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High-temperature inter-mineral potassium isotope fractionation: implications for K-Ca-Ar chronology.

ABSTRACT: 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 a granite. In addition, we also observed smaller but measurable K isotope fractionation (0.280‰±0.030‰) between ternary feldspar phenocrysts and matrix 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 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 phenocryst rim and matrix in the trachyandesite agrees well with the calculated equilibrium isotope fractionation. However, the measured K isotope fractionations between 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 Earth and 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 the variation of 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±0.11ppm) 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.

SUBMITTER: Wilson Kuhnel W

PROVIDER: S-EPMC8740525 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Silicon and Mg in differentiated rocky bodies exhibit heavy isotope enrichments that have been attributed to evaporation of partially or entirely molten planetesimals. We evaluate the mechanisms of planetesimal evaporation in the early solar system and the conditions that controled attendant isotope fractionations. Energy balance at the surface of a body accreted within ~1 Myr of CAI formation and heated from within by 26Al decay results in internal temperatures exceeding the silicate solidus, producing a transient magma ocean with a thin surface boundary layer of order < 1 meter that would be subject to foundering. Bodies that are massive enough to form magma oceans by radioisotope decay (≥ 0.1% M ⊕) can retain hot rock vapor even in the absence of ambient nebular gas. We find that a steady-state rock vapor forms within minutes to hours and results from a balance between rates of magma evaporation and atmospheric escape. Vapor pressure buildup adjacent to the surfaces of the evaporating magmas would have inevitably led to an approach to equilibrium isotope partitioning between the vapor phase and the silicate melt. Numerical simulations of this near-equilibrium evaporation process for a body with a radius of ~ 700 km yield a steady-state far-field vapor pressure of 10-8 bar and a vapor pressure at the surface of 10-4 bar, corresponding to 95% saturation. Approaches to equilibrium isotope fractionation between vapor and melt should have been the norm during planet formation due to the formation of steady-state rock vapor atmospheres and/or the presence of protostellar gas. We model the Si and Mg isotopic composition of bulk Earth as a consequence of accretion of planetesimals that evaporated subject to the conditions described above. The results show that the best fit to bulk Earth is for a carbonaceous chondrite-like source material with about 12% loss of Mg and 15% loss of Si resulting from near-equilibrium evaporation into the solar protostellar disk of H2 on timescales of 104 to 105 years.

Project description:RationaleOxygen isotope exchange between O2 and CO2 in the presence of heated platinum (Pt) is an established technique for determining the δ17 O value of CO2 . However, there is not yet a consensus on the associated fractionation factors at the steady state.MethodsWe determined experimentally the steady-state α17 and α18 fractionation factors for Pt-catalyzed CO2 -O2 oxygen isotope exchange at temperatures ranging from 500 to 1200°C. For comparison, the theoretical α18 equilibrium exchange values reported by Richet et al. (1997) have been updated using the direct sum method for CO2 and the corresponding α17 values were determined. Finally, we examined whether the steady-state fractionation factors depend on the isotopic composition of the reactants, by using CO2 and O2 differing in δ18 O value from -66 ‰ to +4 ‰.ResultsThe experimentally determined steady-state fractionation factors α17 and α18 are lower than those obtained from the updated theoretical calculations (of CO2 -O2 isotope exchange under equilibrium conditions) by 0.0024 ± 0.0001 and 0.0048 ± 0.0002, respectively. The offset is not due to scale incompatibilities between isotope measurements of O2 and CO2 nor to the neglect of non-Born-Oppenheimer effects in the calculations. There is a crossover temperature at which enrichment in the minor isotopes switches from CO2 to O2 . The direct sum evaluation yields a θ value of ~0.54, i.e. higher than the canonical range maximum for a mass-dependent fractionation process.ConclusionsUpdated theoretical values of α18 for equilibrium isotope exchange are lower than those derived from previous work by Richet et al. (1997). The direct sum evaluation for CO2 yields θ values higher than the canonical range maximum for mass-dependent fractionation processes. This demonstrates the need to include anharmonic effects in the calculation and definition of mass-dependent fractionation processes for poly-atomic molecules. The discrepancy between the theory and the experimental α17 and α18 values may be due to thermal diffusion associated with the temperature gradient in the reactor.

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High-temperature inter-mineral potassium isotope fractionation: implications for K-Ca-Ar chronology.

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