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Track density and track length in Durango apatite in relation to crystallographic orientation
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Abstracts-6th International FTD Workshop naturally-occurring apatites in the system. Precise X-ray crystal structure refinements of near end-member compositions (including some of those mentioned in Part I) allow us to speculate on the effect of anion composition on the crystal structure and to correlate these structural variations with observed differences in annealing susceptibility. We have examined the three component polyhedra of the above apatites for variations in bond lengths, bond valence sums on the central cation and polyhedral volume. For all three end-members the Ca(I)09 polyhedron is similar in terms of polyhedral volume (~= 0.6%), mean Ca-o bond length (~= 0.2%) and bond valence sum on the central cation (~= 0.5%). Similarly, the P04 tetrahedra in all three structures do not exhibit large differences resulting from anionic substitution. Polyhedral volumes differ by 1.1 %, mean bond length by 0.3% and the bond valence sums on the central cation differ by 1.4%. In contrast, the Ca(2)061/J1 polyhedron of F- and OH-apatites is distinct from that of CI-apatite. F- and OH-apatites have similar volumes for the polyhedron (~ = 0.9%), but their average polyhedron volume is 5.1 % smaller than that of the CI-apatite polyhedron. F and OH end-members have similar average bond lengths for the Ca(2)061/J, polyhedra (~= 0.4%), but they differ from that of CI-apatite by 3.4%. Finally, bond valence sums for the Ca(2) polyhedron are similar for F- and OH-apatites (~= 0.5%), yet their mean is 4.6% larger than that for CI-apatite. In summary, the similar annealing susceptibility of F- and OH-apatites mirrors the similarity in crystal structure of the two minerals, and differences in the crystal structure ofCI- vs F- and OH-apatites correlate well with observed differences in annealing susceptibility. These variations in physical properties of the end-member apatites appear to be controlled largely by the coordination of Ca(2).
TRACK DENSITY AND TRACK LENGTH IN DURANGO APATITE IN RELATION TO CRYSTALLOGRAPHIC ORIENTATION R. JONCKHEERE,* M. MARS,t M. REBETEZ,t P. VAN DEN HAUTE* and A. CHAMBAUDETt *Laboratorium voor Aardkunde, State University of Gent, B-90oo Gent, Belgium and tLaboratoire de Microanalyses Nucleaires, U.F.R. Sciences, Route de Gray, 25030 Besanrvon Cedex, France SPONTANEOUS and induced fission tracks have been analysed In a number of slices cut parallel to the prismatic face and basal face to several well-developed crystals of Durango apatite. Density and projected length distributions of both spontaneous and induced tracks have been established for different degrees of annealing. From these data it is attempted to deduce the degree of crystallographic control on the track annealing process and possible differences in the annealing behaviour of spontaneous and induced tracks. In addition similar slices have been dated using both the technique based on the annealing of spontaneous tracks and the external detector method with three different detectors (mica, makrofol and kapton). The influence of crystallographic orientation on the fission track ages and on the geometry factor for the external detector method is discussed.
MODELING VARIABLE TEMPERATURE ANNEALING IN APATITE STEVEN M. JONES and Roy K. DOKKA Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A. SEVERAL different equations have been used to describe the thermal annealing of fission tracks in apatite. Up until now, only a first order kinetic model has been used to simulate variable temperature annealing. However, a simple stepwise algorithm can be used to implement the parallel and fanning Arrhenius models of Laslett et al. (1987) and solve for variable temperature thermal histories. This facilitates more extensive comparisons between the models. The equations used do not take into account the effect of chemical composition of apatite on its annealing properties, but when more complete experimental data becomes available this factor can easily be incorporated into the solution. The equations published in Laslett et al. (1987) for the parallel and fanning Arrhenius models predict reduction in mean track length as a function of temperature and time. Although these equations constitute the best description of the experimental annealing of Durango apatite (Green et al., 1986), they cannot be used directly to describe annealing due to variable temperature thermal histories over geologic time scales. To do this, both the variation in temperature from step to step and the effect of track production must be taken into account. The errors incurred in stepwise solutions of the parallel and fanning models depend on the size of the timestep used. Reducing the timestep has two effects: (I) For each individual timestep solution there IS less extrapolation from the experimental data range; (2) the processes of annealing and track production are modeled more accurately. Both of these are beneficial, but as the time step is decreased (or the number of steps is increased) the computation time increases significantly. Preliminary results suggest that timesteps of the order of 0.25-0.5 Ma give stable solutions for model times of 25.0 Ma. A computer program implementing this method of solution will be available for use by participants at the meeting.
