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Title: Direct dating of weathering phenomena by [sup 40]Ar/[sup 39]Ar and K-Ar analysis of supergene K-Mn oxides

Abstract

Potassium-bearing manganese oxides, cryptomelane, K[sub 1-2](Mn[sup 3+]Mn[sup 4+])[sub 8] O[sub 16] [center dot] xH[sub 2]O, and hollandite, (K,Ba)[sub 1-2](Mn[sup 3+],Mn[sup 4+])[sub 8] O[sub 16] [center dot] xH[sub 2]O, are often authigenically precipitated in weathering profiles. Dating of these phases allows timing of the progression of oxidation fronts during weathering and pedogenic processes. Potential problems in manganese oxide dating, such as Ar and/or K losses, excess argon, [sup 39]Ar loss by recoil during neutron irradiation, etc. are addressed. The K-Ar and [sup 40]Ar/[sup 39]Ar analytical results indicate that Ar and/or K losses, excess [sup 40]Ar, and [sup 39]Ar recoil seem not to pose problems in manganese oxide dating. This investigation suggests that the fine scale, laser-probe [sup 40]Ar/[sup 39]Ar technique is most appropriate for dating of weathering phenomena because this technique permits identification of contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles. K-Ar and [sup 40]Ar/[sup 39]Ar dating of supergene K-bearing manganese oxides formed during lateritization of Archean and Proterozoic bedrocks in the Carajas Region, Amazonia, Brazil, indicates that weathering started before 72 [+-] 6 Ma. Petrographic, electron microscope, and electron microprobe investigation reveal multiple generations of manganesemore » oxide precipitation. Age clusters at 65-69, 51-56, 40-43, 33-35, 20, 24, 12-17 Ma, and zero-age (0.2 [+-] 0.2 Ma) suggest episodic precipitation of K-Mn oxides resulting form changing weathering conditions in the Amazon throughout the Cenozoic. K-Ar and [sup 40]Ar/[sup 39]Ar dating of supergene cryptomelane from weathering profiles in eastern Minas Gerais, southeastern Brazil, suggests continuous weathering from 10 to 5.6 Ma ago, possibly reflecting local climatic conditions due to the proximity with the Atlantic Ocean.« less

Authors:
;  [1]; ;  [2]
  1. (Univ. of California, Berkeley, CA (United States))
  2. (Institute of Human Origins, Berkeley, CA (United States))
Publication Date:
OSTI Identifier:
7181219
Resource Type:
Journal Article
Journal Name:
Geochimica et Cosmochimica Acta; (United States)
Additional Journal Information:
Journal Volume: 58:6; Journal ID: ISSN 0016-7037
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; ARGON 39; ISOTOPE RATIO; ARGON 40; MANGANESE OXIDES; ISOTOPE DATING; WEATHERING; PALEOCLIMATOLOGY; AGE ESTIMATION; ARGON ISOTOPES; BETA DECAY RADIOISOTOPES; BETA-MINUS DECAY RADIOISOTOPES; CHALCOGENIDES; EVEN-EVEN NUCLEI; EVEN-ODD NUCLEI; ISOTOPES; LIGHT NUCLEI; MANGANESE COMPOUNDS; NUCLEI; OXIDES; OXYGEN COMPOUNDS; PALEONTOLOGY; RADIOISOTOPES; STABLE ISOTOPES; TRANSITION ELEMENT COMPOUNDS; YEARS LIVING RADIOISOTOPES; 580000* - Geosciences

Citation Formats

Vasconcelos, P.M., Brimhall, G.H., Renne, P.R., and Becker, T.A. Direct dating of weathering phenomena by [sup 40]Ar/[sup 39]Ar and K-Ar analysis of supergene K-Mn oxides. United States: N. p., 1994. Web. doi:10.1016/0016-7037(94)90565-7.
Vasconcelos, P.M., Brimhall, G.H., Renne, P.R., & Becker, T.A. Direct dating of weathering phenomena by [sup 40]Ar/[sup 39]Ar and K-Ar analysis of supergene K-Mn oxides. United States. doi:10.1016/0016-7037(94)90565-7.
Vasconcelos, P.M., Brimhall, G.H., Renne, P.R., and Becker, T.A. Tue . "Direct dating of weathering phenomena by [sup 40]Ar/[sup 39]Ar and K-Ar analysis of supergene K-Mn oxides". United States. doi:10.1016/0016-7037(94)90565-7.
@article{osti_7181219,
title = {Direct dating of weathering phenomena by [sup 40]Ar/[sup 39]Ar and K-Ar analysis of supergene K-Mn oxides},
author = {Vasconcelos, P.M. and Brimhall, G.H. and Renne, P.R. and Becker, T.A.},
abstractNote = {Potassium-bearing manganese oxides, cryptomelane, K[sub 1-2](Mn[sup 3+]Mn[sup 4+])[sub 8] O[sub 16] [center dot] xH[sub 2]O, and hollandite, (K,Ba)[sub 1-2](Mn[sup 3+],Mn[sup 4+])[sub 8] O[sub 16] [center dot] xH[sub 2]O, are often authigenically precipitated in weathering profiles. Dating of these phases allows timing of the progression of oxidation fronts during weathering and pedogenic processes. Potential problems in manganese oxide dating, such as Ar and/or K losses, excess argon, [sup 39]Ar loss by recoil during neutron irradiation, etc. are addressed. The K-Ar and [sup 40]Ar/[sup 39]Ar analytical results indicate that Ar and/or K losses, excess [sup 40]Ar, and [sup 39]Ar recoil seem not to pose problems in manganese oxide dating. This investigation suggests that the fine scale, laser-probe [sup 40]Ar/[sup 39]Ar technique is most appropriate for dating of weathering phenomena because this technique permits identification of contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles. K-Ar and [sup 40]Ar/[sup 39]Ar dating of supergene K-bearing manganese oxides formed during lateritization of Archean and Proterozoic bedrocks in the Carajas Region, Amazonia, Brazil, indicates that weathering started before 72 [+-] 6 Ma. Petrographic, electron microscope, and electron microprobe investigation reveal multiple generations of manganese oxide precipitation. Age clusters at 65-69, 51-56, 40-43, 33-35, 20, 24, 12-17 Ma, and zero-age (0.2 [+-] 0.2 Ma) suggest episodic precipitation of K-Mn oxides resulting form changing weathering conditions in the Amazon throughout the Cenozoic. K-Ar and [sup 40]Ar/[sup 39]Ar dating of supergene cryptomelane from weathering profiles in eastern Minas Gerais, southeastern Brazil, suggests continuous weathering from 10 to 5.6 Ma ago, possibly reflecting local climatic conditions due to the proximity with the Atlantic Ocean.},
doi = {10.1016/0016-7037(94)90565-7},
journal = {Geochimica et Cosmochimica Acta; (United States)},
issn = {0016-7037},
number = ,
volume = 58:6,
place = {United States},
year = {1994},
month = {3}
}