Lecture 8 Radiometric Dating Half-life Radiometric Dating 14C Dating K-Ar Dating U-Pb Dating

Half Life Half-life: the time required for half of an original quantity of an isotope to decay Related to the rate of decay for a radioactive isotope. Greater rate of decay = greater radioactivity = shorter half-life

Half Life Half-life: Is constant and independent of any physical or chemical changes – cannot be altered. Can be calculated at any given moment by measuring rate of decay of a known quantity using a radiation detector.

zero. one-quarter. half. one-eighth. A certain isotope has a half-life of one day. This means the amount of that isotope remaining at the end of three days will be zero. one-quarter. half. one-eighth. D. One-eighth.

Radiometric Dating Radiometric Dating: process that measures the ratio of radioactive isotopes to their decay products in order to determine the numeric age of a natural material. Radioactive Isotope = Parent Isotope Decay Product = Daughter Isotope

14C-Dating Carbon-14 (14C): Decays to Nitrogen-14 (14N). Good to date geologically young organic materials (useful in archeology). Relatively short half-life of 5,760 years. Useful for dating materials within the last 50,000-70,000 years.

14C-Dating Cosmic Rays bombard Earth’s atmosphere resulting in the formation of C-14 from N-14.

14C-Dating C-14 chemically reacts with Oxygen to form CO2 which is taken in by plants. C-14 transmitted to animals by consumption of plants. Eventually, C-14 will decay back to N-14.

14C-Dating When alive, organisms maintain equilibrium with the atmosphere as they replenish C-14 in their systems. When an organism dies, the total C-14 decreases as it decays back to N-14.

5,760 years 11,520 years 17,280 years 2,880 years A gram of carbon is extracted from an ancient tree stump exposed on the beach. The carbon is one fourth as radioactive as a gram of carbon extracted from a currently living tree. How long ago did the tree die? Note: half life of C-14 = 5,760 years. 5,760 years 11,520 years 17,280 years 2,880 years

K-Ar Dating: Potassium-40 (40K): Decays to Argon-40 (40Ar). Useful for determining crystallization age of igneous and metamorphic rocks. Half-life of 1.25 billion years.

K-Ar Dating: K-Ar Dating: Occasionally can be used to date rocks younger than 100,000 years. More commonly used for rocks with ages in the millions of years.

Igneous rock from the Oregon Coast Range is determined to be 32. 5 ± 0 Igneous rock from the Oregon Coast Range is determined to be 32.5 ± 0.5 million years old. K-rich minerals contain abundant 40K which decays to 40Ar that remains trapped within crystal structure. Ratio of 40Ar (Daughter) to 40K (Parent) used to determine age which is based on known decay rate.

The 40Ar-39Ar age dating technique in which samples are subjected to incremental heating is a more accurate version of K-Ar dating widely utilized by geologists.

Samples to be analyzed Furnace for heating and melting samples in order to release gases trapped within minerals.

Gas Extraction Line – removes chemically reactive gases leaving only noble gases such as Argon to be analyzed.

Laser used for heating and melting very small samples such as individual crystal grains. Vacuum chamber containing samples

Mass Spectrometer for measuring ratios of Argon isotopes. Utilizes Newton’s 2nd Law which can be applied to individual atoms

Those with larger mass numbers (40Ar,39Ar) Ions of different Argon isotopes have a force acting on them when they pass through a strong magnetic field. Which would experience the greatest accelerations? Those with larger mass numbers (40Ar,39Ar) Those with smaller mass numbers (37Ar, 36Ar).

While passing through a magnetic field as charged particles (ions): Diagram of how a Mass Spectrometer separates different isotopes While passing through a magnetic field as charged particles (ions): Isotopes that have less inertia (mass) are deflected more (undergo greater acceleration). Lower atomic mass numbers = lower masses.

Uranium-Lead Dating One of the oldest, most refined radiometric dating techniques. Measure ages of rocks of ages ranging from 1 million to 4.5 billion years. Precisions of 0.1 to 1 percent range. Acquired from zircon crystals which are very resistant to chemical and physical alteration.

Uranium-Lead Dating Uranium-238 and -235: Decays to Lead-206 and Lead-207 respectively. No other natural sources exist for the daughter isotopes. Half-lives of 4.47 Billion Years and 704 million years respectively.