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Author Topic:   Questions about Xenoliths and dating
Human Shield
Inactive Member


Message 1 of 13 (264263)
11-29-2005 6:30 PM


Is lava flows the only way to 'zero' a date? I heard that testing that produces older ages then expected is the resault of xenoliths (older rocks in the material). Are these xenoliths easily identified, or are they assumed?

Does lavaflow always reset the clock? If the conditions were different, would cooled lava flows contain different ratios?


Replies to this message:
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AdminNosy
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From: Vancouver, BC, Canada
Joined: 11-11-2003


Message 2 of 13 (264300)
11-29-2005 7:51 PM


Thread moved here from the Proposed New Topics forum.
  
JonF
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Posts: 3483
Joined: 06-23-2003
Member Rating: 2.8


Message 3 of 13 (264319)
11-29-2005 8:27 PM
Reply to: Message 1 by Human Shield
11-29-2005 6:30 PM


Long exposure to high temperature without melting can "zero" a rock's date by homogenization of the isotopic composition.

Xenoliths are a very common cause of error in YEC "dating studies". Often they are known to be present. For example, in Snelling's Radioactive ‘dating’ failure there's no mention of xenoliths. But, in ANDESITE FLOWS AT MT NGAURUHOE, NEW ZEALAND, AND THE IMPLICATIONS FOR POTASSIUM-ARGON "DATING" he writes:

quote:
Steiner [90] stressed that xenoliths are a common constituent of the 1954 Ngauruhoe lava, but also noted that Battey [7] reported the 1949 Ngauruhoe lava was rich in xenoliths. All samples in this study contained xenoliths, including those from the 1975 avalanche material.

but the analyses were performed on "whole rock" samples in which, by definition, no separation of components is performed!. So in this case xenoliths are known to be present, are known to have been included in the analyzed material, and are at least a strong possibility for the reason for the erroneous date … but Snelling never mentions the possibility!

At Young-Earth Creationist 'Dating' of a Mt. St. Helens Dacite: The Failure of Austin and Swenson to Recognize Obviously Ancient Minerals Dr. Henke discusses the possibility of older material being incorporated in Austin's analysis of Mt. St. Helens.


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Human Shield
Inactive Member


Message 4 of 13 (264619)
11-30-2005 11:30 PM
Reply to: Message 3 by JonF
11-29-2005 8:27 PM


Would the heating have to be done while exposed to the surface? And would the properties of the atmosphere matter?

Do they notice xenoliths before testing the sample?


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JonF
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Posts: 3483
Joined: 06-23-2003
Member Rating: 2.8


Message 5 of 13 (264736)
12-01-2005 11:40 AM
Reply to: Message 4 by Human Shield
11-30-2005 11:30 PM


Would the heating have to be done while exposed to the surface?

No, and it essentially never is. In any solid, the atoms have a certain probability of moving from place to place. That probability is strongly dependent on the temperature, getting higher with higher temperature. When the temperature gets high enough for each atom to have a high probability of moving during the time that the material is exposed to that temperature, then most of the atomss will move at some time or another, and many will move multiple times.

If we are looking at "simple accumulation" dating, like potassium-argon (beloved of creationists because it can come out wrong), and the argon atoms that are produced by decay of potassium escape form the material, then the clock is reset to zero. This doesn't happen all that much in solids, 'cause the argon atoms have to move a really long distance (compared to their size) to get out of the material.

What happens more often (but not very often) is "homogenization". For example, in isochron dating we look at how isotopes of elements are distributed within the material. In rubidium-strontium isochron dating, the material starts out (just after solidification) with the ratio of 87Sr to 86Sr the same throughout the material, although the amounts of the materials vary from place to place. As time goes on, the amount of 87Rb decreases due to radioactive decay to 87Sr, and (of course) the amount of 87Sr increases, but the amount of 86Sr stays the same; so the ratio of 87Sr to 86Sr increases by different amounts in different parts of the material because the produced 87Sr atoms are trapped in place, and we can use this to obtain an age and an indication of the reliability of that age. But, if the material is hot enough, the produced 87Sr atoms can move around so the concentration of Sr is the same everywhere, and that zeros out the clock. Clear as mud? .

(That's much simplified but reasonably accurate).

And would the properties of the atmosphere matter?

Not really.

Do they notice xenoliths before testing the sample?

Almost always yes, although they can be tricky to spot. But they are often difficult to separate out even if you can see them.

