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Germanium isotopes and Ge/Si fractionation under extreme tropical weathering of basalts from the Hainan Island, South China TEXT SIZE: A A A
Chemical weathering of silicate rocks controls the fluvial input of dissolved Ge and Si into the ocean, and has substantial influence on the global Ge and Si geochemical cycles. The heavier dissolved Ge isotope compositions in the rivers (relative to the bulk silicate earth) suggest preferential incorporation of light isotopes into secondary weathering products during rock weathering (Baronas et al., 2017a). In this paper, we present the Ge isotope and Ge/Si ratio variations in the solid weathering products (soil and saprolite) from a well-developed basalt weathering profile (>15 m thick, including soil, saprolite, semi-weathered rock and fresh basalt) on the tropical island of Hainan (South China). We discussed the elemental/isotopic fractionation mechanism and the possible influence of major oxide composition on Ge isotope fractionation during extreme weathering of basalts in tropical climate. The Ge content ([Ge] = 2.19-4.12 ppm, 2.93 ppm on average, n = 52) and Ge/Si ratios (5.55-13.7 mu mol/mol, 7.42 mu mol/mol on average, n = 52) of solid weathering products are distinctly higher than those of the fresh basalts (avg. [Ge] = 1.64 ppm, Ge/Si = 2.66 mu mol/mol, n = 5). The delta Ge-74 values of solid weathering products range from -0.02 +/- 0.10%e (2 sigma) to 0.63 +/- 0.10%e (2 sigma), and exhibits complex stratigraphic variation across the weathering profile. There are no distinct correlations between the concentrations of Ge (or the Ge/Si ratio and delta Ge-74 values) and SiO2, Al2O3, Fe2O3 and MnO for most of the soil and saprolite samples. The distinct positive delta Ge-74 vs. Ge (or tau(Ge) value and Ge/Si ratio) correlations for most soil and saprolite samples indicate that the enrichment (or depletion) of Ge content, Ge isotopes and Ge/Si fractionation are controlled by a common sorption process of solid weathering products. The predicted opposite relationships between Ge/Si ratios and delta Ge-74 values for pore water and that for solid weathering products can be evidenced by the positive Ge/Si vs. delta Ge-74 correlation in the Wenchang basalt weathering profile and a negative one in the river water (Baronas et al., 2017a). Moreover, the estimated negative Delta Ge-74(solid-dissolved) (1000 * Ln(alpha)) value (-1.38 +/- 0.28 parts per thousand (2 sigma)) indicates that solid weathering products are a sink for light Ge isotopes, possibly balancing the isotopic budget with heavy Ge isotope compositions in global rivers and oceans. (C) 2019 Elsevier Ltd. All rights reserved.
 

Publication name

 GEOCHIMICA ET COSMOCHIMICA ACTA, 253 249-266; 10.1016/j.gca.2019.03.022 MAY 15 2019

Author(s)

 Qi, Hua-Wen; Hu, Rui-Zhong; Jiang, Ke; Zhou, Ting; Liu, Yue-Fu; Xiong, Yan-Wen

Corresponding author(s) 

 QI Huawen 
 qihuawen@vip.gyig.ac.cn  
 Chinese Acad Sci, Inst Geochem, State Key Lab Ore Deposit Geochem, Guiyang 550081, Guizhou, Peoples R China.

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