Using argon as a temporal tracer of large-scale geologic processes

Abstract

Argon (Ar) has proven to be one of the most useful elements in understanding the timing and duration of geological processes. Because of its chemical and physical properties, Ar is a particularly sensitive indicator of temperature variations associated with large-scale geological events. Ar mobility in minerals is commonly described using solid-state diffusion theory (volume diffusion), which has been used extensively to model diffusion profiles and closure temperatures in minerals. Because 40Ar is the radiogenic daughter product of radioactive 40K and therefore occurs in many common rock-forming minerals, Ar has been used effectively as a temporal tracer to understand a wide variety of geological phenomena. Ar isotopes have been applied with great success in three broad fields of earth science—the origin and evolution of the earth's atmosphere, the tectonothermal history of geologic terranes, and the timing and duration of large-scale tectonic events. From Ar isotopic data, the earth's atmosphere is now known to be mainly secondary in nature, although evolutionary models have undergone a recent paradigm shift due to geophysical evidence, moving away from the earlier notion of a chemically layered upper and lower mantle to a whole-earth mantle structure that may be chemically inhomogeneous. Although recent models are growing in sophistication by integrating additional information and measurements reflecting the behaviour of other noble gases through time, the temporal evolution of the earth's atmosphere still remains ambiguous because measured compositions of ancient atmosphere are sparse and remain difficult to obtain. 40Ar/ 39Ar thermochronology is one of the most commonly used tools in tectonothermal investigations because of the large range of Ar closure temperatures recorded by various common K-bearing minerals; although more recent thermochronological models, such as multidiffusion domain (MDD) modelling, remain controversial, the combination of 40Ar/ 39Ar thermochronology with both higher and lower temperature thermochronological methods utilizing other isotopic systems makes this an extremely versatile method of understanding the temperature–time evolution of geologic regions. Recent developments involving the numerical modelling of Ar diffusion coupled with the geodynamic modelling of orogens have led to new ways of estimating the timing and duration of a complete tectonic cycle of subduction and exhumation; by applying these new methods to rocks from the Bergen Arcs in Norway, studies have shown that this cycle can be so rapid that the subducted and exhumed crust can remain relatively cool, never achieving thermal equilibrium with its ambient environment throughout the entire process. In summary, the versatility of the 40Ar/ 39Ar dating method combined with the effective application of various techniques (e.g. laser spot-dating and step-heating) and a detailed understanding of the diffusional behaviour of Ar has proven to be a powerful research tool in elucidating the timing and duration of many large-scale earth-system processes.