Understanding through Discussion


Welcome! You are not logged in. [ Login ]
EvC Forum active members: 108 (8738 total)
Current session began: 
Page Loaded: 04-28-2017 4:21 AM
415 online now:
kbertsche, PaulK (2 members, 413 visitors)
Chatting now:  Chat room empty
Newest Member: Jayhawker Soule
Post Volume:
Total: 805,589 Year: 10,195/21,208 Month: 3,282/2,674 Week: 698/961 Day: 10/150 Hour: 0/3

Announcements: Reporting debate problems OR discussing moderation actions/inactions


Thread  Details

Email This Thread
Newer Topic | Older Topic
  
RewPrev1
...
1314
15
1617
...
20Next
Author Topic:   Introduction To Geology
dwise1
Member
Posts: 2729
Joined: 05-02-2006
Member Rating: 3.5


Message 211 of 292 (683758)
12-13-2012 3:13 AM
Reply to: Message 209 by morningstar2008
12-12-2012 1:13 AM


Re: K-Ar Dating
Nu, tovarishch! Vy Ruski? Ja niet. Na, yest zdis problema.

You appear to be Russian or something close. There appears to be some attempts to display Cyrillics which is failing completely. This makes your posts even more unreadable.

For example, your "From:" field is unreadable. Then in this message there's this line:

As I in physics am not so strong, but nevertheless there is a wish to understand .

Hopefully, you can see that that last four-letter word after "understand" is gibberish. Even if it is not to you with your PC's settings, it is gibberish to us. By the way (BTW), "gibberish" means that it is complete incomprehensible nonsense.

These are things that you need to take care of before we can even begin to take you seriously. You should work with the forum administrators to resolve these problems.

Mods: I believe that I violated the "off topic" admonision for a very good reason that had nothing to do with the off-topic-niss issue. Lo que pasa!


This message is a reply to:
 Message 209 by morningstar2008, posted 12-12-2012 1:13 AM morningstar2008 has responded

Replies to this message:
 Message 214 by morningstar2008, posted 12-14-2012 6:47 AM dwise1 has not yet responded

    
JonF
Member
Posts: 3513
Joined: 06-23-2003
Member Rating: 3.1


Message 212 of 292 (683784)
12-13-2012 8:12 AM
Reply to: Message 210 by Dr Adequate
12-13-2012 2:27 AM


Re: Rb-Sr Dating
Er, under Confounding Factors you might want to discuss mixing. Mixing isochrons have been found and published, e.g. Age and origin of the rdal dike complex, SW Norway: False isochrons, incomplete mixing, and the origin of Caledonian granites in basement nappes

I assume you are familiar with the isochron page at talkorigins.org.

There is a test for two-factor mixing discussed there, which isn't 100% accurate. Of course we can be sure that few isochrons are mixing isochrons because, were many isochrons mixing isochrons, about half of them would have negative slope.

Some creationist came up with a three-factor mixing scenario which would be nearly impossible to detect. But it required one of the factors to have none of one of the isotopes, a very unlikely occurrence. I don't have a pointer to that one.

Edited by JonF, : No reason given.


This message is a reply to:
 Message 210 by Dr Adequate, posted 12-13-2012 2:27 AM Dr Adequate has responded

Replies to this message:
 Message 213 by Dr Adequate, posted 12-13-2012 10:19 AM JonF has not yet responded
 Message 217 by Dr Adequate, posted 12-14-2012 11:03 AM JonF has responded

  
Dr Adequate
Member
Posts: 15790
Joined: 07-20-2006
Member Rating: 3.1


Message 213 of 292 (683790)
12-13-2012 10:19 AM
Reply to: Message 212 by JonF
12-13-2012 8:12 AM


Re: Rb-Sr Dating
Thank you very much.

There is a test for two-factor mixing discussed there, which isn't 100% accurate. Of course we can be sure that few isochrons are mixing isochrons because, were many isochrons mixing isochrons, about half of them would have negative slope.

A good point which I shall use.


