Feedbacks between the formation of secondary minerals and the infiltration of fluids into the regolith of granitic rocks in different climatic zones (Chilean Coastal Cordillera)
Authors: Hampl Ferdinand J. , Schiperski Ferry , Schwerdhelm Christopher , Stroncik Nicole , Bryce Casey , von
Blanckenburg Friedhelm , Neumann Thomas 
Published: June 2023

Abstract. Subsurface fluid pathways and the climate-dependent infiltration of fluids into the subsurface jointly control the intensity and depth of mineral weathering reactions. The products of these weathering reactions (secondary minerals), such as Fe(III) oxyhydroxides and clay minerals, in turn exert a control on the subsurface fluid flow and hence on the development of weathering profiles. We explored the dependence of mineral transformations on climate during the
weathering of granitic rocks in two 6 m deep weathering profiles in
Mediterranean and humid climate zones along the Chilean Coastal Cordillera.
We used geochemical and mineralogical methods such as (micro-) X-ray
fluorescence (μ-XRF and XRF), oxalate and dithionite extractions, X-ray diffraction (XRD), and electron
microprobe (EMP) mapping to elucidate the transformations involved during
weathering. In the profile of the Mediterranean climate zone, we found a low weathering intensity affecting the profile down to 6 m depth. In the profile of the humid climate zone, we found a high weathering intensity. Based on our results, we propose mechanisms that can intensify the progression of weathering to depth. The most important is weathering-induced fracturing
(WIF) by Fe(II) oxidation in biotite and precipitation of Fe(III)
oxyhydroxides and by the swelling of interstratified smectitic clay minerals that promotes the formation of fluid pathways. We also propose mechanisms that mitigate the development of a deep weathering zone, like the precipitation of secondary minerals (e.g., clay minerals) and amorphous
phases that can impede the subsurface fluid flow. We conclude that the depth and intensity of primary mineral weathering in the profile of the
Mediterranean climate zone is significantly controlled by WIF. It generates
a surface–subsurface connectivity that allows fluid infiltration to great
depth and hence promotes a deep weathering zone. Moreover, the water supply
to the subsurface is limited in the Mediterranean climate, and thus, most of the weathering profile is generally characterized by a low weathering
intensity. The depth and intensity of weathering processes in the profile of the humid climate zone, on the other hand, are controlled by an intense
formation of secondary minerals in the upper section of the weathering
profile. This intense formation arises from pronounced dissolution of
primary minerals due to the high water infiltration (high precipitation
rate) into the subsurface. The secondary minerals, in turn, impede the
infiltration of fluids to great depth and thus mitigate the intensity of
primary mineral weathering at depth. These two settings illustrate that the depth and intensity of primary
mineral weathering in the upper regolith are controlled by positive and
negative feedbacks between the formation of secondary minerals and the
infiltration of fluids.