| Abstract: | 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 a0.6%variation in δ41K values(41K/39K relative to NIST SRM 3141a)from a set of well characterized postArchean Australian shale（PAAS）samples.By contrast,loess samples have relatively homogenous δ41K values（-0.5±0.1%）,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 δ41K value（from-0.5to-0.7%）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 δ41K values up to-0.1%,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. |
