Hi AC
Glad you took some advice and went out to find a real example. Hopefully we can discuss this paper in detail and this topic will remain as civil as it has so far.
I have read and re-read your critique and have also done the same for the paper that you cited.
My conclusions, however, do not match your own. To me the paper reads that the people who carried out the analysis took very great care to make sure that all of their results were "correct". Not fudged but genuinely correct.
I think some of your issues could be due to an incomplete knowledge of the processes involved in the analysis. Roxrkool explained some of it but let me give my slant on the analysis from the point of an analytical chemist. (not a geochronologist but someone who has analyzed a whole lot of rocks for chemical composition)
First I would like to say that there are parts of the paper that definitely lack adequate descriptions of methodology, particularly if you are not very familiar with analytical practices. (of course I have no idea of your own background so please excuse me if I am speaking out of line here)
I just intend to stick to my own area so let's look at the leacheates.
Leacheates are the liquid residue containing dissolved elements after a sample has been left to soak in the liquid for a period of time.
The mineral fractions were leached in warm 1N HCl for 10 minutes prior to digestion.
This is a fairly mild acid but is enough to dissolve some of the Rubidium and Sr which is easily available (not locked into crytaline structures) but doesn't come close to dissolving the locked in isotopes. That is done by the "digestion" that they mention
Why did they feel the need to leach the sample at all I wonder? It certainy isn't standard practice to soak a chunk of rock in warm acid
prior to digestion. There had to be a valid
pre-existing condition which they had noted in the sample in order to make it necessary.
Admittedly they didn't actually tell us the reason but if you know anything about sampling techniques then it is absolutely
certain that they had one or they simply would not have done it.
It turns out that the paper does actually list this reason but you have to search a little to find it.
Rb and Sr in the host silicates may have been
fractionated and partitioned into leachable sites at ~90 Ma.
It is well known that only certain sites within the structure of the rock are
leachable. Features on the rock's surface can often indicate that "shock" or some such event has occurred which would very likely have changed the chemical structure of certain parts of the rock, hence making the sites
leachable.
It seems obvious to me that they must have foreseen that certain sites would be
leachable or they would have had no reason to
leach the rock at all. It would have been extremely
bad science to have ignored this fact and carried on regardless. It should also be noted that leaching like this takes out such an infinitesimal amount of material from the parent rock that, had they not done it, the age of the bulk rock would not have changed by a measurable amount.
Therefore I would say that it speaks volumes for the integrity of these scientists that not only did they perform this "leach" but they reported it
and explained it's
completely predictable deviation from the primary isochron.
After all, there is a slight difference in the solubility between Rb and Sr.
I don't feel that I am qualified to comment in the defense of the actual age measurements. I am studying it in the attempt to become more familiar and knowledgable about this but (unlike chemical analysis techniques) it is still a little out of my field for now.
PY
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