Another creationist favorite dating episode is the KBS Tuff. A tuff is a particular product of a volcanic eruption, and the KBS tuff is named for the initials of the woman who first studied it. Initially (in 1969) it was dated at 2.9 million years by potassium-argon (K-Ar) dating (and this was pushing the envelope at the time, that's pretty young for successful K-Ar dating). But the pig fossils under it didn't seem right for that age, some other groups dated it and found younger ages, still other groups dated it and found the same older age, and a hominid fossil (skull KNM-ER 1470) was found under it. If the KBS tuff were really 2.9 million years old, then that hominid fossil was about 3 million years old, and that would push back the timeline of human evolution significantly. Eventually Ian McDougall (a Grand Old Man of radioisotope dating) and his group determined that the KBS Tuff contained significant xenoliths. The tuff itself is 1.8-1.9 million years old, the xenoliths are much older. The differing age results were a result of samples that contained differing amounts of difficult-to-see xenoliths. After McDougall's results were confirmed by other researchers and other methods, the KBS tuff episode was over in 1980. (Of course the creationists claim that the date was adjusted to fit the presumed age of the hominid fossil, but that's just silly; it was the pig fossils, well known and understood from all over Africa, that really signalled something was wrong, and the scientists beat at the problem until they figured out its source, established the mechanism, and replicated the results many times). There's a good discussion of this incident at Claim CD031 and some abstracts of the final age determinations at Fission track age of the KBS Tuff and associated hominid remains in northern Kenya, K−Ar age estimate for the KBS Tuff, East Turkana, Kenya, and KBS Tuff dating and geochronology of tuffaceous sediments in the Koobi Fora and Shungura Formations, East Africa.


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Coragyps
Member
Posts: 5266
From: Snyder, Texas, USA
Joined: 11-12-2002
Member Rating: 6.1


Message 6 of 13 (264767)
12-01-2005 2:05 PM
Reply to: Message 4 by Human Shield
11-30-2005 11:30 PM


Do they notice xenoliths before testing the sample?

I'll bet a Pilsner Urquell that Austin noticed them and sent the sample in anyway.....


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Minnemooseus
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Message 7 of 13 (264772)
12-01-2005 2:46 PM
Reply to: Message 5 by JonF
12-01-2005 11:40 AM


KBS tuff
I believe there is a topic somewhere, that is fairly specific to the KBS tuff. Unfortunately, I'm currently unable to track it down.

In short, the KBS tuff was a water laid tuff. A rather hybrid volcanic/sedimentary rock. As such, it contained significant contamination from older materials. These contaminants might be considered xenoliths, but I think that would be extending the definition outside of the usual definition.

The contaminents were determined and painstakingly removed from the sample, and a quality dating was the result.

Note: I post this not having read the cited materials. I suspect such is covered in them. I have a Talk Origins reference filed away somewhere, if anyone wishes to demand such. Or you could just go to Talk Origins (or Google) and do a search for "KBS tuff"

Moose

Added by edit: D'oh, you supplied a good Talk Origins reference. It in turn supplied the reference I was thinking of, Radiometric Dating and the Geological Time Scale.

This message has been edited by minnemooseus, 12-01-2005 02:55 PM


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JonF
Member
Posts: 3483
Joined: 06-23-2003
Member Rating: 2.8


Message 8 of 13 (265016)
12-02-2005 9:56 AM
Reply to: Message 7 by Minnemooseus
12-01-2005 2:46 PM


Re: KBS tuff
In short, the KBS tuff was a water laid tuff. A rather hybrid volcanic/sedimentary rock. As such, it contained significant contamination from older materials. These contaminants might be considered xenoliths, but I think that would be extending the definition outside of the usual definition.

My recollection is that the KBS Tuff was a mixture of old and new volcanic material, but you could be right. If so then xenolith is the wrong term, and it just stands as an example of the difficulty in dating mistures.
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JonF
Member
Posts: 3483
Joined: 06-23-2003
Member Rating: 2.8


Message 9 of 13 (265017)
12-02-2005 10:00 AM
Reply to: Message 7 by Minnemooseus
12-01-2005 2:46 PM


Re: KBS tuff
Oh, your "specific to the KBS tuff" thread is probably the one starting at Message 7 or maybe Potassium Argon Dating doesnt work at all.
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Joe Meert
Member (Idle past 3029 days)
Posts: 913
From: Gainesville
Joined: 03-02-2002


Message 10 of 13 (274098)
12-30-2005 6:15 AM
Reply to: Message 6 by Coragyps
12-01-2005 2:05 PM


I don't know if Austin noticed them
But IIRC, he cited an article that specifically discussed the xenoliths in the dacitic rocks he sent for analysis.

Cheers

Joe Meert


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bob200800 
Inactive Suspended Junior Member


Message 11 of 13 (486282)
10-18-2008 12:41 AM


Questions about Xenoliths and dating
spam removed

Edited by AdminNosy, : No reason given.


  
Dr Jack
Member
Posts: 3500
From: Leicester, England
Joined: 07-14-2003


Message 12 of 13 (486286)
10-18-2008 5:34 AM
Reply to: Message 1 by Human Shield
11-29-2005 6:30 PM


Is lava flows the only way to 'zero' a date? I heard that testing that produces older ages then expected is the resault of xenoliths (older rocks in the material). Are these xenoliths easily identified, or are they assumed?

Xenoliths will typically have a different composition and grain size to the rock they are encased in. Large xenoliths are easy to spot; smaller ones less so.


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teddy33 
Suspended Junior Member (Idle past 2426 days)
Posts: 4
Joined: 05-04-2010


Message 13 of 13 (558847)
05-04-2010 11:43 PM


how i s going
how should that going on?

_______________________________________
{Spam links deleted - Adminnemooseus}

Edited by Adminnemooseus, : See above.


    
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