Growth Rate Estimates of Supergene Manganese Nodule by ⁴⁰Ar/³⁹Ar Isotopic Dating

Increasing world-class, high-grade, and metals-enriched supergene manganese ore deposits have been discovered in the last two decades, making them more and more economically important. However, data on the timing and duration of their formation are sparse, mainly due to the difficulties extracting datable minerals suited to traditional radiometric dating methods. Hollandite, cryptomelane, coronadite, todorokite, and manjiroite are common manganese oxide minerals in supergene environments. These minerals host potassium of variable amounts from 0.1 wt% to 5.0 wt% in their structural sites. This geochemical property provides possibility to date supergene manganese ores by using K-Ar and ⁴⁰Ar/³⁹Ar methods. In this study, we perform ⁴⁰Ar/³⁹Ar dating on a 7.1-cm-thick botryoidal manganese nodule from an ancient weathering profile at Mount Tabor, central Queensland, Australia. Laser microprobe incremental analyses of distinct growth bands, from the inner core through the intermediate bands to the outermost crusts of the nodule, have yielded high quality ⁴⁰Ar/³⁹Ar ages at 27.3 Ma, 20.9 Ma, 19.2 Ma, and 16.1 Ma, respectively. The age results permit preliminary estimates on the average growth rates of the nodule varying from 4.7×10^-3 mm/ka to 7.6×10^-3 mm/ka to 9.0×10^-3 mm/ka, from the core to the rim.Resultsof this study are of significance in our understanding of the mode, mechanism, process, and climatic conditions in the formation of supergene manganese ore deposits.