Porosity evolution and crystallization-driven fragmentation during weathering of andesite

Jamtveit, Bjørn; Kobchenko, Maya; Austrheim, Håkon; Malthe‐Sørenssen, Anders; Røyne, Anja; Svensen, Henrik

doi:10.1029/2011jb008649

Cited by 54 publications

(50 citation statements)

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“…Oxidative weathering produces a specific volume increase of the solid material and thus builds up strain that fractures the rock. Similarly, precipitation of secondary clay minerals following dissolution of primary minerals was also proposed to result in strain build-up and rock fracturing (Jamtveit et al, 2011;Jamtveit et al, 2008;Røyne et al, 2008;Rudge et al, 2010). In contrast to pathways that require the generation of strain, the access of reactive phases from the surface to the weathering front might also be promoted by the formation of secondary porosity by primary mineral dissolution and precipitation of secondary minerals with a lower molar volume (Lebedeva et al, 2007;Navarre-Sitchler et al, 2011;Sak et al, 2010;Velbel, 1993).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2015

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Originally published as:Behrens, R.; Bouchez, J.; Schuessler, J. A.; Dultz, S.; von Blanckenburg, F. (2015): Mineralogical transformations set slow weathering rates in low-porosity metamorphic bedrock on mountain slopes in a tropical climate. 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 O 2 is a rate-limiting factor for chemical weathering. Hence, the supply of O 2 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 O 2 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 O 2 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.

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Section: Introductionmentioning

confidence: 99%

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

et al. 2015

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“…Spheroidal weathering, the process that produces concentric, onion-skin type weathering rinds around bedrock corestones (Chapman and Greenfield, 1949), is an extensively explored process converting bedrock into weathered material, and thereby may set the advance rate of the weathering front in a wide range of settings (Sak et al, 2004(Sak et al, , 2010Fletcher et al, 2006;Royne et al, 2008;Buss et al, 2008). Several mechanisms have been suggested for spheroidal weathering, and most of these involve fracturing induced by local strain that is caused by several processes: 1) mineralogical transformations involving volume change such as the oxidation of ferrous minerals, mostly biotite (Fletcher et al, 2006;Lebedeva et al, 2007; 2) the precipitation of secondary minerals therein, thereby generating stress and fracturing (Røyne et al, 2008;Jamtveit et al, 2009Jamtveit et al, , 2011; 3) or the utilization of mesofractures by fungal hyphae that supply water and mineral-dissolving reagents (Graham et al, 2010). The spacing of initial fractures bounding corestones (Fletcher and Brantley, 2010), and of those formed by positive strain during spheroidal weathering (Fletcher et al, 2006) have been identified as a key control over the rate of weathering front advance.…”

Section: Introductionmentioning

confidence: 99%

Slow advance of the weathering front during deep, supply-limited saprolite formation in the tropical Highlands of Sri Lanka

Hewawasam

Blanckenburg

Bouchez

et al. 2013

Geochimica et Cosmochimica Acta

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Silicate weathering -initiated by major mineralogical transformations at the base of ten meters of clayrich saprolite -generates the exceptionally low weathering flux found in streams draining the crystalline rocks of the mountainous and humid tropical Highlands of Sri Lanka. This conclusion is reached from a thorough investigation of the mineralogical, chemical, and Sr isotope compositions of samples within a regolith profile extending >10 m from surface soil through the weathering front in charnockite bedrock (a high-grade metamorphic rock), corestones formed at the weathering front, as well as from the chemical composition of the dissolved loads in nearby streams. Weatherable minerals and soluble elements are fully depleted at the top of the profile, showing that the system is supply-limited, such that weathering fluxes are controlled directly by the supply of fresh minerals. We determine the weathering rates using two independent means: (1) in situ-produced cosmogenic nuclides in surface soil and creek sediments in the close vicinity of the regolith combined with immobile element mass balance across the regolith and (2) river dissolved loads. Silicate weathering rates determined from both approaches range from 16 to 36 t km -2 y -1 , corresponding to a weathering front advance rate of 6 to 14 mm ky -1 . These rates agree across the 10 1 to 10 4 y time scales over which our rate metrics integrate, suggesting that the weathering system operates at steady state. Within error these rates are furthermore compatible with those obtained by modeling the advance rate of the weathering front from chemical gradients and mineral dissolution rates. The silicate weathering flux out of the weathering profile, measured on small creeks, amounts to 84% of the profile's export flux; the remaining 16% is contributed by non-silicate, atmospheric-derived input. The silicate weathering flux, as measured by dissolved loads in large catchments, amounts to ca. 50% of the total dissolved flux; the remainder being contributed by dust, rain, and weathering of local marble bands. Spheroidal weathering is the key processes of converting the fresh bedrock into saprolite at the weathering front. The mineralogical composition of weathering rinds shows that the sequence of mineral decomposition is: pyroxene; plagioclase; biotite; K-feldspar. Observable biotite alteration does not appear to initiate spheroidal weathering within corestones; therefore, we infer that other processes than biotite oxidation, like pyroxene oxidation, clay formation from pyroxene and plagioclase decomposition, the development of secondary porosity by plagioclase dissolution, or even microbiologic processes at depth enable the coupling between slow advance of the weathering front and slow erosion at the surface. The comparison to tectonically more active tropical landscapes lets us conclude that the combination of hard rock with tightly interlocked mineral grains and slow erosion in the absence of tectonically-induced landscape rejuvenation lead to these excep...

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“…Domain III is further subdivided into IIIA and IIIB, before IIIA split into twins IIIAa and IIIAb. The brown coloured striation is a combination of Liesegang bands and onion skin fractures (slightly modified from Jamtveit et al, 2011). Figure 8.2 shows a block of andesite, originally cut out by perpendicular joints probably formed during thermal contraction of the andesitic sill intrusion of which the block is a part.…”

Section: Figure 82mentioning

confidence: 99%

“…The porosity is characterised by a high frequency of narrow pore throats, with a maximum frequency ~20 nm and a maximum value ~200 nm ( Fig. 8.3; more details presented in Jamtveit et al, 2011).…”

Section: Figure 82mentioning

confidence: 99%

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Sculpting of Rocks by Reactive Fluids

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Hammer²

2012

Geochem Persp

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Porosity evolution and crystallization-driven fragmentation during weathering of andesite

Cited by 54 publications

References 30 publications

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

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

Slow advance of the weathering front during deep, supply-limited saprolite formation in the tropical Highlands of Sri Lanka

Sculpting of Rocks by Reactive Fluids

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