This message is a reply to:
 Message 212 by JonF, posted 12-13-2012 8:12 AM JonF has not yet responded

  
morningstar2008
Member (Idle past 1368 days)
Posts: 43
From:
Joined: 12-11-2012


Message 214 of 292 (683863)
12-14-2012 6:47 AM
Reply to: Message 211 by dwise1
12-13-2012 3:13 AM


Re: K-Ar Dating
dwise1

.
. . , . .
. . .
.


This message is a reply to:
 Message 211 by dwise1, posted 12-13-2012 3:13 AM dwise1 has not yet responded

Replies to this message:
 Message 215 by Admin, posted 12-14-2012 9:18 AM morningstar2008 has responded

    
Admin
Director
Posts: 12436
From: EvC Forum
Joined: 06-14-2002
Member Rating: 2.1


Message 215 of 292 (683878)
12-14-2012 9:18 AM
Reply to: Message 214 by morningstar2008
12-14-2012 6:47 AM


Moderator Request
Hi MorningStar,

Could you please post your messages in English? Thank you!


--Percy
EvC Forum Director

This message is a reply to:
 Message 214 by morningstar2008, posted 12-14-2012 6:47 AM morningstar2008 has responded

Replies to this message:
 Message 216 by morningstar2008, posted 12-14-2012 10:30 AM Admin has acknowledged this reply

    
morningstar2008
Member (Idle past 1368 days)
Posts: 43
From:
Joined: 12-11-2012


Message 216 of 292 (683886)
12-14-2012 10:30 AM
Reply to: Message 215 by Admin
12-14-2012 9:18 AM


Re: Moderator Request
Hi MorningStar,
Could you please post your messages in English? Thank you!
--------------------------------------
.
http://www.translate.ru/ .
. . .
. .
This message is a reply to:
 Message 215 by Admin, posted 12-14-2012 9:18 AM Admin has acknowledged this reply

Replies to this message:
 Message 218 by JonF, posted 12-14-2012 12:07 PM morningstar2008 has not yet responded

    
Dr Adequate
Member
Posts: 15790
Joined: 07-20-2006
Member Rating: 3.1


Message 217 of 292 (683889)
12-14-2012 11:03 AM
Reply to: Message 212 by JonF
12-13-2012 8:12 AM


Re: Rb-Sr Dating
I've added a bit on mixing, let me know what you think.

Thank you.


This message is a reply to:
 Message 212 by JonF, posted 12-13-2012 8:12 AM JonF has responded

Replies to this message:
 Message 219 by JonF, posted 12-14-2012 1:02 PM Dr Adequate has not yet responded

  
JonF
Member
Posts: 3513
Joined: 06-23-2003
Member Rating: 3.1


Message 218 of 292 (683896)
12-14-2012 12:07 PM
Reply to: Message 216 by morningstar2008
12-14-2012 10:30 AM


Re: Moderator Request
I wonder if you realize that most of us do not have the appropriate font on our computers and your messages do not appear to contain words in any language? C;lick the image for full size:


This message is a reply to:
 Message 216 by morningstar2008, posted 12-14-2012 10:30 AM morningstar2008 has not yet responded

  
JonF
Member
Posts: 3513
Joined: 06-23-2003
Member Rating: 3.1


Message 219 of 292 (683916)
12-14-2012 1:02 PM
Reply to: Message 217 by Dr Adequate
12-14-2012 11:03 AM


Re: Rb-Sr Dating
Looks good to me...
This message is a reply to:
 Message 217 by Dr Adequate, posted 12-14-2012 11:03 AM Dr Adequate has not yet responded

  
Dr Adequate
Member
Posts: 15790
Joined: 07-20-2006
Member Rating: 3.1


(1)
Message 220 of 292 (683999)
12-14-2012 9:00 PM


Other Isochron Methods
Other isochron methods

Introduction

In this article we shall point out some other dating methods which work the same way as the Rb-Sr method. The reader who has not read the article on the Rb-Sr method will find this present article almost completely incomprehensible, and should go back and read about Rb-Sr.

The isochron method generalized

We have introduced the isochron method in the context of rubidium and strontium. But is there anything particularly special about those two elements? Not really. For the isochron method to work, what we need are three isotopes with the following properties.

