气候变化领域集成服务门户(CCPortal): Phyllosilicate controls on magnesium isotopic fractionation during weathering of granites: Implications for continental weathering and riverine system

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DOI	10.1016/j.epsl.2020.116613
	Phyllosilicate controls on magnesium isotopic fractionation during weathering of granites: Implications for continental weathering and riverine system
	Li M.Y.H.; Teng F.-Z.; Zhou M.-F.
发表日期	2021
ISSN	0012821X
卷号	553
英文摘要	Continental weathering is a fundamental process in releasing magnesium (Mg) from crystalline rocks to the hydrosphere and biosphere. Mg isotopes can be substantially mobilized, re-distributed, and fractionated during weathering, and therefore can be used as a powerful tool to trace the biogeochemical cycle of Mg. Causes of significant Mg isotopic fractionation and behaviors during silicate weathering are still not well understood, hindering further application of the Mg isotopes to probe different geological processes. In this study, we demonstrate that dissolution and formation of phyllosilicates are the main control of Mg isotopic fractionation during sub-tropical weathering of granite. Furthermore, different formation and dissolution mechanisms for the same mineral phase could also cause variations in magnitude and directionality of fractionation. In incipient weathering, supergene phyllosilicates form mainly through topotactic transformation. Vermiculitization of parental chlorite tends to release 24Mg and causes significant 26Mg enrichment in the saprock. In an advanced stage of weathering, Mg isotopic compositions of supergene phyllosilicates are more influenced by the interaction with the soil solutions. Minerals formed mainly through a dissolution-precipitation mechanism with Mg in neoformed phyllosilicates dominantly sourced from the contemporary soil solutions. 26Mg would be firstly incorporated into neoformed phyllosilicates, such as vermiculite, interstratified biotite/vermiculite and chlorite/vermiculite. Therefore, soil solutions became more enriched in 24Mg with depth in the pedolith, from which relatively 24Mg-rich phyllosilicates would form. However, in the saprolite, precipitation of illite may have preferentially scavenged 24Mg, enriching the soil solutions with 26Mg. Varying relative abundances of different phyllosilicate minerals along the profile could cause large variations in the Mg isotopic compositions of regolith. Our study shows that Mg isotopic composition of the slightly weathered materials could be significantly heavy. Hence, entrainment of 26Mg-rich but slightly weathered materials could be an alternative to explain the high δ26Mg as recorded in some sedimentary rocks, especially of aeolian source. Whereas low δ26Mg widely archived in groundwater and river water could be alternatively explained by interaction with the saprock and 26Mg scavenging during phyllosilicate transformation, instead of severe depletion of 26Mg in soil solutions due to intense weathering and vast formation of secondary minerals, as previously suggested. Comprehensive characterization of the weathering processes and the resultant products is essential to interpret the observed Mg isotopic fractionation and trace the biogeochemical cycle of Mg. © 2020 Elsevier B.V.
关键词	clay minerals granite weathering magnesium isotopes Mg biogeochemical cycle phyllosilicates
英文关键词	Biogeochemistry; Clay minerals; Dissolution; Ecology; Granite; Groundwater; Isotopes; Magnesium compounds; Mica; Nanocrystalline materials; Sedimentary rocks; Silicates; Soil moisture; Biogeochemical cycle; Dissolution mechanism; Dissolution precipitations; Isotopic composition; Isotopic fractionations; Silicate weathering; Topotactic transformation; Tropical weathering; Weathering; biogeochemical cycle; chemical weathering; clay mineral; dissolution; enrichment; granite; isotopic fractionation; magnesium; phyllosilicate
语种	英语
来源期刊	Earth and Planetary Science Letters
文献类型	期刊论文
条目标识符	http://gcip.llas.ac.cn/handle/2XKMVOVA/203184
作者单位	Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong; Isotope Laboratory, Department of Earth and Space Sciences, The University of Washington, Seattle, WA 98195, United States
推荐引用方式 GB/T 7714	Li M.Y.H.,Teng F.-Z.,Zhou M.-F.. Phyllosilicate controls on magnesium isotopic fractionation during weathering of granites: Implications for continental weathering and riverine system[J],2021,553.
APA	Li M.Y.H.,Teng F.-Z.,&Zhou M.-F..(2021).Phyllosilicate controls on magnesium isotopic fractionation during weathering of granites: Implications for continental weathering and riverine system.Earth and Planetary Science Letters,553.
MLA	Li M.Y.H.,et al."Phyllosilicate controls on magnesium isotopic fractionation during weathering of granites: Implications for continental weathering and riverine system".Earth and Planetary Science Letters 553(2021).