Fission track

Although fission tracks (FT) are not applied as commonly as the other radiometric dating methods in paleoanthropology, they made significant contributions at some important sites in volcanic regions. They are formed by the spontaneous nuclear fission of uranium. Natural uranium consists of the isotopes 238U (99.3%) and 235U (0.7%), whereby 238U decays by spontaneous fission. The decay rate of this fission is 106 times less than that of the a-decay of the same isotope. During fission, the uranium nucleus splits up into two fragments. Due to their kinetic energy, both fission fragments are expelled in opposite directions and leave along their path a zone of radiation damage. Both branches together form a straight fission track of 10-20 mm in length and several 10~3 mm in diameter. By chemical etching, the fission tracks can be made visible under the optical microscope. In the course of time, the tracks accumulate in the minerals. When all tracks are preserved, their number gives the age of the sample. Since the track number depends also on the uranium content, the latter one needs to be known. For U analysis, the thermal-neutron induced fission of 235U is exploited.

The number of the induced 235U fission tracks is proportional to the U content. Thus, the procedure of fission-track dating essentially involves the counting of spontaneous 238U fission tracks before and induced 235U fission tracks after a neutron irradiation.

The principles and application of fission-track dating were described in detail by Wagner and Van den haute (1992). With fission tracks, one dates either the formation or a secondary heating event at which all previous tracks were erased, i.e., the clock was reset. The fission-track method is applicable to ages of >10 ka. This requires, however, sufficiently high uranium contents above 100 mg/g. Zircon, due to its high uranium content, is most frequently used in the paleoanthropologic age range. This mineral may occur in volcanic rocks. Of particular interest are volcanic ashes that are intercalated in sedimentary sequences containing hominid remains and Paleolithic implements. Also volcanic glass, such as obsidian and pumice, is frequently used for fission-track dating.

A commonly met problem in fission-track dating is track annealing. Latent fission tracks gradually fade over time. The fading is accelerated at elevated temperatures, a process known as annealing. Since annealing reduces the apparent fission-track age, it is of fundamental importance. Fading can be recognized by track-length measurements since annealing shortens the tracks. Fortunately, tracks in zircon are rather stable and do not show any signs of fading over several million years at ambient temperatures, although tracks in natural glasses certainly may fade under such conditions.

For fission-track dating of tephra, mainly zircon grains and, to a lesser degree, also glass shards and apatite as well as titanite grains are used. When relying on heavy minerals, the problem of different provenance of the various grains needs to be taken into consideration, a difficulty already discussed (cf. K-Ar dating). Primary volcanic grains in presence of detrital ones can be identified—apart from mineralogical criteria—by single-grain fission-track data. A good case study is that on the Plio-Pleistocene sedimentary sequence of the Koobi Fora formation, Kenya. It contains several tuff horizons, which primarily consist of glass fragments and pumice cobbles and show signs of redeposition. Of particular interest is the KBS Tuff, which is intercalated in hominid-bearing layers. K-Ar data on the KBS Tuff raised in the 1970s a controversy between supporters of a long chronology (2.61 ± 0.26 Ma) (Fitch and Miller 1970) and those of a short chronology (1.82 ± 0.04 Ma) (Curtis et al. 1975). FT dating on zircon (2.44 ± 0.08 Ma) (Hurford et al. 1976) at first seemed to support the high K-Ar age. A later FT study of zircon from the pumice yielded 1.87 ± 0.04 Ma (Gleadow 1980) in accordance with the low chronology. Besides methodological aspects, the main reasons for the previous fission-track overestimate of the KBS Tuff are detrital, old zircon grains. A far-reaching study on tuffaceous zircon was reported by Morwood et al. (1998). At the site Mata Menge, Flores, Indonesia, a layer with stone tools is intercalated in tuffaceous layers. FT dating on zircon from the lower and the upper layer yielded 880 ± 70 and 800 ± 70 ka, respectively. Provided that these grains are primary and not reworked, these findings imply that at that time H. erectus had already reached the island of Flores from Southeast Asia—a journey that requires an amazing sea-crossing capability, even at periods of lowest sea level.

Ashes at prehistoric fireplaces may contain sufficiently heated grains of apatite, zircon, and titanite. Such case was encountered at Zhoukoudian near Peking, with its numerous remains of H. erectus, the Peking man. From ashes of the layers 10 and 4, several hundred grains of titanite in the size range of 50-300 mm were separated. As criterion for discriminating completely from partially annealed titanite grains, the length of the fission tracks was utilized. Altogether, 100 grains showed complete resetting and gave mean ages of 462 ± 45 ka for layer 10 and 306 ± 56 ka for layer 4 (Guo et al. 1991), being significantly less than the already mentioned uranium series ages for this site (Shen et al. 2001).

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