Goswami, J N

Jha, R

Lal, D

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Indian J. Phys. 64A (3), 251-252 (1990)Fission track blocking temperatures in rock minerals : a re-evaluationJ N Goswami, R Jha and D LaiP h y s ica l R esearch  Laboratory, A h m ed abad-380 00 9 , IndiaThe concept of the closure or blocking temperature was first introduced in geochronology to model the post-metamorphic thermal history of geological samples. The blocking temperature can be considered as the "effective temperature" at which the radiometric clock starts. The apparent radiometric age thus corresponds to the time at which the system cools down to the blocking tempera­ture. The first analytical attempt to introduce the concept of blocking temperature in the field of fission track geochronology was made by Haack (1977). A  prerequisite in estimating the blocking temperature for tracks in rock mineral is to know the manner in which the etchable track length is foreshortened for any prescribed thermal history of the mineral. Haack could only partially consider this aspect in his treatment. Bertagnolli et al (1983) made the first atempt to realistically estimate the continuous change in the length distribution of tracks, upto any given point of a specified thermal history by considering an empirical relation for rate of annealing of tracks proposed by Mark et al (1973). James and Durrani (1986) subsequently extended this approach to estimate fission track blocking temperatures in different rock minerals and found that they are systemati­cally higher than those obtained earlier by Haack. We have shown in an earlier publication (Goswami et al 1984) that the problem of annealing of tracks for any given thermal history and track production rate can in fact be treated quantitatively. The equation describing the track annealing process was primarily based on the assumption of the validity of the Arrhenious equation for annealing of tracks. The validity of the four parameter semi-empirical relation derived by us was demonstrated by carrying out laboratory annealing studies of mineral grains subjected to complex temperature history. We have now used this formalism to calculate the foreshortening of tracks in rock minerals for any prescribed thermal history and used these data to obtain their fission track blocking temperatures. In Table 1, we show  the results for blocking temperature of apatites for several linear cooling history all of which terminate at 300"K. Comparison of these values w ith those obtained by others indicate significant differences. Of course the blocking temperature obtained by both James251252 j  N Goswami R Jha end D Lol Table I. Fission track blocking temperatures for apatite.Source of F.T. dataBlocking temperatures (°C) for cooling rates ofO.r/m.y. 17m.y 107m.y.50%Annealing temp. (°C) (t-1  m.y.)Naeser and 62 78 94 (Haack) 110Foul (1969) 95 107 119 (This work)Wagner and 71 83 95 (Haack) 140Reimer (1972) 125 137 149 (This work)Watt and 64 74 84 (Haack)Durrani (1985) 73 86 99 (James and Durrani) 123113 125 137 (This work)and Durrani (1986) and us are higher than those of Haack, which is to be expected considering the oversimplication in the approach used by Haack. Blocking temperatures for other common rock forming minerals used in fission track studies (e. g. Sphene, epidote, garnet etc) as well as illustrative examples of track length distributions during various stages of prescribed thermal history have also been studied. We believe that our approach is more appropriate, and the blocking temperature estimated here should be a proper guide for the geochronological/ geophysical interpretation of fission track data.ReferencesBertagnolli E, Kiel R and Pahl M 1983 N u c l . T racks  1163Goswami J N, Jha R and Lai D 1984 E a rth  P la n e t. Sci. Lett. 71120Haack U 1977 4m. j.Sci. 277 459James K and Durrani S A 1986 N u c le a r  T racks  12 921Mark E, Pahl M, Purtscheller F and Mark T D 1973 Tjc/iermoks Min. 20131Naeser C W and Faul H 1969 j. G eophys, Res. 74 705Wagner G A  and Reimer G M 1972 Earth P la n e t. S c i. Lett. 14 263Watt S and Durrani S A 1985 N u c l. Tracks  10 349

Fission Track Blocking Temperatures in Rock Minerals : a Re-Evaluation

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Fission Track Blocking Temperatures in Rock Minerals : a Re-Evaluation

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