(1) An unstable isotope. This should have a fairly long half-life if it is to be of any use in dating rocks, but not too long, or it will hardly undergo any decay at all. A figure expressible in billions of years is ideal. In the Rb-Sr method, we used 87Rb.

(2) A stable daughter isotope of isotope (1). In the Rb-Sr method, we used 87Sr.

(3) An isotope which is the same element as isotope (2) and which is neither unstable nor radiogenic, so that in a closed system it remains constant in quantity. In the Rb-Sr method, we used 86Sr.

Given a sets of three such isotopes, we can apply exactly the same reasoning as we did for 87Rb, 87Sr and 86Sr, and it will be equally valid.

The isotopes

The table below shows some sets of three isotopes which can be treated like rubidium and strontium for the purposes of dating, together with the half-life of the parent and its decay mode. The numbers (1) (2) and (3) are as in the section above.

Method(1)half-lifedecay mode(2)(3)
Rb-Sr87Rb48109 yrbeta minus87Sr86Sr
Sm-Nd147Sm106109 yralpha143Nd144Nd
Lu-Hf176Lu36109 yrbeta minus176Hf177Hf
Re-Os187Re43109 yrbeta minus187Os186Os
La-Ba138La105109 yrelectron capture138Ba137Ba
La-Ce138La105109 yrbeta minus138Ce142Ce
K-Ca40K1.2109 yrbeta minus40Ca42Ca
U-Pb238U4.5109 yrdecay chain206Pb204Pb
U-Pb235U0.7109 yrdecay chain207Pb204Pb

Notes

* We said that isotope (3) should be stable. 186Os is not in fact stable, but as it has a half-life of two quadrillion years, it might as well be.

* In the Lu-Hf method, again, 144Nd is unstable, but has a half-life even longer than that of 186Os.

* Similarly in the La-Ce method, neither isotope of cerium used is strictly speaking stable, but their half-lives are so enormously long that for all practical purposes they may be treated as stable.

* In using the K-Ca method, we have to make a slight mathematical adjustment to take into account the fact, mentioned in our article on the K-Ar method, that 40K decays to 40Ar as well as to 40Ca.

* Similarly, 138La can decay two ways, to 138Ce or 138Ba. As you can see from the table, both are susceptible to the isochron method.

* We have noted the peculiarities of the half-life of 187Re in the article on radioactive decay. As we only have to consider how it behaves in rocks, and not in elaborate equipment in physics laboratories, we may take its half-life to be 43 billion years as given in the table.

* There are two entries for U-Pb because there are two parent istopes we can use, 238U and 235U. Each decays to a (different) final stable element of lead by a complex decay chain.

* In practice, the U-Pb decay chain is usually exploited by methods other than isochron dating, for reasons that will be explained in the next article.


  
Dr Adequate
Member
Posts: 15790
Joined: 07-20-2006
Member Rating: 3.1


(1)
Message 221 of 292 (684386)
12-17-2012 12:19 PM


U-Pb, Pb-Pb, And Fission Track Dating
U-Pb, Pb-Pb, and fission track dating

Introduction

In this article we shall discuss the basis of the U-Pb and Pb-Pb methods, and also fission track dating. The reader will find this article much easier to grasp if s/he has already mastered the material in the articles on K-Ar, Ar-Ar, and Rb-Sr dating.

The isotopes

There are a number of isotopes of interest in U-Pb dating.

238U (uranium-238) decays to 206Pb by a complex decay chain. It has a half-life of 4.5 billion years. 235U (uranium-235) decays to 207Pb by an equally complex decay chain, and has a half-life of 0.7 billion years.

It is also useful to know of the existence of 204Pb, which is neither unstable nor radiogenic.

Isochron dating and U-Pb

We can always try U-Pb dating using the isochron method, but this often doesn't work: the compositions of the minerals involved, when plotted on an isochron diagram, fail to lie on a straight line.

There seem to be two reasons for this. First of all, the straight-line property of the isochron diagram is destroyed when the isotopes involved get shuffled between minerals. Now lead and uranium are particularly susceptible to such shuffling in the event of even mild metamorphism. The other problem is that uranium is particularly susceptible to weathering. Now since all rocks are somewhat porous, and since we are pretty much obliged to date rocks from near the surface, it's hard to find instances in which uranium has not been lost.

