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Author Topic:   Dating Question For Members
JonF
Member
Posts: 6174
Joined: 06-23-2003


Message 15 of 77 (610176)
03-27-2011 11:32 AM
Reply to: Message 14 by Buzsaw
03-27-2011 9:46 AM


Re: Related Dating Questions
It's already been said that dating the individual grains would yield the date that the parent rock formed, many years before the sedimentary or metamorphic rock formed. So scientists are smart enough to avoid doing that.

However, there are some methods for dating when sedimentary rock formed. Not surprisingly they involve dating the "glue" that holds the rock together. The individual grains of sedimentary rock don't stick together by themselves; something forms or precipitates between the grains and holds them together.

Examples include U-Pb dating of xenotime grown on zircons (SHRIMP Uranium-Lead Dating of Diagenetic Xenotime in Siliciclastic Sedimentary Rocks) and K-Ar dating of glauconite (Sedimentary geology: an introduction to sedimentary rocks and stratigraphy, pg 400). The latter is not particulary reliable because argon loss is common, leading to dates that are 10-20% too low (but still incompatible with creationist time scales by orders of magnitude).

Most sedimentary rocks, and the fossils they contain, are dated by a combination of stratigraphy, index fossils, and radiometric dating.

The simplest case is a fossil found in a sedimentary layer that lies between two igneous layers. Suppose the igneous layer above is dated at 500 million years and the layer below is dated as 510 million years; the sedimentary layer and fossil are therefore 500 to 510 million years old. This situation is common, such as in the Rift Valley of East Africa, where so many hominid fossils have been found; the area is loaded with volcanic tuff layers.

An example of a slightly more complex case is the use of index fossils to calibrate, not date, layers. An index fossil is a fossil of an organism that has been found to occur only in some narrow range of dates (by many applications of the technique above). FOr example foraminifera, AKA forams, are widely used and are critical to oil well drilling. Let's suppose we know of an index fossil that is reliably known to be 370-380 million years old. Also suppose we find a fossil of some other organism in a layer that's above a layer containing those index fossils and below an igneous layer that dates to 350 million years. The new fossil is therefore 350-370 million years old.

There are lots of scenarios and variations, but the bottom line is that most fossils are dated by radiometric dating of associated igneous layers, the association being established by various means.


This message is a reply to:
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 Message 20 by bluegenes, posted 03-27-2011 12:11 PM JonF has replied

  
JonF
Member
Posts: 6174
Joined: 06-23-2003


Message 27 of 77 (610195)
03-27-2011 4:18 PM
Reply to: Message 20 by bluegenes
03-27-2011 12:11 PM


Re: Related Dating Questions
You can probably tell us something about this, Jon.

I'm not familiar with that particular technique, but it makes sense. Except for the "kind of food a dinosaur eats" bit. I don't see any way for that kind of information to be extracted.

There is all sorts of interesting work with U-Th disequilibrium dating. Uranium is somewhat soluble in water (deposition from groundwater is probably how the uranium gets into the dinosaur bones you mentioned). But thorium is very insoluble in water. So when the decay chain from uranium hits thorium, the thorium drops out of solution. But when the uranium starts building up in a solid such as a bone or coral or a stalactite or stalagmite, the thorium is trapped and starts building up. Since the half-life of the uranium is much much longer than thorium, eventually the thorium concentration reaches "secular equilibrium", in which the number of thorium atoms formed per unit time equals the number of thorium atoms decaying per unit time and the thorium concentration is constant. But you can date things that haven't reached that point yet by measuring how far away the thorium concentration is from secular equilibrium.

That was how they dated the Siloam tunnel in Jerusalem, getting a U-Th disequilibrium date for a stalactite and a 14C date for a leaf trapped in the plaster, which obviously bracketed the construction date.

But that's only good for about a million years at most.

Edited by Admin, : Fix superscript.


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 Message 20 by bluegenes, posted 03-27-2011 12:11 PM bluegenes has seen this message

  
JonF
Member
Posts: 6174
Joined: 06-23-2003


Message 61 of 77 (610415)
03-29-2011 6:12 PM
Reply to: Message 60 by ringo
03-29-2011 5:36 PM


Re: Related Dating Questions
ringo writes:

Only the extrusions would be used as dating layers.

Yeah, this will probably confuse Buz, but intrusive layers are used for dating where appropriate:

The black basalt intrusion in the middle obviously is younger than the rock through which it cut, and can easily be dated.

Buzsaw writes:

If it is the cooled magma rock that is tested by the radiometric dating, does it date differently than the rising magma and if so, why?


The rate of cooling would depend on the initial temperature of the magma/lava, the temperature of the surrounding material, etc. My guess is that lava on the surface would cool more rapidly than magma below the surface.

Oh, typically much slower below the surface. Big plutons can take millions of years to cool to near the temperature of the surrounding rock. There are lots of interesting thing to be said about that, but Buz is confused enough already.


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JonF
Member
Posts: 6174
Joined: 06-23-2003


Message 74 of 77 (630101)
08-22-2011 12:49 PM
Reply to: Message 73 by edge
08-22-2011 10:36 AM


Re: Related Dating Questions
And, of course, once the mineral is well below its closure temperature diffusion is not a problem.

Closure temperature varies by mineral and atomic species. I.e. the temperature at which U is immobilized will be different from the temperature at which Sr is immobilized. This fact is used to track the cooling history of rocks which cooled over times greater than the uncertainty in radiometric ages, by dating different minerals from the rock by different methods. E.g. Tectonic geomorphology.


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