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)

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.</p

Deep weathering in the semi-arid Coastal Cordillera, Chile

The weathering front is the boundary beneath Earth’s surface where pristine rock is converted into weathered rock. It is the base of the “critical zone”, in which the lithosphere, biosphere, and atmosphere interact. Typically, this front is located no more than 20 m deep in granitoid rock in humid climate zones. Its depth and the degree of rock weathering are commonly linked to oxygen transport and fluid flow. By drilling into fractured igneous rock in the semi-arid climate zone of the Coastal Cordillera in Chile we found multiple weathering fronts of which the deepest is 76 m beneath the surface. Rock is weathered to varying degrees, contains core stones, and strongly altered zones featuring intensive iron oxidation and high porosity. Geophysical borehole measurements and chemical weathering indicators reveal more intense weathering where fracturing is extensive, and porosity is higher than in bedrock. Only the top 10 m feature a continuous weathering gradient towards the surface. We suggest that tectonic preconditioning by fracturing provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. Porosity was preserved throughout the weathering process, as secondary minerals were barely formed due to the low fluid flow

Deep weathering in the semi-arid Coastal Cordillera, Chile

The weathering front is the boundary beneath Earth’s surface where pristine rock is converted into weathered rock. It is the base of the “critical zone”, in which the lithosphere, biosphere, and atmosphere interact. Typically, this front is located no more than 20 m deep in granitoid rock in humid climate zones. Its depth and the degree of rock weathering are commonly linked to oxygen transport and fluid flow. By drilling into fractured igneous rock in the semi-arid climate zone of the Coastal Cordillera in Chile we found multiple weathering fronts of which the deepest is 76 m beneath the surface. Rock is weathered to varying degrees, contains core stones, and strongly altered zones featuring intensive iron oxidation and high porosity. Geophysical borehole measurements and chemical weathering indicators reveal more intense weathering where fracturing is extensive, and porosity is higher than in bedrock. Only the top 10 m feature a continuous weathering gradient towards the surface. We suggest that tectonic preconditioning by fracturing provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. Porosity was preserved throughout the weathering process, as secondary minerals were barely formed due to the low fluid flow

Mineralogical, geochemical and magnetic susceptibility data from a deep hydrothermally altered profile in a semi-arid region (Chilean Coastal Cordillera)

This data publication contains mineralogical, geochemical and magnetic susceptibility data of an 87.2 m deep profile of hydrothermally altered plutonic rock in a semi-arid region of the Chilean Coastal Cordillera (Santa Gracia). The profile was recovered during a drilling campaign (March and April 2019) as part of the German Science Foundation (DFG) priority research program SPP-1803 “EarthShape: Earth Surface Shaping by Biota” which aims at understanding weathering of plutonic rock in dependency on different climatic conditions. The goal of the drilling campaign was to recover the entire weathering profile spanning from the surface to the weathering front and to investigate the weathering processes at depth. To this end, we used rock samples obtained by drilling and soil/saprolite samples from a manually dug 2 m deep soil pit next to the borehole. To elucidate the role of iron-bearing minerals for the weathering, we measured the magnetic susceptibility, determined the mineral content and analysed the geochemistry as well as the composition of Fe-bearing minerals (Mössbauer spectroscopy) in selected samples

Mineralogical and geochemical data of two weathering profiles in a Mediterranean and a humid climate region of the Chilean Coastal Cordillera

This publication provides mineralogical and geochemical data of two 6-m-deep weathering profiles formed from granitic rock. They are located in different climate zones (Mediterranean and humid) and are close to the national parks of La Campana and Nahuelbuta in the Chilean Coastal Cordillera. Additional rock samples from adjacent boreholes were used to relate the regolith to the bedrock. The profiles were sampled in February and March 2020 as part of the German Science Foundation (DFG) priority research program SPP-1803 “EarthShape: Earth Surface Shaping by Biota”. The goal of this project is to obtain a holistic view on the interplay of the geosphere and the biosphere under different climatic conditions and to investigate weathering mechanisms. The aim of this publication is to provide the data basis for understanding the weathering processes that control the development of the profiles in relation to different climatic conditions. To this end, we measured the geochemistry with X-ray fluorescence, extracted Fe, Al and Si with oxalate/dithionite, determined the grain sizes by wet sieving and pipetting, measured the magnetic susceptibility, and analysed the mineral content of bulk samples and clay fractions with X-ray diffraction. The data are compiled in one Excel file and all results of the X-ray diffraction measurements are available as RAW- and TXT files

Physical and geochemical data on a drill core from the semi-arid Coastal Cordillera, Chile

This dataset contains petrophysical, geochemical, and mineralogical data from a drilling core from the Coastal Cordillera, Chile. The drilling campaign in the semi-arid field site Reserve Santa Gracia was conducted in the framework of the “EarthShape” project (DFG SPP1803) to study deep weathering along a climate gradient. Previous studies in this area found that the weathering front is located much deeper than expected (Oeser et al., 2018). To explore the weathering profile and the depth of the weathering front, we performed various geochemical, petrophysical, and mineralogical analyses. The drilling campaign was conducted in March and April 2019, using the wireline drilling method with a standard industry truck-mounted PQ3-sized (85 mm core diameter, 123 mm hole diameter) rotary drilling rig (Sondajes Araos E.I.R.L.). A detailed description of the drilling activities is given in Krone et al. (2021). The retrieved core runs with a maximum length of 1.5 m were drilled using potable water, with added contamination control tracer for further microbiological analyses of the rock. As basis for our detailed study of deep weathering we determined the porosity, density, specific surface area, elemental composition, mineralogical composition, Fe oxidation, and the degree of weathering from chemical depletion, volumetric strain, and the weathering rate using the in situ cosmogenic nuclide beryllium-10 (10Be)