Lumberg said:
Only if there was a raging debate about it and you totally owned someone with your knowledge.
[snapback]201883[/snapback]
I suppose you can guess what type of person I was heavily engaged with.
Disclaimer: I am not a geochronologist but my degree is in geology. It is not something I do but I have studied it in college. I know people who do this stuff. If I mistype, misspell, or use the wrong word it doesn’t take away from the bread and butter of the process. They have entire dictionaries devoted to scientific terminology and I don’t walk around with that stuff in my head. Also I’m trying to be brief so some parts may be oversimplified. I’m not writing a thesis here
OK buys and girls get your pen and paper out. Lets talk radiometric dating of rocks.
Lets start with a definition that is core to radiometric dating. An isotope is a radionuclide of an element that either has an excess or deficiency in the number of neutrons in the nucleus. Because of this it’s not a happy element. To get happy it sheds of gains neutrons to become a stable element. This is radioactive decay. The rate of decay is something that can be
measured in the laboratory. Anybody heard of the phrase half-life? These isotopes decay into stable elements, which are called daughter elements. The isotope it comes from is called the parent element. For every daughter element there once was a parent element.
Math is really the only thing that can be proven. Two plus two is four right? Not five, not three, but four. Radioactive decay happens exponentially. This can be expressed in a calculus function that is taught in every first semester Calc class. If you know how many daughter isotopes you have, how many parent isotopes you have, and the decay rate which has been
measured previously, then it’s a pretty simple integration solving for t time.
Geologists date rocks by collecting samples and running them through a mass spectrometer. The samples are dissolved and thrown past a big honking magnet in like a tube. Because different elements have different weights they are attracted differently to the magnet as they pass by. Heavy stuff that gets attracted to the magnet will have an arc and liter stuff will go straighter. Just passin’ through the neighborhood so to speak. Out the backside is like a piece of flypaper type thing that the elements stick to. (I’m really oversimplifying this part there is no flypaper
). They then measure how much deviation is each of the paths with a computer and then they can quantify how much of each element/isotope they have etc compared to previous calculations of known elements. OK enough about mass specs.
There are naturally occurring isotopes all over. Geologists collect rocks and then run them through a mass spec and use whatever methodology matches the mineralogical composition of the sample. A common one used on igneous rocks such as granite etc. is potassium (K) – argon (Ar) dating. It’s useful because potassium is the building block of many minerals found in many igneous rocks. An unstable isotope of K decays into a stable isotope of Ar. The reason that this is so useful and pretty bullet proof is that this is one time that assuming something doesn’t make an ass out of you and me. So how do we know, or assume, that there wasn’t any Ar in the mineral during the formation? Argon is a noble element meaning it don’t like hangin with any of the others. That’s why they use it in MIG welders as a shielding gas. So it’s logical to assume that it didn’t go against every scientific law known to man and jump into the potassium bearing mineral during formation.
So crunch you sample, run it through a mass spec, measure the quantities of everything that spits out the other end, do some integration and solve for t, and presto you have a date.
There are other methods that don’t use elements that had quantities of 0 back when the mineral was formed. They can calculate the amount that was present at the beginning by some wacky isochrones and stuff that makes my head hurt thinking about it so I’m leaving that out now for sanities sake. I’m also basically tired of typing.
This dear friends is how rocks are dated. They’ve been doing this stuff for the better apart of the 20th century so the numbers were dealing with now are pretty damn solid.
The above was paraphased from chapter 16 of
Principles and Applications of Geochemisty 2nd edition by Gunter Faure.
Russell
