1. Bio-Astronomy of the Solar System
Chapter 6.3 Isotopic Dating
Astrobiology: A Multidisciplinary Approach by Jonathan Lunine

2. Isotopic Dating Radioactive decay is probabilistic
Large numbers of atoms
Half-life is independent of the environment
Half-life is well-defined for isotopes abundant on Earth
We can use isotopic abundance to determine age of rocks

3. Radioactive Isotopes P D Di Some common isotopes used in dating
Half-Life (*10^9)
87Rb
87Sr
86Sr
4.8
40K+
40Ar
36Ar
1.25
147Sm
143Nd
144Nd
106
176Lu
176Hf
177Hf
35.9
187Re
187Os
186Os 43
232Th*
208Pb
204Pb 14
238U*
206Pb
4.47
Some common isotopes used in dating
Radioactive decay equation

4. Isochron Diagram
An "isochron" is a set of data points in a plot which all fall on a line representing a single age
Each mineral grain is a data point

5. Isochron Diagram When rock solidifies, the atoms no longer move around
The parent isotope, P, decays into daughter isotope, D. A non-radiogenic isotope of D, Di must also be measured.
We plot the ratios of P/Di today (x) and D/Di today (y)

6. Isochron Diagram The nonradiogenic isotope “normalizes” the equation
The y-intercept is the ratio of D/Di at time zero
The slope of the line is the time since the rock formed

7. Complications in Radioisotopic Dating
Closed system implied
Daughter molecules must stick around
Melting episodes disrupt ages
Daughter isotopes may predate the rock

8. Further Complications
Date is only accurate from last melt
Igneous cycle on Earth
Sedimentary age not always useful
Metamorphic rocks partially melt under pressure
Chain of radioactive decays; more than one daughter molecule to consider

9. Concordia Diagram
Two isotopes of Uranium and one isotope of Thorium decay to Lead (Pb)
We can write two isochron equations and get two independent dates from the U-Pb system
If the two dates are the same, we say they are concordant

10. Concordia Diagram
The concordia diagram shows the values of Pb isotopes that would give concordant dates.
Concordia are useful for dating the mineral Zircon
Zircon has high amounts of U and low amounts of Pb

11. Discordia In a closed system Zircon will plot on the concordia line
But if lead leaks from the system and
If several discordant dates plot on a line
Then it is still possible to determine the age of the rock (t0)

12. Carbon-14 Dating Carbon-14 is a heavy isotope of carbon
Carbon-12 is stable
C-14 has a half-life of 5,730 years
C-14 is continuously produced in Earth’s atmosphere
Carbon-14 is bound into living organisms
The ratio of C-14 to C-12 is constant while the organism is alive

13. Carbon-14 Dating When an organism dies, the C-14 exchange stops
C-14 beta decays back to N-14
Tree trunks, animal bones, cloth, plant fibers, and durable remains are best
Less than 70,000 years old

14. Carbon-14 Dating
Variation of C-14 production adds to uncertainty in the date
Solar wind
Earth’s magnetic field changes
Ocean circulation of carbon dioxide
Nuclear testing
Most accurate at 10,000 years or less

15. The End

16. Carbon-14 dating