Statistical empirical index of chemical weathering in igneous rocks: A new tool for evaluating the degree of weathering

Ohta, Tohru; Arai, Hiroyoshi

doi:10.1016/j.chemgeo.2007.02.017

Cited by 299 publications

(181 citation statements)

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“…3b). This discrepancy may result from variable Nesbitt and Young (1982); PIA: Plagioclase Index of Alteration from Fedo et al (1995); the first principal component (PC1), the second principal component (PC2) and Weathering Index (W) are calculated after Ohta and Arai (2007); PAAS: post-Archean average Australian shales from Taylor and McLennan (1985) and UCC: upper continental crust from Rudnick and Gao (2003).…”

Section: Sample Description and Analytical Methodsmentioning

confidence: 99%

“…To quantitatively depict the source-area weathering, numbers of geochemical indices based on the decomposition of unstable minerals, tracing the mass transfer of active elements or multivariate statistics were proposed (Ohta and Arai, 2007). The Chemical Index of Alteration (CIA; Nesbitt and Young, 1982), logically predicted the decomposition of feldspar, has been widely applied to evaluate the degree of source-area weathering.…”

Section: Source Area Weatheringmentioning

confidence: 99%

“…The degree of source-area weathering can be further evaluated by principal component analysis (PCA) and Weathering Index (W index) which estimate the intensity of weathering by taking into account the behavior of multiple elements (Ohta and Arai, 2007). The first principal component (PC1) and the second principal component (PC2) are the two most important parameters for evaluating the weathering profiles (see Ohta and Arai, 2007).…”

Section: Source Area Weatheringmentioning

confidence: 99%

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Provenance and depositional setting of Lower Silurian siliciclastic rocks on Hainan Island, South China: Implications for a passive margin environment of South China in Gondwana

Zou

et al. 2016

Journal of Asian Earth Sciences

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Section: Sample Description and Analytical Methodsmentioning

confidence: 99%

Section: Source Area Weatheringmentioning

confidence: 99%

Section: Source Area Weatheringmentioning

confidence: 99%

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Provenance and depositional setting of Lower Silurian siliciclastic rocks on Hainan Island, South China: Implications for a passive margin environment of South China in Gondwana

Zou

et al. 2016

Journal of Asian Earth Sciences

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“…component diagram of Ohata and Arai (2007), non-altered ~3800 Ma calc-alkaline meta-plutonic rocks from south of Isua lie along the igneous trend (Fig. 10).…”

Section: In Terms Of the M (Mafic Igneous) -F (Felsic Igneous) -W (Wementioning

confidence: 99%

3806Ma Isua rhyolites and dacites affected by low temperature Eoarchaean surficial alteration: Earth's earliest weathering

