Current Activities
Dating metamorphism and deformation
Isotopic dating of deformation is difficult for many reasons: it
is often hard to tell which minerals formed or recrystallised completely
during deformation; minerals often only become closed systems at
temperatures below the conditions of crystallisation; isotopic equilibration
during deformation and metamorphism is often incomplete. We have overcome
the problems of incomplete isotopic equilibration by using microsamples
selected from specific microstructures; these are drilled from thin sections
under the microscope.This technique has been used to precisely date deformation
and recrystallisation in basement gneisses from the Alps by Rb-Sr isotopic
analysis of microsamples of phengite and feldspar from individual
crenulation hinges (Meffan-Main & Cliff, 1996, Meffan-Main, 1998).
The same approach is now being applied to hornblende and plagioclase from
amphibolite facies shear zones. In a related study of a pelitic schist,
individual allanite porphyroblasts, tied by inclusion fabrics to the deformation
history, have been dated by the Th-Pb method.
Dating speleothems
Precise geochronological techniques for the early Quaternary and latest
Tertiary samples are rare. Calcite precipitated from solution in caves
(speleothems) has been widely used in U-Th geochronology in the age range
up to ca. 0.5Ma but older deposits cannot be dated by this technique. However
the range of uranium concentrations includes values which are high enough
for the 238U-206Pb technique to be used. We have
shown (Richards et al. 1998) that a sample from Winnat's Head Cave
in the English Peak District yields concordant 0.24Ma ages by mass-spectrometric
230Th analysis and by 238U-206Pb; this
concordance confirms that radon loss and other intermediate daughter effects
are not a problem. We are now applying the technique to a range of samples
from Britain, the Alps and southern Africa.