Radiogenic argon and major-element loss from biotite during natural weathering: A geochemical approach to the interpretation of potassium—argon ages of detrital biotite

Abstract

Major-element geochemical data from fresh biotites are used to distinguish an “invariant” parameter, Σ, which is the scaled sum of elemental concentrations, 0.7 (Fe 2+ + Mg + Ti + Al + Si) + 0.3K, having the value 35.3 ± 0.5 wt.%. Analyses of biotites from a sub-tropical modern weathering section of the Beni-Toufout Granite (northern Algeria) (K-Ar age 17.5 ± 0.2 Ma) are used to demonstrate that, during weathering, Σ is reduced in a fashion which is linearly correlated with radiogenic Ar loss. By this means, a quantification of the effect of weathering on the K-Ar age system is established, and used to “correct” conventional K-Ar ages of detrital biotites from Middle Triassic sandstones of the Cheshire Basin, U.K. Following “correction”, the biotite ages are in closer accord with K-Ar ages of coexisting detrital muscovite than they were previously. The “corrected” ages support the view that the palaeo-drainage system transporting the micas originated in the Armorican uplands lying to the south, where both the Cadomian and Variscan orogenic belts are represented. No evidence is found to indicate significant “Caledonian” input to the “Keuper” of the Cheshire Basin.