[meteorite-list] Re: hunting (and radiometric dating)

From: Frank Prochaska <fprochas_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:43 2004
Message-ID: <NDBBICFKNKHAAEEJLDALIEAECIAA.fprochas_at_premier1.net>

Hello all,

This is a very good summary of many of the issues involved in terrestrial
dating meteorites. I've wondered for some time whether anyone has done
studies to try to understand the problem better, like using isotopes to
estimate the ages of a number of known falls, or doing a sizable number of
samples from a large strewn field where you'd presumably be looking at
different portions of a large preatmospheric mass, etc. Does anyone on this
list know of any studies like this?

Frank Prochaska




-----Original Message-----
From: meteorite-list-admin_at_meteoritecentral.com
[mailto:meteorite-list-admin_at_meteoritecentral.com]On Behalf Of Kelly
Webb
Sent: Monday, March 26, 2001 12:17 AM
To: meteorites_at_space.com
Cc: meteorite-list_at_meteoritecentral.com
Subject: [meteorite-list] Re: hunting (and radiometric dating)


Hi, Steve,
    There was a longish thread on the List some months back about the find
story for Lafayette, about whether it was a fall or a find, who the finder
was, etc. As I recall, the story of it's being found in a creek or lake bank
at a fishing spot was brought out years later by Nininger's inquiries, but
the location of that fishing spot could never be determined. Hell of a
shame, because I would love to go hunting, er, I mean, fishing there!
    The 13,000 year age is often cited as proof it wasn't a fall. I find it
curious that that age so closely matches the retreat of the ice from
northern Indiana. Of course, as a Martian basalt having formed in 0.38 g, it
would have a resistance to weathering far beyond that of an ordinary
chondrite. Having formed in a weak (or nearly non-existent) gravity field,
chondrites are fragile, poorly consolidated, porous stones when compared to
a planetary rock.
    The usual method of dating the terrestial age of a meteorite depends on
comparing the amounts of various unstable isotopes produced by cosmic ray
exposure. When the stone is in space, cosmic radiation produces a number of
unstable isotopes, which are both continuously decaying according to the
half life of each and being continuously produced. When the stone lands on
earth, the cosmic radiation is shut off and with it, isotope production, but
decay continues. By comparing abundances of short-lifed with long-lifed
isotopes in a given stone with other similar meteorites of recent arrival,
the time-on-earth can be estimated from the shortfalls in short-lifed
isotopes.
    Presumably, the comparison stone for Lafayette is Nakhla, about whose
fall date there is no dispute and whose estimated CRE age is nearly
identical (11,600,000 years vs. 11,400,000 years). Presumably, there are
adjustments that were made for the differences in bulk composition of Nakhla
and Lafayette, which, while they are similar, are not identical. For the
best results, the levels of as many different decaying isotopes as possible
should be measured. The procedure works best for the oldest (in
time-on-earth) stones. For a stone to be dated at 13,000 years
time-on-earth, we are probably talking about shortfalls in most isotopes of
at most only 1% or 2%.
    Then, there are relevant factors which cannot be known. The penetration
of even very energetic cosmic radiation falls off sharply in distances of
less than a meter of stone. How do we know where in the original unablated
meteroid the portion that became Lafayette (or Nakhla either for that
matter) came from. Answer: we don't. The geometry (shape) of the original
meteoroid is also a major consideration. These factors alone could probably
account for small differences in abundances.
     Another annoying item is that so many references (like the Catalogue)
usually give these dates without the requisite +/- of error range. This can
be very misleading, as it suggests a precision that is illusory. Got to see
those error bars. Even with the precision noted, however, there's still
another problem.
    That other part of the problem is the confusion between precision and
accuracy. Atomic assay can be both very precise and of poor accuracy at the
same time. No conflict at all. This is not well understood. While the
time-on-earth method uses short lifed isotopes, in general, the most used
isotopes for cosmic dating are long lifed (it's an old universe) like Rb/Sr
decay.
    I've seen very precise determinations of Rb/Sr isotopes that yielded
formation ages for a sample that were negative, that is, the stone wouldn't
form until a few million years from now. Not accurate, but the results were
very precise, fixing its future formation time to within 50,000 years! Of
course, what we're actually dating is not the sample itself, but some event
