Details
| Original language | English |
|---|---|
| Pages (from-to) | 141-163 |
| Number of pages | 23 |
| Journal | Isotopes in Environmental and Health Studies |
| Volume | 52 |
| Issue number | 1-2 |
| Publication status | Published - 3 Mar 2016 |
Abstract
In this study, the U isotope composition, n(238U)/n(235U), of major components of the upper continental crust, including granitic rocks of different age and post-Archaean shales, as well as that of rivers (the major U source to the oceans) was investigated. Furthermore, U isotope fractionation during the removal of U at mid-ocean ridges, an important sink for U from the oceans, was investigated by the analyses of hydrothermal water samples (including low- and high-temperature fluids), low-temperature altered basalts and calcium carbonate veins. All analysed rock samples from the continental crust fall into a limited range of δ238U between −0.45 and −0.21 ‰ (relative to NBL CRM 112-A), with an average of −0.30 ± 0.15 ‰ (2 SD, N = 11). Despite differences in catchment lithologies, all major rivers define a relatively narrow range between −0.31 and −0.13 ‰, with a weighted mean isotope composition of −0.27 ‰, which is indistinguishable from the estimate for the upper continental crust (−0.30 ‰). Only some tributary rivers from the Swiss Alps display a slightly larger range in δ238U (−0.29 to +0.01 ‰) and lower U concentrations (0.87–3.08 nmol/kg) compared to the investigated major rivers (5.19–11.69 nmol/kg). These findings indicate that only minor net U isotope fractionation occurs during weathering and transport of material from the continental crust to the oceans. Altered basalts display moderately enriched U concentrations (by a factor of 3–18) compared to those typically observed for normal mid-ocean ridge basalts. These, and carbonate veins within altered basalts, show large U isotope fractionation towards both heavy and light U isotope compositions (ranging from −0.63 to +0.27 ‰). Hydrothermal water samples display low U concentrations (0.3–1 nmol/kg) and only limited variations in their U isotope composition (−0.43 ± 0.25 ‰) around the seawater value. Nevertheless, two of the investigated fluids display significantly lower δ238U (−0.55 and −0.59 ‰) than seawater (−0.38 ‰). These findings, together with the heavier U isotope composition observed for some altered basalts and carbonate veins support a model, in which redox processes mostly drive U isotope fractionation. This may result in a slightly heavier U isotope composition of U that is removed from seawater during hydrothermal seafloor alteration compared to that of seawater. Using the estimated isotope compositions of rivers and all U sinks from the ocean (of this study and the literature) for modelling of the isotopic U mass balance, this gives reasonable results for recent estimates of the oceanic U budget. It furthermore provides additional constraints on the relative size of the diverse U sinks and respective net isotope fractionation during U removal.
Keywords
- hydrothermal seafloor alteration, isotope geochemistry, oceanic U cycle, rivers, uranium isotope composition, uranium-238
ASJC Scopus subject areas
- Environmental Science(all)
- Environmental Chemistry
- Environmental Science(all)
- General Environmental Science
- Chemistry(all)
- Inorganic Chemistry
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In: Isotopes in Environmental and Health Studies, Vol. 52, No. 1-2, 03.03.2016, p. 141-163.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - 238U/235U isotope ratios of crustal material, rivers and products of hydrothermal alteration
T2 - new insights on the oceanic U isotope mass balance
AU - Noordmann, Janine
AU - Weyer, Stefan
AU - Georg, R. Bastian
AU - Jöns, Svenja
AU - Sharma, Mukul
N1 - Funding information: Sampling of the Swiss Rivers was supported by ETH Zurich and the Swiss National Funds [No.: 2000 20/101 780]. Funding was provided by the Deutsche Forschungsgemeinschaft [DFG, WE 2850/6].
