Biochronology (relative dating, limited to fossil record)
Every parent atom of a particular type has the same probability of decaying to a specified daughter atom. Decay only depends on structure of atomic nucleus - NOT on external conditions. If N = number of parent atoms at time "t" and N 0 is number of parent atoms present at start where is the DECAY CONSTANT which is constant for a particular parent-daughter system. The basic principle
Rb-> beta decay->Sr Sr has several isotopes: 84, 85, 86, 87, 88 Only 87 Sr has a radiogenic source – 87 Rb
How much 87 Sr was at the beginning and how much came from 87 Rb? For this method, t 1/2 = 47 Byr
Rb-Sr Method An example from some of the oldest rocks on Earth. A suite of different samples from a group of granites and gneisses were analyzed. These had different concentrations of parent rubidium 87 and so generated different amounts of the daughter (strontium 87). The resulting data plot on a straight line, called an isochron. This means that all samples have seen the same amount of time. If different samples had formed at different times (a random collection of rocks on earth) then the data would not fall on a straight line (no single isochron).
Dating meteorites with the 87 Rb 87 Sr method – the age of the Solar system
K-> beta decay->Ar 40 K – 89% -> 40 Ca, 11% -> 40 Ar For this method, t 1/2 = 1.93Byr
K-> beta decay->Ar Parent against daughter concentrations. In this hypothetical example there are no daughter atoms present in the original rock but a certain amount of parent atoms. Consequently a line passing through this point to the origin has zero slope. After time a number of parent atoms have decayed and the number of daughters increase. Eventually there will be no parents and only daughters - the composition will plot on the daughter axis. Note therefore the gradient of line connecting any composition with the origin increases with time. Or - the gradient of the line is a function of time and also a function of the decay constant.
Carbon dating Carbon has 3 isotopes: 12 C – stable 13 C – stable 12 C: 13 C = 98.89 : 1.11 14 C – radioactive Abundance:
RadiocarbonForms: in the upper atmosphere in the upper atmosphereDecays: t ½ = 5730 yr.
Half life time Measured Constant Calculated ???
Dating Dating methods: *Conventional way *Accelerator mass spectrometry (AMS) Bowman S., 1990
Assumptions *Constant production rate *Constant and rapid mixing *Constant exchange rate *Constant reservoirs size *Same concentration in all parts of the biosphere *Decay determines the radiocarbon concentration
Problems concerning the radiocarbon dating: -Atmospheric 14 C variations -Alteration effects -Contamination -Source or reservoir effects