Half-life (not the game)

[Yom]

Unless it had decaying potasium in it.

Exactly.

I am not questioning what you are saying. I am simply trying to understand how we can assume that none of the potasium already decayed before the orgonism was covered with lava.

If it were decayed, then the Argon would probably find a way to be expelled from the organism with other gases. Potassium has such a low probability of decaying though (hence the long half-life), that it is unlikely that a significant amount would decay (or at least enough to skew the results very much). I would think that that's why Carbon-14 can't be used for dating past 50,000 years (Nitrogen-14 isn't that rare, so it could already be in the organism, and already decayed Carbon-14 could exist in the organism).

 

[Cheetah]

I think you meant to say "the time it takes" instead of "the probability of."

Yes, that's probably more correct.

The probability of that happening is very very small, akin to flipping a coin 10^500 times and getting a head every time (to use Renata's analogy). We know that the percentage of C-14 atoms in nature is very small, so it's all down probabilities. (can work out percentage of C-14 based on, e.g. ... something [I forgotten the proper word, but it's where the atoms are ionised by removing 1 electron and subsequently deflected by a EM field - the heavier particles are deflected less, and the mass/charge ratio is measured])

@ Cheetah -- I'm afraid I can't figure out what you're trying to ask.

I think Mise already answered, but I'll try to clarify just to be on the safe side:

Say there were an organism living 500 million years ago. But at that time, the levels of Argon in the food it ate were ten times greater than today. When it was found this year, its date was meassured. But because of the very high levels of Argon relative to Potassium, won't the date of the fossil be calculated wrongly?

Now, if I understood Mise correctly, it's simply that the probability for that organism is so small that we can assume that the levels of Argon for it was just as it is for us today?

 

[Souron]

Exactly.


If it were decayed, then the Argon would probably find a way to be expelled from the organism with other gases. Potassium has such a low probability of decaying though (hence the long half-life), that it is unlikely that a significant amount would decay (or at least enough to skew the results very much). I would think that that's why Carbon-14 can't be used for dating past 50,000 years (Nitrogen-14 isn't that rare, so it could already be in the organism, and already decayed Carbon-14 could exist in the organism).

Well Potasium has as high a possibility of decay in the rock as it does in the orgonism. But it makes sence that Argon would not remain in the orgonism for long when it's alive.

As for Carbon 14, as I understand it, there is generally a balance of Carbon-14 that is maintained withthe atmosphere. When the Carbon ceases to have access to the atmosphere, it ceases to balance itself, thus we can caluclate the ratio of carbon 13 to carbon 14 and use that to determine the age.

 

[HighlandWarrior]

You would know because I defined it as such. Dating works the other way around. If you find 2.5 moles of Potassium and 2.5 moles of Argon, then it would probably be around 1.3 billion years old (having started with 5 moles of Potassium). Since Argon isn't nearly as widespread as Potassium, there's little confusion with having Argon that isn't from decayed Potassium (plus decayed Argon is an isotope of Argon that I believe is different from the type in the atmosphere).

as potassium decays, it turns into an argon potassium compound? hmm that's pretty weird.

 

[Yom]

as potassium decays, it turns into an argon potassium compound? hmm that's pretty weird.

Um...no. When did I say that? 40-K (Potassium) decays into 40-Ar (an isotope of Argon) by electron capture and positron emission.

 

[HighlandWarrior]

Um...no. When did I say that? 40-K (Potassium) decays into 40-Ar (an isotope of Argon) by electron capture and positron emission.

potassium has 1 valence electron, so the potassium loses the elctron and turns into argon? or does it take 18 potassiums losing their valence electron and turning into an argon? it's been a little while since my chemistry class so excuse me if i said something ignorant.

 

[Yom]

potassium has 1 valence electron, so the potassium loses the elctron and turns into argon? or does it take 18 potassiums losing their valence electron and turning into an argon? it's been a little while since my chemistry class so excuse me if i said something ignorant.

40-K can decay to 40-Ar by electron capture and positron emission. In the electron capture method, an electron and proton merge to create a neutron and neutrino. In the positron emission method, a proton decays to a neutron, a positron, and a neutrino.

 

[Mise]

I think Mise already answered, but I'll try to clarify just to be on the safe side:

Say there were an organism living 500 million years ago. But at that time, the levels of Argon in the food it ate were ten times greater than today. When it was found this year, its date was meassured. But because of the very high levels of Argon relative to Potassium, won't the date of the fossil be calculated wrongly?

