Abstract 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 δ 26 Mg as recorded in some sedimentary rocks, especially of aeolian source. Whereas low δ 26 Mg 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.

中文翻译：

---

花岗岩风化过程中层状硅酸盐对镁同位素分馏的控制：对大陆风化和河流系统的影响

摘要 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大量移动、重新分布和分馏，因此可以用作追踪镁生物地球化学循环的有力工具。硅酸盐风化过程中显着 Mg 同位素分馏和行为的原因仍不清楚，阻碍了 Mg 同位素进一步应用于探测不同的地质过程。在这项研究中，我们证明了页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幅度和方向性的变化。在初期风化中，表生层状硅酸盐主要通过拓扑转化形成。母体绿泥石的蛭石化往往会释放 24 Mg 并导致基岩中 26 Mg 显着富集。在风化的后期，表生层状硅酸盐的镁同位素组成更受与土壤溶液相互作用的影响。矿物主要通过溶解-沉淀机制形成，其中 Mg 主要来自当代土壤溶液的新形成的页硅酸盐中。26Mg 将首先掺入新形成的页硅酸盐中，如蛭石、层间黑云母/蛭石和绿泥石/蛭石。所以，随着基石深度的增加，土壤溶液中 24Mg 的含量越来越高，从中会形成相对富含 24Mg 的层状硅酸盐。然而，在腐泥土中，伊利石的沉淀可能优先清除了 24Mg，使土壤溶液富含 26Mg。沿剖面不同层状硅酸盐矿物的相对丰度变化可能导致风化层镁同位素组成的巨大变化。我们的研究表明，轻微风化材料的 Mg 同位素组成可能非常重。因此，夹带富含 26Mg 但轻微风化的物质可以替代解释某些沉积岩中记录的高 δ 26 Mg，尤其是风成源。而广泛存在于地下水和河水中的低 δ 26 Mg 可以通过与叶状硅酸盐转化过程中的叶质和 26Mg 清除相互作用来替代解释，而不是像以前那样由于强烈的风化和大量次生矿物的形成而导致土壤溶液中的 26Mg 严重枯竭建议。风化过程和所得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。