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Ra-excess dating: Hopes and limitations
Goldschmidt Conference Abstracts 2006
A251
Ab initio and experimental studies of equilibrium isotopic fractionation in aqueous ferric chloride complexes
Ra-excess dating: Hopes and limitations
P.S. HILL, E.A. SCHAUBLE, A. SHAHAR, E. TONUI, E.D. YOUNG
GEOTOP-UQAM & McGill, POB 8888, Montreal, Que., Canada H3C 3P8
Department of Earth and Space Sciences, University of California, Los Angeles, USA ([email protected])
Among U-series isotopes, radium 226 has attracted much attention as it potentially permits (i) to confront 14C-dates spanning the interval 0–8 kyr BP (226Ra-excess dating), (ii) the precise dating of the 0–100 yr interval (210Pb-excess), and (iii) also provides useful information on geochemical processes (226Ra–Ba–Ca) and about 226Ra–230Th concordias in the 10–50 ka range. Applications for the study of speleothems, corals, mollusks, volcanic rocks, ground waters, soils have been explored. The 226Ra-excess method is usually based on the assumption of a constant initial excess and/or 226Ra/Ba ratio. Here, we discuss 210 Pb–226Ra–230Th–U systematics and Ra–Ba geochemistry in aragonitic Scleractinian deep corals, calcitic and organic Gorgonian corals and sea water from North Atlantic sites. One gram (Ra,Pb) to 0.1 g (U, Ba, Ca) of carbonate samples, and from 1 ml (U, Ca, Ba) to 250–500 ml (Ra) of water, are usually needed to perform TIMS measurements (Ra, U), isotope dilution ICPMS analyses (Ba), and 210Po alpha-counting (210Pb analysis). Ra/Ba ratios in water samples show distinct entry functions for these two elements, besides radioactive decay of 226Ra, particularly in marginal marine settings and near deep hydrothermal sites. By combining 230Th-234U-238U and 226Ra measurements, initial 226 Ra-excesses in carbonate minerals are examined. They indicate specific Ra/Ba fractionation, variable and possibly thermodependent Ra-uptake rates, thus raise concern about the reliability of 226 Ra-excess ages. Applications for the validation of 230Th-ages of recent deep-sea corals are presented. Finally, 210Pb–226Ra ratios in biogenic carbonate minerals as well as initial 210Pb-excesses in organics corals are shown to provide good age estimates for the calculation of recent growth rates.
Both iron and chlorine are ubiquitous in natural systems, but not much is known about isotopic fractionation among ferric chloride complexes. To determine the extent of this fractionation, we did integrated theoretical and experimental studies of fractionation among the aqueous ferric chloride complexes: Fe(H2O)63+, FeCl(H2O)52+, FeCl2(H2O)4+, FeCl3(H2O)3, FeCl3(H2O)2, FeCl4 , FeCl5H2O2 , FeCl52 , FeCl63 . We developed four sets of ab initio models (UHF and modified DFT with different basis sets) for each complex. All sets of models showed a linear decrease in the isotopic fractionation factor as the number of Cl ions per Fe+3 ion increased, with slopes 0.8& to 1.0& per Cl , depending on model. At 20 °C, 1000 ln b56–54 (relative to a dissociated Fe atom) values ranged from 8.93& to 9.73& for Fe(H2O)63+, 8.04–9.12& for FeCl(H2O)52+, 7.61– 8.73& for FeCl2(H2O)4+, 7.14–8.25& for FeCl4 , and 3.09– 4.41& for FeCl63 . To test these models, we mixed two series of low pH solutions of ferric chloride (total Fe = 0.08 and 0.17 M) with total chlorine = 0–5 M. Each solution equilibrated in the presence of immiscible diethyl ether. Only FeCl4 dissolves in the ether phase. DFe56–54 = d56Fe (total Fe remaining in aqueous phase) d56Fe (FeCl4 in ether phase) was determined for each solution via MC-ICPMS analysis. Relative amounts of each complex for each experiment were calculated from a speciation model and then used with the ab initio values to calculate the theoretical DFe56–54 for each experiment. (d56Feaqueous equals the sum of d56Fe of each complex times its fractional portion of the whole.) According to the speciation model, Fe+3 dominates for [Cl ] < 0.75 M; FeCl+2 for [Cl ] < 1.9 M; and FeCl2+ for 1.9 M < [Cl ] < 5 M. Theoretical values of DFe56–54 for each ab initio model sets were compared to the experimental DFe56–54 values. Both the experimental data and the theoretical models show the same downward trend: DFe56–54 is highest for low chlorinity where Fe(H2O)6+3 is the dominant species and lowest at high chlorinity where FeCl2(H2O)4+1 is dominant. The theoretical models predict 0.5& to 0.7& decrease in DFe56–54 (depending on the model) for the range of chlorinity studied (0.5–5 M). The experimental DFe56–54 ranged from 1& at [Cl ] = 0.5 M to 0.3& at [Cl ] = 5 M. The experimental results are in good agreement with the theoretical predictions, suggesting the existence of equilibrium isotopic fractionation among aqueous ferric chloride complexes and a decrease in 56Fe/54Fe as the number of Cl ions per Fe3+ ion increases. doi:10.1016/j.gca.2006.06.505
