Geological Society of America

T8 Evolution of Earth’s Surface: Honoring Xiao-Ming Liu, Recipient of the 2021 Mineralogy, Geochemistry, Petrology, and Volcanology Division’s Early Geological Career Award

Sunday, 10 October 2021
8:00 AM - 12:00 PM
Portland Ballroom 252 (Hybrid Room)

LITHIUM AND OXYGEN ISOTOPIC COMPOSITIONS OF ANCIENT SHALES TRACE CHEMICAL WEATHERING ACROSS THE ARCHEAN-PROTEROZOIC BOUNDARY

Paper number: T8-7
Program: Technical Sessions
Time: 9:25 AM – 9:40 AM

Authors

Su Li Presenting Author
Lanzhou University, Lanzhou
Roberta Rudnick
University of California Santa Barbara, Santa Barbara, CA
Lin Qiu
IVL Swedish Environmental Research Institute, Shanghai
Claire Zurkowski
University of Chicago, Chicago
Rich Gaschnig
University of Massachusetts Lowell, Lowell, MA
John Valley
University of Wisconsin-Madison, Madison, WI
Bradley M. Guy
University of Johannesburg, Auckland Park
Nicolas J. Beukes
University of Johannesburg, Johannesburg

Abstract

To investigate the compositional evolution of continental crust and weathering conditions across the Archean-Proterozoic boundary, major and trace elements, Li and O isotopes were analyzed for thirty-four shales from the West Rand and Central Rand Groups of the Witwatersrand Supergroup and the Pretoria Group of the Transvaal Supergroup, Kaapvaal craton, South Africa, with depositional ages ranging from 2960 to 2250 Ma. The δ⁷Li values of the shales, ranging from -4.3 to +3.1, are lower than those of average MORB (+3.7±2, 2σ) and the low δ⁷Li values are generally accompanied by high CIA (Chemical Index of Alteration), δ¹⁸O and [Li] values, reflecting a weathering signature. A generally positive correlation between CIA and [Li] indicates uptake of lithium by clay minerals during chemical weathering. Lack of correlation between diagnostic trace element ratios (i.e., La/Yb, Th/La, Ni/Co and Th/Sc) and d⁷Li, d¹⁸O, CIA values suggests that the mafic/felsic characteristics of the provenance has negligible influence on the isotopic compositions of the shales, though high Ni/Co shales (indicating mafic/ultramafic provenance) have uniformly high CIA. The δ⁷Li and [Li] averages (δ⁷Li = -0.9±3, 2σ; [Li] = 30 ± 22, 1σ) of the shales are lower than those of PAAS (post Archean Australian shales, δ⁷Li = -0.3 ± 2.6, 1σ; [Li] = 63 ± 25 ppmw, 1σ), and the CIA average of 82 for Archean shales is much higher than that of PAAS (CIA = 69), far above the range of fresh basalts (40-45) and the average upper continental crust (59-65). Combining shale data in this study with literature data for sedimentary rocks (e.g., shales, tillites, pelites), we observe that the Archean crust experienced more intense weathering than post-Archean crust (Li et al., 2016, GCA), but Archean shales have lower δ¹⁸O values (6.8 ± 1.4, 1σ) than those of post-Archean shales (7.5 ± 2.4, 1σ). As in previous works, we attribute the increasing trend of δ¹⁸O across the Archean – Proterozoic boundary to enhanced sedimentary recycling, possibly related to the wide-spread emergence of continents.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-367061

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