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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Phyllosilicate controls on magnesium isotopic fractionation during weathering of granites: Implications for continental weathering and riverine system
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Phyllosilicate controls on magnesium isotopic fractionation during weathering of granites: Implications for continental weathering and riverine system

机译：花岗岩风化过程中的镁同位素分馏的Phyllosinalate对照：对大陆风化和河流系统的影响

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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 Mg-24 and causes significant Mg-26 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. Mg-26 would be firstly incorporated into neoformed phyllosilicates, such as vermiculite, interstratified biotite/vermiculite and chlorite/vermiculite. Therefore, soil solutions became more enriched in Mg-24 with depth in the pedolith, from which relatively Mg-24-rich phyllosilicates would form. However, in the saprolite, precipitation of illite may have preferentially scavenged Mg-24, enriching the soil solutions with Mg-26. 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 Mg-26-rich but slightly weathered materials could be an alternative to explain the high delta Mg-26 as recorded in some sedimentary rocks, especially of aeolian source. Whereas low delta Mg-26 widely archived in groundwater and river water could be alternatively explained by interaction with the saprock and Mg-26 scavenging during phyllosilicate transformation, instead of severe depletion of Mg-26 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. (C) 2020 Elsevier B.V. All rights reserved.
机译：大陆风化是将镁（Mg）从结晶岩释放到水圈和生物圈的基本过程。镁同位素在风化过程中可以被充分调动、重新分布和分馏，因此可以作为追踪镁的生物地球化学循环的有力工具。镁同位素分馏的原因和硅酸盐风化过程中的行为尚不清楚，这阻碍了镁同位素在探测不同地质过程中的进一步应用。在这项研究中，我们证明了层状硅酸盐的溶解和形成是花岗岩亚热带风化过程中镁同位素分馏的主要控制因素。此外，同一矿物相的不同形成和溶解机制也可能导致分馏程度和方向的变化。在早期风化中，表生层状硅酸盐主要通过地形转化形成。母绿泥石的蛭化倾向于释放Mg-24，并导致腐岩中Mg-26显著富集。在风化后期，表生层状硅酸盐的镁同位素组成更多地受到与土壤溶液相互作用的影响。主要通过溶解-沉淀机制形成的矿物，新形成的层状硅酸盐中的镁主要来源于当代土壤溶液。Mg-26将首先并入新形成的层状硅酸盐中，如蛭石、层间黑云母/蛭石和绿泥石/蛭石。因此，土壤溶液中的Mg-24含量随着土壤深度的增加而增加，从而形成相对富含Mg-24的层状硅酸盐。然而，在腐泥中，伊利石的沉淀可能优先清除了Mg-24，使土壤溶液中的Mg-26含量丰富。沿剖面不同层状硅酸盐矿物的相对丰度不同，可能会导致表土的镁同位素组成发生较大变化。我们的研究表明，轻度风化物质的镁同位素组成可能非常重。因此，夹带富含Mg-26但轻度风化的物质可能是解释某些沉积岩（尤其是风成岩）中记录的高δMg-26的一种替代方法。然而，地下水和河水中广泛存在的低δMg-26可以通过与腐岩的相互作用和层状硅酸盐转化过程中的Mg-26清除来解释，而不是如前所述，由于强烈风化和大量次生矿物的形成，土壤溶液中的Mg-26严重枯竭。风化过程及其产物的综合表征对于解释观察到的镁同位素分馏和追踪镁的生物地球化学循环至关重要。（C） 2020爱思唯尔B.V.版权所有。

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