40Ar/39Ar geochronological constraints for the Urucum Iron and Manganese-deposits, Brazil

⁴⁰Ar/³⁹Ar geochronology of cryptomelane (KMn₈O₁₆.xH₂O) interbedded with the Urucum late Proterozoic banded iron-formation (BIF) reveals a minimum deposition age of 587 ± 7 Ma. All cryptomelane samples yield climbing ⁴⁰Ar/³⁹Ar incremental heating spectra suggestive of partial ⁴⁰Ar* loss after mineral precipitation, indicating that the samples were heated at the cryptomelane Ar partial retention zone. Inverse isochron calculated for all cryptomelane grains reveals a minimum age of 578 ± 2 Ma, with an initial ⁴⁰Ar/³⁶Ar ratio of 274.0 ± 1.7. While the age is compatible with the minimum age determined from the incremental heating spectra, the initial ⁴⁰Ar/³⁶Ar ratio is significantly different from the present value of 298.6 ± 0.3. ⁴⁰Ar/³⁹Ar geochronology for braunite coexisting with cryptomelane yield complex saddle-shaped age spectra where plateau-like segments are followed by an age increase at the high temperature steps. This degassing pattern may reflect an excess Ar component in braunite, which is aggravated by its low K content and, therefore, intrinsically low radiogenic ⁴⁰Ar* component. Despite these complexities, the plateau-like segments yield ages varying from 547 ± 8 Ma to 513 ± 8 Ma, which are compatible with the ages estimated for peak metamorphism of the BIF sequence. An inverse isochron for all braunite analyzed yield a well-defined array with an age of 524 ± 3 Ma and an initial ⁴⁰Ar/³⁶Ar ratio of 271 ± 2, compatible with the initial ratios obtained for the cryptomelane samples. ⁴⁰Ar/³⁹Ar geochronology for two metamorphic muscovite single crystals from meta-arkoses interbedded with the BIFs yielded reproducible plateau ages of 512 ± 3 Ma and 512 ± 5 Ma, probably recording the timing of peak Brasiliano metamorphism. The incremental heating spectra for the cryptomelane and braunite samples suggest that the Urucum Rapitan-age BIF is at least 587 ± 7 Ma. The initial ⁴⁰Ar/³⁶Ar ratios for both cryptomelane and braunite suggest a lower atmospheric ⁴⁰Ar/³⁶Ar for the late Proterozoic, consistent with previous estimates for the palaeoatmospheric ⁴⁰Ar/³⁶Ar ratios determined (Cadogan, 1977, Hanes et al., 1985), suggesting that the Earth's atmosphere may not have reached its present ⁴⁰Ar/³⁶Ar until the late Paleozoic. The evidence indicating partial Ar loss for the cryptomelane samples suggest that the Urucum BIF may have been buried and heated to significant temperatures in the late Proterozoic and that the present mineralogy and isotopic composition of the BIF may not reflect the original isotopic composition of the depositional environment. Cadogan PH. 1977. Palaeoatmospheric argon in Rhynie chert. Nature, 268, 38-41. Hanes JA, York D & Hall CM. 1985. An ⁴⁰Ar/³⁹Ar geochronological and electron microprobe investigation of an Archaean pyroxenite and its bearing on ancient atmospheric compositions. Canadian Journal of Earth Sciences, 22, 947-958.