[meteorite-list] Ordovician Meteorites...was New or maybe oldQUESTION???

From: star-bits at tx.rr.com <star-bits_at_meteoritecentral.com>
Date: Mon, 5 May 2008 20:40:47 -0400
Message-ID: <10945003.88141210034447568.JavaMail.root_at_cdptpa-web06-z02>

---- "Sterling K. Webb" <sterling_k_webb at sbcglobal.net> wrote:

<Whoa, Eric! You're confusing two sets of dating. The dates you're talking about are the K/Ar dates.>

Except that I never mentioned or implied K/Ar dating which does reset from shock or melting unlike Rb/Sr.

<.) 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.>

Yes I agree. All the isotope dating schemes use isochrons and they provide valid data if interpreted correctly.

< 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?>

Probably not. A major impact melts significantly more terrestrial material than there is impactor material. So unless the impactor has a ratio that is significantly different than terrestrial, the ratios change very little from the average terrestrial ratio.

< 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. >

Which indicates the Sr87/Sr86 ratio is the average of the terrestrial material that was melted.

< 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.>

Which also fits with a young terrestrial age.

< 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.>

Unless the explanation is the Sr87/86 measures the average age of the terrestrial melt material which seems like an explanation, even a good one, to me.

< BTW, I never said nor even implied tektites formed from L chondrites; you just jumped there.>

?Formed from L chondrites?, no you never said or implied that, nor did I. You also never said that L chondrites were the impactors which caused the formation of tektites however that seemed to be infered from the following:

< A one-hundred-fold increase in
meteorites, five ice ages in 100 million years, one of them
the worst in Earth history ("Snowball Earth"), massive
breakups of major bodies, the complete re-surfacing of
Venus (surface age 480 million years). And all tektites
have an original Rb/Sr melt date of 440-480 million years
ago. All just a coincidence, of course... >

All a coincidence? No. The average terrestrial melt/tektite Rb/Sr date? Yep.

--
Eric Olson
7682 Firethorn Dr
Fayetteville, NC 28311
http://www.star-bits.com

Received on Mon 05 May 2008 08:40:47 PM PDT