No. Arnaud et Ea. Eide, Brecciation-related argon redistribution in alkali feldspars: An in naturocrushing study, GEOCH COS A, 64(18), 2000, pp. 3201-3215
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.