Solar System Chronology - Methods

Question 1

in radioactive decay, a radioactive parent isotope decaus to a _____ daughter

Answer 1

radiogenic

Question 2

after how many half-lives is radioactive dating not possible anymore

Answer 2

5 half-lives, by which only 3% of the parent isotopes originally present remain

Question 3

what is the half-life?

Answer 3

the time taken for the number of radioactive isotopes to decrease to half their initial population

Question 4

in long-lived decay systems, what are the abundances of radiogenic daughter and radioactive parent measured relative to, to get a much more precise isotope ratio?

Answer 4

a stable non-radiogenic daughter isotope, e.g. 86Sr for 87Sr -> 87Rb

Question 5

given a basic decay equation in long-lived decay system, what information do we need to determine t? e.g. for 87Sr -> 87Rb

Answer 5

1. the decay constant
2. the present day 87Sr/86Sr ratio (through mass spec)
3. the present day 87Sr/86Sr

Question 6

what is the unknown in calculating long-lived decay ages?

Answer 6

the initial ratio of radioactive isotope and stable non-radiogenic isotope it is is measured against

Question 7

how do we get around the problem of the unknown isotope composition of a sample e.g. 87Sr/86Sr

Answer 7

can analyse two or more cogenetic samples, for several minerals or whole rock samples from one meterotie or several meteorites from a single parent body

Question 8

what characteristics should cogenetic samples have when using them to calculate the initial isotope ratio of a sample?

Answer 8

1. they formed at the same time
2. they remained close systems since formation
3. they had the same initial isotope ratio e.g. 87Sr/86Sr due to the isotopic homogenisation
4. different parent daughter ratios (so they can plot on an isochron)

Question 9

what is a cogenetic sample?

Answer 9

mineral or whole rock samples from one meteorite or several meteorites from a single parent body

Question 10

what is an isochron?

Answer 10

the straight line defining the age of a sample. the isochron becomes steeper with time as each sample produces more daughter radiogenic nuclides and decreases their parent radioactive isotope ratio.

Question 11

in extinct decay systems what is sought to be measured?

Answer 11

the initial parent radioactive isotope ratio e.g. (26Al/27Al)

Question 12

do isochrons based on extinct radionuclides (directly) provide absolute age information?

Answer 12

no - the different isochron slopes between two meteorites can be used to calculate an age difference between them using the deltaT equation

Question 13

due to their short half-lives (<1Ma), what are extinct nuclide systems best used for?

Answer 13

providing very precise relative ages for the early solar system objects/processes

Question 14

how can absolute ages for extinct decay systems be calculated?

Answer 14

if the absolute age of one of the samples if known (from dating with a long-lived radionuclide scheme), then the age difference calculated from the extinct radionuclide system can be used to obtain an absolute age for the second sample

Question 15

what information dos the differences in slope of an extinct radionuclide isochron provide?

Answer 15

differences in the initial parent isotope ratio