New method to date fossils using micro-laser carbonate uranium-lead (U-Pb) dating

[dlamberth]

"It is the first time that dinosaur eggs have been dated using carbonate uranium-lead (U-Pb) dating."
“We fired a micro-laser at eggshell samples, vaporizing carbonate minerals into aerosol.
"This is analyzed by a mass spectrometer to count uranium and lead atoms.
"Since uranium decays into lead at a fixed rate, we were able to calculate the age by measuring accumulated lead - it’s like an atomic clock for fossils."

Basic article found on MSN.
MSN

Here's a link to several papers on micro-laser U-Pb dating.
GChron – Special issue – In situ carbonate U–Pb geochronology

 

[Warden_of_the_Storm]

Oh, that's so cool.

 

[Ophiolite]

As the precision and sensitivity of technology increases, new avenues of dating will continue to emerge. On the other hand a select group will continue to think it all hinges on carbon datating. Ah well!

 

[Tuur]

Someone help me out there:

It's radiometric dating. I get that. Uranium decays at a known rate, producing thorium, and thorium producing radium, and so forth until you get to the stable element lead. Different isotopes of uranium produce different isotopes of lead. I get that. Looking at the ratio of a particular isotope of uranium with a particular isotope of lead and applying the rate of decay gives how long it's been since the uranium started to decay. I get that.

What I don't understand is how pre-existing lead is ruled out of the equation. Is lead stable but some lead is more stable than others? It's still lead but becomes a different isotope of lead? If that's the case, is there a ceiling to the time range uranium-lead dating can be uses, sort of like there's a floor to carbon dating? What I'm getting at is if a the isotope of lead they compare to an isotope of uranium would have already decayed into a different isotope of lead by the time someone measures the ratios.

Side question: Am I right in assuming that the uranium came from fossilization minerals and not from the animal? Thinking of how strontium 90 ends up in bones, but that isn't what's happening with uranium-lead dating.

 

[sjastro]

Someone help me out there:

It's radiometric dating. I get that. Uranium decays at a known rate, producing thorium, and thorium producing radium, and so forth until you get to the stable element lead. Different isotopes of uranium produce different isotopes of lead. I get that. Looking at the ratio of a particular isotope of uranium with a particular isotope of lead and applying the rate of decay gives how long it's been since the uranium started to decay. I get that.

What I don't understand is how pre-existing lead is ruled out of the equation. Is lead stable but some lead is more stable than others? It's still lead but becomes a different isotope of lead? If that's the case, is there a ceiling to the time range uranium-lead dating can be uses, sort of like there's a floor to carbon dating? What I'm getting at is if a the isotope of lead they compare to an isotope of uranium would have already decayed into a different isotope of lead by the time someone measures the ratios.

When carbonate is precipitated or comes out of solution in the form of calcite the crystal structure is too small for Pb (lead) to reside in as the ionic radius of Pb²⁺ is large.
The ionic radius of uranium U⁴⁺ is small enough to fit inside the crystal structure.
The decay products for uranium ²⁰⁶Pb and ²⁰⁷Pb reside inside the crystal, if there is non-radiogenic ²⁰⁴Pb present it means lead has found its way into the crystallization process but can be factored out.

Side question: Am I right in assuming that the uranium came from fossilization minerals and not from the animal? Thinking of how strontium 90 ends up in bones, but that isn't what's happening with uranium-lead dating.

The uranium comes from uranium ions dissolved in water.
When the carbonate comes out of solution uranium ions are incorporated into the calcite.

 

[Tuur]

When carbonate is precipitated or comes out of solution in the form of calcite the crystal structure is too small for Pb (lead) to reside in as the ionic radius of Pb²⁺ is large.
The ionic radius of uranium U⁴⁺ is small enough to fit inside the crystal structure.
The decay products for uranium ²⁰⁶Pb and ²⁰⁷Pb reside inside the crystal, if there is non-radiogenic ²⁰⁴Pb present it means lead has found its way into the crystallization process but can be factored out.

That's pretty neat. That eliminates the issue of pre-existing lead isotopes.

The uranium comes from uranium ions dissolved in water.
When the carbonate comes out of solution uranium ions are incorporated into the calcite.

Umm...hard egg shells are calcite. This implies ingested uranium. Of course, it's not like eating yellow cake but it's somewhat unsettling, even in trace amounts. Radium I knew about, and it does show up naturally in some aquifers. But we get radium from thorium (I'm assuming a shorter-lived isotope) and thorium from uranium, but haven't heard anything about this showing up in well water. As a quick check, looked up the EWG of one municipality that had problems with it, and it lists radium (well below the maximum allowable amount), but not uranium and thorium.

Off-Topic Sidebar Ahead:
Why my interest? Something called the Apalachicola Embayment. This cuts across part of the Florida Panhandle and across part of Georgia, and was the old channel of the Gulfstream. Have the report in PDF, but without looking at it, conditions were such that it allowed radium to be precipitated out and trapped in the channel. This was all shaking out when I had college chemistry, mainly about it showing up in high amounts in some municipalities, and was discussed in class. Remediation consisted of drilling new wells and filtration.

Note: A search of Apalachicola Embayment radium turned up a report with the date of 1990, but this was discussed in class way before then. Skimming over it, there's the implication of focusing on radium because it's a high alpha emitter. I don't know if the tests aren't looking for uranium and thorium, or if it's not present. The implication is that they are.

Note II: Since this is off-topic, I haven't posted a link. The title is Hydrology of the Gulf Trough - Apalachicola Embayment Area, Georgia.

If it hadn't come up in a chemistry class, probably wouldn't even think about it.

 

[sjastro]

That's pretty neat. That eliminates the issue of pre-existing lead isotopes.


Umm...hard egg shells are calcite. This implies ingested uranium. Of course, it's not like eating yellow cake but it's somewhat unsettling, even in trace amounts. Radium I knew about, and it does show up naturally in some aquifers. But we get radium from thorium (I'm assuming a shorter-lived isotope) and thorium from uranium, but haven't heard anything about this showing up in well water. As a quick check, looked up the EWG of one municipality that had problems with it, and it lists radium (well below the maximum allowable amount), but not uranium and thorium.

It's a result of a hydrothermal pathway.
The source of uranium is in magmatic rock below the surface where there is also superheated water.
The superheated water partially dissolves uranium bearing minerals such as uraninite and pitchblende and is carried to the surface through vents where it mixes with surface water.
This becomes the source for uranium ions in water and eventually uranium in calcite when carbonate comes out of solution during the fossilization process.