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

From: Kelly Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:43 2004
Message-ID: <3ABE7C6A.5C55A95B_at_bhil.com>

Hi, Steve,
    There was a longish thread on the List some months back about the fin=
d story for Lafayette, about whether it was a fall or a find, who the fin=
der was, etc. As I recall, the story of it's being found in a creek or la=
ke bank at a fishing spot was brought out years later by Nininger's inqui=
ries, but the location of that fishing spot could never be determined. He=
ll of a shame, because I would love to go hunting, er, I mean, fishing th=
ere!
    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 n=
orthern 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 c=
hondrite. 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 num=
ber 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 pro=
duction, but decay continues. By comparing abundances of short-lifed with=
 long-lifed isotopes in a given stone with other similar meteorites of re=
cent arrival, the time-on-earth can be estimated from the shortfalls in s=
hort-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 iden=
tical (11,600,000 years vs. 11,400,000 years). Presumably, there are adju=
stments 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 possib=
le should be measured. The procedure works best for the oldest (in time-o=
n-earth) stones. For a stone to be dated at 13,000 years time-on-earth, w=
e 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 penetrati=
on 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 unab=
lated meteroid the portion that became Lafayette (or Nakhla either for th=
at matter) came from. Answer: we don't. The geometry (shape) of the origi=
nal meteoroid is also a major consideration. These factors alone could pr=
obably 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 use=
d isotopes for cosmic dating are long lifed (it's an old universe) like R=
b/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 yea=
rs! Of course, what we're actually dating is not the sample itself, but s=
ome 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 fractionat=
ion events in seemingly "good" samples that have pushed the values up and=
 down and back and forth in undetermined and unforeseen ways, in effect s=
mearing out the values, blurring the data. This would show up as variatio=
ns in the values found in supposedly "identical" samples, which often hap=
pens.
    Say, for example, that you had ten samples of objects you knew were e=
qual aged, like stones from the same strewn field. You might get Sr87/Sr8=
6 ratios whose precision was +/- 0.00005 for each individual item (very p=
recise), 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 de=
cay. 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 margi=
n 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 for=
mation 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 "ide=
ntical" 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 pr=
otected from minor events that would have obliterated its initial variati=
on! 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 m=
akes unruly data lie down flat and neat like reducing it to order of magn=
itude 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 ho=
w much of Lafayette is likely to be handed over to be reduced to post-tes=
t 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 l=
ong and full of disasters. Radiometric dating has to be evaluated within =
its limitations and constraints, compared with data from as many other da=
ting 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 no=
w 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 inves=
tigated is how long does it take for a meteorite in say average condition=
s to survive and still be recognizable as meteorites? Also, many of the =
Sahara meteorites may have fallen during a time when the environment ther=
e 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 inter=
esting 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 ar=
e 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... T=
hey say that these are at least 20,000 years old, and look at their condi=
tion-- 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
Received on Sun 25 Mar 2001 06:16:59 PM PST