PY - 2016/3/3
Y1 - 2016/3/3
N2 - In this study, the U isotope composition, n(238U)/n(235U), of major components of the upper continental crust, including granitic rocks of different age and post-Archaean shales, as well as that of rivers (the major U source to the oceans) was investigated. Furthermore, U isotope fractionation during the removal of U at mid-ocean ridges, an important sink for U from the oceans, was investigated by the analyses of hydrothermal water samples (including low- and high-temperature fluids), low-temperature altered basalts and calcium carbonate veins. All analysed rock samples from the continental crust fall into a limited range of δ238U between −0.45 and −0.21 ‰ (relative to NBL CRM 112-A), with an average of −0.30 ± 0.15 ‰ (2 SD, N = 11). Despite differences in catchment lithologies, all major rivers define a relatively narrow range between −0.31 and −0.13 ‰, with a weighted mean isotope composition of −0.27 ‰, which is indistinguishable from the estimate for the upper continental crust (−0.30 ‰). Only some tributary rivers from the Swiss Alps display a slightly larger range in δ238U (−0.29 to +0.01 ‰) and lower U concentrations (0.87–3.08 nmol/kg) compared to the investigated major rivers (5.19–11.69 nmol/kg). These findings indicate that only minor net U isotope fractionation occurs during weathering and transport of material from the continental crust to the oceans. Altered basalts display moderately enriched U concentrations (by a factor of 3–18) compared to those typically observed for normal mid-ocean ridge basalts. These, and carbonate veins within altered basalts, show large U isotope fractionation towards both heavy and light U isotope compositions (ranging from −0.63 to +0.27 ‰). Hydrothermal water samples display low U concentrations (0.3–1 nmol/kg) and only limited variations in their U isotope composition (−0.43 ± 0.25 ‰) around the seawater value. Nevertheless, two of the investigated fluids display significantly lower δ238U (−0.55 and −0.59 ‰) than seawater (−0.38 ‰). These findings, together with the heavier U isotope composition observed for some altered basalts and carbonate veins support a model, in which redox processes mostly drive U isotope fractionation. This may result in a slightly heavier U isotope composition of U that is removed from seawater during hydrothermal seafloor alteration compared to that of seawater. Using the estimated isotope compositions of rivers and all U sinks from the ocean (of this study and the literature) for modelling of the isotopic U mass balance, this gives reasonable results for recent estimates of the oceanic U budget. It furthermore provides additional constraints on the relative size of the diverse U sinks and respective net isotope fractionation during U removal.
AB - In this study, the U isotope composition, n(238U)/n(235U), of major components of the upper continental crust, including granitic rocks of different age and post-Archaean shales, as well as that of rivers (the major U source to the oceans) was investigated. Furthermore, U isotope fractionation during the removal of U at mid-ocean ridges, an important sink for U from the oceans, was investigated by the analyses of hydrothermal water samples (including low- and high-temperature fluids), low-temperature altered basalts and calcium carbonate veins. All analysed rock samples from the continental crust fall into a limited range of δ238U between −0.45 and −0.21 ‰ (relative to NBL CRM 112-A), with an average of −0.30 ± 0.15 ‰ (2 SD, N = 11). Despite differences in catchment lithologies, all major rivers define a relatively narrow range between −0.31 and −0.13 ‰, with a weighted mean isotope composition of −0.27 ‰, which is indistinguishable from the estimate for the upper continental crust (−0.30 ‰). Only some tributary rivers from the Swiss Alps display a slightly larger range in δ238U (−0.29 to +0.01 ‰) and lower U concentrations (0.87–3.08 nmol/kg) compared to the investigated major rivers (5.19–11.69 nmol/kg). These findings indicate that only minor net U isotope fractionation occurs during weathering and transport of material from the continental crust to the oceans. Altered basalts display moderately enriched U concentrations (by a factor of 3–18) compared to those typically observed for normal mid-ocean ridge basalts. These, and carbonate veins within altered basalts, show large U isotope fractionation towards both heavy and light U isotope compositions (ranging from −0.63 to +0.27 ‰). Hydrothermal water samples display low U concentrations (0.3–1 nmol/kg) and only limited variations in their U isotope composition (−0.43 ± 0.25 ‰) around the seawater value. Nevertheless, two of the investigated fluids display significantly lower δ238U (−0.55 and −0.59 ‰) than seawater (−0.38 ‰). These findings, together with the heavier U isotope composition observed for some altered basalts and carbonate veins support a model, in which redox processes mostly drive U isotope fractionation. This may result in a slightly heavier U isotope composition of U that is removed from seawater during hydrothermal seafloor alteration compared to that of seawater. Using the estimated isotope compositions of rivers and all U sinks from the ocean (of this study and the literature) for modelling of the isotopic U mass balance, this gives reasonable results for recent estimates of the oceanic U budget. It furthermore provides additional constraints on the relative size of the diverse U sinks and respective net isotope fractionation during U removal.
KW - hydrothermal seafloor alteration
KW - isotope geochemistry
KW - oceanic U cycle
KW - rivers
KW - uranium isotope composition
KW - uranium-238
UR - http://www.scopus.com/inward/record.url?scp=84931380069&partnerID=8YFLogxK
U2 - 10.1080/10256016.2015.1047449
DO - 10.1080/10256016.2015.1047449
M3 - Article
C2 - 26085006
AN - SCOPUS:84931380069
VL - 52
SP - 141
EP - 163
JO - Isotopes in Environmental and Health Studies
JF - Isotopes in Environmental and Health Studies
SN - 1025-6016
IS - 1-2
ER -