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- Source: Dr. Paul Pellas and the Musee National d’Histoire Naturelle, Paris.)There are a number of isotope pairs used in radioisotopic dating, some of
ISOTOPE PAIRS
Parent Daughter Half-life
147Sm 143Nd 106Gy
87Rb 87Sr 48.8Gy
232Th 208Pb 14.0Gy
238U 206Pb 4.47Gy
40K 40Ar 1.31Gy
A method of meteorite dating is chosen on the basis of its application to
finding a desired type of age (e.g. cosmic ray exposure, formation, etc.), as
well as its suitability for the chemical composition of the meteorite ( you
can not measure the amount of an isotope that is not contained in your
sample).
The most useful for determining the ages of meteorites is 87Rb- 87Sr
(Rubidium - Strontium) method. The amount of the daughter isotope now
present in the sample is equal to the initial amount of the daughter isotope
plus the amount generated by the radioactive isotope (87Rb) over time. This
is illustrated by the equation below, where k is the half-life of 87Rb and t
is the time since 87Rb started to decay (Morrison and Owen, 1996).
87Sr = 87Sr initial + 87Rb (ekt-1)
Determination of the initial amount of the radiogenic isotope is difficult.
Considering that all isotopes of the same element have the same chemical
properties, (but different atomic weight) so the initial amount of 87Sr must
be proportional to 86Sr. For each grain the ratio of the 87Sr to 86Sr is
plotted against the ratio of 87Rb to 86Sr. If a line is drawn through the
data plotted an isochron (iso - same, chron - time; since all points
represent a rock at a specific age) is created (see Figure 2; The Rb-Sr
isochron for the Guarena H6 chrondrite. Source: Morrison and Owen, 1996).
The slope of this line represents the age of the meteorite that is
independent of any assumption of the initial quantity of 87Sr. As the
sample ages the amount of the radioactive isotope (87Rb) decreases but at the
same time the daughter isotope (87Sr) increases and so does its ratio to 87Sr
, therefore the slope of the isochron lessens. The older the sample is, the
more the isochron shifts clockwise towards the horizontal position. The
equation used to calculate the age of the meteorite (therefore the slope of
the isochron) is presented below, where l is the half-life of 87 Rb (Dickin,
1995)
t = 1/l ln{1+ 86Sr/87Rb[(87Sr/86Sr)present - (87Sr/86Sr)initial]}
As mentioned before not all meteorites can be dated through 87Rb-87Sr
method. The achrondrites for example do not have very much of Rubidium, but
have high abundances of other elements through which Sm-Nd
(Samarium-Neodymium) method can be used. 147Sm has a sufficiently short
half-life (106 Gy) that the small amounts of its daughter isotope 143Nd can
be measured (Dickin, 1995). In a process similar to the one for Rb-Sr dating
the Sm-Nd age is determined. Sm-Nd isotopic system is more resistant to
postformational heating than Rb-Sr system.
