Age dating rocks

This is called stratigraphy and is most commonly used for sedimentary layers, laid down successively on top of one another. Different radioactive decay systems have different probabilities and these are expressed as their DECAY CONSTANT. potassium 40 goes to argon 40) and its unique decay constant, the number of daughter atoms created depends on the amount of time and the original number of parent atoms. In practice the determination of ages uses ratios between different isotopes, measured with great precision in modern mass spectrometers.

The results can be interpreted graphically on something called an isochron plot.

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Note however, these problems also work to our advantage.

We can use the leaky nature of rocks and minerals to isotopic diffusion to estimate the cooling history of rocks - which is very important in tracking the passage of rocks to the surface as their overburden is eroded.

Isochron plots for the rubidium-strontium system applied to old rocks from Greenland and for chrondritic meteorites.

In practice great care is necessary in applying isotopic methods to date rocks.

Using the SAM mass spectrometer to measure the abundance of three isotopes that result from cosmic-ray bombardment—helium-3, neon-21, and argon-36—Farley and his colleagues calculated that the mudstone at Yellowknife Bay has been exposed at the surface for about 80 million years.

"All three of the isotopes give exactly the same answer; they all have their independent sources of uncertainty and complications, but they all give exactly the same answer.

That is probably the most remarkable thing I've ever seen as a scientist, given the difficulty of the analyses," Farley says.

This also helps researchers looking for evidence of past life on Mars.

There, the sample was heated to temperatures high enough that the gasses within the rock were released and could be analyzed by an onboard mass spectrometer.

Farley and his colleagues determined the age of the mudstone to be about 3.86 to 4.56 billion years old.

So while the mudstone indicates the existence of an ancient lake—and a habitable environment some time in the planet's distant past—neither crater counting nor potassium-argon dating can directly determine exactly when this was.

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