---- "Sterling K. Webb" <[EMAIL PROTECTED]> 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
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