FISSION TRACK DATING OF PHOSPHATE MINERALS

Gleadow, Belton, Kohn Brown FISSION TRACK DATING OF PHOSPHATE MINERALS 

Relatively little work has been carried out on fission track dating of phosphate minerals other than apatite, and merrillite, P-Ca3(P04)2, is the only other example to have received any significant attention. Two early studies examined the potential of monazite, (Ce,La,Y,Th)P04, and pyromorphite Pb5(P04)3Cl, but these minerals have not been studied further. Pyromorphite was shown by Haack (1973) to contain numerous spontaneous fission tracks, but these were very unevenly distributed, making its use in dating very difficult. 

Merrillite, an anhydrous calcium phosphate mineral often coexisting with apatite in lunar and meteorite samples, has been used for a number of fission track dating studies of extraterrestrial materials. Following a paper by Fuchs (1962) this mineral was most commonly identified in meteorites as whitlockite, but Dowty (1977) has shown that it exhibits significant differences to terrestrial whitlockite, (Ca,Mg)3(P04)2, and should be distinguished from it. As a result, earlier publications use whitlockite while later ones apply the name merrillite , for the same mineral. Merrillite is now the appropriate species name for the high-temperature phosphate mineral found predominantly in 

Two Other factors unique to these extraterrestrial samples must also be accounted for. The first is that tracks from heavy cosmic ray particles (mainly Fe-group nuclei) and other nuclear interactions with cosmic rays may also be present, in addition to fission tracks. The simplest method for correcting for the cosmic ray background is to measure the track density in adjacent silicate mineral grains, such as feldspar and pyroxene, which do not contain uranium and are therefore free of fission tracks. The second factor is that the samples are so old as to contain tracks from the spontaneous fission of now-extinct transuranic elements, particularly with a half-life of 82 Myr. Such Pu tracks will only be present in samples older than about 3.9 Gyr (Crozaz and Tasker 1981). 

The occurrence of extinct Pu tracks first became apparent when a large excess of fission tracks over that which would be expected from spontaneous fission alone was discovered (e g., Burnett et al. 1971). If calculated as a fission track age in the usual 

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Koul, SL (1979) On the fission track dating and annealing behavior of accessory minerals of eastern Ghats (Andlua Pradesh, India). Radiation Effects 40 187-192 Kryii ova AI, Kulikov lA, Artem eva GY (1992) Crystalline phosphates of the NaZr2(P04)3 family radiation stability. Radiochem 34 82-89. 

Fission track geochronology is a method for dating minerals containing uranium, particularly apatites and zircon. Apatite is a calcium phosphate that is common in granites and metamorphic rocks. Zircon is a zirconium silicate that is also common in similar rocks. The sample age is determined by counting fission tracks in the material from spontaneous fission of These tracks are a function of the U content and the age since closure when the fission track clock started. A research reactor is then used to irradiate the samples and induce fission in the present in the sample. By comparison of the before and after track count, the U content in the sample is determined. 

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