(unknown) which produced a fractionation of a volatile (the Rb) that left
behind a refractory (the Sr). When you get a really anomalous date, like a
negative date, you throw it out because obviously there were other events
(equally unknown) in the life of the sample, so you can no longer be certain
you are dating a single unique event anymore.
    However, this also means that there may well be undetected fractionation
events in seemingly "good" samples that have pushed the values up and down
and back and forth in undetermined and unforeseen ways, in effect smearing
out the values, blurring the data. This would show up as variations in the
values found in supposedly "identical" samples, which often happens.
    Say, for example, that you had ten samples of objects you knew were
equal aged, like stones from the same strewn field. You might get Sr87/Sr86
ratios whose precision was +/- 0.00005 for each individual item (very
precise), but the value of the ratio for each item might differ from other
items in the same assemblage by +/- 0.001.
    So, while your precision is one part in 20,000, your accuracy is only
equal to the total spread in sample values times the half-life of the decay.
Since Rb87/Sr87 decay has a half life of 48,800,000,000 years (yeah, that's
50 billion years), your accuracy in this imaginary case is +/- 50 million
years (+/- 0.001 x 50,000,000,000), which is a much larger margin of error
than what the precision suggests.
    Pinning down an age somewhere in a 100 million year span is accurate or
not depending on the span being dated. If you're talking about the formation
age of the solar system, that's moderately accurate. On the other hand, if
the sample were relatively young, like Australian tektites, this level of
"accuracy" would not cast much light into the darkness!
    Even worse is the fact that we cannot be sure that variations in
"identical" samples are due to a variety of minor fractionation events in
the life history of the sample. The variations may have been there from the
beginning and instead of having been altered, the sample may have been
protected from minor events that would have obliterated its initial
variation! There's simply no way to tell.
    Next, of course, the data is most often presented in a notation (like
sigma's) or a format (log graphs) that is exponential, because nothing makes
unruly data lie down flat and neat like reducing it to order of magnitude
values. Like if I owed you $99.00 but I paid you $10 because I was doing my
accounting by order of magnitude.
    In the case of Lafayette it would be useful to burrow into it looking
for signs of (and measuring the depth of) aqueous alteration from 13,000
years of Indiana weather, but there's the practical matter of exactly how
much of Lafayette is likely to be handed over to be reduced to post-test
rubble!
    At any rate, if you wanted to ignore Lafayette's supposed 13,000 year
date as a practical matter, you'd probably be on reasonably good ground.
    Radiometric dating is not a flawed model. The model (the random decay
clock) is perfect. But it's a sloppy universe, very messy, and time is long
and full of disasters. Radiometric dating has to be evaluated within its
limitations and constraints, compared with data from as many other dating
techniques as possible, and, most important of all, be considered in
context, and contexts are often best established by other means.

Sterling K. Webb

meteorites_at_space.com wrote:

> These are very good points, Kelly. I have read that much of what is now
the hard baked Sahara was 10,000 years ago a lush green land supporting a
wide variety of wildlife and flora. The question that should be
investigated is how long does it take for a meteorite in say average
conditions to survive and still be recognizable as meteorites? Also, many
of the Sahara meteorites may have fallen during a time when the environment
there was like the US Midwest, and maybe even before that. I think
meteorites can survive longer than what was here-to-fore believe. Look at
all the big finds that Nininger made in the semi-arid Midwest. It would be
interesting to see what the terrestrial age dates come out on those. But
then again I think that the terrestrial age date model is flawed, as there
are at least one clearly recent fall-find that was dated at 13,000+ years
(Lafayette, IN) But perhaps on a statistical average it might work.
>
> Then, look at the meteorites that are being gathered at Gold Basin... They
say that these are at least 20,000 years old, and look at their condition--
they are not too bad. (And I suspect that many of the so called "Gold
Basin" meteorites are in fact many different falls)
>
> Steve Schoner, AMS


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Received on Sun 25 Mar 2001 10:39:56 AM PST