Nutman

Bennett

Chivas

et al. 2015

Precambrian Research

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This paper reports evidence for Earth's oldest-recognised low temperature alteration, at ∼3800 Ma. Potassic felsic schists with a protolith age of 3806 ± 2 Ma form a ∼30 km long unit in the amphibolite facies, deformed, Isua supracrustal belt (West Greenland). At a single locality, boudinaged layers (nodules) within the schists are low strain zones: they are fine-grained, weakly feldspar-phyric, contain quartz amygdules and have fiammelike structures, all supporting a volcanic protolith. The nodules and surrounding schistose matrix contain abundant, 100-50 μm, euhedral, oscillatory zoned 3806 Ma zircons. The rare earth patterns of the zircons indicate crystallisation was magmatic. Some zircons contain axial lobate voids indicating that they grew at low pressure as the magma exsolved a fluid. Ti-in-zircon thermometry indicates crystallisation temperatures of 750-650 °C. Taken together, these zircon features indicates growth at eutectic temperatures in a hypabyssal chamber as the magma was exsolving a fluid phase. The magmatic zircons have ɛHf initial values of ∼0 and δ18OVSMOW of +5.0‰ ( Hiess et al., 2009), showing that the source of the volcanic rocks was devoid of assimilated markedly older or weathered crustal material, and probably essentially juvenile. In contrast, the whole rock δ18OVSMOW values are elevated at +14.7 to +16.2‰, indicative of superimposed lowtemperature alteration processes. The nodules and matrix schists have non-igneous bulk compositions, exemplified by strong enrichment in K2O and depletion in Na2O. They are depleted in Sr, have no negative Eu anomalies, but have high Rb/Sr, with an Rb-Sr age of 3760 ± 140 Ma ( Jacobsen and Dymek, 1988). This indicates that the alteration involving strong degradation of plagioclase occurred in the Eoarchaean. Trace element compositions and establishment of alteration vectors suggest the protoliths were likely rhyolitic and dacitic in composition. The strongest-modified matrix schist compositions contain biotite ± calcite ± dolomite with increase in MgO relative to the nodules, which indicates early magnesian carbonate growth. The whole-rock chemistry, decoupling of the igneous zircon and whole-rock oxygen isotope signatures and the Rb-Sr dating indicate that after eruption, the 3806 Ma felsic volcanic rocks underwent Eoarchaean lowtemperature potassic alteration with weathering and groundwater circulation the most likely process. The geochemistry of the Isua felsic schists is compared with that of better-preserved volcanic rocks where the alteration conditions are known. This suggests a subaerial environment. The carbonatisation of the Isua felsic schists demonstrates drawdown of atmospheric CO2 into rocks made porous by the weathering.

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“…The quantitative evaluation of the degree of chemical weathering in continents and paleoclimate reconstruction is becoming of increased importance. Many studies have proposed different chemical weathering indices based on sediment geochemical analyses [8][9][10][11][12][13][14][15][16]. Nesbitt and Young [9] first proposed the chemical index of alteration (CIA) in their study of early Proterozoic lutite in Canada to calculate the degree of feldspar mineral weathering to clay.…”

mentioning

confidence: 99%

Does chemical index of alteration (CIA) reflect silicate weathering and monsoonal climate in the Changjiang River basin?

Jingqing

Yang

2012

Chin. Sci. Bull.

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Chemical weathering of continental silicates significantly influences global climate change, earth surface processes, material cycling and oceanic chemical composition. How to quantitatively reconstruct chemical weathering history has become an important issue in global change research. Chemical index of alteration (CIA) has been widely used as a quantitative indicator for estimating the degree of silicate weathering. However, its method of calculation and the limitations of its application are not well understood. In this study, we calculated CIA values from suspended particulate matter collected from the mainstream and major tributaries of the Changjiang River. The values yielded considerable variations at different temporal and spatial scales. The average CIA values increased from the upper to middle-lower reaches, and were lower in the suspended samples taken during the flood than in the dry season. The spatial variation in the CIA is predominantly controlled by basinal monsoon climate. In contrast, the temporal variation in the Changjiang River basin is controlled mainly by the changing provenance of suspended samples in relation to the shift of the precipitation zone. The CIA probably indicates the integrated weathering history in the river basin, and thus, cannot be used as a reliable proxy of instantaneous chemical weathering. Furthermore, the calculation method and hydrodynamic sorting also influence the CIA values. Therefore, caution should be taken when using the CIA as a proxy for studying chemical weathering from different regions.

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Statistical empirical index of chemical weathering in igneous rocks: A new tool for evaluating the degree of weathering

Cited by 299 publications

References 68 publications

Provenance and depositional setting of Lower Silurian siliciclastic rocks on Hainan Island, South China: Implications for a passive margin environment of South China in Gondwana

Provenance and depositional setting of Lower Silurian siliciclastic rocks on Hainan Island, South China: Implications for a passive margin environment of South China in Gondwana

3806Ma Isua rhyolites and dacites affected by low temperature Eoarchaean surficial alteration: Earth's earliest weathering

Does chemical index of alteration (CIA) reflect silicate weathering and monsoonal climate in the Changjiang River basin?

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