Deep Time

[AV1611VET]

Then post something having to do with a geologic dating method and ask questions about it.

Sure.

All in situ [tree] fossils can be polystrate fossils, but not all polystrate fossils are in situ fossils.

What's your opinion?

Note: The reason I said "can be" is because I would assume for an in situ tree fossil to be polystrate, it would have to permeate a different layer of strata.

If it doesn't, I assume it is in situ, but not polystrate.

Am I wrong?

 

[RickG]

Sure.

All in situ [tree] fossils can be polystrate fossils, but not all polystrate fossils are in situ fossils.

What's your opinion?

Note: The reason I said "can be" is because I would assume for an in situ tree fossil to be polystrate, it would have to permeate a different layer of strata.

If it doesn't, I assume it is in situ, but not polystrate.

Am I wrong?

My opinion is that you are having difficulty in discussing the topic of this thread. Let me elaborate a bit. Geochronology is the science of dating and determining the time sequence of events in the history of the Earth. In doing this there are numerous methods that are utilized with specific applications and limitations unique to each situation. What you just posted has absolutely nothing to do with this thread. There are several threads in the Science forums of the CF which deal with your question concerning "in situ/polystrate" fossils, this is not one of them. Perhaps you may address your question from a different perspective. For example, how do we date fossils and what are some of the methods for dating them. Blessings.

 

[AV1611VET]

For example, how do we date fossils and what are some of the methods for dating them.

Okay.

How does radiometric dating account for processes that occur within magma chambers that can change the parent/daughter ratio of [igneous?] rock formed from lava?

Wouldn't below-the-surface (plutonic) processes mess with above-the-surface (volcanic) processes?

 

[RickG]

Okay.

How does radiometric dating account for processes that occur within magma chambers that can change the parent/daughter ratio of [igneous?] rock formed from lava?

Radiometric dating can only be performed after magma or lava solidifies into solid rock.

Wouldn't below-the-surface (plutonic) processes mess with above-the-surface (volcanic) processes?

No, not in the least. Can you be more specific?

 

[AV1611VET]

Radiometric dating can only be performed after magma or lava solidifies into solid rock.

And what if parent/daughter elements are present in the rock, but got there separately -- that is, they really aren't parent/daughter, but two separate elements?

If one atom of uranium is found with one atom of lead, wouldn't scientists say that molecule (or rock) has been there for 704,000,000 years?

 

[RickG]

And what if parent/daughter elements are present in the rock, but got there separately -- that is, they really aren't parent/daughter, but two separate elements?

The fact that we know that that can happen with some specific isotopes demonstrates that we not only can detect it if it happens, but it can also be quantified so the excess daughter can be excluded.

If one atom of uranium is found with one atom of lead, wouldn't scientists say that molecule (or rock) has been there for 704,000,000 years?

No they would not. The decay of uranium isotopes into lead isotopes is a decay series, not a straight decay.

Thus many ratios between the isotopes. Excess daughters are daughters are detectable and measurable.

 

[AV1611VET]

Excess daughters are daughters are detectable and measurable.

And that's my point.

They aren't daughters at all.

They are separate elements.

Not one element that changed into another over time.

Using your chart, suppose there was a bunch of Uranium on the surface, and some Thorium came along and got trapped in it.

Wouldn't that cause the element to return a false age?

 

[RickG]

And that's my point.

They aren't daughters at all.

They are separate elements.

Not one element that changed into another over time.

Using your chart, suppose there was a bunch of Uranium on the surface, and some Thorium came along and got trapped in it.

Wouldn't that cause the element to return a false age?

However, your point is a misunderstanding. The ratios between those daughters will show any excess daughter and where if there is any. This can also be confirmed using other isotope methods other than the uranium or thorium series as well.

 

[Astrophile]

Hello all.

How did science arrive at a date for the formation of the earth?

Let's begin at the beginning. The first radiometric dating method to be used was the uranium-lead method. Various minerals contain uranium-238 (U-238), which decays into lead-206 (Pb-206) with a half-life of 4.468 billion years; they also contain U-235, which decays into Pb-207 with a half-life of 703.8 million years.

