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Controlling factors of potassium (K) isotopic fractionation in K-feldspar remain poorly constrained. In this study, we analyzed the K isotopic compositions of 11 K-feldspar samples from diverse lithological compositions. The degree of Al/Si order ranged from 0.22 to 0.94 (1.0 = completely ordered). Analyzed samples are mixtures of K-feldspar (>70 wt%) and coexisting albite. The relative contribution of K2O from the K-feldspar phase of the sample was over 98%, indicating that the K isotopic composition (δ41K) derives mainly from K-feldspar and hence reflects its behavior. The δ41K values of these samples range from –0.710 to –0.075‰, which are slightly correlated with the degree of Al/Si order. The correlations of δ41K with SiO2 and Al2O3 contents and the corresponding Al/Si mole ratios reveal that Al and Si play a significant role in the K isotopic behavior of K-feldspar. The correlations of δ41K with SiO2 and Al2O3 contents are attributed to the difference in K-O bond strengths. Compared to K-feldspar, the K content could be a better proxy for constraining the δ41K of plagioclase. Our results demonstrate that the δ41K of K-feldspar is dependent on its mineral chemistry, and its K isotopic composition may be insensitive to other factors, such as the source heterogeneity. The inference is further confirmed by comparing the δ41K values in this study with published δ41K values of K-feldspar from different sources. [ABSTRACT FROM AUTHOR]

For the first time, cathodoluminescence (CL) was used to show oscillatory zoning in perthitic K‑feldspars from the equigranular Toki granite, central Japan. Based on the CL patterns, two types of zoning are identified: single core oscillatory zoning (SCOZ) and multiple core oscillatory zoning (MCOZ). The SCOZ is defined by oscillatory zoning around a single-crystal core within the K‑feldspar crystal, whereas the MCOZ depicts two or more such crystal cores. The crystal cores displayed in CL images reflect the nucleation parts of magmatic K‑feldspar. The existence of MCOZ patterns in K‑feldspars indicates multiple nuclei. CL patterns reveal crystal growth behavior of magmatic K‑feldspar in the equigranular Toki granite. CL intensities are positively correlated with titanium and barium concentrations, indicating that the CL variations depend on two factors: (1) titanium concentration as a CL activator and (2) density of Al-O−-Al structural defects. The analysis of CL images revealed that albite-rich phases in microperthite and patchperthite with low-luminescence intensities cut across the CL bands of the oscillatory zoning, indicating that the oscillatory zoning in the orthoclase-rich host phase of K‑feldspar was not perturbed by the formation of microperthite and patchperthite in the post-crystallization stage. The luminescence intensities of albite-rich phases in patchperthite are lower than those in microperthite, which is due to the diferences in titanium and barium concentrations between them. In the post-crystallization stage, the mass transfer of titanium and barium occurred during the formation of microperthite and patchperthite. Therefore, the diference in the luminescence intensities between microperthite and patchperthite lamellae reflects their diferent formation mechanisms between exsolution coarsening and dissolution-precipitation coarsening. In summary, CL analyses can be used for the evaluation of the nucleation and growth not only of anhedral K‑feldspar crystals in equigranular granite but also of K‑feldspar phenocrysts/megacrysts in porphyritic granite. It can reveal the spatial extent of element partitioning between the melt and crystal, along with that of mass transfer from the melt into crystals during the magma evolution. Moreover, the CL analyses can also be used for the interpretation of K‑feldspar textural development during the post-crystallization stage. [ABSTRACT FROM AUTHOR]

The K/Cs ratios of K-feldspars from granitic pegmatites are compared to models derived from the Rayleigh equation. The K/Cs and K/Rb ratios of K-feldspars and micas exhibit decreasing values when plotted against their Cs or Rb contents across cogenetic suites of granites and pegmatites and from margin to core of individual bodies. The trends in elemental ratios conform to Rayleigh fractionation for the crystallization of feldspars and micas from a silicate melt. Within two individual pegmatite bodies, the K/Cs ratio of K-feldspar initially falls more rapidly than the Rayleigh model predicts. That might reflect a local increase in the concentration of Cs relative to K due to the pile-up of incompatible elements in a boundary layer of melt adjacent to the crystal growth front. The addition of an aqueous solution to the Rayleigh model (i.e., the simultaneous crystallization of K-feldspars from melt and from aqueous solution) predicts high and increasing K/Cs ratios of K-feldspars that are not observed in natural rock suites, except when K-feldspars crystallize in miarolitic cavities or when primary K-feldspar recrystallizes to microcline perthite in an open hydrothermal system. In those cases, the Cs content of K-feldspars falls to nil because of the high solubility of Cs in aqueous solution and low compatibility of Cs in K-feldspar. Otherwise, the observed patterns of K/Rb or K/Cs in K-feldspar and micas in pegmatites conform to crystal-melt fractionation in which an aqueous solution plays no part. From the viewpoint of the geochemistry of Cs in pegmatites, these observations give support to the model proposed by Cameron et al. (1949) and endorsed by Jahns (1953a, 1953b). [ABSTRACT FROM AUTHOR]