Fission Track Dating
Wednesday, 4. March 2009, 15:26:01
Fission track dating has brought great contributions to the advancement of knowledge in different disciplines, such as archaeology, geochemistry and geology. This method of dating rocks was proposed in 1963 - so it is not exactly new.
In uranium bearing minerals (e.g. apatite) spontaneous fission of uranium-238 (the most abundant isotope of uranium) releases energy, with the two fission products (daughter products) speeding away in opposite directions and electrons fleeing away from their path and disrupting the crystal lattice. The fission products travel about 7 µm in opposite directions, leaving a single trail of damage through the medium which is about 15 µm long. By counting the number of tracks you can in principle calculate the age of the mineral - typically, fission-tracks are etched with acid so they become large enough to see (and count) with an optical microscope. There is however a small problem. As with any radiometric dating method the “age” is not necessarily the time elapsed since the mineral first crystallised. It dates the time at which the relevant “geological clock” (radiometric dating clock / isotopic clock) was reset to zero, that is in this case the time at which the fission tracks began to accumulate.
Fission-tracks are sensitive to heat. If the temperature is sufficiently high for a sufficiently long time, any existing fission track will disappear, and the “clock” will be reset. The temperature is different for different minerals, and is generally called the minerals (specific) “closure temperature” aka “blocking temperature” - the temperature below which isotopes in a mineral are no longer free to move. I have seen differing closing temperatures quoted for each particular of the two minerals most commonly used for fission track analysis, namely apatite and zircon. They are a bit complicated to calculate and always given with a certain margin (e.g. 100° ± 20°C). As I am only writing about the principles it should be sufficient here to say that apatite has a closure temperature of about 90°C and zircon around 300°C.
(About a year ago Apparent Dip had an excellent post on closure temperature if you want to go into greater depth)
Originally the temperature dependance was seen as a problem for the dating. Later it was seen as a great advantage for certain applications. such as measure of cooling, uplift or burial processes.
Let me just quote a few random examples of use of fission track analyses:
“Fission-track evidence for Quaternary uplift of the Nanga Parbat region, Pakistan”
“In divergent tectonic regimes, the record of uplift associated with rifting has been recorded by fission track ages in the southeastern Australian margin and around the Red Sea. In an intra-plate tectonic setting, our current fission track reconnaissance study in the British Isles is revealing a hitherto unrecognised thermal history for crystalline and sediment alike.”
“Systematic zircon and apatite fission-track analyses across the southern part of the mountains unraveled the uplift history of the Hidaka Mountains.”
“Apatite and zircon fission-track cooling ages constrain the Tertiary cooling and uplift history of the eastern Cordillera and Altiplano of Bolivia.”
As for the study of the uplift of mountains / bodies of rock: Due to the increase of temperature with depth in the Earth, a body of rock which is raised/uplifted cools through the various mineral closure temperatures, recording the times at which each temperature was reached. Fission track ages of samples from different altitudes can be used to derive the rate at which uplift occurred.
Most current research using fission tracks are aimed at understanding:
a) the evolution of mountain belts
b) the source or provenance of sediments
c) the thermal evolution of basins
d) determining the age of poorly dated strata
e) dating and provenance determination of archeological artefacts (clock reset during heating of pottery, glass etc. over a fire).
• http://www.britannica.com/EBchecked/topic/208805/fission-track-dating
• http://www.onafarawayday.com/Radiogenic/Ch16/Ch16-1.htm
• http://faculty.plattsburgh.edu/mary.rodentice/research/Fission_Track.html
In uranium bearing minerals (e.g. apatite) spontaneous fission of uranium-238 (the most abundant isotope of uranium) releases energy, with the two fission products (daughter products) speeding away in opposite directions and electrons fleeing away from their path and disrupting the crystal lattice. The fission products travel about 7 µm in opposite directions, leaving a single trail of damage through the medium which is about 15 µm long. By counting the number of tracks you can in principle calculate the age of the mineral - typically, fission-tracks are etched with acid so they become large enough to see (and count) with an optical microscope. There is however a small problem. As with any radiometric dating method the “age” is not necessarily the time elapsed since the mineral first crystallised. It dates the time at which the relevant “geological clock” (radiometric dating clock / isotopic clock) was reset to zero, that is in this case the time at which the fission tracks began to accumulate.
Fission-tracks are sensitive to heat. If the temperature is sufficiently high for a sufficiently long time, any existing fission track will disappear, and the “clock” will be reset. The temperature is different for different minerals, and is generally called the minerals (specific) “closure temperature” aka “blocking temperature” - the temperature below which isotopes in a mineral are no longer free to move. I have seen differing closing temperatures quoted for each particular of the two minerals most commonly used for fission track analysis, namely apatite and zircon. They are a bit complicated to calculate and always given with a certain margin (e.g. 100° ± 20°C). As I am only writing about the principles it should be sufficient here to say that apatite has a closure temperature of about 90°C and zircon around 300°C.
(About a year ago Apparent Dip had an excellent post on closure temperature if you want to go into greater depth)
Originally the temperature dependance was seen as a problem for the dating. Later it was seen as a great advantage for certain applications. such as measure of cooling, uplift or burial processes.
Let me just quote a few random examples of use of fission track analyses:
“Fission-track evidence for Quaternary uplift of the Nanga Parbat region, Pakistan”
“In divergent tectonic regimes, the record of uplift associated with rifting has been recorded by fission track ages in the southeastern Australian margin and around the Red Sea. In an intra-plate tectonic setting, our current fission track reconnaissance study in the British Isles is revealing a hitherto unrecognised thermal history for crystalline and sediment alike.”
“Systematic zircon and apatite fission-track analyses across the southern part of the mountains unraveled the uplift history of the Hidaka Mountains.”
“Apatite and zircon fission-track cooling ages constrain the Tertiary cooling and uplift history of the eastern Cordillera and Altiplano of Bolivia.”
As for the study of the uplift of mountains / bodies of rock: Due to the increase of temperature with depth in the Earth, a body of rock which is raised/uplifted cools through the various mineral closure temperatures, recording the times at which each temperature was reached. Fission track ages of samples from different altitudes can be used to derive the rate at which uplift occurred.
Most current research using fission tracks are aimed at understanding:
a) the evolution of mountain belts
b) the source or provenance of sediments
c) the thermal evolution of basins
d) determining the age of poorly dated strata
e) dating and provenance determination of archeological artefacts (clock reset during heating of pottery, glass etc. over a fire).
• http://www.britannica.com/EBchecked/topic/208805/fission-track-dating
• http://www.onafarawayday.com/Radiogenic/Ch16/Ch16-1.htm
• http://faculty.plattsburgh.edu/mary.rodentice/research/Fission_Track.html
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