Phyllosilicate controls on magnesium isotopic fractionation during weathering of granites: Implications for continental weathering and riverine system

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.

Automated summary

Continental weathering plays a crucial role in releasing magnesium from rocks to the hydrosphere and biosphere. Magnesium isotopes can be used as a powerful tool to trace the biogeochemical cycle of magnesium, but the causes of significant isotopic fractionation during silicate weathering are not well understood. This study demonstrates that the dissolution and formation of phyllosilicates are the main factors controlling magnesium isotopic fractionation during sub-tropical weathering of granite, and different mechanisms of formation and dissolution could cause variations in fractionation.