Detecting Deep Rock Weathering

The weathering front, the interface beneath Earth’s surface where unweathered bedrock is converted into weathered rock, is a zone where chemical disequilibrium results in some of the most intense mineralogical transformations. These are focused into a narrow zone; yet its depth is poorly known due to its inaccessible nature deep beneath the Earth’s surface. Studies in humid and temperate climate suggest a maximum depth of 20 m for the weathering front in granitoid rock (Hayes et al., 2020).

To explore whether this depth is unique to humid climate we drilled into fractured rock in the semi-arid climate zone of the Coastal Cordillera of Chile. We found deep weathering down to 76 m below the surface which represents one of the deepest weathering fronts ever found. To characterise and quantify rock weathering, we investigated mineralogical and geochemical transformations. Iron (Fe) oxidation and related porosity formation is the first weathering process taking place and hence an indicator for the onset of weathering (Buss et al., 2008). Elemental (τ) and bulk loss (chemical depletion fraction, CDF) calculated from the chemical composition reveal multiple zones with more intense weathering compared to bedrock, and where the specific surface area also increases due to formation of secondary solids. Fracturing and the related increase in macro-porosity thus induce these mineralogical and chemical transformations. Below 76 m, bedrock is devoid of weathering features. We suggest that tectonic pre-fracturing in this geologically active region provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. This porosity was preserved throughout the weathering process, as secondary minerals that might fill pores were not formed due to the low fluid flow.

Hayes, N. R., Buss, H. L., Moore, O. W., Krám, P. and Pancost, R. D. (2020): Controls on granitic weathering fronts in contrasting climates. Chemical Geology, 535, 119450.

Buss, H.L., Sak, P. B., Webb, S. M. and Brantley, S. L. (2008): Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing. Geochimica et Cosmochimica Acta, 72 (18), 4488-4507.