FACTORS AFFECTING ISOTOPIC DATING Works best when a rock or mineral represents a “closed” system. Parent and daughter isotopes cannot move in or out of a mineral or rock. Igneous rocks best fit this criteria. Radioisotopic Methods for Dating Rocks

FACTORS AFFECTING ISOTOPIC DATING Metamorphic rocks are not always closed systems. During metamorphism, heat, pressure, and circulating fluids affect mineral grains. Daughter isotopes are generally lost in the process. Dating metamorphic rocks provides the age of the metamorphic event rather than the age of the rocks themselves. Radioisotopic Methods for Dating Rocks

FACTORS AFFECTING ISOTOPIC DATING Accuracy of isotope dating also depends on the condition of the material dated Fractured or weathered rock is not a good candidate. Age of the rocks being considered also presents some problems. Very young rocks may not have had enough time to accumulate enough daughter isotope to measure. Need to choose a radioactive isotope with t ½ that fits the approximate age of the rock. Radioisotopic Methods for Dating Rocks

FACTORS AFFECTING ISOTOPIC DATING The minerals in the rock also determine which isotope that is best for dating the rock. Radioisotopic Methods for Dating Rocks

TYPES OF ISOTOPIC DATING TECHNIQUES Uranium (U) - Thorium (Th) - Lead (Pb) Dating 238 U decays to 206 Pb 235 U decays to 207 Pb 232 Th decays to 208 Pb Rocks containing Uranium provide three possible techniques. Because all three occur together, it allows a method to cross-check the dates. Radioisotopic Methods for Dating Rocks

Concordia Diagram

Radioisotopic Methods for Dating Rocks Discordant Points not on curve

TYPES OF ISOTOPIC DATING TECHNIQUES Uranium (U) - Thorium (Th) - Lead (Pb) Dating 238 U decays to 206 Pb Radioisotopic Methods for Dating Rocks

TYPES OF ISOTOPIC DATING TECHNIQUES Uranium (U) - Thorium (Th) - Lead (Pb) Dating 238 U decays to 206 Pb Half-life (t 1/2 ) is 4.5 billion years. Can be applied to igneous and metamorphic rocks. Uses zircons, uraninite and uranium ores. Radioisotopic Methods for Dating Rocks

TYPES OF ISOTOPIC DATING TECHNIQUES Uranium (U) - Thorium (Th) - Lead (Pb) Dating 235 U decays to 207 Pb Half-life (t 1/2 ) is 713 million years. Can be applied to igneous and metamorphic rocks. Uses zircons, uraninite and uranium ores. Radioisotopic Methods for Dating Rocks

TYPES OF ISOTOPIC DATING TECHNIQUES Uranium (U) - Thorium (Th) - Lead (Pb) Dating 232 Th decays to 208 Pb Half-life (t 1/2 ) is 14.1 billion years. Can be applied to igneous and metamorphic rocks. Uses zircons, uraninite and uranium ores. Radioisotopic Methods for Dating Rocks

TYPES OF ISOTOPIC DATING TECHNIQUES Potassium (K) - Argon (Ar) Dating Potassium (K) is an extremely common element. One isotope, 40 K, is radioactive. Found in muscovite, biotite, orthoclase and glauconite. Used to date volcanic rocks. Radioisotopic Methods for Dating Rocks

TYPES OF ISOTOPIC DATING TECHNIQUES Potassium (K) - Argon (Ar) Dating Produced by electron or beta (  ) capture. Half-life (t 1/2 ) is 1.3 billion years. Range is 100,000 to 4.6 billion years. Useful for relatively young and very old rocks. Radioisotopic Methods for Dating Rocks

TYPES OF ISOTOPIC DATING TECHNIQUES Potassium (K) - Argon (Ar) Dating Problem with K-Ar dating is that the Argon produced is a gas and with fracturing, weathering, or metamorphism, the gas can be lost, resetting the clock. Radioisotopic Methods for Dating Rocks

TYPES OF ISOTOPIC DATING TECHNIQUES Rubidium (Rb) - Strontium (Sr) Dating Rubidium (Rb) decays to Strontium (Sr). Half-life (t 1/2 ) is 47 billion years. Found in muscovite, biotite, feldspars and hornblende. Used to date volcanic and metamorphic rocks. Because of large half-life, rocks between 10 million and 4.6 billion years can be dated. Radioisotopic Methods for Dating Rocks

TYPES OF ISOTOPIC DATING TECHNIQUES Rubidium (Rb) - Strontium (Sr) Dating Whole rock analysis Radioisotopic Methods for Dating Rocks Isochron Ratio of 87 Rb/ 86 Sr is graphed Against the 87 Sr/ 86 Sr The older the rocks, the Greater the slope of the isochron

Radioisotopic Methods for Dating Rocks 14 Carbon (C) Dating Produced by Beta (  ) decay. Half-life (t 1/2 ) is 5,730 years. Age range is 100 to 70,000 (really ~50,000) years. Used to date carbon-based remains like bones, plant remains (wood, pollen, seeds), shells, cloth, paper and charcoal.

Radioisotopic Methods for Dating Rocks 14 Carbon (C) Dating 14 C is produced in the atmosphere. Cosmic rays hit other atoms in atmosphere, giving off neutrons. Neutrons hit 14 N and  decay occurs producing 14 C.

Radioisotopic Methods for Dating Rocks 14 C in atmosphere combines with O 2 to produce 14 CO 2. Plants and animals ingest or breathe in 14 CO 2 and it becomes incorporated in the organism. Upon death, 14 C decays back into 14 N. The rate of cosmic ray bombardment has varied over time. Needs to be calibrated with other techniques. 14 Carbon (C) Dating

FISSION is the division of radioactive nuclei (usually 238 U) into two equally-sized fragments. Process releases  and  particles. When splitting occurs, particles rip through the mineral lattice (crystal structure) producing tracks or tears in the lattice. Occurs continuously in minerals with radioactive substances. OTHER NUMERICAL DATING TECHNIQUES FISSION-TRACK DATING

The older the mineral, the more tracks are produced. Age range is 50,000 to billions of years. Can be applied to volcanic glass, zircons and apatites. Limitations do exist. Temperatures above 250  C cause tracks to heal. Can’t be used to date medium- to high-grade metamorphic rocks. Fills the gap between 14 C and K-Ar techniques. (between 50,000 and 1,000,000 years) FISSION-TRACK DATING OTHER NUMERICAL DATING TECHNIQUES

FISSION-TRACK DATING 13.5  m 1  m = mm OTHER NUMERICAL DATING TECHNIQUES

Trees in temperate regions produce light and dark annual growth rings. By counting the rings, the tree’s age can be determined. DENDROCHRONOLOGY OTHER NUMERICAL DATING TECHNIQUES

Climate and other events are also recorded. By comparing the ring counts and chronology from living and fossil trees a dendrochronology for a region can be formed. Goes back about 9000 years. DENDROCHRONOLOGY OTHER NUMERICAL DATING TECHNIQUES