Abstracts-6th International FTD Workshop naturally-occurring apatites in the system. Precise X-ray crystal structure refinements of near end-member compositions (including some of those mentioned in Part I) allow us to speculate on the effect of anion composition on the crystal structure and to correlate these structural variations with observed differences in annealing susceptibility. We have examined the three component polyhedra of the above apatites for variations in bond lengths, bond valence sums on the central cation and polyhedral volume. For all three end-members the Ca(I)09 polyhedron is similar in terms of polyhedral volume (~= 0.6%), mean Ca-o bond length (~= 0.2%) and bond valence sum on the central cation (~= 0.5%). Similarly, the P04 tetrahedra in all three structures do not exhibit large differences resulting from anionic substitution. Polyhedral volumes differ by 1.1 %, mean bond length by 0.3% and the bond valence sums on the central cation differ by 1.4%. In contrast, the Ca(2)061/J1 polyhedron of F- and OH-apatites is distinct from that of CI-apatite. F- and OH-apatites have similar volumes for the polyhedron (~ = 0.9%), but their average polyhedron volume is 5.1 % smaller than that of the CI-apatite polyhedron. F and OH end-members have similar average bond lengths for the Ca(2)061/J, polyhedra (~= 0.4%), but they differ from that of CI-apatite by 3.4%. Finally, bond valence sums for the Ca(2) polyhedron are similar for F- and OH-apatites (~= 0.5%), yet their mean is 4.6% larger than that for CI-apatite. In summary, the similar annealing susceptibility of F- and OH-apatites mirrors the similarity in crystal structure of the two minerals, and differences in the crystal structure ofCI- vs F- and OH-apatites correlate well with observed differences in annealing susceptibility. These variations in physical properties of the end-member apatites appear to be controlled largely by the coordination of Ca(2).
TRACK DENSITY AND TRACK LENGTH IN DURANGO APATITE IN RELATION TO CRYSTALLOGRAPHIC ORIENTATION R. JONCKHEERE,* M. MARS,t M. REBETEZ,t P. VAN DEN HAUTE* and A. CHAMBAUDETt *Laboratorium voor Aardkunde, State University of Gent, B-90oo Gent, Belgium and tLaboratoire de Microanalyses Nucleaires, U.F.R. Sciences, Route de Gray, 25030 Besanrvon Cedex, France SPONTANEOUS and induced fission tracks have been analysed In a number of slices cut parallel to the prismatic face and basal face to several well-developed crystals of Durango apatite. Density and projected length distributions of both spontaneous and induced tracks have been established for different degrees of annealing. From these data it is attempted to deduce the degree of crystallographic control on the track annealing process and possible differences in the annealing behaviour of spontaneous and induced tracks. In addition similar slices have been dated using both the technique based on the annealing of spontaneous tracks and the external detector method with three different detectors (mica, makrofol and kapton). The influence of crystallographic orientation on the fission track ages and on the geometry factor for the external detector method is discussed.
MODELING VARIABLE TEMPERATURE ANNEALING IN APATITE STEVEN M. JONES and Roy K. DOKKA Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A. SEVERAL different equations have been used to describe the thermal annealing of fission tracks in apatite. Up until now, only a first order kinetic model has been used to simulate variable temperature annealing. However, a simple stepwise algorithm can be used to implement the parallel and fanning Arrhenius models of Laslett et al. (1987) and solve for variable temperature thermal histories. This facilitates more extensive comparisons between the models. The equations used do not take into account the effect of chemical composition of apatite on its annealing properties, but when more complete experimental data becomes available this factor can easily be incorporated into the solution. The equations published in Laslett et al. (1987) for the parallel and fanning Arrhenius models predict reduction in mean track length as a function of temperature and time. Although these equations constitute the best description of the experimental annealing of Durango apatite (Green et al., 1986), they cannot be used directly to describe annealing due to variable temperature thermal histories over geologic time scales. To do this, both the variation in temperature from step to step and the effect of track production must be taken into account. The errors incurred in stepwise solutions of the parallel and fanning models depend on the size of the timestep used. Reducing the timestep has two effects: (I) For each individual timestep solution there IS less extrapolation from the experimental data range; (2) the processes of annealing and track production are modeled more accurately. Both of these are beneficial, but as the time step is decreased (or the number of steps is increased) the computation time increases significantly. Preliminary results suggest that timesteps of the order of 0.25-0.5 Ma give stable solutions for model times of 25.0 Ma. A computer program implementing this method of solution will be available for use by participants at the meeting.