Preprints
https://doi.org/10.5194/esurf-2022-71
https://doi.org/10.5194/esurf-2022-71
 
17 Jan 2023
17 Jan 2023
Status: this preprint is currently under review for the journal ESurf.

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)

Ferdinand J. Hampl1, Ferry Schiperski1, Christopher Schwerdhelm2, Nicole Stroncik3, Casey Bryce4, Friedhelm von Blanckenburg3,5, and Thomas Neumann1 Ferdinand J. Hampl et al.
  • 1Department of Applied Geochemistry, Technische Universität Berlin, Ernst-Reuter-Platz 1, 10587 Berlin, Germany
  • 2Geomicrobiology Group, Eberhard-Karls-University Tübingen, Schnarrenbergstrasse 94-96, 72076 Tübingen, Germany
  • 3Earth Surface Geochemistry, GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
  • 4School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, United Kingdom
  • 5Institute of Geological Sciences, Freie Universität Berlin, Malteserstrasse 74-100, 12249 Berlin, Germany

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, oxalate/dithionite extractions, X-ray diffraction and electron microprobe 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 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.

Ferdinand J. Hampl et al.

Status: open (until 08 Mar 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on esurf-2022-71', Peter Finke, 27 Jan 2023 reply

Ferdinand J. Hampl et al.

Ferdinand J. Hampl et al.

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Short summary
We investigated the mineral content and geochemical composition of the upper 6 meters of regolith, formed by weathering of granitic rocks in Mediterranean and humid climate. We found that the development of the upper regolith in the Mediterranean climate is controlled by secondary minerals which cause fracturing and thus facilitate fluid infiltration to depth. The upper regolith in the humid climate is controlled by secondary minerals that cause a reduction of fluid infiltration to depth.