Confusion About Radiometric Dating

[earthist]

Here's a confusion that's been bugging me for a while now:

 

I get the gist of radiomentric dating: namely that unstable isotopes decay into stable ones over a period of time, and thus, by comparing the ratio of unstable to stable, it's possible to calculate how long the decay has been in progress.

 

That raises 2 issues for me:

 

1) Using the uranium to lead scenario as an example, are we saying that any and all lead found is necessarily from uranium decay? In other words, does lead never occur directly in nature without having been derived from unranium? (and similar question for other such scenarios)?

 

2) If we say, based on radiometric dating that a certain rock is X years old, what do we mean by that? For instance, it's my understanding that heavy elements -- uranium, for instance -- are created in nova events. Given that, then any decay presumably begins immediately, and the rock we say is X years old, would actually reflect a date that greatly precedes it's formation. The date we are reading would be the date of the nova event, wouldn't it?

 

I assume there are aspects of radiometric dating I don't understand. I have no thought that radiometric dating is wrong. Still, it would be nice to know how it really works.

 

Thanks in advance for your help.

[Moontanman]

Here's a confusion that's been bugging me for a while now:

 

I get the gist of radiomentric dating: namely that unstable isotopes decay into stable ones over a period of time, and thus, by comparing the ratio of unstable to stable, it's possible to calculate how long the decay has been in progress.

 

That raises 2 issues for me:

 

Ok, I'll try to tackle this one but I am sure there are others here more qualified

 

 

1) Using the uranium to lead scenario as an example, are we saying that any and all lead found is necessarily from uranium decay? In other words, does lead never occur directly in nature without having been derived from unranium? (and similar question for other such scenarios)?

 

Lead is always the result of some form of radioactive decay, in nature when rocks vaporise or melt some elements are separated from others by chemical processes or gravitational sifting. All elements heavier than iron are the result of radioactive decay of some sort at some point.

 

2) If we say, based on radiometric dating that a certain rock is X years old, what do we mean by that? For instance, it's my understanding that heavy elements -- uranium, for instance -- are created in nova events. Given that, then any decay presumably begins immediately, and the rock we say is X years old, would actually reflect a date that greatly precedes it's formation. The date we are reading would be the date of the nova event, wouldn't it?

 

What it means is that the rock solidified at some point and the age of the rock is when it solidified. yes a Nova event of the right kind would result in tiny amounts of uranium and it would immediately start to decay into daughter products it is only after the uranium has solidified can we begin to date it. the oldest rocks on the earth solidified around 4 billion years ago, some calculation suggest it took about a half billion years for the Earth to cool to the state where we could begin to start the radioactive decay clock. The time the uranium spent in interstellar space cannot be measured in that manner. We do know that some meteroites date to close to 4.5 billion years or so as well and the earth was made up of these type objects but melting always resets the clock.

 

I assume there are aspects of radiometric dating I don't understand. I have no thought that radiometric dating is wrong. Still, it would be nice to know how it really works.

 

Thanks in advance for your help.

 

 

No problem I hope i helped.

[earthist]

Thanks, Moontanman, I appreciate it. Here's the thought process this invoked in me (in case it invokes something in someone else, too):

 

I'm still wrestling a bit, with lead in particular. As a kid, I used to have lead soldiers; not to mention a pretty hefty chunk of lead. Pipes were also made of lead for the most part. Point being that lead was ubiquitous, and cheap. I have no idea what percentage of uranium was contained in the ore from which these things were derived, but I'm thinking it was pretty small. Anyway, I'm thinking that any ore containing a high percentage of lead must necessarily be some pretty old stuff, right?

 

I'm also thinking that lead seems fairly abundant on Earth as opposed to uranium. If it all derives from decayed uranium, then there must have been a lot of uranium on Earth at some point in the distant past. Furthermore, if the U-238 to Pb-206 decay has a half-life of 4.5 billion years, then half the total amount of U-238 that was ever on Earth must still be around in undecayed state.

 

All-in-all, what I'm trying to say is that, strictly from an intuitive standpoint, it seems like there's an awful lot of lead lying around. I suppose doing the math would set me straight, but intuitively, it just seems mind-blowing to imagine so much uranium decaying away over the eons, and to wonder what effects that may have had on the evolution of life.

 

So, OK, math is not my strong suit, but assuming I did my decimals right, it turns out that after 4.5 billion years, >98% of the U-235 on the planet would have decayed to lead, so I suppose it really does make sense. When all else fails, do the math. (doh).

 

I'm still thinking Earth must have been a pretty hot place (radioactively-speaking) at the beginning. It's a real eye-opener, so, thanks again!

[Moontanman]

I don't think it's really proper to say that all the lead ore on the planet was at one time uranium ore and that it decayed into lead ore. Geological and chemical processes separate out lead from uranium on an ongoing basis, they are chemically different enough to be separated quite easily. Much if not most of the uranium is thought to have been dragged toward the earths core by geochemical and gravitational sifting while lead is kept near the surface by chemical processes that over ride gravity. Only in rocks that have remained unmelted can be dated or they are dated at the time they solidified. The crust of the earth is consistently turning over and being melted as it subducted into the mantle. Very few rocks are left that have not been melted at least once and most have been melted many times. BTW, there is a theory that there is a ball of uranium and thorium at the Earth's center, maybe 5 to 10 miles across and it is a seething liquid metal nuclear reactor kept from exploding by the gravity of the earth and the over burden of giga tons of iron nickle and rock. The Earth is still quite radioactive and the deeper you go the more radioactive and physically hotter it becomes. Radioactive elements are a significant part of the Earth and there is much more than just uranium. Radioactive heat drives the earths processes like plate tectonics and volcanism.

