Causes of spheroidal weathering

Ollier, Clifford

doi:10.1016/0012-8252(71)90005-5

Cited by 66 publications

(26 citation statements)

References 15 publications

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“…It is known that the formation of normal patterns of spheroidal weathering sensu stricto involves the following sequence of events: rectilinear systems of microfractures rectangular blocks rounding of their edges and angularities (Chapman & Greenfield, 1949;Ollier, 1971;Chatterjee & Raymahashay, 1998;Fletcher et al, 2006;Buss et al, 2008). Indeed, no rindlets or pore spaces within a zone of rindlets, or a specific system of microfractures, are observed in our case, nor is there anything that could be ascribed to the known mechanism of normal spheroidal weathering.…”

Section: Factors Of Spheroidal Weathering At Moncheplutonmentioning

confidence: 56%

A novel mechanism of spheroidal weathering: a case study from the Monchepluton layered complex, Kola Peninsula, Russia

Arkov¹,

Aa²,

Artin³

2015

Bull. Geol. Soc. Finland

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We describe an unusual pattern of spheroidally weathered surface developed in a mineralized harzburgite of Mount Kumuzhya, Monchepluton layered complex of Early Proterozoic age, Kola Peninsula, Russia. This rock is composed of a matrix and spheroids, which differ in their paragenesis and textures. The relief spheroids consist principally of large oikocrysts (up to 3-4 cm) of orthopyroxene (En 86 ), enclosing aggregates of finegrained chromian spinel (chromite-magnesiochromite). The matrix is mostly composed of olivine (Fo 90.5 ), surrounded by patches of chromite. We infer that this pattern represents a new variety of spheroidal weathering of rock surface, which is not related to "normal" spheroidal weathering. Presumably, it has resulted mainly from the primary magmatic characteristics, whereas other factors, such as differential rates of weathering of olivine and orthopyroxene, were of secondary importance. AbstractA novel mechanism of spheroidal weathering: a case study from the Monchepluton layered complex, Kola Peninsula, Russia

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Section: Factors Of Spheroidal Weathering At Moncheplutonmentioning

confidence: 56%

A novel mechanism of spheroidal weathering: a case study from the Monchepluton layered complex, Kola Peninsula, Russia

Arkov¹,

Aa²,

Artin³

2015

Bull. Geol. Soc. Finland

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“…The formation of "normal patterns" of spheroidal weathering is generally based the following sequence of events: rectilinear systems of microfractures → rectangular blocks → rounding of their edges and angularities (Chapman & Greenfield, 1949;Ollier, 1971;Chatterjee & Raymahashay, 1998;Fletcher et al, 2006;Buss et al, 2008). No rindlets or pore spaces within a zone of rindlets, or a specific system of microfractures, are observed in our cases.…”

Section: Factors Favoring Spheroidal Weatheringmentioning

confidence: 71%

“…Spheroidal weathering, related to chemical weathering, is expressed by the appearance corestones, i.e., spheroids of relict bedrock of various compositions and sizes, which are mantled by concentric shells or rindlets arranged in zones (e.g., Chapman & Greenfield, 1949;Ollier, 1971;Chatterjee & Raymahashay, 1998;Fletcher et al, 2006).…”

Section: Introductionmentioning

confidence: 99%

The origin of spheroidal patterns of weathering in the Pados-Tundra mafic-ultramafic complex, Kola Peninsula, Russia

Barkov¹,

Aa²,

Halkoaho³

et al. 2016

Bull Geol Soc Finland

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We document a new and unusual occurrence of patterns of protruding spheroidal weathering developed in a dunitic rock of the Pados-Tundra mafic-ultramafic complex of Early Proterozoic age, Kola Peninsula, Russia. It provides an example similar to that reported recently from a mineralized harzburgite in the Monchepluton layered complex in the same region. These patterns are genetically different from common results of "normal spheroidal weathering" sensu stricto. The spheroidally weathered dunite at Pados-Tundra consists of a high-Fo olivine, Ol (Fo 87.5 ), which is, in fact, not altered. Accessory grains of aluminous chromite are present. Relief spheroids (1.5 to 4 cm in diameter; up to ~5 vol. %) are distributed sparsely and heterogeneously. They are hosted by the olivine matrix and composed of talc, Tlc, and tremolite, Tr, (Mg# = 95-96) formed presumably at the expense of orthopyroxene, Opx, (i.e., pre-existing oikocrysts) during a deuteric (autometasomatic) alteration. In contrast, oikocrystic Opx (En 86.0 ) is quite fresh in related spheroids at Monchepluton, in which only minor deuteric alteration (Tlc + Tr) are observed. We infer that (1) the ball-shaped morphology of the weathered surface is a reflection of the presence of oikocrysts of Opx, which crystallized after Ol at the magmatic stage; they were entirely replaced by the deuterically induced Tlc + Tr at Pados-Tundra. (2) Differential rates of weathering are implied for rock-forming minerals in these ultramafic rocks, with a higher resistance of Opx vs. Fo-rich Ol, and Tlc + Tr vs. Fo-rich Ol. (3) The ball-like shape of the large spheroids, produced by magmatic processes, may likely represent an additional factor of their higher stability to weathering in the superficial environment. Similar patterns can be expected in other mafic-ultramafic complexes, especially in layered intrusions.

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“…1a) of intact but weathered rock form sequentially by fracture of weakly altered bedrock [4]. Spheroidal weathering has been observed on almost all rock types including gneiss, schist, andesite, sandstone, and greywacke and in almost every climate; however, the phenomenon is most common in homogeneous, jointed, coherent rocks, primarily granites and basalts [5][6][7][8].…”

Section: Introductionmentioning

confidence: 99%

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

Fletcher

Buss

Brantley

2006

Earth and Planetary Science Letters

211

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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Causes of spheroidal weathering

Cited by 66 publications

References 15 publications

A novel mechanism of spheroidal weathering: a case study from the Monchepluton layered complex, Kola Peninsula, Russia

A novel mechanism of spheroidal weathering: a case study from the Monchepluton layered complex, Kola Peninsula, Russia

The origin of spheroidal patterns of weathering in the Pados-Tundra mafic-ultramafic complex, Kola Peninsula, Russia

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

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