Mineralogical transformations set slow weathering rates in low-porosity metamorphic bedrock on mountain slopes in a tropical climate

authored by

Ricarda Behrens, Julien Bouchez, Jan A. Schuessler, Stefan Dultz, Tilak Hewawasam, Friedhelm Von Blanckenburg

Abstract

In the Sri Lankan Highlands erosion and chemical weathering rates are among the lowest for global mountain denudation. In this tropical humid setting, highly weathered deep saprolite profiles have developed from high-grade metamorphic charnockite during spheroidal weathering of the bedrock. The spheroidal weathering produces rounded corestones and spalled rindlets at the rock-saprolite interface. We used detailed textural, mineralogical and chemical analyses to reconstruct the sequence of weathering reactions and their causes. The first mineral attacked by weathering was found to be pyroxene initiated by in situ Fe oxidation. Volumetric calculations suggest that this oxidation leads to the generation of porosity due to the formation of micro-fractures allowing for fluid transport and subsequent dissolution of biotite and plagioclase. The rapid ensuing plagioclase weathering leads to formation of high secondary porosity in the corestone over a distance of only a few cm and eventually to the final disaggregation of bedrock to saprolite. The first secondary phases are oxides or amorphous precipitates from which secondary minerals (mainly gibbsite, kaolinite and goethite) form. As oxidation is the first weathering reaction, the supply of O₂ is a rate-limiting factor for chemical weathering. Hence, the supply of O₂ and its consumption at depth connects processes at the weathering front with those at the Earth's surface in a feedback mechanism. The strength of the feedback depends on the relative weight of advective versus diffusive transport of O₂ through the weathering profile. The feedback will be stronger with dominating diffusive transport. The low weathering rate is explained by the nature of this feedback that is ultimately dependent on the transport of O₂ through the whole regolith, and on lithological factors such as low bedrock porosity and the amount of Fe-bearing primary minerals. Tectonic quiescence in this region and low pre-development erosion rate (attributed to a dense vegetation cover) minimize the rejuvenation of the thick and cohesive regolith column, finally leading to low denudation rates.

Organisation(s)

Institute of Soil Science
Section Soil Chemistry

External Organisation(s)

Helmholtz Centre Potsdam - German Research Centre for Geosciences
University of Potsdam
Institut de Physique du Globe de Paris (IPGP)
University of Peradeniya

Type

Article

Journal

Chemical geology

Volume

411

Pages

283-298

No. of pages

16

ISSN

0009-2541

Publication date

04.09.2015

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Geology, Geochemistry and Petrology

Electronic version(s)

https://doi.org/10.1016/j.chemgeo.2015.07.008 (Access: Unknown)