[earthist]

OK, I think I get what you're saying, but check me out to be sure:

 

All lead derives from decayed uranium. Later, the derived lead often gets separated from the remaining, undecayed uranium such that it is not necessarily radioactive any more.

 

What I was calculating earlier was meant to give me a feeling for how much planetary lead there is (at or near the surface) and to relate that to how much uranium there is (again, at or near the surface). I did not mean to suggest that all lead ore would necessarily contain uranium.

 

Your comment about radioactivity at the center of the planet is, of course, interesting. I have heard it before, and don't doubt it's true, but we (certainly I) have too little actual, measurable, data to discuss it much. However, the idea that the surface has "cooled off" so much over 4.5 billion years, leads me to assume that the core has also cooled significantly (not quite as much as the surface); likely a lot more than had ever occurred to me in the past.

 

I have seen the models of planetary history that say Earth started out essentially molten-hot. It supposedly took a couple billion years to cool down enough to have liquid water, and eventually life. Calculating (in very simplisitc, limited terms) that one isotope is now at a mere 2% of it's original quantity gave me a sense of reality as to just how much things might have cooled. When I said it's a real eye-opener, what I meant was that hit me like a slap in the face just how real the model would be, and how it actually worked. It takes things from a mere "concept" and makes them "solid" in my mind.

 

 

It also raises questions as to how much plate tectonic movement might have slowed, and how much longer it might continue before slowing enough not to matter any more. Then it evokes thoughts and questions as to how that process will affect (has affected) volcanism, earthquakes, weather, etc.; and life itself. In all the "how the Earth could end" scenarios that seem so popular on the History Channel lately, radioactive death is never presented. Now that I think of it, such a presentation would necessarily depict radioactivity as a positive force. That might not be a bad idea. Hmmm......

 

 

On the other hand, I'm confident we're talking at least thousands of years, if not millions, into the future, so I don't plan to give it much attention at the moment.

[Turtle]

OK, I think I get what you're saying, but check me out to be sure:

 

...Your comment about radioactivity at the center of the planet is, of course, interesting. I have heard it before, and don't doubt it's true, but we (certainly I) have too little actual, measurable, data to discuss it much. However, the idea that the surface has "cooled off" so much over 4.5 billion years, leads me to assume that the core has also cooled significantly (not quite as much as the surface); likely a lot more than had ever occurred to me in the past.

 

I have seen the models of planetary history that say Earth started out essentially molten-hot. It supposedly took a couple billion years to cool down enough to have liquid water, and eventually life. Calculating (in very simplisitc, limited terms) that one isotope is now at a mere 2% of it's original quantity gave me a sense of reality as to just how much things might have cooled. When I said it's a real eye-opener, what I meant was that hit me like a slap in the face just how real the model would be, and how it actually worked. It takes things from a mere "concept" and makes them "solid" in my mind.

 

 

It also raises questions as to how much plate tectonic movement might have slowed, and how much longer it might continue before slowing enough not to matter any more. Then it evokes thoughts and questions as to how that process will affect (has affected) volcanism, earthquakes, weather, etc.; and life itself. In all the "how the Earth could end" scenarios that seem so popular on the History Channel lately, radioactive death is never presented. Now that I think of it, such a presentation would necessarily depict radioactivity as a positive force. That might not be a bad idea. Hmmm......

 

 

On the other hand, I'm confident we're talking at least thousands of years, if not millions, into the future, so I don't plan to give it much attention at the moment.

 

you might find this helpful. :)

full article: >> Probing Question: What heats the earth's core? @ physorg.com

...For all this, however, Marone says, the vast majority of the heat in Earth's interior—up to 90 percent—is fueled by the decaying of radioactive isotopes like Potassium 40, Uranium 238, 235, and Thorium 232 contained within the mantle. These isotopes radiate heat as they shed excess energy and move toward stability. "The amount of heat caused by this radiation is almost the same as the total heat measured emanating from the Earth."

 

Radioactivity is present not only in the mantle, but in the rocks of Earth's crust. For example, Marone explains, a 1-kilogram block of granite on the surface emanates a tiny but measurable amount of heat (about as much as a .000000001 watt light bulb) through radioactive decay.

 

That may not seem like much. But considering the vastness of the mantle, it adds up, Marone says.

 

Sometime billions of years in the future, he predicts, the core and mantle could cool and solidify enough to meet the crust. If that happens, Earth will become a cold, dead planet like the moon.

 

Long before such an occurrence, however, the Sun will likely have evolved into a red-giant star, and grown large enough to engulf our fair planet. At that point, whatever heat is left in the mantle will hardly matter.

[Moontanman]

Another kind of odd way to look at is that a cubic meter of the Earths crust contains enough thorium to power your personal energy needs for the lifetime of an average human, 75 years or so I think it is...

 

See this thread for an explanation of how that works.

 

http://scienceforums.com/topic/23723-liquid-fluoride-thorium-reactor/

[earthist]

Thanks, Turtle and Moontanman! I love learning things! :)

 

Moon, I have bookmarked your vid re: LFTR reactor, and will watch it in the next day or two.