Brecciation-related argon redistribution in alkali feldspars: An in naturocrushing study

Alkali feldspar thermochronologic modeling with the Ar-40/Ar-39 method has generated marked advances in knowledge of the mechanisms for argon diffusio n in feldspars. While the goal in many cases has been to extrapolate the ob served and modeled argon behavior in feldspars to natural geological settin gs, scientific debate surrounding the true feasibility of such extrapolatio ns and indeed, the validity of thermochronologic modeling in itself, have p rovided much impetus to improve laboratory techniques to test, and increase basic understanding of, argon diffusion. Two cornerstones for the debate o ver the feasibility of alkali feldspar thermochronology for modeling natura l, geologic processes have been: 1. is volume diffusion the main mechanism for argon movement in feldspars? and 2. if volume diffusion is a viable mechanism, does argon then reside in dis crete 'domains' within the feldspar lattice? We describe a study of alkali feldspars from a profile through a well-contr olled brittle fault zone in western Norway; the feldspars document argon lo ss during deformation and strongly suggest the existence of argon 'domains' within the feldspars, at least during laboratory step heating. The progres sive change in the character of argon diffusion is recognizable in the logr /r(o) diffusion data from the feldspars and is mimicked by physical changes observed optically in the feldspars through progressive degrees of brittle deformation. Modeling results indicate a reduction in size of the biggest domains and the appearance of smaller domains during the strongest stages o f deformation. Whether or not this reveals the existence and the transforma tion of the domain structure in nature is difficult to prove from our data alone, but interestingly, this behaviour corresponds directly to the physic al (optical) appearance of more intense crack networks and subgrains in pro gressively more brecciated feldspars. Because the thermochronologic histori es derived from modeling the feldspar data conform very well to the known t ectonic history of the area, the feldspars appear to have successfully reta ined physical (optical and isotopic) records of episodic tectonic processes operating from ductile through low-temperature brittle regimes in rocks wi th a Caledonian history overprinted by several later (younger) geologic eve nts. However, because the 'cold' brecciation is the last tectonothermal eve nt recorded by these rocks, it is impossible to truly test for the existenc e of diffusion domains in nature. Argon loss appears to have been effective only in the most highly brecciated (deformed) samples where the combinatio n of the connected crack network, increased fluid flow and higher temperatu res enhanced diffusion via fast diffusion pathways and thus, volume diffusi on from the lattice. Only minor argon loss occurred in zones of lower britt le strain, although some development of cracks and brittle features is evid ent. Independent of the existence of diffusion domains, this study highligh ts the possible pitfalls when cooling histories are deduced from brecciated feldspars in which age and diffusion charateristics have been decoupled: w hile the geochronological memory has survived and is identical to that of n onbrecciated feldspars (suggesting no loss and minor effects of deformation ), the diffusion characteristics have been completely transposed by breccia tion and the appearance of new domains. Modeling feldspars with these latte r characteristics would effectively utilise a new feldspar diffusion struct ure with an 'old' (relict) age memory. Copyright (C) 2000 Elsevier Science Ltd.