Integration 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.

As 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.

References

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.