Tracing continental evolution by potassium isotopes

Abstract

The Earth’s continental crust has diverse lithologies and preserves a rich geologic history of Earth’s evolution, which is vital for understanding the origin and evolution of our planet. Potassium (K) and its isotopes can provide new insights into the continent formation and evolution. This dissertation focuses on the K isotope system in the continental crust and its application in tracing continental evolution.The dissertation begins with a review of the analytical protocol for high-precision K isotope measurements using MC-ICPMS in our laboratory, which makes it possible to resolve K isotope variations in natural terrestrial samples. Then, the first paper constrains the K isotopic composition of the upper continental crust. A variety of upper crustal samples, including granite, loess, shale, and upper crustal composites, were measured, which display significant K isotopic variations ranging from 0.68 to -0.12‰. Variations in sedimentary rocks reflect continental weathering, whereas variations in granites may reflect either source heterogeneity or magmatic differentiation. Although the upper crust is isotopically heterogeneous, it has a mantle-like average K isotopic composition, with an average δ41K of 0.44 ± 0.05‰ (2SD, n = 88). The next paper further investigates K isotope behaviors during granitic magmatism to explain the origin of K isotopic variations in granites. Large K isotope fractionations are observed among the granitic minerals, with plagioclase being highly enriched in heavy isotopes compared to other minerals (e.g., K-feldspar, mica, and hornblende). We measured two sets of co-genetic granitoids and their K-bearing minerals. Our results suggest that the differentiated melts may be isotopically light, while plagioclase cumulates are expected to have heavy K isotopic compositions. The third paper aims to investigate the temporal K isotopic variation of the continental crust. We analyzed K isotopes in glacial diamictites from Archean to the present, representing average crustal compositions through time. We found large K isotopic variations in the Archean samples but more homogeneous compositions in younger samples. This change in the magnitude of K isotopic variation is interpreted as reflecting a transition from Na-rich to K-rich continental crust and decreasing weathering intensity over time. Collectively, this dissertation reveals the heterogeneous K isotopic compositions of the upper continental crust, confirms the occurrence of K isotope fractionation during granitic magmatic differentiation, and highlights the importance of K isotopes in tracing continental evolution. The results from this dissertation contribute to the application of K isotopes to trace the Earth’s history.