Regolith production and chemical weathering of granitic rocks in central Chile

Abstract

We have evaluated the long-term chemical and physical weathering mass balance of an anomalously thick saprolite profile (>30m thickness) developed in granitic rocks near Curacaví, Coastal Range of central Chile. The results of geochemical mass balance calculations for the regolith profile point to a moderate depletion for Na and Sr due to the plagioclase dissolution and precipitation of kaolinite. Furthermore, silicon shows a predominantly constant behaviour along the profile that is indicative of incongruent weathering of plagioclase, strongly suggesting that quartz dissolution conditions are not reached. In addition, low depletion of K, Mg, Ca, Rb, and Ba in the saprolite indicates a low degree of K-feldspar, biotite and hornblende weathering. Denudation rates calculated using cosmogenic 10Be data range from 20 to 70m/Ma. If the saprolite thickness has reached a steady-state condition, the reported denudation rates lead to mineral residence times in the upper part of the saprolite profile of about 0.5 to 1.8Ma. With such duration a higher weathering degree than observed at Curacaví would be expected. This apparent contradiction might be explained if all of the weatherable plagioclase has been dissolved under the present-day precipitation and temperature conditions of the Curacavi region. Therefore, we propose that the Curacaví profile could be in local equilibrium with respect to the weatherable fraction of plagioclase. A local equilibrium regime may be reached when the overall degree of depletion is small in a weathering profile. Uniform elemental distribution and low weathering degree in the thick weathering profile at Curacaví defy the traditional notion of weathering regimes and reveal the importance of exploratory studies in order to better understand weathering process in Mediterranean and semi-arid climate regions.