Igneous / Magmatic rocks-Part 03

1.Basic

     1.1.Volcanic or extrusive rock.

     1.2.Plutonic or intrusive rock 

2.Advanced

     2.1 Stage of igneous or magmatic cycle.

𝟑.𝐄𝐱𝐩𝐞𝐫𝐭

     𝟑.𝟏.𝐙𝐢𝐫𝐜𝐨𝐧 𝐭𝐨 𝐜𝐚𝐥𝐜𝐮𝐥𝐚𝐭𝐞 𝐭𝐡𝐞 𝐚𝐠𝐞 𝐨𝐟 𝐭𝐡𝐞 𝐞𝐚𝐫𝐭𝐡.

Today we are talking about highlighted topics.

Expert

Zircon to calculate the age of the earth

Zircon forms in granites deep in the earth's crust (plutonic rock). Through movement of tectonic plates, this granite is brought to the surface and starts mountain building. Through erosion, the granite (and the contained zircon) builds sediments which will eventually be buried deep enough to transform into metamorphic rocks.

Zircon has two important properties:

Relative high hardness

Resistance to chemical attacks.

Due to its hardness of 7.5 on the scale of Mohs, the zircons usually survive the sedimentary process intact. Because of its resistance to chemical attacks, zircon will survive the contact metamorphism process which is trying to attack it with heat and pressure. The latter is important as the liquid mass surrounding the zircon will cause a new rim to be formed around the old zircon, just like the formation of tree rings. This first cycle usually will take hundreds of million years.

The old crystal with its newly formed rim is then pushed up again through tectonic plate interactions, then this geologic cycle repeats itself.

A third and well known property of zircon is that it can accommodate radioactive elements like uranium. Uranium has the ability to decay into lead at a fixed time rate. Calculation of the uranium-lead ratio may give a clue of the age of the zircon (and thus Earth's age). However, lead may leak from the zircon, disturbing the uranium-lead ratio.

Fortunately, scientists found a new way of calculation by using isotopes. The uranium-238 isotope (with a half-life of 4.468 billion years) decays to lead-206, while uranium-235 (with a half-life of 703.8 million years) decays to lead-207. The "secondary ion mass spectrometry" (or SIMS) technique is used for this kind of measurement. By measuring both 'parent-daughter' decays of both these processes, an accurate age of the zircon can be calculated -- that is, if both measurements are consistent (which is not always the case).

The Isua rocks on Greenland are a source for this way of dating the earth's age. Current calculations indicate Earth is 4.6 billion years old.

This zircon, which is about twice the width of a human hair, is now confirmed to be the oldest bit of the Earth's crust ever discovered. It was found in the Jack Hills region of Australia. (Courtesy John Valley/University of Wisconsin-Madison)