Absolute Dating Using Radioactivity Slide 2 Absolute Age dating was major advance in Historical Geology More accurate reconstruction of geologic events Mostly based on radioactivity Defined as spontaneous decay of one element to another Pierre and Marie Curry -1903- decay produces heat Slide 3 What Are Absolute Dates The age of a rock, fossil, or geologic event expressed in units, such as years For example, your birthday... You were born on a specific day, month and year Absolute age can be determined by radioactive decay Slide 4 The Atom Fundamental unit of matter Made up of components called subatomic particles Protons (positive charge) Neutron (no electrical charge) Electron (negative charge Slide 5 Radioactive Decay Stable Atoms An atom is generally stable if the number of protons equals the number of neutrons in the nucleus Atomic Number An elements identifying # Equals # of protons in atoms nucleus Mass Number Equals # of protons and neutrons in an atoms nucleus Slide 6 Radioactive Decay - Isotopes Variant of the same parent atom Differ in the # of neutrons Result in different mass # than parent For example: Carbon-14 (C-14) Types of carbon C-12 (stable) C-13 (unstable) C-14 (unstable) Slide 7 Radioactive Decay The nucleus of an atom (decays) changes into a new element. The proton number (atomic number) changes Parent unstable radioactive isotope Daughter isotopes resulting from decay of parent 14 C 14 N 67 Slide 8 Types of Radioactive Decay Alpha emission Emission of 2 p+ and 2 n ( particle) Mass # by 4 Atomic # by 2 Beta emission N emission of e- ( particle) Mass # remains unchanged Atomic # by 1 Electron Capture An e- captured by p e- + p = n Mass # remains unchanged Atomic # by 1 Slide 9 Radioactive IsotopesDetermine Decay Type Alpha emission Emission of 2 p+ and 2 n ( particle) Mass # by 4 Atomic # by 2 Beta emission N emission of e- ( particle) Mass # remains unchanged Atomic # by 1 Electron Capture An e- captured by p e- + p = n Mass # remains unchanged Atomic # by 1 U 238 Radioactive Decay Series Slide 10 How Long Does Radioactive Decay Take? Half Life - time required for one-half of radioactive nuclei in a sample to decay The half life of C-14 is 5,730 years 1.If start with 10,000 atoms of Carbon-14, how many will you have after 5 half-lives? 2.How old is sample? Slide 11 Carbon-14 Dating The Carbon Cycle Carbon dating is common Only for young samples DECAY PROCESS FOR CARBON IS DIFFERENT FROM OTHER ISOTOPES! Slide 12 When Does the Clock Start?Carbon Dating When a plant or animal dies, the clock starts. Organism dies No more C-14 intake C-14 begins to decay Slide 13 How the Carbon Clock Works There are two types of carbon used in the dating process C-12 (stable does not decay) C-14 (radioactive decays) When an organism is alive, it has the same ratio (C-12 to C-14) that is found in the atmosphere (1 trillion to 1) A living starfish has the same ratio as the atmosphere A fossilized starfish has a different ratio Slide 14 How the C-12:C-14 Ratio Works Amount of stable C-12 Amount of unstable C-14 RatioYears dead# of half-lives 100 trillion1001-T to 100 100 trillion502-T to 15,7301 100 trillion254-T to 111,4602 100 trillion12.58-T to 117,1903 100 trillion616-T to 122,9204 100 trillion332-T to 128,6505 Carbon-14 can date elements up to approximately 100,000 years Used to date very recent events Important tool for anthropologists, archeologists and geologists Slide 15 Principles of Radioactive Dating Percentage of radioactive atoms that decay during one half-life is always the same (50 %) However, the actual # of atoms that decays continually decreases Comparing the ratio of parent to daughter yields the age of the sample Slide 16 Radioactive Isotopes Frequently used isotopes in Radiometric Dating Slide 17 Radioactive Isotopes U 238 Radioactive Decay Series Slide 18 Magma Crystallization and Clock Begins Slide 19 Radiometric Dating Sources of Error A closed system is required Only fresh, non-weathered, unaltered or non-deformed rock samples should be used For example, Metamorphism Reheats samples Sample measures younger than should be Can cross check age using other isotopes Slide 20 Isotope Used For Dating U-Pb & Th-Pb most common Used for ancient samplesinstrusives, lunar rocks, meteroites Rb-Sr used for oldest rocks K-Ar used for fine grained volcanic rocks Ar is gas so sample must be fresh Can also be used for metamorphic rocks Slide 21 Importance of Radiometric Dating A complex procedure that requires precise measurement Rocks from several localities have been dated at more than 3 billion years Confirms the idea that geologic time is immense Slide 22 Radiometric Dating Dating Sedimentary Strata How old are Dakota Sandstone, Mancos Shale and Mesaver?