Geochemistry and cosmochemistry of potassium stable isotopes

Wang, Kun; Li, Weiqiang; Li, Shilei; Tian, Zhengkun; Koefoed, Piers; Zheng, Xin-Yuan

doi:10.1016/j.chemer.2021.125786

Cited by 43 publications

(30 citation statements)

References 198 publications

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“…Alkaline rocks derived from the MLMR can offer opportunities to investigate the interaction of the MLMR and asthenosphere 14 . The K, Sr, and Nd isotopic compositions of alkaline rocks are similar to those of the subducted sediments 3,15 , and should be distinctly different from those of the asthenosphere which has a limited K isotopic composition range at the initial stage 5 . Therefore, the isotopic compositions (K, Sr, and Nd) of alkaline rocks derived from the MLMR are good proxies for evaluating the extent and nature of the interaction between the MLMR and asthenosphere.…”

Section: Introductionmentioning

confidence: 88%

Decoupling of K isotope from Sr-Nd isotope in the Saima alkaline complex, NE China: interactions of cratonic roots and asthenosphere

Gao

Zhu

et al. 2022

Preprint

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We report high-precision K isotopes, apatite U-Pb ages, whole-rock elements, and Sr-Nd isotopes of Saima nepheline syenite in the North China Craton. Trace element and Sr-Nd-Hf-O isotope data indicate the presence of subducting sediments in source region, while K isotopic compositions showed a narrow range between –0.54 ‰ and –0.31 ‰, with an average of –0.41 ± 0.06 ‰, identical to the value of asthenosphere. The nearly identical K isotopic compositions are low probability events compared with K isotopic compositions of the island arc lavas reported previously (–1.55 ‰ to +0.2 ‰). Although crustal contamination is consistent with Sr-Nd-K isotopic data, alternatively we propose the isotopic data also reconcile to the interaction between cratonic roots and underlying convective asthenosphere, if this interaction is over prolonged periods of time. Numerical simulations successfully reproduced the observed data, if the metasomatized lithospheric mantle root, the source of the Saima, occurred 500 Ma ago. Our study reveals that the isotopic compositions of fast-diffusion components in a metasomatized lithospheric mantle by ancient subducting melts can be effectively homogenized by convective asthenosphere through diffusion in a long-time interval.

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Section: Introductionmentioning

confidence: 88%

Decoupling of K isotope from Sr-Nd isotope in the Saima alkaline complex, NE China: interactions of cratonic roots and asthenosphere

Gao

Zhu

et al. 2022

Preprint

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“…Potassium has two stable isotopes, 39 K (93.3%) and 41 K (6.7%), and there is a large (5%) relative mass difference between the two isotopes. 41 K/ 39 K ratios are remarkably variable in nature, and significant K isotope fractionations have been documented in various processes, 1 including planetary evaporation and condensation, 2–8 continental weathering, 9–17 marine diagenesis, 16,18 hydrothermal alteration, 12,13,19–21 metamorphic dehydration, 22 mineral precipitation, 18,23–25 and biological uptake 26–30 . Stable K isotope geochemistry is becoming a vibrant field of research in recent years 1 …”

Section: Introductionmentioning

confidence: 99%

“…Potassium (K) is a major element that plays important roles in a plethora of cosmochemical, geological, geochemical, and biological processes due to its moderately volatile, lithophile, water‐soluble, and biophile properties 1 . Potassium has two stable isotopes, 39 K (93.3%) and 41 K (6.7%), and there is a large (5%) relative mass difference between the two isotopes.…”

Section: Introductionmentioning

confidence: 99%

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Precise measurement of ⁴¹K/³⁹K ratios by high‐resolution multicollector inductively coupled plasma mass spectrometry under a dry and hot plasma setting

Luo

2022

Rapid Comm Mass Spectrometry

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Rationale: Stable K isotope geochemistry is becoming an important tool for various applications. Developments in analytical methods for K isotopes based on multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) without collision cell will bring research capability of K isotopes to many existing MC-ICP-MS labs.Methods: Stable K isotopes were analyzed without applications of "cold plasma" and collision cell on a Nu 1700 Sapphire high-resolution multicollector inductively coupled plasma mass spectrometer. A conventional dry and hot plasma setting is used for analysis to maintain high K sensitivity and signal stability, and high mass resolution was applied to provide interference-free shoulders of 39 K + for isotopic measurement of 41 K/ 39 K ratios. 40 Ar + ion beam generated in ICP was neutralized in the ion guide rail for the Daly detector.Results: Under such operating conditions, an external reproducibility of <±0.1‰(2 standard deviation) for 41 K/ 39 K is achieved for K solutions of 1 ppm or above.Tests were carried out to evaluate the influence of total K loading, K concentration and acid molarity mismatch, matrix effects, and 40 Ar + and 40 Ar 1 H + tailing on K isotope analysis. We found that the accuracy of K isotope analysis can be compromised by concentration mismatch of sample and standard K, by 0.007‰ in δ 41 K per 1% mismatch of K content. By contrast, mismatch of HNO 3 molarity or existence of HCl in HNO 3 exerts negligible influences on the analytical precision and accuracy of K isotope analysis. Furthermore, K isotope analytical results remain accurate when Na/K, Mg/K, Ca/K, Rb/K, V/K, and Cr/K ratios are below 3%. Conclusions:The high-precision K isotope analytical method reported here is robust for studies on K isotopic variations in geological, cosmochemical, and biological samples. The f 41 K values of six international geostandards measured using our method are consistent with data measured using different analytical methods from other laboratories. | INTRODUCTIONPotassium (K) is a major element that plays important roles in a plethora of cosmochemical, geological, geochemical, and biological processes due to its moderately volatile, lithophile, water-soluble, and biophile properties. 1 Potassium has two stable isotopes, 39 K (93.3%) and 41 K (6.7%), and there is a large (5%) relative mass difference between the two isotopes. 41 K/ 39 K ratios are remarkably variable in nature, and significant K isotope fractionations have been documented in various processes, 1 including planetary evaporation