Every atom of U-238 that decays yields exactly one atom of Pb-206; every atom of U-235 that decays yields exactly one atom of Pb-207. Thus the sum of the number of atoms of U-238 and Pb-206 is the original number of U-238 atoms when the rock was first formed. (The same goes for U-235 and Pb-207.) Knowing both the present number and the original number of U-238 atoms yields the time when decay started, i.e. the time when the U-containing mineral crystallised from the magma.

Now comes an interesting bit. You must know from your geochemistry and mineralogy lessons that different elements have different chemical characteristics and are associated with different types of minerals. Uranium is chemically lithophile and occurs in silicate minerals. Lead is chalcophile and occurs in sulphide minerals. For example, as everybody knows, the main ore of lead is galena, or lead sulphide (PbS). Thus most of the lead in silicate minerals comes from the radioactive decay of uranium. Zircon (zirconium silicate) is particularly useful in this respect, since it practically excludes lead from its crystal structure. Notice also that lead ores contain essentially no uranium, so that the isotopic composition of the lead has not changed since the ore was deposited.

Moreover, the isotope lead-204 (Pb-204) is not produced by radioactive decay; all the Pb-204 in a mineral is primordial, it was in the mineral from the beginning. Knowing the amount of Pb-204 in a mineral and the ratios of Pb-206 and Pb-207 to Pb-204 makes it possible to subtract the primordial Pb, so that one is left with the Pb-206 and Pb-207 that is produced by decay.

Even the first U-Pb measurements of terrestrial rocks showed that the Earth was much older than Lord Kelvin had thought. By the early 1930s, the oldest known rocks had yielded ages of 1300 million years. However, these measurements could give only a minimum age for the Earth itself.

Now we come to the really interesting part, the determination of the age of the Earth as a planet. Because U-238 and U-235 have different half-lives, they produce Pb-206 and Pb-207 at different rates. Therefore the Pb-207/Pb-206 ratio in terrestrial lead ores has changed during the Earth's history. Let me make this quite clear. A Mesozoic lead ore will still have the lead isotopic composition that it had when it was deposited, since it contains no uranium, but its isotopic composition will be different from an early Proterozoic lead ore, since the terrestrial lead reservoir from which it was formed has had time to accumulate more Pb-206 and Pb-207. This difference would not exist if the Earth was young and lead ores were therefore all the same age.

Knowing the present Pb-207/Pb-206 ratio, and knowing the production rates of Pb-207 and Pb-206, we can calculate the time when the two lead isotopes began to be formed by the radioactive decay of uranium if we know the original Pb-207/Pb-206 ratio when the solar system was formed. This was what Clair Patterson did in the 1950s. He very carefully measured lead isotopes in the Canyon Diablo iron meteorite; iron meteorites do not contain uranium, so the lead isotopic composition in the meteorite has remained the same since it was formed. Knowing the original Pb-207/Pb-206 ratio and the present ratio, Patterson was able to calculate an age of 4550 million years for the Earth and meteorites.

This has been a long explanation, but I hope that you have been able to follow it. I have done my best to explain clearly and in detail how radiometric dating works, to show how possible errors can be avoided, and to distinguish between radiometric dating of rocks and measurement of the age of the Earth as a whole. However, if you are interested and want to learn more, you ought to read books on the subject; they are much better than the inevitably short and superficial explanations that you will find on Internet forums.

 

[Astrophile]

I'd start by asking myself if your dating is really accurate?

Would you at least accept that the fossil bone, or fossil ammonite, was older than the lava flow?

 

[klutedavid]

Remnants from a prior supernovae according to astrophysicists, which formed an interstellar molecular cloud and collapsed under gravitational forces.



The oldest earth rocks, moon rocks, and meteorites are of very similar age. As for leaching of a specific radionuclide, there are specific methods that can verify whether any leaching has occurred.

Hello Rick.

The oldest earth rocks, moon rocks, and meteorites are of very similar age.

The age of these rocks is based on theoretical estimates. Science cannot prove that the age
of these rocks, is in the order of billions of years.

This is an assumed, theoretical estimation of age, the age of these rocks is unknown.

Radiometric clocks are set when each rock forms.

If an igneous or other rock is metamorphosed, its radiometric clock is reset.

If a heat source diffuses any parent isotope, and daughter isotopes, then the age of the rock is altered.

