Geochronological evidence for pervasive miocene weathering, Minas Gerais, Brazil

Ar-40/Ar-39 laser incremental-heating analyses of 22 individual grains of supergene cryptomelane from three weathering profiles, up to 400 km apart, in the Rio Doce valley and Barbacena regions at Minas Gerais, Brazil, show that the formation of weathering profiles in these regions is contemporaneous, suggesting a strong weathering event in the Middle to Late Miocene (10-8 Ma). The preservation of these Miocene samples at or near the present surface suggests that either erosion rates have been very low in the region since the Miocene or that a much thicker weathering mantle was present in the region originally. Assuming a constant thickness of weathering profiles in the region throughout the Tertiary, we may calculate weathering front propagation rates of 4-8 m Myr(-1) during the past 10 Ma. Copyright (C) 2004 John Wiley Sons, Ltd.

Weathering geochronology by (U-Th)/He dating of goethite

Nine samples of supergene goethite (FeOOH) from Brazil and Australia were selected to rest the suitability of this mineral for (U-Th)/He dating. Measured He ages ranged from 61 to 8 Ma and were reproducible to better than a few percent despite very large Variations in [U] and [Th]. In all Samples with internal stratigraphy or independent age constraints, the He ages corroborated the expected relationship's. These data demonstrate that internally consistent He ages can be obtained on goethite. but do not prove quantitative 4 He retention. To assess possible diffusive He loss, stepped-heating experiments were performed on two goethite samples that were subjected to proton irradiation to produce a homogeneous distribution of spallogenic He-3. The He-3 release pattern indicates the presence of at least two diffusion domains, one with high helium retentivity and the other with very low retentivity at Earth surface conditions. The low retentivity domain, which accounts for similar to 5% of He-3, contains no natural He-4 and may represent poorly crystalline or intergranular material which has lost all radiogenic He-4 by diffusion in nature. Diffusive loss of He-3 from the high retentivity domain is independent of the macroscopic dimensions of the analyzed polycrystalline aggregate, so probably represents diffusion from individual micrometer-size goethite crystals. The He-2/He-3 evolution during the incremental heating experiments shows that the high retentivity domain has retained 90%-95% of its radiogenic helium. This degree of retentivity is in excellent agreement with that independently predicted from the helium diffusion coefficients extrapolated to Earth surface temperature and held for the appropriate duration. Considering both the high and low retentivity domains, these data indicate that one of the samples retained 90% of its radiogenic He-4 over 47.5 Ma and the other retained 86% over 12.3 Ma. Thus while diffusive-loss corrections to supergene goethite He ages are required. these initial results indicate that the corrections are not extremely large and can be rigorously quantified using the proton-irradiation He-4/He-3 method. Copyright (C) 2005 Elsevier Ltd.

40Ar/39Ar geochronological constraints on the evolution of lateritic iron deposits in the Quadrilatero Ferrifero, Minas Gerais, Brazil

Weathering profiles overlying the Sapecado, Pico and Andaime iron ore deposits, Quadrilátero Ferrífero (QF), Minas Gerais, Brazil, reach depths of 150–400 m and host world-class supergene iron orebodies. In addition to hosting supergene ore bodies of global economic significance, weathered banded iron-formations at the Quadrilátero Ferrífero and elsewhere (e.g., Carajás, Hamersley) are postulated to underlie some of the most ancient continuously exposed weathering profiles on earth. Laser incremental-heating 40Ar/39Ar results for 69 grains of hollandite-group manganese oxides extracted from 23 samples collected at depths ranging from 5 to 150 m at the Sapecado, Pico and Andaime deposits reveal ages ranging from ca. 62 to 14 Ma. Older Mn-oxides occur near the surface, while younger Mn-oxides occur at depth. However, many samples collected at the weathering–bedrock interface yield ages in the 51–41 Ma range, suggesting that the weathering profiles in the Quadrilátero Ferrífero had already reached their present depth in the Paleogene. The antiquity of the weathering profiles in the Quadrilátero Ferrífero is comparable to the antiquity of dated weathering profiles on banded iron-formations in the Carajás Region (Brazil) and the Hamersley Province, Western Australia. The age versus depth distributions obtained in this study, but not available for other regions containing similar supergene iron deposits, suggest that little further advance of the weathering front has occurred in the Quadrilátero Ferrífero lateritic profiles during the Neogene. The results suggest that weathering in some of these ancient landscapes is not controlled by the steady-state advance of weathering fronts through time, but may reflect climatic and geomorphological conditions prevailing in a remote past. The geochronological results also confirm that the ancient landsurfaces in the Quadrilátero Ferrífero probably remained immune to erosion for tens of millions of years. Deep weathering, mostly in the Paleogene, combined with low erosion rates, account for the abundance and widespread distribution of supergene iron, manganese, and aluminum orebodies in this region.

(U-Th)/He and 40Ar/39Ar Geochronology of Weathering, Hamersley Province, Australia: Implications for weathering history and landscape Evolution

(U–Th)/He dating of goethite, when combined with quantification of diffusive 4He loss by the 4He/3He methodology, provides reliable corrected ages for minerals precipitated in weathering profiles. We have combined (U–Th)/He dating of supergene goethite with 40Ar/39Ar dating of supergene manganese oxides to study the weathering history and landscape evolution in the Hamersley Province, northwestern Australia. Incremental heating 40Ar/39Ar analysis of 187 grains of Mn oxides from 65 samples (44 hand specimens) collected from weathering profiles at seven field sites across the Hamersley Province yield precipitation ages ranging from 63.4 ± 0.9 to 1.5 ± 0.2 Ma. These results, combined with previous results of 40Ar/39Ar dating of Mn oxides (Vasconcelos, 1998 Vasconcelos, P.V., 1998. Unpub. report, pp. 1–278.Vasconcelos, 1998 and Cochrane, 2003), reveal a protracted and episodic history of weathering and landscape evolution, which was already ongoing in Late Cretaceous and spans the Palaeogene and Neogene. Seventy-three grains of goethite from 39 samples extracted from 21 hand specimens, collected from the same field sites where the Mn oxides originated, were dated by the (U–Th)/He method. Internally consistent (U–Th)/He ages, which range from 84.3 ± 12.2 to 3.3 ± 0.5 Ma, have been obtained for most samples when corrections are applied for 10% helium diffusive loss. The geochronological results obtained show remarkable similarity in the distribution of ages associated with supergene mineral precipitation. The widespread occurrence of iron oxides such as goethite in soils and weathering profiles and the successful application of (U–Th)/He dating of goethite offers great opportunities for extracting the wealth of palaeoclimatic and palaeoenvironmental information recorded by these profiles on the surface of terrestrial planets such as Earth and Mars.