Rates of weathering rind formation on Costa Rican basalt 1 1Associate editor: E. H. Oelkers

Sak, Peter B.; Fisher, Donald M.; Gardner, Thomas W.; Murphy, Katherine R.; Brantley, Susan L.

doi:10.1016/j.gca.2003.09.007

Cited by 142 publications

(147 citation statements)

References 59 publications

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“…As pointed out earlier, for the spheroidally fracturing system and for a given c R , once the reacting mineral is depleted from volume fraction f o to 0 in the outer alteration zone, a quasi-stationary state is established [33] wherein the entire reaction front remains unchanging in shape but moves inward into the corestone at a steady-state rate ω = W/t crack . Importantly, while weathering advance rates that are constant in time have generally been interpreted in the literature as rate limitation not by transport but by the mineral-water interface reaction, results presented here can explain why transport-limited weathering can still yield weathering advance rates that are linear with respect to time [41].…”

Section: Weathering and Denudationmentioning

confidence: 56%

A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

Fletcher

Buss

Brantley

2006

Earth and Planetary Science Letters

202

229

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Spheroidal weathering, a common mechanism that initiates the transformation of bedrock to saprolite, creates concentric fractures demarcating relatively unaltered corestones and progressively more altered rindlets. In the spheroidally weathering Rio Blanco quartz diorite (Puerto Rico), diffusion of oxygen into corestones initiates oxidation of ferrous minerals and precipitation of ferric oxides. A positive ∆V of reaction results in the buildup of elastic strain energy in the rock. Formation of each fracture is postulated to occur when the strain energy in a layer equals the fracture surface energy. The rate of spheroidal weathering is thus a function of the concentration of reactants, the reaction rate, the rate of transport, and the mechanical properties of the rock. Substitution of reasonable values for the parameters involved in the model produces results consistent with the observed thickness of rindlets in the Rio Icacos bedrock (≈ 2-3 cm) and a time interval between fractures (≈ 200-300 a) based on an assumption of steady-state denudation at the measured rate of 0.01 cm/a. Averaged over times longer than this interval, the rate of advance of the bedrock-saprolite interface during spheroidal weathering (the weathering advance rate) is constant with time. Assuming that the oxygen concentration at the bedrock-saprolite interface varies with the thickness of soil/saprolite yields predictive equations for how weathering advance rate and steadystate saprolite/soil thickness depend upon atmospheric oxygen levels and upon denudation rate. The denudation and weathering advance rates at steady state are therefore related through a condition on the concentration of porewater oxygen at the base of the saprolite. In our model for spheroidal weathering of the Rio Blanco quartz diorite, fractures occur every ~ 250 years, ferric oxide is fully depleted over a four rindlet set in ~ 1000 years, and saprolitization is completed in ~ 5000 years in the zone 2 containing ~ 20 rindlets. Spheroidal weathering thus allows weathering to keep up with the high rate of denudation by enhancing access of bedrock to reactants by fracturing.Coupling of denudation and weathering advance rates can also occur for the case that weathering occurs without spheroidal fractures, but for the same kinetics and transport parameters, the maximum rate of saprolitization achieved would be far smaller than the rate of denudation for the Rio Blanco system. The spheroidal weathering model provides a quantitative picture of how physical and chemical processes can be coupled explicitly during bedrock alteration to soil to explain weathering advance rates that are constant in time.

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Section: Weathering and Denudationmentioning

confidence: 56%

A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

Fletcher

Buss

Brantley

2006

Earth and Planetary Science Letters

202

229

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“…1B shows that the fractionation factor of 26 Mg/ 24 Mg between aqueous Mg and Mg in the weathered residue (␣ aqueous residue ) should be Ͻ0.9997. We chose a value of 0.5 wt.% MgO for the residue because it approaches values observed for highly weathered and Mgdepleted soils (35). The integrated weathering residues, however, will be represented by detrital sediments, which are mixtures of soil residues and authigenic clays, the latter of which have higher Mg contents.…”

Section: Resultsmentioning

confidence: 99%

The Mg isotopic systematics of granitoids in continental arcs and implications for the role of chemical weathering in crust formation

Shen

Jacobsen

Lee

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

112

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Continental crust is too Si-rich and Mg-poor to derive directly from mantle melting, which generates basaltic rather than felsic magmas. Converting basalt to more felsic compositions requires a second step involving Mg loss, which is thought to be dominated by internal igneous differentiation. However, igneous differentiation alone may not be able to generate granites, the most silicic endmember making up the upper continental crust. Here, we show that granites from the eastern Peninsular Ranges Batholith ( batholith ͉ granite ͉ isotope ͉ magnesium ͉ California

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“…As such, corestones include a core of bedrock and several layers around this core with increasing weathering intensities outward. They serve as small natural laboratories on which incipient weathering can be studied in detail (Buss et al, 2008;Ma et al, 2012;Sak et al, 2004;Sak et al, 2010). In fact, as the fronts of the respective weathering reactions move inwards with time, the spatial alignment also represents the state of weathering at different times, with outer layers having been exposed to weathering for longer times than inner layers.…”

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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Rates of weathering rind formation on Costa Rican basalt 1 1Associate editor: E. H. Oelkers

Cited by 142 publications

References 59 publications

A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

The Mg isotopic systematics of granitoids in continental arcs and implications for the role of chemical weathering in crust formation

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

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