How would anyone know Rick, whether any specific sample has been heated at some stage in the past,
and on multiple occassions. We do not need a firm reset of the clock but a partial, heat induced influence
on the clock?

 

[RickG]

Hello Rick.

Hello klutedavid, I'm glad to have your participation in this thread.

The age of these rocks is based on theoretical estimates.

No they are not theoretical estimates, they are actual measurements of ratios of specific isotopes present in the sample.

Science cannot prove that the age of these rocks, is in the order of billions of years.

Well, just to be technical, science does not deal with proofs. Proofs are specific to mathematics. What science does with respect to the age of rocks (meteorites in this case) is to quantify as accurately as possible their age which does contain a statistical margin of error, which is generally less than 2%.

This is an assumed, theoretical estimation of age, the age of these rocks is unknown.

Direct measurements of isotope ratios are not theoretical assumptions. They are direct measurements. Do you understand what I am saying and the difference I am trying to explain between the two.

Radiometric clocks are set when each rock forms.

Spot on.

If an igneous or other rock is metamorphosed, its radiometric clock is reset.

That is correct, and in such a case only the age of the last metamorphic event can be dated.

If a heat source diffuses any parent isotope, and daughter isotopes, then the age of the rock is altered.

Yes, as you previously and correctly stated, the clock is reset to the newly formed parent isotopes. BTW, in the metamorphic process all previous isotopes are reset.

How would anyone know Rick, whether any specific sample has been heated at some stage in the past,
and on multiple occassions. We do not need a firm reset of the clock but a partial, heat induced influence
on the clock?

That is a very good question. There is a specific discipline within the field of geology called Petrology, which is the study of the formation and processes of formation of rocks. It is also subdivided into the area of sedimentary, igneous and metamorphic rocks. Also covering those fields of study is the field of geochemistry. Through this knowledge the degree of metamorphism can be recognized and determined. Also, the type of rock itself is also important as metamorphism is very problematic with some rocks and not so problematic with others depending upon the isotopes being tested. The U-Pb concordia-discordia method is one that works quite well in metamorphic rocks as opposed to the K-Ar method where Ar can be lost. It is very important to understand though, that that does not invalidate the K-Ar method as all dating methods have their limitations. Outside of metamorphic rocks, the K-Ar method works quite well.

 

[Astrophile]

Are these meteorites remnants from a former planet, within or external to our solar system?

Some meteorites come from Mars, others come from the Moon. There have also been speculative suggestions that enstatite chondrites and enstatite achondrites come from Mercury. No meteorites are known to have come from outside the solar system, although some meteorites contain pre-solar dust grains.

Are they remnants of an asteroid, if so, was this internal or external to our solar system?

Most meteorites come from asteroids; in particular, most achondrite meteorites come from the asteroid Vesta. Other asteroids have been tentatively identified as the parent bodies of different classes of chondritic meteorites. As before, no meteorites are known to have come from outside the solar system.

For each meteorite, the former state of it's environment, its geology needs to be described in detail. The problem with any single meteorite is knowing the initial ratio of the isotopes.

As I have explained before, Patterson measured the initial lead isotopic ratios for solar system material during the 1950s. For Rb-Sr dating, scientists use isochron methods to determine both the initial Rb-87/Sr-87 ratio and the initial Sr-87/Sr-86 ratios.

We would need to know whether the meteorite in its original environment, was subject to water for example, i.e., in the case of U238, has radioactive leaching occurred?

Most meteorites (specifically, ordinary and enstatite chondrites, achondrites, stony-irons and irons) do not contain any water and do not show any evidence of aqueous alteration.

 

[The Cadet]

If an igneous or other rock is metamorphosed, its radiometric clock is reset.

Okay, but wouldn't this lead to, at best, a measured age which is younger than the actual age? This doesn't help the case of those looking for a young earth.

 

[klutedavid]

Hello Rick.

No they are not theoretical estimates, they are actual measurements of ratios of specific isotopes
present in the sample.

The decay of a parent isotope into a daughter isotope has to do with the calculated half life. Where the
half life is beyond that which is verfiable by observation, i.e., a half life of say millions of years. Then
we are only ever assuming that this parent to daughter half life, will decay according to the estimated
half life.

