The role of iron-bearing minerals for the deep weathering of a hydrothermally altered plutonic rock in semi-arid climate (Chilean Coastal Cordillera)

Abstract

Weathering is a fundamental process that controls the development of Earth's surface by the formation of erodible material and the release of mineral-bound nutrients. Weathering at depth has been predominantly studied in humid environments, where fluid flow sets mineralogical transformations, whereas (semi-)arid regions were barely investigated in this respect. In many studies the weathering of iron-bearing minerals was shown to play a major role for the advance of weathering at depth. Weathering of iron-bearing minerals is often superimposed on factors that precondition rocks for weathering such as hydrothermal alteration and fracturing by tectonic stress. The low degree of water-rock interactions in (semi-)arid climates offers the opportunity to single out the preconditioning factors and explore the role of iron-bearing minerals for weathering in detail. To this end the interactions of tectonic fracturing, hydrothermal alteration and weathering were investigated in a deep, weakly weathered profile of plutonic rock in a semi-arid region (Santa Gracia, Northern Chile). The profile was recovered by an 87-m-deep drill core and a 2-m-deep soil pit which were used to investigate the mineral composition, magnetic susceptibility, geochemical gradients, and Fe redox-state.Non-cemented fractures of tectonic origin were identified as pathways for the transport of fluids and reagents to depth that are fuelling the initial weathering reactions. Previous hydrothermal fluids mainly migrated along the same fractures and altered the properties of the original bedrock. Therefore, the weathering features along fractures depend on the preconditioning by hydrothermal alteration and tectonic stress which affected the vicinity of these fractures. From the fracture surfaces, water and O2 diffuse into the rock and initiate oxidation and dissolution-reprecipitation reactions. Our study shows that the transformation of iron-bearing silicates (especially biotite) to Fe(III) (oxyhydr)oxides and clay minerals is the dominant weathering-promoting process as it produces stress which leads to micro-fracturing of the rock and finally to the formation of saprolite.Deciphering the links between tectonic fracturing, hydrothermal alteration, and weathering-induced fracturing by iron-bearing silicates in semi-arid settings provides unique insights into the controls of deep weathering in regions with limited fluid flow.