[meteorite-list] Ordovician Meteorites...was New or maybe oldQUESTION???
From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Mon, 5 May 2008 15:56:12 -0500 Message-ID: <139401c8aef2$7428d1d0$db45e146_at_ATARIENGINE> Hi, List, Whoa, Eric! You're confusing two sets of dating. The dates you're talking about are the K/Ar dates, the secondary melt dates, at impact when the K/Ar clock was reset. At impact melt, the argon-40 formed by decay of potassium-40 is released; the melt solidifies and argon-40 begins to re-accumulate. All the Australites have the same K/Ar date, which is different from the Moldavite dates (which are all the same as each other), and so forth. K/Ar dates are the date of the last melting/vaporizing event, the impact. That's one kind of "how old?" But obviously the material existed before the impact! Rb/Sr dating is used to date the initial formation time of the material. Argon is a volatile gas, easily released by an energetic event, but rubidium and strontium are more refractory and are not that "flighty." It takes something bigger than an impact. With formation or complete thermal metamorphism, the Rb/Sr "clock" is set. It defines the formation date of the material, the initial melt. Rb87 decays into Sr87 which decays into Sr86, a more complicated arrangement than K40 decaying into Ar40. The material has some Sr87 before the Rb87 starts to decay into Sr87. That Sr87 is decaying into Sr86 at the same time the Rb87 is decaying into Sr87. With two decays going on at once, how can you ever date anything with that? The method used is called the isochron method. Here's a great explanation of how this method works: http://www.talkorigins.org/faqs/isochron-dating.html (It was written to convince Young Earth Creationists of the validity of isotopic dating, so it's very thorough and laboriously detailed.) You get two pieces of information from the Rb/Sr isochron. One is an age of igneous formation ("How old are these mountains?"), set by the zero point of the isochron. The other is that you can determine the initial Sr87/Sr86 ratio of the material at the zero point of the isochron. That initial Sr87/Sr86 ratio is one of the pieces of data that vary from one solar system body to another. All the initial Sr87/Sr86 ratios of terrestrial material are very similar, clustered around the value for this planet, but material from other bodies have different initial Sr87/Sr86 ratios based on the different make-up of their original composition. Different classes of meteorites, for example, have different initial Sr87/Sr86 ratios: basaltic achondrites 0.6990, H chondrites 0.6986, E chondrites 0.6990, L and LL chondrites 0.7000. This range corresponds to original formation ages of 4.46 and 4.60 billion years. Clearly these Rb/Sr dates have "coarser" resolution, but the result is a correct one. Rb/Sr systematics are very long-scale. They show, for example, that the original "formation times" of the various elements found on Earth range from 4.6 billion years to about 11 billion years. This result is a strong independent confirmation of the age of the Galaxy and the universe. (For the first few billion years, the universe was a hydrogen-helium-lithium heaven until the "heavy" elements evolved, giving an age for the universe of 13.0 to 13.5 billion years which fits with the Hubble parameter.) Tektites have a very tight specific cluster of initial Sr87/Sr86 ratios which is not like any known terrestrial material. Does it tell us something about the material of the impactor? Well, that initial Sr87/Sr86 ratio is not like any meteorites, either. In fact, it's not the same as any other known initial Sr87/Sr86 ratio from any solar system body. The actual value of the initial Sr87/Sr86 ratio is a different datum from the age determination which comes from the position of the zero point on the isochron, not its value. BTW, I never said nor even implied tektites formed from L chondrites; you just jumped there. But the shock features that most L chondrites show have re-set their Rb/Sr dates to the same date as the isochronic formation date of tektites. (It's just a puzzle.) The initial Sr87/Sr86 ratio of tektites gives an age of 460 +/-50 million years for the time of the primary melt formation of the tektite material. Rb/Sr dating is now combined with Hf/Sm (and other) dating methods (the 12-point test) in geo- and cosmochemistry. Hf/Sm dating has been applied to dating the age of the presumably "surface" materials melted into tektites, as a way of vaguely "confirming" that tektites are derived from local surface materials. It is very rough and sloppy, but everybody seems happy with it. > Whether a tektite is formed depends on the impacted material... The problem is that tektites have been widely suggested to derive from sandy surficial materials, or from loess, or off-shore sediments -- you know the list. They are all erosion products and, when mixed together, they have roughly similar ages to the very age suggested by the initial Sr87/Sr86 ratio of tektites, just less precise, so the Hf/Sm dates are similar to the Rb/Sr ones. Only small selected portions of the Earth's surface are very, very old, and the surface is always littered with the more recent debris. It's cruder data, but it fits the current opinion better. (See, I held the sarcasm off for quite a while.) There's nothing unusual or odd-ball about the isochron method; it's widely used and completely valid. So, why isn't the Sr87/86 value point of tektites better known? Well, it strongly suggests that tektites are not formed from local surface materials of the Earth, for one thing, but from a single material with a single unique and non-terrestrial origin, and not one of the ones we know about or have sampled yet. There simply is no explanation for that, not even a bad one. The tektite Rb/Sr isochron is just ignored. Sometimes it is described as "a coincidence." Sometimes it's suggested that the data-scatter is too great for a "good" isochron. This would be reasonable, unless you compare the tektite isochron data with other isochrons regarded as "acceptable." There are many, many worse cases of scatter in the literature and regarded as "good," but they're not so hard to explain. It's what I call an "orphan fact." I'm trying to find it a home, i.e., an explanation. I'm still looking, but I'm not ignoring data. Sterling K. Webb ------------------------------------------------------------------------ PS: I'll reign in the sarcasm; you work on controlling the use of the dogmatism club... Deal? ------------------------------------------------------------------------ ----- Original Message ----- From: <star-bits at tx.rr.com> To: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net> Cc: "Meteorite List" <meteorite-list at meteoritecentral.com> Sent: Monday, May 05, 2008 11:01 AM Subject: Re: [meteorite-list] Ordovician Meteorites...was New or maybe oldQUESTION??? ---- "Sterling K. Webb" <sterling_k_webb at sbcglobal.net> wrote: <And all tektites have an original Rb/Sr melt date of 440-480 million years ago. All just a coincidence, of course... > Where does this information come from? It does not seem even remotely possible. No tektites are this old. All tektites originated as melted terrestrial material. How do you postulate a "melt date" which precedes a later melting? You can't even get a date by backing out the Rb/Sr from the known terrestrial impacted material because the SE Asia impact site is not known. Additionally this means that all tektite forming impacts were L chondrites, no irons, no H chondrites, no carbonaceous etc. This is certainly possible but seems unlikely. Whether a tektite is formed depends on the impacted material and not on the composition of the impactor. -- Eric Olson 7682 Firethorn Dr Fayetteville, NC 28311 http://www.star-bits.com ______________________________________________ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list at meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list Received on Mon 05 May 2008 04:56:12 PM PDT |
 StumbleUpon del.icio.us reddit Yahoo MyWeb
|