TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE FACTORS AFFECTING ISOTOPIC DATING Most useful in igneous rocks. As minerals crystallize, radioactive isotopes become incorporated in the minerals. No daughter isotopes at that time. Crystallization sets the isotopic “clock”. Doesn’t work in sedimentary rocks. How come?

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE 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.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE 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.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE 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.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE FACTORS AFFECTING ISOTOPIC DATING The minerals in the rock also determine which isotope that is best for dating the rock.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE 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.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE TYPES OF ISOTOPIC DATING TECHNIQUES Uranium (U) - Thorium (Th) - Lead (Pb) Dating 238 U decays to 206 Pb

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE 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.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE 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.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE 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.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE 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.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE 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.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE 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.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE 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.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE TYPES OF ISOTOPIC DATING TECHNIQUES 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.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE TYPES OF ISOTOPIC DATING TECHNIQUES 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.

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. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE TYPES OF ISOTOPIC DATING TECHNIQUES 14 Carbon (C) Dating

FISSION is the division of radioactive nuclei 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. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE 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. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE OTHER NUMERICAL DATING TECHNIQUES FISSION-TRACK DATING

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE OTHER NUMERICAL DATING TECHNIQUES FISSION-TRACK DATING 13.5  m 1  m = mm

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

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. TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE OTHER NUMERICAL DATING TECHNIQUES DENDROCHRONOLOGY

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE OTHER NUMERICAL DATING TECHNIQUES VARVE CHRONOLOGY Lakes can produce annual layers. Usually occur in glacial lakes or those that freeze over in winter. Coarser sediments are deposited in summer. Winter-summer layers are called COUPLETS. Couplets in lakes are known as VARVES. Count the couplets back from the sediment surface to determine numerical age.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE OTHER NUMERICAL DATING TECHNIQUES VARVE CHRONOLOGY

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE OTHER NUMERICAL DATING TECHNIQUES LICHENOMETRY Lichens are plant-like organisms that grow on rocks. Grow at a measurable rate. By measuring size on items of known date, the size is plotted against size on unknown aged objects. Good for the last 9000 years.

TELLING TIME GEOLOGICALLY DETERMINING NUMERICAL OR ABSOLUTE AGE OTHER NUMERICAL DATING TECHNIQUES LICHENOMETRY