Ages provided by Sm-Nd are older then the once provided by Rb-Sr therefore
Rb-Sr ages are thought to be the result of postcrystalization impact events
(Dalrymple,1991). The composition of basaltic achrondrites favors the
207Pb/235U system for dating (Tilton, 1988). If the daughter ratio
(207Pb/204Pb) and the parent - daughter ratio (235U/204Pb) are only available
for one sample, than the initial amount (207Pb/204Pb) is assumed (Podosek and
Cassen, 1994). The result is called "model age". This is another dating
method that makes use of the isochron diagram. The ordinate intercept is the
initial daughter abundance. The degree for which the data departs form a
straight isochron line shows the accuracy of the assumption of the initial
daughter abundance (Podosek and Cassen, 1994). A parent-daughter system that
is appropriate for dating most meteorites is the 129I/129Xe. It is
applicable to most meteorites because it the daughter is a scarce noble gas
(Podosek and Cassen, 1994). The ages of meteorites determined by the use of
this method, almost surly provide a record of parent body metamorphic
events. However, "the ages do not correlate with the chemical classification
(presumably parent body identification) or the metamorphic grade". This
points to a methodological flaw in this method (Podosek and Cassen, 1994). A
convenient method for determining cooling age is the 40Ar/39Ar method. This
system that measures the abundance of 40Ar (radiogenic isotope of 40K) and
39Ar (radiogenic isotope of 39K). In the use of this method only one sample
is needed (which naturally makes things less complicated for meteorite
dating). The sample is packaged in a aluminum capsule and activated with
fast neutrons in a nuclear reactor. During the activation time a neutron can
collide with 39K, this causes 39K to decay to 39Ar (the reaction is 39K +
n Æ 39Ar + p; abbreviated notation is: 39K(n,p)39Ar). The 40Ar/39Ar ratio is
measured directly on a mass spectrometer and this information combined with
the neutron dose is enough to deduce the age (Huntley,1985). The results can
be plotted on an isochron diagram (the y-axis will represent the ratio of
40Ar/36Ar and the x-axis will represent the ratio of 39Ar/36Ar) and the age
can be determined from the isochron’s slope. Another procedure for 40Ar/39Ar
dating is the step heating method (Smith, personal communication). The
sample is heated little by little to release small amounts of 39Ar. For each
step the age is calculated and the results are plotted on the age spectrum
diagram (see Figure 3; This is an age spectrum diagram of vacuum-stored
komatiite sample B40A. The sizes of the boxes represent 1s errors, where
they are large enough to be shown. Age spectra for meteorites look similar.
Source: Martinez et al, 1984). One would expect that the results of the age
calculation for the outer part of the sample (which are determined first, due
to the nature of the step heating method) will be younger then the ages
provided from steps closer to the inside of the sample. This is in fact true
because of the nature of the crystal growth and the result for the first
steps are scattered on the age spectrum diagram. A portion of the age
results for further steps should agree and form a plateau. If, however, the
result do not form a plateau they are unreliable.
Finally, not all meteorites can be dated. One type of meteorites that are
very problematic are the irons. In order to date them, the sample must have
inclusions containing minerals that are datable. An example of a meteorite
for which efforts to date it did not succeed is Chassigny. This is one of
the "snicks" (SNC meteorites), a group that is thought to have originated on
Mars. Chassigny is composed entirely of olivine and contains no datable
minerals (Dalrymple, 1991).
Meteorites have provided much information needed for our understanding of
various cosmological processed. They are the only source of extraterrestrial
matter that is available to us. Radiometric dating of the most primitive
meteorites (once that have not changed since they were formed) supplied the
age of the Solar system. Although the radiometric dating methods might not
be a hundred per cent accurate, without their use this valuable information
would not have been obtainable. Our civilization’s technology is not yet
capable of providing an efficient method of transportation that would allow
us to travel great distances in space. This is the greatest obstacle that
prevents us from collecting intergalactic samples our selves. This further
underlines the importance of meteorite dating and other meteoritic research.
References
Martinez, M. L., York, D., Hall, C. M., Hanes, J. A., 1984. Oldest reliable
40Ar/39Ar ages for terrestrial rocks: Barberton Mountain Komatiites. Nature,
Vol. 307, No. 5949, pp. 352-354.
Huntley, D. J., 1985. An introduction to dating, analysis and locating in
archaeology ” D. J. Huntley.
Dalrymple, G. B., 1991. The age of the Earth. Stanford University Press.
Hartmann, W. K., 1993. The moons and the planets. Third edition. Wadsworth
Publishng Company.
Podosek, F. A. and Cassen P.,1994. Theoretical, observational, and isotopic
estimates of the lifetime of the solar nebula. Meteoritics Vol.29, pp.6-25.
Tilton, G. R., 1988. Age of the solar system. In Meteorites and the early
solar system, pp-259-275. Eds. Kerridge, J. F. and Matthews, M. S. The
University of Arizona Press.
Morrison, D. and Owen, T.,1996. The planetary system. Second edition.
Addison-Wesley Publishing Company.
Dickin, A. P., 1995. Radiogenic Isotope Geology. Cambridge University
Press.
Smith, P.,1997. Personal communication.
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