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

Springer Science and Business Media LLC - Tập 38 - Trang 508-516 - 2019
Weiqiang Li1, Shilei Li2, Brian L. Beard3,4
1State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing, People’s Republic of China
2MOE Key Laboratory of Surficial Geochemistry, School of Earth Sciences and Engineering, Nanjing University, Nanjing, People’s Republic of China
3Department of Geoscience, University of Wisconsin-Madison, Madison, USA
4NASA Astrobiology Institute, University of Wisconsin–Madison, Madison, USA

Tóm tắt

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 ‰ variation in δ41K values (41K/39K relative to NIST SRM 3141a) from a set of well characterized post-Archean 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 − 0.5 to − 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.

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