Zircons

Zircon is the mineral ZrSiO4; as you can see from its chemical formula, it is one of the silicate minerals. Although it is not abundant in igneous rocks, it is sufficiently common to be used for the purposes of radiometric dating.

It has two properties which make it useful for this purpose.

First of all, uranium will readily substitute for the zirconium (Zr) in the mineral, whereas lead is strongly rejected. For this reason we expect zircons, when formed, to contain some uranium, but virtually no lead.

Second, zircons are durable and chemically inert, able to resist chemical weathering and even high-grade metamorphism up to about 900C.

Zircons, then, are relatively immune to the problems that make isochron U-Pb dating so difficult. But of course for isochron dating we need more than one mineral; zircons alone would not be enough.

However, these facts about zircons, combined with what we know about uranium, suggest an alternative method of dating.

The method

If there is no lead in the zircon originally, and if no lead or uranium has been added or subtracted to the zircon since its formation, then the following formula will hold:

t238 = h238 log2(1 + R238)

where t238 is the age of the zircon, h238 is the half-life of 238U and R238 is the ratio of 206Pb to 238U.

Not only that, but since we have two uranium isotopes to work with, we will also have

t235 = h235 log2(1 + R235)

where t235 is the age of the zircon, h235 is the half-life of 235U and R235 is the ratio of 207Pb to 235U.

Now because the zircon has only one age, it follows that if no lead or uranium has been added or subtracted from the zircon since its formation, we will have t238 = t235, in which case the two t values are said to be concordant; whereas if lead and/or uranium has been added or subtracted, then it would require some sort of statistical fluke for the two t values to end up identical. So analysis of both the 206Pb/238U ratio and the 207Pb/235U ratio acts as a check on the correctness of the date we come up with in the same way that step heating does in the Ar-Ar method and the plotting of several minerals on an isochron diagram does for the Rb-Sr and related methods: it allows us to find out if the isotope ratios have been affected by something other than the passage of time, and to reject any "dates" calculated from the isotope ratios if this turns out to be the case.

It is possible to refine this date still further. If we suspect that the zircon, despite its chemical properties, still managed to incorporate a little lead at or after its formation, then since all lead isotopes are chemically the same, we can measure the amount of 204Pb the zircon contains. Since we know the ratios in which the various lead isotopes are usually found, we can then apply the same sort of correction we used to account for atmospheric argon in the K-Ar method.

While zircon has been the most popular mineral for U-Pb dating, other minerals have been employed, including apatite, monazite, titanite, allanite and, most interesting of all, xenotime.

Xenotime

There is a difficulty in using radiometric dating to put an age on sedimentary rock. The problem is that sediment is made up of fragments of some parent rock, and when we date these fragments, we are in effect dating the parent rock rather than the the sediment as such. If, for example, we apply U-Pb dating to a grain of zircon found in sandstone, we aren't dating the formation of the sandstone, we're dating the formation of the granite that the zircon came from; all we could say about the sandstone is that it must be younger than that.

However, it is possible to put a date on some sedimentary rocks using the mineral xenotime (YPO4). Uranium can and often does substitute for the yttrium, whereas lead cannot, making xenotime suitable for radiometric dating.

The key fact about xenotime is that since it has the same crystal structure as zircon, it can grow on zircon crystals, forming a crust; and this process, of course, cannot begin to take place while the zircon crystal is still locked inside its parent rock. The zircon will only start acquiring its xenotime crust after weathering and erosion have freed it from its parent rock and it becomes sediment.

So by dating the xenotime crust, we can find out when the zircon it's growing on became sediment; dating the zircon itself would tell you the age of the parent rock.

Speleothems

A speleothem, more colloquially known as a cave formation, is formed when minerals dissolved in water precipitate out of the water as it drips, seeps, or flows into a cave. The reader will probably be familiar with stalagmites and stalagtites; more speleothems are shown in the labeled photograph below.

