Geological cycling of potassium and the K isotopic response: insights from loess and shales

Shales are a major sink for K into seawater delivered from continental weathering, and are potential recorders of K cycling. High precision K isotope analyses reveal a > 0.6 parts per thousand variation in delta K-41 values (K-41/K-39 relative to NIST SRM 3141a) from a set of well characterized post-Archean Australian shale (PAAS) samples. By contrast, loess samples have relatively homogenous delta K-41 values (-0.5 +/- 0.1 parts per thousand), which may represent the average K composition of upper continental crust. Most of the shales analyzed in this study have experienced K enrichment relative to average continental crust, and the majority of them define a trend of decreasing delta K-41 value (from -0.5 to -0.7 parts per thousand) with increasing K content and K/Na ratio, indicating cation exchange in clays minerals is accompanied by K isotope fractionation. Several shale samples do not follow the trend and have elevated delta K-41 values up to -0.1 parts per thousand, and these samples are characterized by variable Fe isotope compositions, which reflect post-depositional processes. The K isotope variability observed in shales, in combination with recent findings about K isotope fractionation during continental weathering, indicates that K isotopes fractionate during cycling of K between different reservoirs, and K isotopes in sediments may be used to trace geological cycling of K.