HIGH-T TOPIC 1: RADIOGENIC ISOTOPES & DATING

Radiogenic isotope dating principle

radioactive parent isotopes decay to daughter isotopes at a constant rate allowing absolute geological ages to be calculated

Why radiogenic isotopes are powerful

unlike relative dating they provide absolute time constraints and record time-integrated geological processes

Radioactive decay equation

N = N₀e⁻ˡᵗ where λ is the decay constant and t is time

Half-life concept

half-life controls which systems are suitable for different geological timescales

Closed system assumption

dating assumes no gain or loss of parent or daughter isotopes after formation

Why closed systems matter

isotope loss resets or partially resets ages producing incorrect results

Isochron method purpose

removes need to know initial daughter isotope and tests closed-system behaviour

Isochron slope meaning

slope = e^(λt) − 1 which directly yields the age

Isochron intercept meaning

intercept represents initial daughter/normalising isotope ratio

Rb–Sr system behaviour

Rb is incompatible and Sr is compatible making system sensitive to crustal processes

Sm–Nd system behaviour

Sm and Nd behave similarly making the system resistant to alteration and ideal for mantle studies

U–Pb system strength

dual decay schemes provide internal checks and very precise ages

Zircon importance

incorporates U but excludes Pb and is highly resistant to metamorphism

Fun exam fact

zircon can preserve multiple geological events as age zones within one crystal

Diagram card – Radiogenic isotopes

draw an isochron diagram with slope and intercept labelled

Calculation card – Age

rearrange slope = e^(λt) − 1 to solve for t = (1/λ) ln(slope + 1)