40 Ar/ 39 Ar geochronology constraints on late miocene weathering rates in Minas Gerais, Brazil Isabela de Oliveira Carmo a,1 , Paulo Marcos Vasconcelos b, * a Departamento de Geologia-UFRJ Av. Brigadeiro Trompowsky, s/n, CidadeUniversita ´ria, Ilha do Funda ˜o 21949-900, Rio de Janeiro, RJ, Brasil b The University of Queensland Earth Sciences, Steele Building Brisbane, Qld 4072 Australia Received 13 October 2004; received in revised form 8 September 2005; accepted 19 September 2005 Available online 21 November 2005 Editor: K. Farley Abstract 40 Ar/ 39 Ar incremental heating ages for twenty one grains of cryptomelane, collected at 0, 42, 45, and 60 m 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 40 Ar/ 39 Ar plateau ages obtained decrease from the top to the bottom of the profile (12.7 F 0.1 to 7.6 F 0.1 Ma at surface; 7.6 F 0.2 to 6.1 F 0.2 Ma at 42 m; and 7.1 F 0.2 to 5.9 F 0.1 Ma at 45 m; 6.6 F 0.1 to 5.2 F 0.1 Ma at 60 m), yielding a weathering front propagation rate of 8.9 F 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 schists. The measured weathering front propagation rate yields a saprolitization rate of 24.9 F 3.1 t/km 2 /yr. This average long-term (N 10 Ma) saprolitization rate is consistent with mass balance calculations results for present saprolitization rates in weathering watersheds. These results are also consistent with long- term saprolitization rates estimated by combining cosmogenic isotope denudation rates with mass balance calculations. D 2005 Elsevier B.V. All rights reserved. Keywords: Weathering rate; chemical denudation; 40 Ar/ 39 Ar geochronology; Miocene; SE Brazil 1. Introduction Determining weathering rates is essential in under- standing global elemental cycles, CO 2 sequestration dur- ing weathering, weathering control on landscape evolution, and other surficial phenomena. Weathering rates are commonly measured by mass balance calcula- tions of the solute flux from drainage basins [1–17], by studying mass loss from known age objects (tombstone, buildings, etc.) [18–22], by measuring weathering rind thickness in boulders and cobbles [23–28], by laboratory experiments [29–31], and by the analysis of soil thick- ness in dated profiles [32–34]. Recently, Small et al. [35] and Riebe et al. [36] showed that erosion rates obtained from cosmogenic isotope measurements can be com- bined with mass balance calculations to estimate long- term weathering rates. Vasconcelos and Conroy [37] have also shown that weathering rates can be obtained by the age versus elevation relationships obtained from weathering profiles associated with stepped landscapes. Weathering rates measured from the solution and suspended fluxes in drainage basins record an instanta- 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.09.056 * Corresponding author. Tel.: +61 7 3365 2297; fax: +61 7 3365 1277. E-mail addresses: isacarmo@oi.com.br (I. de Oliveira Carmo), paulo@earth.uq.edu.au (P.M. Vasconcelos). 1 Tel.: +55 21 2540 5597; fax: +55 21 2598 9465. Earth and Planetary Science Letters 241 (2006) 80 – 94 www.elsevier.com/locate/epsl