Heterogeneous potassium isotopic composition of the upper continental crust. (1st June 2020)

• Record Type: Journal Article
• Authors: Huang, Tian-Yi; Teng, Fang-Zhen; Rudnick, Roberta L.; Chen, Xin-Yang; Hu, Yan; Liu, Yong-Sheng; Wu, Fu-Yuan
• Is Part Of: Geochimica et cosmochimica acta. Volume 278(2020)
• Page Start: 122
• Page End: 136
• Publication Date: 2020-06-01
• Subjects: Potassium isotopes -- Granite -- Loess -- Shale -- Upper continental crust; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2019.05.022
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Assessing the K isotopic composition of the upper continental crust is important for understanding the processes by which the crust is generated and modified, and constraining the K isotopic budget of the silicate Earth. High-precision K isotopic data are reported for 70 well-characterized individual and composite samples from the upper continental crust, including diorite, granodiorite, granite, loess, shale, graywacke, pelite, and tillite, to constrain its K isotopic composition. δ 41 K varies significantly in eight I-type and two S-type granites from Australia (−0.57 to −0.40‰), nine A-type granites (−0.53 to −0.38‰), and three granitoid composites including diorite, granodiorite, and granite (−0.50 to −0.37‰) from China, mainly reflecting source heterogeneity. The 22 post-Archean Australian shales (PAAS) (δ 41 K = −0.68 to −0.12‰) and the 12 sedimentary composites including graywacke, pelite, and tillite from China (δ 41 K = −0.57 to −0.23‰) have heterogeneous K isotopic compositions while the 12 loess samples from around the world display more limited K isotopic variation (δ 41 K = −0.47 to −0.35‰). δ 41 K values display a smaller dispersion in loess compared to shales, which have comparatively more intense weathering and higher chemical index of alteration (CIA). δ 41 K correlates with Fe2 O3 /Al2 O3 and Fe2 O3 /K2 O in shales and Al2 O3 /SiO2, K2 O/Al2 O3, and δ 7 Li in loess, suggesting that K isotopes are fractionated during chemical weathering. Overall, the upper continental crust has a heterogeneous K isotopic composition, ranging from −0.68 to −0.12‰ with an average δ 41 K of −0.44 ± 0.05‰ (2SD, n = 88), which is indistinguishable from the mantle.