The K-Feldspar thermochronometer: a test of
the recrystallisation hypothesis
W. J. Dunlap
K-feldspars
are one the most widely used geochronometers in earth science research because
they are common in continental crustal rocks, and they retain radiogenic argon
quantitatively in a variety of situations. Sanidines from silicic volcanics,
for example, do not exhibit age gradients, and their ages can in most cases be
interpreted as the age of eruption. Slowly cooled K-feldspar from granites and
gneisses, on the other hand, almost always exhibit evidence for internal age
gradients. The multidomain theory of K-feldspar thermochronometry was
developed to explain the strong age gradients commonly observed in the 40Ar/39Ar
age spectra of slowly cooled K-feldspars (cf. Lovera et al. 1989). Despite the
apparent success of this method in elucidating crustal thermal histories, an
alternative theory refutes the basis of the multidomain method and holds that
the age gradients are a direct reflection of late-stage low temperature
recrystallization of a large percentage of the original lattice (Parsons et
al., 1888). In recent this has been one of the most hotly debated issues in
geochronology.
The
effect of recrystallization on the K-feldspar thermochronometer has been
assessed by comparing 40Ar/39Ar and textural data for two
samples from Toftoy, Norway, where feldspathic gneisses are cut by later
epidote-filled fractures. Fracture opening was accompanied by discolouration
and recrystallization of adjacent wallrock K-feldspar within 3-5 cm of the
fracture. Although light microscopy clearly shows that the outline of the
original (millimetre scale) gneissic grains remain intact, SEM studies of the
recrystallised K-feldspar indicate that a network of crystallographically
controlled dissolution channels developed, mostly probably along high strain
zones during deformation (Figure x). The remaining K-feldspar between the
channels was pervasively twinned and recrystallised on a very fine scale.
The recrystallized K-feldspar yields an age spectrum with
ages between ~230-290 Ma, and a bulk age of 282 Ma (Figure xx). In contrast,
the unrecrystallized, cryptoperthitic K-feldspar from the unaffected gneiss
only centimeters away preserves ages between ~280-430 Ma, and a bulk age of 393
Ma. The differences in age and texture between the gneissic and recrystallized
K-feldspars are most readily explained by argon loss related to dissolution,
recrystallisation and pervasive checkerboard twinning of the remaining
K-feldspar during deformation. I believe that the original K-feldspar lattice
remains, although it has probably been swept through by microstructures that
have allowed, at least locally, complete escape of accumulated radiogenic
argon. Moreover, the younger age limit of single fragment fusion ages for the
recrystallised sample coincides with the suspected age of fracture initiation,
during formation of the immediately adjacent Viking Graben and opening of the
North Sea (Figure xxx).
In
the absence of significant optical evidence for recrystallization, the age
gradients preserved in slowly cooled K-feldspars are most simply and elegantly
interpreted as resulting from multiple diffusion domains that record detailed
thermal closure to argon loss. It is not a coincidence that a large fraction
of slowly cooled K-feldspars analysed by the 40Ar/39Ar
method yield ages between higher temperature (K-Ar of micas) and lower
temperature (Fission track) thermochronometers. This indicates that
low-temperature recrystallisation, although an acknowledged and well-documented
process, is generally not volumetrically significant enough to affect the 40Ar/39Ar
thermochronometer of most fresh, "garden variety", crustal
K-feldspars.
Figure x: SEM studies of the
recrystallised K-feldspar indicate that a network of crystallographically
controlled dissolution channels developed, mostly probably along high strain
zones during deformation.
Figure xx: The recrystallized
K-feldspar yields an age spectrum with ages between ~230-290 Ma, and a bulk age
of 282 Ma
Figure xxx: the
younger age limit of single fragment fusion ages for the recrystallised sample
coincides with the suspected age of fracture initiation, during formation of
the immediately adjacent Viking Graben and opening of the North Sea.