Chapter 21
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University of Guelph**We aren't endorsed by this school
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ENVS 3180
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Geology
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Jan 8, 2025
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Chapter 21 Dating and Correlation Techniques Radiometric Dating •Overview: Radiometric dating measures the decay of radioactive isotopes to determine the age of rocks. Best for igneous rocks but limited for sedimentary rocks. •Key Principles: oRadioactive Decay: Parent isotopes decay into daughter isotopes at a known rate (half-life). oClosed System: Assumes no loss or gain of isotopes during rock formation. oApplications: Igneous rocks (e.g., volcanic layers in sedimentary sequences), metamorphic rocks (dating metamorphism), and some authigenic minerals like glauconite. Radiometric Techniques 1. Potassium-40 (40K^{40}\text{K}40K) → Argon-40 (40Ar^{40}\text{Ar}40Ar): •Half-life: 1.25 billion years. •Dating Range: 1 million to >4.5 billion years. •Usage: Previously widely used for dating older rocks but is now considered less reliable due to potential complications such as argon loss. 2. Rubidium-87 (87Rb^{87}\text{Rb}87Rb) → Strontium-87 (87Sr^{87}\text{Sr}87Sr): •Half-life: 48.8 billion years. •Dating Range: Up to >4.5 billion years. •Usage: Effective for very old rocks but less commonly used today compared to more robust methods like uranium-lead dating. 3. Samarium-147 (147Sm^{147}\text{Sm}147Sm) → Neodymium-143 (143Nd^{143}\text{Nd}143Nd): •Half-life: 1.06 billion years. •Dating Range: Over 200 million years. •Usage: Useful for dating older rocks and for geochemical studies. 4. Rhenium-187 (187Re^{187}\text{Re}187Re) → Osmium-187 (187Os^{187}\text{Os}187Os): •Half-life: 42 billion years. •Dating Range: 10 million to >4.5 billion years. •Usage: Gaining popularity for dating non-igneous materials like sedimentary rocks, which other methods struggle with. 5. Thorium-232 (232Th^{232}\text{Th}232Th) → Lead-208 (208Pb^{208}\text{Pb}208Pb): •Half-life: 14.01 billion years. •Dating Range: 10 million to >4.5 billion years. •Usage: Common for dating very ancient rocks. 6. Uranium-238 (238U^{238}\text{U}238U) → Lead-206 (206Pb^{206}\text{Pb}206Pb):
•Half-life: 4.468 billion years. •Dating Range: 10 million to >4.5 billion years. •Usage: One of the most robust methods for dating ancient rocks due to high precision. 7. Carbon-14 (14C^{14}\text{C}14C) → Nitrogen-14 (14N^{14}\text{N}14N): •Half-life: 5,730 years. •Dating Range: Up to ~60,000 years (Holocene deposits and archeological artifacts). •Usage: Ideal for dating recent organic materials like bones, wood, or charcoal. Chemostratigraphy •Definition: Uses variations in chemical composition of sediments to correlate layers. •Techniques: oBulk Chemical Analysis: Identifies provenance changes in sediments. oHeavy Mineral Assemblages: specific minerals to figure out the sediment's source over time. oClay Mineral Analysis: identifies the types of tiny clay minerals, like kaolinite or smectite, in a sediment or rock to understand the environment. Magnetostratigraphy •Definition: Uses Earth's magnetic fields, recorded in rocks to find their age and correlate strata (match them). •Key Points: oEarth's magnetic field reverses periodically. oMagnetic minerals in rocks record the field’s orientation at formation.oOften used in fine-grained sedimentary rocks. •Applications: oHigh-resolution correlation, especially in continental successions lacking fossils. oRequires tie-points to geological timescales (e.g., biostratigraphy or radiometric dating). Dating Quaternary Deposits 1.Carbon-14 Dating: oDates organic materials up to ~70,000 years old. oMeasures decay of 14C^{14}C14C, which forms in the atmosphere. oLimited to recent geological timescales. 2.Uranium-Series Dating: oDates marine carbonates (e.g., corals) up to 500,000 years. oMeasures ratios of 234U^{234}U234U to 238U^{238}U238U. 3.Oxygen Isotope Stratigraphy: oTracks 18O/16O^{18}O/^{16}O18O/16O ratios in seawater to infer climate changes. oDefines marine isotope stages for Quaternary deposits. 4.Luminescence Dating: oDates buried sediments by measuring trapped energy from natural radiation.
oTechniques: Thermoluminescence (TL) and Optical Stimulating Luminescence (OSL). oEffective up to ~150,000 years. 5.Cosmogenic Isotopes: oMeasures isotopes (e.g., 10Be^{10}Be10Be, 26Al^{26}Al26Al) formed by cosmic ray bombardment. oDates surface exposure of rocks and sediments. 6.Amino-Acid Racemization: oMeasures conversion of "L" to "D" forms of amino acids in tissues post-mortem. oTemperature-sensitive, works best for well-preserved organic material. 7.Annual Cycles: oTree Rings: Count rings to date wood and correlate with climate changes. oGlacial Lake Varves: Count sediment layers deposited seasonally to estimate age.