Now, compounds of uranium are often highly soluble in water (this, indeed, is one of the major problems with U-Pb isochron dating) whereas compounds of lead are stubbornly insoluble. As a result, we expect speleothems when they are first formed to contain some uranium but little or no lead --- just like zircons. So we can apply the same technique to speleothems as we do to zircons.

Pb-Pb dating

We can exploit our double system of 238U-206Pb and 235U-207Pb in another way. Suppose we separate the minerals in a rock, and plot a graph showing their 206Pb/204Pb ratios on one axis and their 207Pb/204Pb ratios on another (similar, though not identical, to what we did when constructing isochron diagrams). It can be shown mathematically that if the rock has been undisturbed, so that the isotope ratios reflect nothing but the passage of time, then just as with the isochron diagrams we've already discussed (though for a different reason) the minerals so plotted will lie on a straight line on the graph; and the age of the rock can be calculated from the slope of the line.

Unlike the ordinary isochron methods such as Rb-Sr, the Pb-Pb method does not allow us to deduce the original proportions of the various lead isotopes from the data acquired from the sample. Instead, we need to find this out some other way.

We can do this by finding minerals that contain lead but never contained any uranium, or only ever contained it in negligible quantities. Troilite (FeS) from iron-rich meteorites fits the bill: its present ratio of uranium to lead is so tiny that either the solar system and indeed the universe is many many times older than cosmologists think, or, given the long half-life (4.5 billion years) of 238U, there can hardly have been any uranium in the meteorites to start with, and so its decay can hardly have affected the lead isotope ratios of these meteorites.

You might perhaps doubt that meteorites would have the same initial lead isotope ratios as the Earth. Planetary scientists maintain that they should, for reasons which are somewhat beyond the scope of this textbook. Another reason for believing it is that if we calculate Pb-Pb dates on this basis, the dates we get are in agreement with dates produced by other methods where they can be applied: this would hardly be possible if we were using the wrong figures for the initial lead isotope ratios. So taking the figure derived from the troilite as an "anchor" for our calculations, we can then go ahead and apply the Pb-Pb method to rocks which do contain significant quantities of uranium.

Now, recalling that we began this article by explaining that the isochron method is no use for U-Pb, you may wonder why this Pb-Pb isochron should be any better. However, recall that one of the major problems with the U-Pb isochron is that uranium compounds are highly soluble and are easily removed from the rock by weathering. But when that occurs, the lead will still remain and can be used for Pb-Pb dating. What's more, even if some lead is also removed, then since all the lead isotopes are the same element, having the same merely chemical properties, there will be no tendency for one isotope to be lost in a greater proportion to the others, and so the isotope ratios will remain the same.

It is true, of course, that the removal of the uranium by weathering will slow down and even, if all the uranium is removed, completely stop the radiometric clock, so that we will not have an accurate measurement of the time after the weathering began, and Pb-Pb dating will therefore tell us that the rock is a little younger than it is. But only a little younger, because a typical chunk of igneous rock will only have spent a relatively short amount of time being exposed to chemical weathering compared to the period of time when it was not.

As with the isochron methods we've already met, the Pb-Pb isochron method carries its own built-in check on its correctness: if the rock has been seriously disturbed, so that the isotope ratios depend on something other than the passage of time, then when we plot the minerals on our graph, they will almost certainly not lie on a straight line, and we will not obtain a date.

Fission track dating

Finally, we should mention fission track dating. The decay chain by which uranium decays involves the emission of alpha particles, and as these particles travel through the rock they produce microscopic scars (fission tracks) in the minerals they pass through, which can be revealed by cutting and polishing the minerals and inspecting them through a microscope. A number of minerals are suitable for this process, including apatite, zircon, and titanite.

The number of fission tracks in the minerals will depend on the quantity of uranium and the amount of time it's had to do damage. So, conversely, if we count the fission tracks and we measure the amount of uranium, then we can figure out how much time it must have taken to produce the fission tracks.

