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Geochronology, radiogenics. Recall the four forces of contemporary physical theory; we have, so-far, explored the geophysics of only two. Now lets look at the two remaining:. Weak: mediates β -decay and β -capture processes Strong: mediates nuclear fission and α -decay. Radioactivity.
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Geochronology, radiogenics Recall the four forces of contemporary physical theory; we have, so-far, explored the geophysics of only two. Now lets look at the two remaining: Weak: mediatesβ-decay and β-capture processes Strong: mediates nuclear fission and α-decay
Radioactivity Radioactivity was discovered in 1896 by the French scientist Henri Becquerel; during his work on the phosphorescence of uranium salts; science now had a method for quantifying the age-history of the Earth, planets and Solar System. Within a little more than a decade, radioactive decay sequences were being used in geochronological dating. Ernest Rutherford, then at McGill, encouraged B. Boltwood to determine the age of minerals by their lead accummulation in 1907... work done partially is what is now the Shulich Physical Sciences and Engineering Library, then the MacDonald Physics Building.
Decay modes Radioactive decay is spontaneous: generally, we cannot stimulate a radionucleus to decay though the condition of the atom is known or suspected to bear on certain decay modes: Electron (beta) capture: 26Al + e- --> 26Mg + νe 40K + e- --> 40Ar + νe The decay rate of these processes is slightly affected by the ionization state of the mother nucleus (e.g. the 40K ) and by the atom's chemical bond state which may be further affected by extremely high pressures.
Decay modes - II Beta, beta+ (positron) decay: 40K --> 40Ca + e- + νe 40K --> 40Ar + e+ + νe Neutron emission: 13Be --> 12Be + n0 5He --> 4He + n0 5He -->α2+ + n0
Decay modes - III Alpha decay: 238U --> 234Th + α2+ 235U --> 231Th + α2+ Spontaneous fission: 235U, 238U --> 90-100X + 130-140Y + (x)n0 Neutron-forced fission: (x)n0 + 235U --> 90-100X + 130-140Y + (x)n0
Decay modes - IV Neutron-forced fission products: (x)n0 + 235U --> 90-100X + 130-140Y + (x)n0
Decay modes - Exotic Proton emission (in elements that have been artificially created with major neutron deficits): 151Lu --> 150Yb + p+ + ... (?) 147Tm --> 146Er + p+ + ... (?) Proton decay (The “standard model” of particle physics predicts spontaneous proton decay – never observed): with a half-life of 1036 to 1040 years! p+ --> π0 + e+ + ... π0 --> 2γ
Decay transmutations Proton number: p+ Neutron number: n0
Interpreting λ The decay constant, λ, might best be seen as a “measure” of the probability of decay during some time interval and λN(t)as the number of decay events per unit time. It is sometimes called the “activity” of the radionucleide. The Bq (becquerel) corresponds to one decay per second. The activity of one litre of normal seawater is about 12Bq, or a human body, about 5000-10000Bq.
A generic decay clock ½ If we know N(t=0), N(t) and the decay constant, λ, and if we know that the system holding our radionuclei is “closed”, this simple equation allows us to determine the passage of time. A closed system is one in which the decay daughters along with the current mothers are preserved, typically within a crystal or rock mass in the geological context.
Decay daughters The decay daughters may not be distinguishable from any identical isotopes that had existed within our crystal or rock mass at time t = 0; i.e., D(t = 0). Either we need to choose the system under analysis carefully, with D(t = 0) known, or find some redundancy in the radiogenic clock or some other reference in order to establish quantities at time t = 0.
Referencing to stable isotopes Except for promethium, Pm, uranium and thorium all naturally occurring elements have stable isotopes. We may use a stable isotope of the same element as our mother and daughter radiogenic isotopes for reference.
14C-dating Cosmic rays convert 14N to 14C in the upper atmosphere: 14N + n0 --> 14C + p+ 14C is radioactive with λ = 1.21 x 10-4/yr or τ½ = 5730 yr : 14C --> 14N + e- + υe 12C is the common stable isotope of carbon.
14C-dating - II At the moment an animal stops metabolising food or a plant stops photosynthesizing, it establishes a 14C/12C ratio of about 10-12. Very sophisticated systems are required for MS analysis.
14C-dating – III (Calibration) At the moment an animal stops metabolising food or a plant stops photosynthesizing, it establishes a 14C/12C ratio of about10-12. We know, however, that the cosmic ray excitation of 14N to 14C has varied with time. We use tight archeological data correlations and tree-ring data from long-lived Bristlecone Pines and their correlatable now-dead ancestors to calibrate the 14C/12C ratio as a function of time. Future 14C-dating will have to contend with a special calibration for the very large influx of 14C into the atmosphere caused by the uncontained nuclear weapons testing of the 1950s and 1960s.
Rubidium-strontium system For many geological or geophysical purposes, 14C-dating is not very useful because the short 5730-year half-life. Within a period of 10 half-lives, only 0.1% of the original 14C is left for comparison. We are led to systems with much slower decay rates: 87Rb --> 87Sr with a half-life of 47.5 x 109 years! 86Sr is stable and derives from no other radio-decay, so we develop ratios of the original 87Rb mother and 87Sr daughter to 86Sr: an equation of a straight line, the “isochron”.
Potassium-Argon dual decay system 40Kdecays by two modes into40Caand40Ar : 40K --> 40Ca + e- + νe 40K --> 40Ar + e+ + νe 40K --> 40Ca : λ= 4.96 x 10-10/yr τ½ = 1.497 x 109 yr λ = 5.81 x 10-11/yr τ½= 11.93 x 109 yr 40K --> 40Ar :
Uranium-lead dating – dual mothers! 238U --> 206Pb + ... Radium decay series: λ= 1.55 x 10-10/yr τ½ = 4.471 x 109 yr Actinium decay series: λ = 9.85 x 10-10/yr τ½= 7.037 x 108 yr We have two internal clocks. One checks the other! 235U --> 207Pb + ...
Uranium-lead decay sequence Source website... check it out: click here!
Counting forward from the beginning: 146Sm - 142Nd 146Sm --> 142Nd + α2+ λ= 6.73 x 10-9/yr τ½ = 1.030 x 106 yr Essentially, all the original146Sm that accreted with the Earth has already transmuted into142Nd. We may, though, compare rocks according to their142Nd/144Nd ratios. The short half-life leads us to conclude that146Sm became extinct very early in Earth's history. To find rocks with a low 142Nd/144Nd ratio suggests that146Sm was already depleted in the host and that the rock's host had not “mixed” into the mantle. A low Sm/Nd (all isotopes) ratio for the rock's reservoir argues for a very old reservoir.
4.280 (+53,-81) x 109 yr “Faux amphibolites”
