2 Absolute Age dating was major advance in Historical Geology More accurate reconstruction of geologic eventsMostly based on radioactivityDefined as spontaneous decay of one element to anotherPierre and Marie Curry decay produces heat
3 What Are Absolute Dates The age of a rock, fossil, or geologic event expressed in units, such as yearsFor example, your birthday . . .You were born on a specific day, month and yearAbsolute age can be determined by radioactive decay
4 The Atom Fundamental unit of matter Made up of components called subatomic particlesProtons (positive charge)Neutron (no electrical charge)Electron (negative charge
5 Radioactive Decay – Stable Atoms An atom is generally stable if the number of protons equals the number of neutrons in the nucleusAtomic NumberAn element’s identifying #Equals # of protons in atom’s nucleusMass NumberEquals # of protons and neutrons in an atom’s nucleus
6 Radioactive Decay - Isotopes Variant of the same parent atomDiffer in the # of neutronsResult in different mass # than parentFor example:Carbon-14 (C-14)Types of carbonC-12 (stable)C-13 (unstable)C-14 (unstable)
7 Radioactive DecayThe nucleus of an atom (decays) changes into a new element.The proton number (atomic number) changesParent – unstable radioactive isotopeDaughter – isotopes resulting from decay of parent14C 14N67
8 Types of Radioactive Decay Alpha emissionEmission of 2 p+ and 2 n (α particle)Mass # ↓ by 4Atomic # ↓ by 2Beta emissionN emission of e- (β particle)Mass # remains unchangedAtomic # ↑ by 1Electron CaptureAn e- captured by pe- + p = nAtomic # ↓ by 1
9 Radioactive Isotopes—Determine Decay Type U238 Radioactive Decay SeriesAlpha emissionEmission of 2 p+ and 2 n (α particle)Mass # ↓ by 4Atomic # ↓ by 2Beta emissionN emission of e- (β particle)Mass # remains unchangedAtomic # ↑ by 1Electron CaptureAn e- captured by pe- + p = nAtomic # ↓ by 1
10 How Long Does Radioactive Decay Take? Half Life - time required for one-half of radioactive nuclei in a sample to decayThe half life of C-14 is 5,730 yearsIf start with 10,000 atoms of Carbon-14, how many will you have after 5 half-lives?How old is sample?
11 Carbon-14 Dating – The Carbon Cycle Carbon dating is commonOnly for young samplesDECAY PROCESS FOR CARBON IS DIFFERENT FROM OTHER ISOTOPES!
12 When Does the Clock Start?—Carbon Dating When a plant or animal dies, the clock starts.Organism dies No more C-14 intakeC-14 beginsto decay
13 How the Carbon Clock Works There are two types of carbon used in the dating processC-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 atmosphereA fossilized starfish has a different ratio
14 How the C-12:C-14 Ratio Works Amount of stable C-12Amount of unstable C-14RatioYears dead# of half-lives100 trillion1001-T to 1502-T to 15,7301254-T to 111,460212.58-T to 117,1903616-T to 122,920432-T to 128,6505Carbon-14 can date elements up to approximately 100,000 yearsUsed to date very recent eventsImportant tool for anthropologists, archeologists and geologists
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 decreasesComparing the ratio of parent to daughter yields the age of the sample
16 Radioactive IsotopesFrequently used isotopes in Radiometric Dating
17 Radioactive IsotopesU238 Radioactive Decay Series
19 Radiometric Dating Sources of Error A closed system is requiredOnly fresh, non-weathered, unaltered or non-deformed rock samples should be usedFor example,MetamorphismReheats samplesSample measures younger than should beCan cross check age using other isotopes
20 Isotope Used For Dating U-Pb & Th-Pb most commonUsed for ancient samples—instrusives, lunar rocks, meteroitesRb-Sr used for oldest rocksK-Ar used for fine grained volcanic rocksAr is gas so sample must be freshCan also be used for metamorphic rocks
21 Importance of Radiometric Dating A complex procedure that requires precise measurementRocks from several localities have been dated at more than 3 billion yearsConfirms the idea that geologic time is immense
22 Radiometric Dating Dating Sedimentary Strata How old are Dakota Sandstone, Mancos Shale and Mesaver?