7.3 K-Ar, Ar-Ar, and U-Pb dating methods

Radiometric dating techniques like K-Ar, Ar-Ar, and U-Pb are powerful tools for determining the age of rocks and minerals. These methods rely on the decay of radioactive isotopes to stable ones, allowing scientists to measure time since rock formation.

Each technique has unique advantages and applications. K-Ar and Ar-Ar dating work well for volcanic rocks, while U-Pb excels at dating ancient zircons. Understanding these methods is crucial for unraveling Earth's geological history.

Potassium-Argon and Argon-Argon Dating

Potassium-40 Decay and Argon-40 Accumulation

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• Potassium-40 is a radioactive isotope of potassium that decays to argon-40 through electron capture and positron emission with a half-life of 1.25 billion years
• Argon-40 is a stable isotope of argon that accumulates in minerals as a result of potassium-40 decay
• The ratio of potassium-40 to argon-40 in a mineral can be used to determine the age of the mineral since the closure temperature was reached (when the mineral became a closed system)
• Potassium is a common element in many minerals (feldspars, micas), making this dating method widely applicable

Argon-Argon Dating and Step-Heating

• Argon-argon dating is a variation of the potassium-argon method that involves irradiating the sample with neutrons to convert some of the potassium-39 to argon-39
• The ratio of argon-40 to argon-39 is then measured, which eliminates the need to measure the potassium content directly
• Step-heating involves incrementally heating the sample and measuring the argon isotopes released at each step
  • This technique can reveal if the sample has been disturbed or if there has been any argon loss, providing a more accurate age determination
• The closure temperature is the temperature below which a mineral becomes a closed system for a particular isotopic system (potassium-argon)
  • For most minerals dated with potassium-argon and argon-argon methods, the closure temperature is between 150°C and 500°C

Uranium-Lead Dating

Uranium Decay and Lead Accumulation

• Uranium-lead dating is based on the decay of uranium-238 and uranium-235 to stable isotopes of lead (lead-206 and lead-207, respectively)
• Uranium-238 has a half-life of 4.47 billion years, while uranium-235 has a half-life of 704 million years
• The ratio of uranium to lead in a mineral can be used to determine the age of the mineral since it cooled below its closure temperature
• Zircon is a common mineral used for uranium-lead dating because it incorporates uranium but not lead when it crystallizes, and it has a high closure temperature (>900°C)

Concordia Diagrams and Discordance

• A concordia diagram is a graphical representation of the relationship between the two uranium-lead decay systems (uranium-238 to lead-206 and uranium-235 to lead-207)
• In an undisturbed system, the ages determined by both decay systems should agree, plotting on a curve called the concordia
• Discordance occurs when the ages determined by the two decay systems do not agree, often due to lead loss or uranium gain after the mineral formed
  • Discordant ages can still provide valuable information about the geological history of the sample (metamorphic events, weathering)

Geochronology Applications

Dating Geological Events and Processes

• Geochronology is the study of the age of rocks, sediments, and fossils using various dating methods, including radiometric dating
• Radiometric dating techniques like potassium-argon, argon-argon, and uranium-lead can be used to determine the age of igneous and metamorphic rocks, as well as some sedimentary minerals (zircon)
• These methods can help constrain the timing of important geological events (volcanic eruptions, mountain building, meteorite impacts) and processes (plate tectonics, erosion, sedimentation)

Closure Temperature and Mineral Selection

• The closure temperature is a critical factor in selecting the appropriate mineral and dating method for a given application
• Minerals with high closure temperatures (zircon, hornblende) are better suited for dating high-temperature events (magma crystallization), while minerals with lower closure temperatures (biotite, potassium feldspar) are better for dating low-temperature events (metamorphism, cooling)
• By using multiple minerals with different closure temperatures from the same rock, it is possible to reconstruct the cooling history of the rock and the timing of various geological events