Now, if I understood Mise correctly, it's simply that the probability for that organism is so small that we can assume that the levels of Argon for it was just as it is for us today?

Sorry I think I misunderstood your question -- I thought you were asking about the level of the particular isotope of Argon being much higher in a particular species.

Well, fossils from 500,000,000 years ago tend not to have very much pottassium in it anyway, because that's not their diet so much, so it's normally used to determine the age of rocks etc.

So to answer your question, I don't know how they knew the original levels of K and Ar (and the original levels I think would make a difference), but they can tell the starting levels of C-14 from looking at our atmosphere and how it has changed over the past X years (which we can determine from other things, such as CO2 levels trapped in ice, and other weird and wonderful techniques).

 

[embitteredpoet]

potassium has 1 valence electron, so the potassium loses the elctron and turns into argon? or does it take 18 potassiums losing their valence electron and turning into an argon? it's been a little while since my chemistry class so excuse me if i said something ignorant.

Elements are determined by the number of protons they have in their nucleus. Thus the different isotopes (for eg.) Carbon all have 6 protons, but have 6,7 or 8 or neutrons, thus giving Carbon 12,13 or 14.
Electrons have a vanishingly small mass compared to protons and neutrons.

 

[Renata]

So to answer your question, I don't know how they knew the original levels of K and Ar (and the original levels I think would make a difference), but they can tell the starting levels of C-14 from looking at our atmosphere and how it has changed over the past X years (which we can determine from other things, such as CO2 levels trapped in ice, and other weird and wonderful techniques).

All I know about potassium-argon testing (and that's not much) is how it's applied to rocks -- the rough outline, anyway. Argon is a noble gas. As others have pointed out, it doesn't make compounds, and it is never a component of the rocks themselves. When rocks crystallize, though, they can trap gasses within the crystal matrix; similarly, gasses produced by radioactive decay can be trapped. Atmospheric gas doesn't have much argon in it -- about 1%. If a scientist tests a likely-looking rock and detects much more argon than that, the only place it could have come from is the decay of potassium-40 within the rock. They can then compare the amount of potassium-40 still there to the amount of argon detected and calculate how much of the potassium-40 has decayed, which directly yields the approximate age of the rock.

The error induced by argon present initially within the rock sample can be large or small depending on the age of the rock and probably other factors I'm not aware of. In the best case, with a sufficiently old rock, the amount of argon produced by radioactive decay will be so large relative to any conceivable amount that could have been present at the rock's formation that the error becomes negligible. The error gets worse with younger samples; beyond a certain point (which I don't know off the top of my head), K-Ar dating shouldn't be used.

There's a more recent technique, though, argon-argon dating, which is not affected by argon initially present in the rock. It's complicated, and I haven't figured it all out yet. But apparently somebody took it on himself a while back to use the argon-argon method to measure the age of lava flows from the historical eruption of Mount Vesuvius about 2000 years ago, and was only off by 7 years. Such recent dates could never be obtained accurately by K-Ar.

Renata

 

[Cheetah]

Just to clarify again, I used Potassium and Argon only as an example.

I'm thankfull for the answers so far, but is it correct for me to conclude that we just assume that the levels of Argon is the same now as then, or that we know the levels at that time?

 

[Renata]

No.

Potassium-Argon dating (old style) assumes that there is no argon in the sample at all when it is formed; in other words, that all argon detected is the product of radioactive decay of 40-K. For many types of rock, this is apparently perfectly accurate, from what I've been reading. For others, it is not. (And in those cases, K-Ar dating is therefore considered not appropriate.) It depends on what conditions the crystals form under. But in any case, for an old enough sample, any conceivable amount of starting argon is irrelevent, because the radiogenic argon will outweigh it by so much.

Ar-Ar dating (newer version of K-Ar dating that has apparently all but replaced it entirely) is independent of the starting amount of argon in the sample. It works and will give an accurate result no matter whether the sample started out with massive amounts of atmospheric argon or with none at all. No assumptions are made about the amount of argon the rock started out with -- the technique itself is capable of determining that and also of determining whether the rock has been disturbed (by weathering, for example) in such a way as to lose argon during its history.

Renata