EXCESS ARGON IN K-AR AND AR-AR GEOCHRONOLOGY

Abstract

The K-Ar and Ar-Ar dating techniques occasionally produce anomalously old ages attributed to excess argon, and such data is often rejected as not offering any insight into the age, thermal history or geochemistry of the rock. However, improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems. Solubility data for argon in hydrous fluids, melts and emerging data for minerals can be used to understand the behaviour of excess argon, and provide valuable insights into the environment in which the samples cooled to their argon retention or 'closure' temperature. Treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature, in deeply buried dry systems such as eclogites, blueschists, granulites and even in the lithospheric mantle. Extremely low partition coefficients between K-feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid-poor systems. Variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock. However, excess argon can also be introduced or removed from minerals by varying temperature, without the need for fluid flow. High mineral/melt and mineral/fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals. Partition coefficients between minerals and hydrous fluids as low as 10-6 lead fluid inclusions to dominate the radiogenic argon budget, particularly in low potassium minerals. Melt inclusions are less dominant but become critical in dating younger samples.