One weakness of this method is that the fission tracks will heal over if the rock is heated to about 200C, so the fission track clock will be reset even by the mildest metamorphism.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


Replies to this message:
 Message 222 by JonF, posted 12-17-2012 2:00 PM Dr Adequate has not yet responded
 Message 223 by JonF, posted 12-17-2012 2:23 PM Dr Adequate has responded
 Message 225 by RAZD, posted 12-17-2012 3:41 PM Dr Adequate has responded

  
JonF
Member
Posts: 3513
Joined: 06-23-2003
Member Rating: 3.1


Message 222 of 292 (684398)
12-17-2012 2:00 PM
Reply to: Message 221 by Dr Adequate
12-17-2012 12:19 PM


Re: U-Pb, Pb-Pb, And Fission Track Dating
Picky-picky, but there are ways of correcting for "primordial lead" (AKA "common lead") without measuring 204Pb, e.g. Correction of common lead in UPb analyses that do not report 204Pb.

Are you going to cover discordia? (Whoopsie, I see you did, but didn't call it that). Although sample selection and preparation methods, along with analyzing incredibly tiny samples such as a spot 10 microns diameter and 1 micron deep from a zircon, have increased the number of concordant dates dramatically, there's still room for discordia. E.g. Evidence from Detrital Zircons for the Existence of Continental Crust and Oceans on the Earth 4.4 Gyr Ago which, as of the last time I looked, covers the oldest minerals found on Earth and has an incredibly clear discordia line:

Edited by JonF, : Add whoopsie

Edited by JonF, : No reason given.


This message is a reply to:
 Message 221 by Dr Adequate, posted 12-17-2012 12:19 PM Dr Adequate has not yet responded

  
JonF
Member
Posts: 3513
Joined: 06-23-2003
Member Rating: 3.1


Message 223 of 292 (684408)
12-17-2012 2:23 PM
Reply to: Message 221 by Dr Adequate
12-17-2012 12:19 PM


Re: U-Pb, Pb-Pb, And Fission Track Dating
Hum. On reflection it looks as if you didn't cover discordia, you did a Pb-Pb isochron section. I suppose you don't want to go into the issue of anchoring the Pb-Pb isochron, which is fundamentally different from other isochrons, and the role of the Canyon Diablo meteorite in that anchoring?
This message is a reply to:
 Message 221 by Dr Adequate, posted 12-17-2012 12:19 PM Dr Adequate has responded

Replies to this message:
 Message 224 by Dr Adequate, posted 12-17-2012 3:17 PM JonF has responded

  
Dr Adequate
Member
Posts: 15790
Joined: 07-20-2006
Member Rating: 3.1


Message 224 of 292 (684434)
12-17-2012 3:17 PM
Reply to: Message 223 by JonF
12-17-2012 2:23 PM


Re: U-Pb, Pb-Pb, And Fission Track Dating
I don't think I follow you. Anchoring? Does the math of the Pb-Pb isochron require that we know the initial Pb-Pb ratio of the solar system?

I do know that the Pb-Pb isochron is different, perhaps I didn't make that sufficiently clear in the article.


This message is a reply to:
 Message 223 by JonF, posted 12-17-2012 2:23 PM JonF has responded

Replies to this message:
 Message 227 by JonF, posted 12-17-2012 5:46 PM Dr Adequate has responded

  
RAZD
Member
Posts: 18257
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 3.2


Message 225 of 292 (684446)
12-17-2012 3:41 PM
Reply to: Message 221 by Dr Adequate
12-17-2012 12:19 PM


Re: U-Pb, Pb-Pb, And Fission Track Dating
Hi Dr Adequate,

typo?:

and plot a graph showing their 206Pb/204Pb ratios on one axis and their 206Pb/204Pb ratios on another

Shouldn't one of those be 207Pb/204Pb

thanks


we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
to share.


Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click)

This message is a reply to:
 Message 221 by Dr Adequate, posted 12-17-2012 12:19 PM Dr Adequate has responded

Replies to this message:
 Message 226 by Dr Adequate, posted 12-17-2012 4:28 PM RAZD has acknowledged this reply

  
RewPrev1
...
1314
15
1617
...
20Next
Newer Topic | Older Topic
Jump to:


Copyright 2001-2015 by EvC Forum, All Rights Reserved

™ Version 4.0 Beta
Innovative software from Qwixotic © 2017