of vitrinite reflectance data with AFTA
Like Apatite Fission Track Analysis (AFTA), Vitrinite
reflectance (VR) data can be used to assess the maximum paleotemperature
experienced by a sample. It is a time-temperature indicator governed
by a kinetic response in a similar manner to the annealing of fission
tracks in apatite. Vitrinite reflectance data are interpreted at Geotrack
on the basis of the distributed activation energy model describing the
evolution of VR with temperature and time described by Burnham and Sweeney
(1989). In a considerable number of wells from around the world, in
which AFTA has been used to constrain the thermal history, we have found
that the Burnham and Sweeney (1989) model gives good agreement between
predicted and observed VR data, in a variety of tectonic settings.
in the case of fission track annealing, it is clear from the chemical
kinetic description embodied in equation 2 of Burnham and Sweeney (1989)
that temperature is more important than time in controlling the increase
of vitrinite reflectance. If the Burnham and Sweeney (1989) distributed
activation energy model is expressed in the form of an Arrhenius plot
(a plot of the logarithm of time versus inverse absolute temperature),
then the slopes of lines defining contours of equal vitrinite reflectance
in such a plot are very similar to those describing the kinetic description
of annealing of fission tracks in Durango apatite developed by Laslett
et al. (1987), which is used to interpret AFTA data. This feature of
the two quite independent approaches to paleotemperature analysis means
that for a particular sample, a given degree of fission track annealing
in apatite (of Durango composition) will be associated with the same
value of vitrinite reflectance regardless of the heating rate experienced
by a sample. Thus paleotemperature estimates based on either AFTA or
VR data sets should be equivalent, regardless of the duration of heating.
One practical consequence of this relationship between
AFTA and VR is, for example, that a VR value of 0.7% is associated with
total annealing of all fission tracks in apatite of Durango composition,
and that total annealing of all fission tracks in apatites of more Chlorine-rich
composition is accomplished between VR values of 0.7 and ~0.9%.
Furthermore, because vitrinite reflectance continues
to increase progressively with increasing temperature, VR data allow
direct estimation of maximum paleotemperatures in the range where fission
tracks in apatite are totally annealed (generally above ~110°C) and
where therefore AFTA only provides minimum estimates. Maximum paleotemperature
estimates based on vitrinite reflectance data from a well in which most
AFTA samples were totally annealed will allow constraints on the paleogeothermal
gradient that would not be possible from AFTA alone. In such cases the
AFTA data should allow tight constraints to be placed on the time of
cooling and also the cooling history, since AFTA parameters will be
dominated by the effects of tracks formed after cooling from maximum
paleotemperatures. Even in situations where AFTA samples were not totally
annealed, integration of AFTA and VR can allow paleotemperature control
over a greater range of depth, e.g., by combining AFTA from sand-dominated
units with VR from other parts of the section, thereby providing tighter
constraint on the paleogeothermal gradient and ultimately the burial
and thermal history.
Burnham, A.K. and Sweeney, J.J. (1989) A chemical kinetic
model of vitrinite reflectance maturation. Geochim. et Cosmochim.
Acta, 53, 2649-2657.
Laslett, G.M., Green, P.F., Duddy, I.R. and Gleadow,
A.J.W. (1987) Thermal annealing of fission tracks in apatite 2. A quantitative
analysis. Chem. Geol. (Isot. Geosci.Sect.), 65, 1-13.