40Ar/39Ar geochronology constraints on late miocene weathering rates in Minas Gerais, Brazil

Carmo, I. D. and Vasconcelos, P. M. (2006) 40Ar/39Ar geochronology constraints on late miocene weathering rates in Minas Gerais, Brazil. Earth And Planetary Science Letters, 241 1-2: 80-94.


Author Carmo, I. D.
Vasconcelos, P. M.
Title 40Ar/39Ar geochronology constraints on late miocene weathering rates in Minas Gerais, Brazil
Formatted title 40Ar / 39Ar geochronology constraints on late miocene weathering rates in Minas Gerais, Brazil
Journal name Earth And Planetary Science Letters   Check publisher's open access policy
ISSN 0012-821X
Publication date 2006-01-15
Sub-type Article (original research)
DOI 10.1016/j.epsl.2005.09.056
Volume 241
Issue 1-2
Start page 80
End page 94
Total pages 15
Editor H. Elderfield
V. Courtillot
Place of publication Netherlands
Publisher Elsevier BV
Collection year 2006
Language eng
Subject C1
260109 Geochronology
780104 Earth sciences
040303 Geochronology
040202 Inorganic Geochemistry
Abstract Ar-40/Ar-39 incremental heating ages for twenty one grains of cryptomelane, collected at 0, 42, 45, and 60 in depths in the Cachoeira Mine weathering profile, Minas Gerais, permit calculating long-term (10 Ma time scale) weathering rate (saprolitization rate) in SE Brazil. Pure well-crystallized cryptomelane grains with high K contents (3-5 wt.%) yield reliable geochronological results. The Ar-40/Ar-39 plateau ages obtained decrease from the top to the bottom of the profile (12.7 +/- 0.1 to 7.6 +/- 0.1 Ma at surface; 7.6 +/- 0.2 to 6.1 +/- 0.2 Ma at 42 m; and 7.1 +/- 0.2 to 5.9 +/- 0.1 Ma at 45 in; 6.6 +/- 0.1 to 5.2 +/- 0.1 Ma at 60 in), yielding a weathering front propagation rate of 8.9 +/- 1.1 m/m.y. From the geochronological results and the mineral transformations implicit by the current mineralogy in the weathering profiles, it is possible to calculate the saprolitization rate for the Cachoeira Mine lithologies and for adjacent weathering profiles developed on granodiorites and scbists. The measured weathering front propagation rate yields a saprolitization rate of 24.9 +/- 3.1 t/km(2)/yr. This average long-term (> 10 Ma) saprolitization rate is consistent with mass balance calculations results for present saprolitization rates in weathering watersheds. These results are also consistent with longterm saprolitization rates estimated by combining cosmogenic isotope denudation rates with mass balance calculations. (c) 2005 Elsevier B.V All rights reserved.
Keyword Geochemistry & Geophysics
Weathering Rate
Chemical Denudation
Ar-40/ar-39 Geochronology
Miocene
Se Brazil
Geochemical Mass-balance
Long-term
River Geochemistry
Water Geochemistry
Physical Erosion
Denudation Rates
Co2 Consumption
French-guiana
Basin
Kaolinite
Q-Index Code C1

 
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