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

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

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

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

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)

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 http://www.talkorigins.org/faqs/isochron-dating/AnimatedIsochron.html

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

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

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

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

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)

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

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

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

The End

Carbon-14 dating