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☄️ How we know the age of a meteorite

A meteorite has no single birthday. Follow one from the museum tray to the mass spectrometer: the mineral that keeps the count, the straight line that tests it, and the five rocks that gave Clair Patterson the age of the Earth in 1956.

4
lessons
~18 min
to learn
🔬 Science
subject
Adults
level
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What you’ll learn

  1. From stone to specimenExplain why a meteorite holds several histories, and how classification and a thin section pick out the mineral a date could belong to.A meteorite is an assembly of materials with different histories, so the first job is choosing the right object, the right mineral and the right question.
  2. The clocks inside atomsDescribe radioactive decay and half-life, and explain why different isotope systems answer different geological questions.Parent and daughter isotopes make clocks whose half-lives and host minerals have to match the event being dated.
  3. Making a clean measurementTrace how a laboratory separates tiny grains, measures isotope ratios on a mass spectrometer, and tests the result with an isochron.Clean chemistry, mass spectrometry and the straight line of an isochron are what turn specks of mineral into a defensible number.
  4. From a number to a historyInterpret meteorite ages as evidence about formation, resetting and the age of the solar system and the Earth.A date is a conditional claim about one event, and cross-checks are what connect a meteorite to Earth's own beginning.

Questions this course answers

A meteorite's radiometric age is almost never the date it landed. Why not?

Radiometric systems date events in a mineral's own history. The fall typically comes billions of years later, and the few seconds of atmospheric heating only affect a thin fusion crust.

What does classifying a meteorite as a chondrite, an achondrite or an iron actually buy the laboratory?

The class is not the date. It narrows down which minerals are worth targeting and which event — melting, cooling, impact — a resulting age could plausibly refer to.

What does a half-life describe?

Individual atoms are unpredictable. A half-life is a statistical property of a large population: after one, half the parent remains; after two, a quarter.

Why is it useful that uranium-238 and uranium-235 decay to different lead isotopes?

Uranium-238 reaches lead-206 with a half-life of about 4.5 billion years and uranium-235 reaches lead-207 in about 704 million, so one sample carries two independent clocks.

Why do laboratories treat sample preparation as part of the measurement rather than as a chore before it?

A dated grain may hold only picograms of lead. Modern lead picked up anywhere along the chemistry would swamp it, which is why papers report their procedural blanks.

Grains that formed together plot as a straight line on an isochron diagram. What do the slope and the intercept tell you?

That is the point of the method: the initial daughter composition falls out of the data instead of being guessed, and scatter around the line is itself evidence of a disturbed history.

Grounded in trusted sources

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