Potassium and argon loss patterns in weathered micas: Implications for detrital mineral studies, with particular reference to the triassic palaeogeography of the British Isles

Abstract

Potassium-argon ages are reported for 16 muscovite and 14 biotite samples separated from Triassic sandstones of the Manx-Furness, Carlisle-Solway Firth and Lough Neagh-Arran Basins. Disturbance of the K-Ar isotopic system in these minerals during weathering, erosion, transport and lithification is discussed and loss of potassium shown to be as important a parameter as argon loss. Published data from tropical weathering profiles are used to establish the pattern of loss for these two elements during the formation of clay minerals from biotite, the data being displayed in a “potassium-argon loss diagram”. The loss curve so produced is then used to interpret our new biotite data and previously published age data for detrital biotite from the Cheshire Basin. The limit of interdigitation of northerly and southerly derived materials in the Irish Sea Basins is shown to lie north of the Fylde district of Lancashire. Age data from detrital muscovite are used to establish a potassum-argon loss curve for muscovite. The features of the two curves are markedly similar. Little effect on the apparent K-Ar age is observed until about 20% loss of potassium occurs. At this point massive argon loss takes place with little loss of potassium, due to either potassium “stabilisation” or structural expansion. The suitability of micas as K-Ar chronometers may be a consequence of the correlated potassium-argon loss rather than intrinsic high argon retentivity. If both mica curves are applied to detrial micas of the Namurian Millstone Grit series, a source area of age of ca. 900 Ma is indicated for both muscovite and biotite, indicating a probable pre-Cambrian age source area, probably in Scandinavia.