Progress on supergene mineral dating utilising the 40Ar/39Ar technique and terrestrial weathering in Great Britain & Ireland

Constraining the weathering history of the British Isles in the Cenozoic is limited by the sparse distribution of terrestrial rock units of appropriate age.  This has prompted many workers to rely on examination of the regional marine record to make inferences about terrestrial weathering and climate in this era.  We have begun a project to date supergene mineral deposits across the region to provide direct temporal information about the timing and extent of weathering processes.  We show first results of dating with the ⁴⁰Ar/³⁹Ar technique on cryptomelane (KMn8O16) from Scotland, suggesting a late Miocene age.  A full sample suite from across Great Britain and Ireland is currently being analysed.

In addition to dating of cryptomelane and other Hollandite group minerals, the NEIF argon isotope laboratory at SUERC has developed the capability of dating difficult hydrous sulphate minerals alunite and jarosite that occur across the region and will be the subject of future weathering studies. Sample preparation remains a challenging aspect of dating supergene minerals.  This is because of the fine-grained nature of the material coupled with the intergrowth of potentially complicating phases such as clay, feldspar or quartz.  HF leaching of materials to remove silicate impurities have shown promise, suggesting a reduction in the budget of trapped atmospheric argon, and reproducible ages have been obtained for samples as young as Pleistocene.  Attempts at micro-sampling, for example, growth layers in cryptomelane using microdrill techniques, have met with limited success.  Future work will look at laser micro-sampling coupled with high precision and high sensitivity ⁴⁰Ar/³⁹Ar analysis on the next-generation THERMO ARGUS VI mass spectrometer.