

Subduction is not the end - kryptiskt
http://all-geo.org/metageologist/2014/11/subduction-is-not-the-end/

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Patient0
Can anyone explain something that's always puzzled me: when dating the ages of
rocks, geologists look at the proportions of the isotopes that are in the
rocks. Because isotopes decay at a well known rate (they have a half life),
then by knowing the proportion of the isotopes that the rock would have had
when it formed and comparing to the observed ratio now, they can tell how long
ago the rock formed.

[http://en.m.wikipedia.org/wiki/Isotopic_dating](http://en.m.wikipedia.org/wiki/Isotopic_dating)

By "formed" I guess this means "cooled down" so it's not magma or lava
anymore.

But _how_ do they know the initial isotope proportion? When an "old" rock with
some ratio of isotopes melts and becomes magma or lava again, why does this
"reset" the proportion of its isotopes? To put it another way: the isotope
will have also been decaying when it was part of the magma wouldn't it? Why
don't new rocks have the same isotopic ratios as old rocks?

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jofer
Great question!

In a nutshell, it's because we choose minerals whose crystal structure can
only bond with the original element and not with it's "daughter" that's
produced after radioactive decay.

For example, Uranium/Lead dating of zircons is one of the most common methods
(particularly for very old rocks). There's a reason we use zircons for
uranium/lead dating, and not just any mineral that contains trace amounts
uranium.

In zircon (ZrSiO4), uranium can (and does, in small amounts) substitute for
zirconium in the crystal structure and bond with the silica tetrahedra.
However, lead cannot bond with the silica tetrahedra in zirconium's place.
Therefore, the zircon crystal structure "rejects" lead, and any lead in the
crystal lattice at the present day must be the result of radioactive decay.

If we know the rate of radioactive decay of the uranium isotopes in question
(there are two uranium isotopes that decay to two different lead isotopes),
all we need to date the rock is the present-day ratio of a particular uranium
isotope to the corresponding lead isotope.

In practice, you also exploit the fact that there are two uranium isotopes
that independently decay to two different lead isotopes. This gives two
independent dates, and disagreements between them can be use to test for
contamination, etc. (There are also a few other checks I won't get into... I'm
not a geochronologist, and each different isotopic system and instrument used
to measure it has slightly different protocols. Reality is messy, but this
gives the basic picture.)

~~~
Patient0
Thanks so much! Such a great answer! This is why I love Hacker News :-)

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arethuza
"Sediments formed billions of years ago in a now alien atmosphere sit
thousands of kilometres under our feet, subtly modulating the length of the
day."

Banded iron formations are fascinating - while browsing around I noticed an
example on the relevant Wikipedia page of a rock that has layers that are
thought to correspond to a day-night-cycle. The rock is 3.5 billion years old!

The idea that we have things to correspond to individual _days_ from 3.5
billion years ago is stunning.

[http://en.wikipedia.org/wiki/Banded_iron_formation](http://en.wikipedia.org/wiki/Banded_iron_formation)

