QUANTIFICATION OF THE FACTORS CONTROLLING THE PRESENCE OF EXCESS 40AR OR 4HE

Abstract

A quantitative physical model is presented which includes the factors that control the presence, or absence, of internally derived excess 40Ar or excess 4He in geological systems. In particular, the model incorporates the transport and partitioning properties of the rock surrounding the mineral of thermochronologic interest and illuminates the related effects on the amount of excess 40Ar or 4He preserved in the system. Modeling of a simplified 1-D rock column bounded by an external sink for 40Ar or 4He shows that a steady-state excess 40Ar or 4He profile develops, the magnitude of which is determined by a system parameter called the 'transmissive timescale', τT. The characteristic time required to reach this steady state depends upon τT and the 'total local sink capacity', TLSC, wherein the important role of local matrix mineral and fluid phases is incorporated. Together, these two system parameters (τT and TLSC) determine the evolution of excess 40Ar or 4He buildup within a system above the closure temperatures of all minerals involved. An analytical expression for the 1-D system describing the evolution of excess 40Ar (or by analogy 4He) in a particular potassium-bearing (or U-Th-bearing) mineral located at a distance, L, from an external sink has been derived empirically from model results:40Arage-equivalent(L,t)#τTAr21-exp52.tτTAr( 1+TLSCAr)Local matrix minerals, perhaps most notably quartz, may act as important sinks for 40Ar (except in the most fluid-rich systems where fluids dominate) and thus are fundamental in controlling, and limiting, thermochronologically problematic excess 40Ar in neighboring potassium-bearing minerals. In general, the model provides a rigorous means of predicting excess noble gas content, residence, release, and transport within the compositionally variable and thermally evolving crust.