DOI: 10.5880/fidgeo.2021.037
title: Mineralogical, geochemical and magnetic susceptibility data from a deep hydrothermally altered profile in a semi-arid region (Chilean Coastal Cordillera)

description: 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.
description: Two boreholes (N1 and N1C) and one soil pit were used to investigate the weathering profile. The soil pit was dug manually next to the drilling site and reached a depth of 2 m. Twelve bulk soil pit samples of approximately 3 kg were taken in different depth intervals to cover the entire 2 m deep soil pit profile. Aliquots were ground in a porcelain and an agate mortar to obtain a grain size of &lt;10 μm (S1-S12). The clay-size fraction of the untreated soil pit samples was obtained via centrifugation. Wireline rotary drilling with a PQ3-sized crown (Ø ~85 mm) and potable water was used for borehole N1 (87.2 m deep) to obtain core runs of up to 1.5 m. A hammer sampler equipped with a core catcher (up to 50 cm long core runs, Ø 80 mm) was used for the first 5.6 meters of borehole N1C before switching to the technique used in N1 to reach the final depth of 10 m. Rock field samples of approximately 20 cm length were separated from the core runs of N1 by using sterilized angle grinder and tools. They were trimmed to discs and divided into three different parts at the Technische Universität Berlin, Germany. The composite of the outer parts of each disk was used as the material for analyses of the respective field sample. Rock powders for analyses were obtained by using a jaw crusher and a ball mill at the German Research Centre for Geosciences in Potsdam (Germany). The loose material of N1C was sampled with sterilized tools and was also milled. Ball-mill processed aliquots of ~10 g (N1 and N1C samples) were ultimately ground in an agate mortar to achieve a grain size of &lt;10 μm. The resulting 33 powder samples (C1-C33) were used for the XRD, magnetic susceptibility and Mössbauer spectroscopy measurements of the profile. Two core samples with fracture surfaces and alteration features were cut in slabs and grooved with a diamond drill bit. The grooves had a width of 5 mm and were ca. 3 mm deep. The obtained rock powders were used for a mineralogical and geochemical characterisation of the alteration (F1-1 to F1-10 and F3-1 to F3-12).
description: The DFG Priority Program 1803 "EarthShape - Earth Surface Shaping by Biota" (2016-2022; https://www.earthshape.net/) explored between scientific disciplines and includes geoscientists and biologists to study from different viewpoints the complex question how microorganisms, animals, and plants influence the shape and development of the Earth’s surface over time scales from the present-day to the young geologic past. All study sites are located in the north-to-south trending Coastal Cordillera mountains of Chile, South America. These sites span from the Atacama Desert in the north to the Araucaria forests approximately 1300 km to the south. The site selection contains a large ecological and climate gradient ranging from very dry to humid climate conditions.