C. HILLAIRE-MARCEL, B. GHALEB, E. PONS-BRANCHU
doi:10.1016/j.gca.2006.06.506
A251
Ab initio and experimental studies of equilibrium isotopic fractionation in aqueous ferric chloride complexes
Ra-excess dating: Hopes and limitations
P.S. HILL, E.A. SCHAUBLE, A. SHAHAR, E. TONUI, E.D. YOUNG
GEOTOP-UQAM & McGill, POB 8888, Montreal, Que., Canada H3C 3P8
Department of Earth and Space Sciences, University of California, Los Angeles, USA ([email protected])
Among U-series isotopes, radium 226 has attracted much attention as it potentially permits (i) to confront 14C-dates spanning the interval 0–8 kyr BP (226Ra-excess dating), (ii) the precise dating of the 0–100 yr interval (210Pb-excess), and (iii) also provides useful information on geochemical processes (226Ra–Ba–Ca) and about 226Ra–230Th concordias in the 10–50 ka range. Applications for the study of speleothems, corals, mollusks, volcanic rocks, ground waters, soils have been explored. The 226Ra-excess method is usually based on the assumption of a constant initial excess and/or 226Ra/Ba ratio. Here, we discuss 210 Pb–226Ra–230Th–U systematics and Ra–Ba geochemistry in aragonitic Scleractinian deep corals, calcitic and organic Gorgonian corals and sea water from North Atlantic sites. One gram (Ra,Pb) to 0.1 g (U, Ba, Ca) of carbonate samples, and from 1 ml (U, Ca, Ba) to 250–500 ml (Ra) of water, are usually needed to perform TIMS measurements (Ra, U), isotope dilution ICPMS analyses (Ba), and 210Po alpha-counting (210Pb analysis). Ra/Ba ratios in water samples show distinct entry functions for these two elements, besides radioactive decay of 226Ra, particularly in marginal marine settings and near deep hydrothermal sites. By combining 230Th-234U-238U and 226Ra measurements, initial 226 Ra-excesses in carbonate minerals are examined. They indicate specific Ra/Ba fractionation, variable and possibly thermodependent Ra-uptake rates, thus raise concern about the reliability of 226 Ra-excess ages. Applications for the validation of 230Th-ages of recent deep-sea corals are presented. Finally, 210Pb–226Ra ratios in biogenic carbonate minerals as well as initial 210Pb-excesses in organics corals are shown to provide good age estimates for the calculation of recent growth rates.
Both iron and chlorine are ubiquitous in natural systems, but not much is known about isotopic fractionation among ferric chloride complexes. To determine the extent of this fractionation, we did integrated theoretical and experimental studies of fractionation among the aqueous ferric chloride complexes: Fe(H2O)63+, FeCl(H2O)52+, FeCl2(H2O)4+, FeCl3(H2O)3, FeCl3(H2O)2, FeCl4 , FeCl5H2O2 , FeCl52 , FeCl63 . We developed four sets of ab initio models (UHF and modified DFT with different basis sets) for each complex. All sets of models showed a linear decrease in the isotopic fractionation factor as the number of Cl ions per Fe+3 ion increased, with slopes 0.8& to 1.0& per Cl , depending on model. At 20 °C, 1000 ln b56–54 (relative to a dissociated Fe atom) values ranged from 8.93& to 9.73& for Fe(H2O)63+, 8.04–9.12& for FeCl(H2O)52+, 7.61– 8.73& for FeCl2(H2O)4+, 7.14–8.25& for FeCl4 , and 3.09– 4.41& for FeCl63 . To test these models, we mixed two series of low pH solutions of ferric chloride (total Fe = 0.08 and 0.17 M) with total chlorine = 0–5 M. Each solution equilibrated in the presence of immiscible diethyl ether. Only FeCl4 dissolves in the ether phase. DFe56–54 = d56Fe (total Fe remaining in aqueous phase) d56Fe (FeCl4 in ether phase) was determined for each solution via MC-ICPMS analysis. Relative amounts of each complex for each experiment were calculated from a speciation model and then used with the ab initio values to calculate the theoretical DFe56–54 for each experiment. (d56Feaqueous equals the sum of d56Fe of each complex times its fractional portion of the whole.) According to the speciation model, Fe+3 dominates for [Cl ] < 0.75 M; FeCl+2 for [Cl ] < 1.9 M; and FeCl2+ for 1.9 M < [Cl ] < 5 M. Theoretical values of DFe56–54 for each ab initio model sets were compared to the experimental DFe56–54 values. Both the experimental data and the theoretical models show the same downward trend: DFe56–54 is highest for low chlorinity where Fe(H2O)6+3 is the dominant species and lowest at high chlorinity where FeCl2(H2O)4+1 is dominant. The theoretical models predict 0.5& to 0.7& decrease in DFe56–54 (depending on the model) for the range of chlorinity studied (0.5–5 M). The experimental DFe56–54 ranged from 1& at [Cl ] = 0.5 M to 0.3& at [Cl ] = 5 M. The experimental results are in good agreement with the theoretical predictions, suggesting the existence of equilibrium isotopic fractionation among aqueous ferric chloride complexes and a decrease in 56Fe/54Fe as the number of Cl ions per Fe3+ ion increases. doi:10.1016/j.gca.2006.06.505
C. HILLAIRE-MARCEL, B. GHALEB, E. PONS-BRANCHU
doi:10.1016/j.gca.2006.06.506