Potassium isotope fractionation during granitic magmatic differentiation: Mineral-pair perspectives

Geochimica et Cosmochimica Acta(2023)

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To constrain the behavior of K isotopes during granitic magmatic differentiation, we present high-precision K isotope data for bulk granitoids and their K-bearing minerals from Dabie and Himalayan orogens in China. Plagioclase displays extremely large K isotopic variation, with delta K-41 ranging from -0.56 to 1.91 parts per thousand. Other minerals also have heterogeneous K isotopic compositions, with delta K-41 varying from -0.56 to -0.24 parts per thousand in hornblende, from -0.72 to -0.39 parts per thousand in biotite, from -0.79 to -0.47 parts per thousand in K-feldspar, and from -0.74 to -0.61 parts per thousand in muscovite. The general trend of K-41 enrichment follows the order of plagioclase >> hornblende > biotite approximate to K-feldspar >= muscovite, which can be attributed to the difference in K-O bond strengths. Both delta K-41 of plagioclase and K isotope fractionation factors between plagioclase and other minerals are negatively correlated with K concentration in plagioclase, suggesting a compositional control on isotope fractionations. Inter-mineral K isotope fractionations among other minerals are roughly temperature-dependent. Potassium isotopic compositions of whole rocks display measurable variations (delta K-41 = -0.69 to -0.28 parts per thousand), which are correlated with proxies for plagioclase fractionation (e.g., Sr and Rb/Sr). The correlation indicates that fractional crystallization may account for the whole-rock K isotopic variations observed in high-silica igneous rocks, which is supported by modeling of fractional crystallization using the apparent isotope fractionation factors determined in this work. Our study demonstrates the existence of large K isotope fractionation during granitic magmatism, making K isotopes a potential tool for studying differentiation of felsic magmas. (c) 2022 Elsevier Ltd. All rights reserved.
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