[meteorite-list] Re: hunting (and radiometric dating)
From: meteorites_at_space.com <meteorites_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:43 2004
Very interesting post, especially the implications that the margin of error with the technique is so great that 13,000 years for the reputed age of Lafayette is-- well let me say dubious.
There are, however, two flaws in your reasoning with regards to the nature of Lafayette.
It is a very porous stone as are the other two Nakhlites. For this reason, it would not last long in the 13,000 freeze thaw cycles over that length of time if the interior became waterlogged.
Also, it has a large amount of olivine, and this is very easily weathered in an earth environment to be reduced to clays. Now, Lafayette does have such clays due to weathering-- However, it has been determined that the weathering occurred on Mars, not on the earth.
No terrestrial weathering consistent with a 13,000 year terrestrial date has been found with regards to the Lafayette Meteorite.
The fusion crust is absolutely fresh, and the weathered grains are truncated by this fusion crust. (Under magnification, such can even be seen in my .87 g fusion crusted specimen http://www.geocities.com/meteorite_identification/LAFAYETTE.htm ).
So for the uncertainty with respect to the possible margin of error in the age dating techniques, and the obvious circumstantial evidence that Lafayette is a very fresh specimen-- I think it best to disregard the 13,000 year date.
Steve Schoner, AMS
On Sun, 25 March 2001, Kelly Webb wrote:
> 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 09:28:54 PM PST