41 olivine-hosted melt inclusions from nine Samoan basalts (taken from 5 Samoan volcanoes) using laser ablation multi-collector (LA-MC) ICPMS. 87Sr/86Sr ratios are corrected for mass bias after eliminating major isobaric interferences from Rb and Kr. The external precision averages ±320 ppm (2r) for the 87Sr/86Sr ratios on natural Samoan basalt glass standards of a similar composition to the melt inclusions. Surprisingly, all of the Sr-isotope ratios measured by LA-MCICPMS on Samoan melt inclusions fall within the range recorded in whole-rocks using conventional methods. However, melt inclusions (grey diamonds and open squares) from two Samoan basalt bulk rock samples are extremely heterogeneous in 87Sr/86Sr (0.70459–0.70926). Melt inclusions from a third high 3He/4He Samoan basalt (open circles) are isotopically homogeneous, exhibit 87Sr/86Sr values from 0.70434 to 0.70469, and mark the unradiogenic extreme in the Samoan array. The isotopic variability (or lack thereof) is mirrored in Rb/Sr from the same inclusions (Fig. 1). Several melt inclusions yield 87Sr/86Sr ratios higher than their host rock, indicating that assimilation of oceanic crust and lithosphere is not the likely mechanism contributing to the isotopic variability in these melt inclusions. Additionally, none of the 41 melt inclusions analyzed exhibit 87Sr/86Sr ratios lower than the least radiogenic basalts in Samoa (87Sr/86Sr = 0.7044), providing an additional argument against assimilation of oceanic crust and lithosphere as the source of the isotopic diversity in the melt inclusions. Instead, the source sampled by the high 3He/4He Samoan basalt may be a more appropriate, common unradiogenic endmember that is the source of much of the isotopic heterogeneity in melt inclusions observed in other Samoan basalts.
Fig. 1. Sr-isotope and Rb/Sr ratios in Samoan basalts (light field) and melt inclusions (dark fields, representing melt inclusions from 9 basalt samples). Internal (error bars on data points) and external (bars on periphery) precision are 2r.
doi:10.1016/j.gca.2006.06.576
1
Water Science and Technology Directorate, Department of the Environment, Burlington, Ont., Canada L7R 4A6 (t.a.jackson @ec.gc.ca) 2 Department of Fisheries and Oceans, Burlington, Ont., Canada L7R 4A6 ([email protected]) 3 Water Science and Technology Directorate, Department of the Environment, Saskatoon, Sask., Canada S7N 3H5 (marlene. [email protected]) Variations in the stable isotope composition of mercury (Hg) in food web organisms (crustaceans, forage fish, and top predator fish) and sediment cores from three different lakes were measured by multicollector inductively coupled plasma mass spectrometry (performed by Activation Laboratories Ltd.). Total Hg and methylmercury (MeHg) analyses were done as well. Our purpose was to investigate possible biotic fractionation of Hg isotopes. The 198 Hg/202Hg, 199Hg/202Hg, 200Hg/202Hg, and 201Hg/202Hg ratios of Hg in standard solutions and sample digests (adjusted to concentrations of 2–5 ng mL 1 and analysed in the order standard– sample–standard following reduction of Hg(II)–Hg(0) with SnCl2) were used to calculate d198Hg, d199Hg, d200Hg, and d201Hg values. Evidence for mass-independent fractionation of Hg isotopes with odd mass numbers (199Hg and 201Hg) was found in the biotic assemblages from all three lakes, but only mass-dependent effects were observed in the sediments. Thus, in plots of 198Hg/202Hg against 200Hg/202Hg the organisms and sediments formed a single linear regression line, but plots of 199Hg/202Hg and 201Hg/202Hg against 200Hg/202Hg showed that the organisms were anomalously rich in 199Hg and 201Hg relative to sediments. Similarly, the d199Hg and d201Hg values of each biotic assemblage were higher than d198Hg and d200Hg and had a different pattern of variation. The deviations of the 199Hg/202Hg and 201Hg/202Hg ratios of the organisms, and the corresponding d199Hg and d201Hg values as well, increased with MeHg concentration, whereas d198Hg and d200Hg gave inverse correlations. Analysis of MeHg and inorganic Hg fractions of fish proved that MeHg was highly enriched in 199 Hg and 201Hg, as implied by the data for whole organisms. Moreover, isotopic signatures of organisms showed effects of habitat and diet (e.g., isotopic differences between planktonic and benthic crustaceans and fish that prey on them) and a distinctive anomaly found consistently in lake whitefish. We conclude that biological processes involved in the biogeochemical cycle of MeHg in aquatic ecosystems mediate mass-independent fractionation of Hg isotopes. This phenomenon is attributable to effects of nuclear spin and penetration of the inner electron shells of Hg by valence electrons of ligands owing to the high nuclear charge of Hg and inefficient shielding of it by d- and f-electrons. doi:10.1016/j.gca.2006.06.577