Radiometric Dating: Age Determination of Rocks, Shells, and Meteorites
Key Concepts:
- Radiometric Dating: A method to determine the age of rocks, shells, and meteorites using radioactive decay.
- Radioactive Parent Material: The original radioactive element present in the rock that decays into a daughter product.
- Half-Life: The time required for half of the radioactive parent material to decay into the daughter product.
Common Radioactive Pairs:
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Carbon-14 to Nitrogen-14
- Half-life: 5,730 years.
- Suitable for dating relatively young rocks or organic materials.
- Not suitable for very old rocks, such as those over 100 million years.
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Uranium-238 to Lead-206
- Half-life: 4.5 billion years.
- Suitable for dating meteorites and older rocks.
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Uranium-235 to Lead-207
- Half-life: 700 million years.
- Also suitable for dating older rocks and meteorites.
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Potassium-40 to Argon-40
- Half-life: 1.4 billion years.
Steps in Radiometric Dating:
Step 1: Identify a Suitable Radioactive Pair
- Select a radioactive decay pair based on the material's composition and the estimated age of the sample.
Step 2: Ensure a Closed System
- Confirm no migration of parent or daughter isotopes into or out of the rock.
- Check for evidence of closed systems, such as lack of weathering or melting.
Example: Dating a Meteorite
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Process:
- Identify the radioactive pair (e.g., Uranium-238 to Lead-206).
- Ensure the sample is a closed system.
- Measure the parent-daughter ratio using a mass spectrometer.
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Findings:
- Meteorite formed roughly 4.5 billion years ago.
- Ratio close to 1:1 indicates one half-life of Uranium-238 has passed.
Additional Considerations:
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Multiple Isotope Pairs:
- Dating accuracy can be confirmed using multiple radioactive isotope pairs.
- Example: Uranium-235 to Lead-207 ratio calculations show a fraction of parent remaining consistent with the age.
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Equation for Precision:
- Fraction of parent remaining = e^(-T / 1.443), where T is the number of half-lives passed.
Conclusion:
Radiometric dating relies on understanding the decay processes of isotopes and ensuring that the samples have remained closed systems. By analyzing parent-daughter ratios, we can accurately determine the ages of rocks and meteorites. More detailed studies and methods are available for deeper understanding.