What influence will solar flares have on decay rates, especially over vast periods of time?

What influence will neutrinos from any other source, have on any assumed, constant half life, of any
parent or daughter isotope?

Have all sources that could influnce the half lives of isotopes been identified?

Unless these questions can be absolutely answered, then all half lives are theoretical estimates.

 

[RickG]

Hello Rick.

Greetings

The decay of a parent isotope into a daughter isotope has to do with the calculated half life. Where the
half life is beyond that which is verfiable by observation, i.e., a half life of say millions of years. Then
we are only ever assuming that this parent to daughter half life, will decay according to the estimated
half life.

Yes, I understand that concern, however there are ways to verify the validity of those half-lifes (decay rates). First off, it is straight forward physics and chemistry. Elements, and their isotopes are dependent upon their physical structures and behave accordingly. Had their physics changed at any point in time we would see these changes as we go back in time. The fact that there is no chemistry change seen in any rocks no matter how old they date to. Second, Using multiple dating methods with different isotopes yield the same results. If there had been any changes in decay rates of any radionuclides they would not check out. Third, when supernovae occur they emit gamma rays of specific isotopes. Some of these observations are from supernovae millions of light years distant, thus what is being observed and measured is what occurred millions of years ago. Numerous isotope decay rates have been measured and found to be in agreement with what is measured today.

What influence will solar flares have on decay rates, especially over vast periods of time?

That influence would affect only those radionuclides of cosmogenic origin in Earth's atmosphere. This does occur and is very minor.

What influence will neutrinos from any other source, have on any assumed, constant half life, of any
parent or daughter isotope?

They are part of the decay process.

Have all sources that could influnce the half lives of isotopes been identified?

I am unaware of any influences that have not been observed and identified.

Unless these questions can be absolutely answered, then all half lives are theoretical estimates.[/QUOTE]

 

[lasthero]

Hi, Rick.

If it's not too far away from the topic, could you talk a bit about correlations in respect to radiometric dating and the island chain of Hawaii. I've always had a little trouble wrapping my head around it.

 

[46AND2]

Okay, but wouldn't this lead to, at best, a measured age which is younger than the actual age? This doesn't help the case of those looking for a young earth.

Well, an igneous rock is only as old as the last time it's clock was reset. In other words, the last time it was molten lava, then cooled to closing temperature of isotope in question. So, yes. It only tells us that the earth is AT LEAST such and such an age.

A major benefit to measuring lunar rocks and meteorites is that they are not subjected to the kind of geological forces as terrestrial rocks, and therefore avoid having their clock reset.

However, some meteorites have been found to have had some metamorphic events, likely due to collisions with other space debris.

The difference in measurement between the lunar rocks and the oldest terrestrial samples is less than 500,000 years.

 

[46AND2]

Hi, Rick.

If it's not too far away from the topic, could you talk a bit about correlations in respect to radiometric dating and the island chain of Hawaii. I've always had a little trouble wrapping my head around it.

The Hawaiian Islands are on a tectonic plate which is moving to the northwest. The islands are created as the plate moves across a "hotspot" where volcanic eruptions are forming them.

When the lava cools, that is the "birth" of the rock. So, any lava that is cool from Kilauea, presently, should come up with a "zero" age, or more accurately, it is not testable, since there is not enough radioactive byproduct accumulated to measure.

What we would expect to find as we measure samples on the different islands, is that there should be sequentially higher ratios of daughter isotope the farther we get from Kilauea (since the Big Island is currently located over the hotspot), since it has been longer since that island was over the hotspot.

This is exactly what we see. In addition to that, the measurements through radiometric dating are consistent with the observed speed at which the plate is moving over the hotspot. In other words, the radiometric date matches with the date we would expect through dividing the distance of Oahu from the hotspot, by the rate at which the plate is moving.

The chain continues on past the Hawaiian Islands on to the Emperor's Seamounts, and if I remember correctly, these have also been tested and also corroborate the pattern.

 

[RickG]

Hi, Rick.

If it's not too far away from the topic, could you talk a bit about correlations in respect to radiometric dating and the island chain of Hawaii. I've always had a little trouble wrapping my head around it.

Hi lasthero, I think 46AND2 covered it quite well. Is there a specific aspect you would like discussed in more detail?