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 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.

Extensive crystal fractionation of high-silica magmas revealed by K isotopes

“…Considering that the K─O bond length in silicate glass (3.00 to 3.06 Å) is close to that in orthoclase (2.94 Å) ( 68 ), we used the Δ 41 K mineral−orthoclase as an approximation to the Δ 41 K mineral−melt in our model. The δ 41 K value of feldspar is inversely correlated with its orthoclase content ( 23 , 24 , 26 ); orthoclase-poor plagioclase [Or = 3 mole percent (mol %)] can be isotopically heavier than the coexisting K-rich feldspar in equilibrium (Or > 50 mol %) by 2.5‰ ( 24 ). By contrast, K isotope fractionation between mica minerals and K-rich feldspar (Or > 50%) is near zero at magmatic temperatures ( 23 – 25 , 54 ).…”

“…Production of leucogranite by anatexis of metasediments is mainly governed by the following reaction: muscovite + plagioclase + quartz ± xH 2 O → melt ± K-feldspar ± biotite ± aluminum-silicate, where the proportions of muscovite and plagioclase contributing to the melt strongly depend on the activity of H 2 O (36). Given the large K isotopic difference between plagioclase and mica (23)(24)(25)(26), incongruent melting of muscovite or plagioclase may fractionate K isotopes between melt and its source. Here, we quantitatively evaluated the extent of K isotope fractionation during anatexis using the same nonmodal partial melting model as previously used to quantify Sr and Nd isotopes (see Materials and Methods for details) (43).…”

“…The most likely process responsible for the progressive decrease in  41 K value from two-mica leucogranite to muscovite leucogranite is the continuous separation of plagioclase during magmatic differentiation, which is the only known phase that could be significantly enriched in heavy K isotopes (23)(24)(25)(26). This is supported by the positive correlations of  41 K value with sensitive proxies for plagioclase crystallization, e.g., Sr and Eu/Eu*, and crystallization temperature in most of our leucogranite samples (Fig.…”

“…Thus, any change in the modes of plagioclase and K-feldspar will affect the geochemical behavior of K during fractional crystallization of high-silica magmas, leading to the nonsystematic variation in K concentration commonly observed in high-silica igneous rocks [e.g., (18,20,21)]. Recent studies revealed large K isotope fractionation among minerals in igneous rocks (22)(23)(24). In particular, plagioclase is significantly enriched in 41 K compared to other coexisting rock-forming minerals [e.g., K-feldspar, hornblende, and mica; (23,24)] due to the decrease in K─O bond length with (Na + Ca)/K in feldspars (25,26).…”

“…Recent studies revealed large K isotope fractionation among minerals in igneous rocks (22)(23)(24). In particular, plagioclase is significantly enriched in 41 K compared to other coexisting rock-forming minerals [e.g., K-feldspar, hornblende, and mica; (23,24)] due to the decrease in K─O bond length with (Na + Ca)/K in feldspars (25,26). Accordingly, systematic K isotopic variation might be expected in melts that experience different degrees of plagioclase crystallization.…”