How do we date sedimentary rocks?

[juvenissun]

Please quote the part where they discuss dating sedimentary rock. I know they talk about drilling through oceanic sedimentary rock, but they didn't date it as far as I can tell. You should quote the relevant sentence.

Such an average age would not really be useful. If you're looking at a rock that was deposited 75Ma but contains grains that are from the Precambrian (say 1Ga) and others that are from 250Ma, your average would be 625Ma. That average is not a useful number.
Please explain in detail how such an average is useful.

It will be useful if you know the source of sediments. Sometimes, critically useful. That is the art of geological research.

 

[Atheos canadensis]

All right. That article did not say it. My mistake.

This one surely does.

This article (for which I would appreciate a title, author and publication information if you have those) does indeed discuss the radiometric dating of shales. And though it does cite Compston and Pidgeon (1962) who considered the method to be successful in one instance, as far as I can tell the author deems it unfeasible:

"Therefore, Rb[FONT=&quot])[/FONT]Sr dating of shales cannot be considered a reliable technique for dating sedimentary deposition."

The Compston and Pidgeon paper is from 1962; a more recent perspective on the technique seems to indicate that it can be reliable in a fairly narrow range of circumstances although there are still questions about its use on sedimentary rocks.

 

[Mr Strawberry]

This article (for which I would appreciate a title, author and publication information if you have those) does indeed discuss the radiometric dating of shales. And though it does cite Compston and Pidgeon (1962) who considered the method to be successful in one instance, as far as I can tell the author deems it unfeasible:

"Therefore, Rb[FONT=&quot])[/FONT]Sr dating of shales cannot be considered a reliable technique for dating sedimentary deposition."

The Compston and Pidgeon paper is from 1962; a more recent perspective on the technique seems to indicate that it can be reliable in a fairly narrow range of circumstances although there are still questions about its use on sedimentary rocks.

None of which juve has the faintest idea about, he's just randomly googling trying to find something to back up his incorrect statement.

 

[Atheos canadensis]

None of which juve has the faintest idea about, he's just randomly googling trying to find something to back up his incorrect statement.

I'm giving him the benefit of the doubt, though I admit it's odd that he's posted three papers now (counting from the old thread) that don't say what he thinks they do.

 

[juvenissun]

This article (for which I would appreciate a title, author and publication information if you have those) does indeed discuss the radiometric dating of shales. And though it does cite Compston and Pidgeon (1962) who considered the method to be successful in one instance, as far as I can tell the author deems it unfeasible:

"Therefore, Rb[FONT=&quot])[/FONT]Sr dating of shales cannot be considered a reliable technique for dating sedimentary deposition."

The Compston and Pidgeon paper is from 1962; a more recent perspective on the technique seems to indicate that it can be reliable in a fairly narrow range of circumstances although there are still questions about its use on sedimentary rocks.

You might get a precise "age" from dating a tree bark. But you need to have a good reason to do all that. That is my message from the very beginning.

 

[juvenissun]

I'm giving him the benefit of the doubt, though I admit it's odd that he's posted three papers now (counting from the old thread) that don't say what he thinks they do.

What they tried to do is not my business.
You can pick up a rock in your backyard and date it 100 times. I don't care a bit on what you are trying to do.

 

[PsychoSarah]

You might get a precise "age" from dating a tree bark. But you need to have a good reason to do all that. That is my message from the very beginning.

We aren't exactly short of reasons.

 

[juvenissun]

We aren't exactly short of reasons.

That is good. May be the young U-Pb date of a tree could be used to argue for YEC.

 

[Loudmouth]

You might get a precise "age" from dating a tree bark. But you need to have a good reason to do all that. That is my message from the very beginning.

The reason is pretty simple. When igneous rock is eroded, deposited, and then cemented into a sedimentary rock, it's radiometric clock is not reset. In the vast majority of cases, the crystals used for radiometric dating remain a closed system. In order to date an event, you need a mechanism that resets the radiometric clocks at that event. No such event occurs during the production of sedimentary rock.

 

[Loudmouth]

That is good. May be the young U-Pb date of a tree could be used to argue for YEC.

How would you use U/Pb to date a tree?

 

[juvenissun]

How would you use U/Pb to date a tree?

Find a tree which contains some U-238.

 

[PsychoSarah]

Find a tree which contains some U-238.

Good luck finding one.

 

[Loudmouth]

Find a tree which contains some U-238.

And? Keep going.

When a tree forms, does it also have Pb?

How do you relate ratios of U/Pb to age?

 

[juvenissun]

The reason is pretty simple. When igneous rock is eroded, deposited, and then cemented into a sedimentary rock, it's radiometric clock is not reset. In the vast majority of cases, the crystals used for radiometric dating remain a closed system. In order to date an event, you need a mechanism that resets the radiometric clocks at that event. No such event occurs during the production of sedimentary rock.

Un-zeroed mineral still give an "age". Why should it be reset?

(I am reasoning in circles and probably could never get out of it. Sigh.)

 

[Loudmouth]

Un-zeroed mineral still give an "age".

An age of what?

Why should it be reset?

That's my entire point. If the U/Pb clock does not reset when the sediment forms, then you can't use U/Pb dating to give an age for the sedimentary event.

 

[juvenissun]

And? Keep going.

When a tree forms, does it also have Pb?

How do you relate ratios of U/Pb to age?

You calculate it. (Is that your favorite part?)

 

[PsychoSarah]

Un-zeroed mineral still give an "age". Why should it be reset?

(I am reasoning in circles and probably could never get out of it. Sigh.)

Because the age it gives isn't set at 0, you can't tell when it was "set", thus you don't no how much time it existed before it became sedimentary rock, thus you can't estimate when that rock formation developed, thus its age is useless for determining the age of a rock formation.

 

[Loudmouth]

You calculate it. (Is that your favorite part?)

How?

How do you calculate the age of a tree from the measured U/Pb ratio?

 

[juvenissun]

Because the age it gives isn't set at 0, you can't tell when it was "set", thus you don't no how much time it existed before it became sedimentary rock, thus you can't estimate when that rock formation developed, thus its age is useless for determining the age of a rock formation.

OK, the age is useless. But, it is still an age. Somebody will find an use of it. Because it is measured in an expensive laboratory.

 

[juvenissun]

How?

How do you calculate the age of a tree from the measured U/Pb ratio?

I forgot. I am sure you know how. Or, you can push a button on the computer screen. Nowadays, one does not even need to do that.