[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
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Received on Mon 05 May 2008 04:56:12 PM PDT


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