Can anyone help me please? I have a sample of granite, which has been determined to have a U238-Pb date of 1120 Ma. How can I find out how this was determined? I have been trying to calculate this but my brain has frozen, so any help would be appreciated! I don't expect the answer but if I could be advised of the equation to use that would be great. Thank you!

I take it this is S103 TMA08 Q2(b) (10 marks) "The age of a granite can be determined using radiometric dating. Explain the basis for the determination of a 238U-Pb radiometric date of 1120 Ma for a granite."

The key word here is "explain". There is nothing to calculate.

The best source of information is the course books. Try Section 8.5 B10.

What is a name?

To determine the U238-Pb date of a granite, radiometric dating is used. Radiometric dating relies on the principle that certain isotopes decay at a specific rate over time. In this case, U238 (uranium-238) decays into Pb206 (lead-206) through a series of intermediate isotopes.

The half-life of U238 is approximately 4.47 billion years, which means that after this time, half of the original amount of U238 will have decayed into Pb206. Knowing the half-life, we can determine the age of a sample by measuring the ratio of the parent isotope (U238) to the daughter isotope (Pb206) in the specimen.

In the case of the granite with a U238-Pb date of 1120 Ma (million years), it means that the ratio of U238 to Pb206 in the granite indicates an age of 1120 million years.

To calculate this age, we can use the equation known as the age equation:

Age = (ln(Nd/N0))/λ,

where Age is the age of the sample in years, Nd is the current amount of the daughter isotope (Pb206), N0 is the original amount of the parent isotope (U238), and λ is the decay constant, which is equal to the natural logarithm of 2 divided by the half-life.

However, since you mentioned that you don't need the actual calculation, understanding the process and principle of radiometric dating should be sufficient to explain the basis for the determination of the U238-Pb radiometric date.