Geochronology primer?
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07-12-2014, 09:01 AM
Geochronology primer?
Does TTA have a resource explaining the basics of geochronology?

If not, would such an item be useful?

---
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07-12-2014, 09:08 AM
RE: Geochronology primer?
(07-12-2014 09:01 AM)Paleophyte Wrote:  Does TTA have a resource explaining the basics of geochronology?

If not, would such an item be useful?

Yeah, I would love to read up on this, I'm really interested in thermoluminescence dating, even though I have a basic understanding of the process, I haven't found anything that breaks down what is going on at the atomic or molecular level.

Gods derive their power from post-hoc rationalizations. -The Inquisition

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07-12-2014, 10:46 AM
RE: Geochronology primer?
(07-12-2014 09:08 AM)TheInquisition Wrote:  
(07-12-2014 09:01 AM)Paleophyte Wrote:  Does TTA have a resource explaining the basics of geochronology?

If not, would such an item be useful?
Yeah, I would love to read up on this, I'm really interested in thermoluminescence dating, even though I have a basic understanding of the process, I haven't found anything that breaks down what is going on at the atomic or molecular level.

I've only really ever scratched the surface of TL. Its "short" (hundred thousand years) useable duration is smaller than the uncertainties on most of my measurements. It's more useful for archeological applications, though it's finding uses in some branches of geology that study comparatively recent events. Here's a quick run down on it though:
  • All crystalline materials have defects, even the best lab-grown synthetics. Natural crystals are full of them. These crystalline defects can act as traps for a variety of different types of energy.
  • In crystals that are good electrical insulators, such as quartz and feldspar, these defects can trap free electrons. These are charge-balanced by electron-deficient atoms in the crystal lattice of the defects. This is handy because quartz and feldspar are two of the most common minerals in the crust.
  • At T=0 there is a zeroing event, involving heat or light. The pottery is fired, the sediment is exposed to sunlight, the volcanic ash is erupted. The heat or light provides enough energy that any free electrons trapped in the crystalline defects are liberated and your TL clock is reset to 0.
  • Ionizing radiation from cosmic rays and the decay of radioisotope sends high-energy particles ricocheting through the crystal matrix. Electrons are liberated and while most of them find a home in an electron-deficient atom a few will be trapped in the crystal defects.
  • Over time, these trapped electrons accumulate, proportional to the amount of time that has passed and the ambient ionizing radiation. The latter can be measured fairly simply.
  • Here's where we run into the problem that limits TL's utility. The crystal defects saturate. The more free electrons that build up in them, the less likely it is that further electrons will be trapped. Initial trapping rates are comparitively rapid but as electrons accumulate the rate slows and approaches zero. Your TL clock slows to the point that it effectively stops.
  • Now some curious, over-evolved ape-descendant digs up the object of interest and conveys it back to the lab. Here it is placed in a very dark oven with a sensitive photometer and warmed gently. This is another zeroing event that liberates the trapped electrons from the crystal defects and allows them to recombine with the electron-deficient atoms that have been keeping them charge balanced. As the electrons trickle down through the various orbitals toward their final rests states photons are given off. This luminescence is measured and is proportional to the abundance of free electrons that were trapped. That in turn is proportional to the ambient ionizing radiation (already measured) and the time that has elapsed since the last zeroing event, which can now be calculated.
That's the quick and dirty explanation. I hope that I didn't get it too wrong as it's really not my field.

---
Flesh and blood of a dead star, slain in the apocalypse of supernova, resurrected by four billion years of continuous autocatalytic reaction and crowned with the emergent property of sentience in the dream that the universe might one day understand itself.
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07-12-2014, 11:01 AM
RE: Geochronology primer?
(07-12-2014 10:46 AM)Paleophyte Wrote:  
(07-12-2014 09:08 AM)TheInquisition Wrote:  Yeah, I would love to read up on this, I'm really interested in thermoluminescence dating, even though I have a basic understanding of the process, I haven't found anything that breaks down what is going on at the atomic or molecular level.

I've only really ever scratched the surface of TL. Its "short" (hundred thousand years) useable duration is smaller than the uncertainties on most of my measurements. It's more useful for archeological applications, though it's finding uses in some branches of geology that study comparatively recent events. Here's a quick run down on it though:
  • All crystalline materials have defects, even the best lab-grown synthetics. Natural crystals are full of them. These crystalline defects can act as traps for a variety of different types of energy.
  • In crystals that are good electrical insulators, such as quartz and feldspar, these defects can trap free electrons. These are charge-balanced by electron-deficient atoms in the crystal lattice of the defects. This is handy because quartz and feldspar are two of the most common minerals in the crust.
  • At T=0 there is a zeroing event, involving heat or light. The pottery is fired, the sediment is exposed to sunlight, the volcanic ash is erupted. The heat or light provides enough energy that any free electrons trapped in the crystalline defects are liberated and your TL clock is reset to 0.
  • Ionizing radiation from cosmic rays and the decay of radioisotope sends high-energy particles ricocheting through the crystal matrix. Electrons are liberated and while most of them find a home in an electron-deficient atom a few will be trapped in the crystal defects.
  • Over time, these trapped electrons accumulate, proportional to the amount of time that has passed and the ambient ionizing radiation. The latter can be measured fairly simply.
  • Here's where we run into the problem that limits TL's utility. The crystal defects saturate. The more free electrons that build up in them, the less likely it is that further electrons will be trapped. Initial trapping rates are comparitively rapid but as electrons accumulate the rate slows and approaches zero. Your TL clock slows to the point that it effectively stops.
  • Now some curious, over-evolved ape-descendant digs up the object of interest and conveys it back to the lab. Here it is placed in a very dark oven with a sensitive photometer and warmed gently. This is another zeroing event that liberates the trapped electrons from the crystal defects and allows them to recombine with the electron-deficient atoms that have been keeping them charge balanced. As the electrons trickle down through the various orbitals toward their final rests states photons are given off. This luminescence is measured and is proportional to the abundance of free electrons that were trapped. That in turn is proportional to the ambient ionizing radiation (already measured) and the time that has elapsed since the last zeroing event, which can now be calculated.
That's the quick and dirty explanation. I hope that I didn't get it too wrong as it's really not my field.

Awesome! Thanks for that!

Gods derive their power from post-hoc rationalizations. -The Inquisition

Using the supernatural to explain events in your life is a failure of the intellect to comprehend the world around you. -The Inquisition
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07-12-2014, 11:52 AM
RE: Geochronology primer?
(07-12-2014 11:01 AM)TheInquisition Wrote:  Awesome! Thanks for that!

No prob. I think that it was mostly right.

If you're looking for geochron tools that are handy for measuring dates over relatively short time spans (<1 Ma or so) then you might be interested in U and Th secular equilibrium and disequilibrium.

---
Flesh and blood of a dead star, slain in the apocalypse of supernova, resurrected by four billion years of continuous autocatalytic reaction and crowned with the emergent property of sentience in the dream that the universe might one day understand itself.
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