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“…The high-precision measurements of K isotopes can be traced back to the 1990s, when an extensive survey of geological samples by Humayun and Clayton found no measurable K isotopic difference on Earth. However, with the development of multiple-collector inductively coupled-plasma mass-spectrometry (MC-ICP-MS), the measurements of K isotopes have been significantly improved, and considerable isotopic variations of K have been observed in bulk igneous and sedimentary rocks, clay, evaporite minerals, and seawater samples. − The K isotopic variations observed on Earth have been mostly attributed to low-temperature geochemical processes such as continental weathering, hydrothermal alteration, and authigenic clay formation. ,− However, it is still unknown whether there is any significant inter-mineral K isotope fractionation during high-temperature igneous processes. Any inter-mineral fractionation of K isotopes is important for the accuracy of K–Ca–Ar dating because current calculations assume a constant 40 K abundance across different samples and minerals (cf.…”

Section: Introductionmentioning

confidence: 99%

High-Temperature Inter-Mineral Potassium Isotope Fractionation: Implications for K–Ca–Ar Chronology

Kuhnel

Jacobsen

et al. 2021

ACS Earth Space Chem.

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Recent advances in high-precision potassium (K) isotopic analysis have found considerable isotopic variation in rock samples of the Earth’s continental and oceanic crusts; however, it is still uncertain whether there is any resolvable inter-mineral and mineral-melt K isotopic fractionation during igneous and metamorphic processes. Here, we report K isotope compositions of mineral separates from three extremely well-preserved igneous rocks (intrusive/extrusive and mafic/intermediate/felsic) in order to investigate possible inter-mineral and mineral-melt K isotopic fractionation at magmatic temperatures. For the first time, we found large inter-mineral fractionation of K isotopes in natural samples (up to 1.072‰), where plagioclase displays a significant enrichment of heavier K isotopes compared to potassium feldspar and biotite in granite. In addition, we also observed smaller but measurable K isotope fractionation (0.280 ± 0.030‰) between ternary feldspar phenocrysts and matrices in a trachyandesite, as well as a comparable isotope fractionation (0.331 ± 0.010‰) between plagioclase and the bulk in a gabbroic intrusive rock. We also evaluated such results by comparing the theoretically calculated equilibrium K isotope fractionation factors between relevant igneous minerals in the literature and this study. In general, the measured inter-mineral fractionations are consistent with the theoretical calculations (i.e., plagioclase is enriched in heavier isotopes compared to potassium feldspar). Specifically, the measured K isotope fractionation between the phenocryst rim and matrix in the trachyandesite agrees well with the calculated equilibrium isotope fractionation. However, the measured K isotope fractionations between the phenocryst core and matrix as well as between plagioclase and K-feldspar are significantly larger (by a factor of ∼2–3) than the calculated isotope fractionations, which suggest isotopic disequilibrium due to kinetic processes. Using a range of plagioclase-melt isotope fractionation factors inferred from the theoretical calculations in this study, we modeled the K isotopic fractionation during the formation of lunar anorthositic crust, and the result shows a negligible effect on the K isotopic compositions in both lunar crust and mantle. The K isotopic difference between the Earth and the Moon, therefore, cannot be the result of lunar magma ocean differentiation. Finally, we evaluate the effect of observed inter-mineral fractionations on K–Ar and 40Ar–39Ar dating. This study indicates that the variation of the 40K/K ratio would contribute a maximum 0.08% error to the K–Ar and 40Ar–39Ar age uncertainties. We propose a refined 40K/total K ratio as 0.00011664 ± 0.00000011 (116.64 ± 0.11 ppm) instead of the conventional value, 0.0001167(2), for the present Earth. Because some minerals fractionate K isotopes, ultrahigh-precision age dating with the K–Ca–Ar dating systems must measure the K isotope fractionation in the same mineral fractions used for age dating.

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Geochemistry and cosmochemistry of potassium stable isotopes

Cited by 43 publications

References 198 publications

Decoupling of K isotope from Sr-Nd isotope in the Saima alkaline complex, NE China: interactions of cratonic roots and asthenosphere

Decoupling of K isotope from Sr-Nd isotope in the Saima alkaline complex, NE China: interactions of cratonic roots and asthenosphere

Precise measurement of ⁴¹K/³⁹K ratios by high‐resolution multicollector inductively coupled plasma mass spectrometry under a dry and hot plasma setting

High-Temperature Inter-Mineral Potassium Isotope Fractionation: Implications for K–Ca–Ar Chronology

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Geochemistry and cosmochemistry of potassium stable isotopes

Cited by 43 publications

References 198 publications

Decoupling of K isotope from Sr-Nd isotope in the Saima alkaline complex, NE China: interactions of cratonic roots and asthenosphere

Decoupling of K isotope from Sr-Nd isotope in the Saima alkaline complex, NE China: interactions of cratonic roots and asthenosphere

Precise measurement of 41K/39K ratios by high‐resolution multicollector inductively coupled plasma mass spectrometry under a dry and hot plasma setting

High-Temperature Inter-Mineral Potassium Isotope Fractionation: Implications for K–Ca–Ar Chronology

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Precise measurement of ⁴¹K/³⁹K ratios by high‐resolution multicollector inductively coupled plasma mass spectrometry under a dry and hot plasma setting