[meteorite-list] 26-AL isotope counting

From: E.L. Jones <jonee_at_meteoritecentral.com>
Date: Thu Apr 22 10:22:36 2004
Message-ID: <3EE6D299.8010806_at_epix.net>

Hello List and Larry,

Since no one else has shouldered the question I'll try to comment.

You asked several questions and made several observations. In regards
to those, Aluminum26(Al26) is not a test for meteorites, per se. There
is no single test that I am aware of for making a determination if a
sample is meteoritic. The paucity or abundance of cosmic ray tracks,
Carbon 14, Al26 all go to determine that a sample has or has not gone
through a period of space transit. Al26 analysis was one of several
tests on Shirokovsky to reach a preponderance of evidence.

Because similar terms/concepts are used in different situations while
analyzing AL26 , I've broken some of them out as follows:
CASE 1: Al26 which was present in the solar nebula has long since
decayed to Magnesium26(Mg26). Finding a Mg atom in a molecule within a
crystal matrix where an Al atom should be, shows that the crystal hasn't
been reset since the formation of the solar system.

CASE 1A: If the "clock" has been reset via differentiation of the parent
body/ impact melting, etc. The ratio of Mg24 to Mg26 might be used to
estimate the petrographic age of a sample by comparing what the ratio
would be between the original/cosmic abundance of Al26 and the present
quantity of the Al26 daughter product(Mg26).(see the McSween Quote for
contrast)

CASE 2: Al26 is "replenished" by cosmic rays/irradiation. Exposure can
convert Phosphorus (P26) or Silicon (Si26) into Al26 even on earth.
However the ratios of Al26 from a terrestrial vs non terrestrial sample
are very large. So a sample having a certain BUT low overall Al27/Al26
ratio could indicate a long or a short terrestrial exposure but still
very far from the levels that would be formed during unshielded space
exposure. This could be one of the terrestrial exposure tests used in
lieu of C14 and others if the terrestrial age is over than 50-60 K years.

CASE 2A: Like case 1 above, finding an Al26 atom in a SiO2 (silica) Xtal
matrix for example, is evidence of recent cosmic ray
exposure/irradiation. This is being used to date certain terrestrial
events such as more precise dating of glaciation coverage.
'
CAVEAT: The size/diameter of the meteoroid affects the distribution of
replenished radionucleides With more being found in the outer portions
than those portions from the interior.(shielding) That is a whole
treatise in itself.

Stating that Shirokovskys' low levels of Al26 is an absolute fool proof
test might be true but oversimplified. As I recall, Larry Taylor's
research showed that the AL26 levels fell within the terrestrial ratio
band. There is probably a technical reason that the presence of AL26
makes calculating the Mg24/Mg26 ratio impractical such as nearly
identical X-ray diffraction patterns. From the quotation it is unclear
if the writer meant the current or primordial existence of Al26.

Your question on Allende, I assume is rhetorical. Of course it is a
meteorite Al26 analysis or not. There are lots of other characteristics
used to identify meteorites. See the Caveat above, however, I would
think that some Al26 should have been found in all the research that has
been conducted on Allende.

I don't understand Hap McSween's quote on Vesta and the absence of Mg26
out of context. Taken as it is it doesn't seem accurate to me either.

I hope this was at least another look at your observations with out
confusing folks more than we already were. Not by your post but by the
complexity of it all. I agree that on casual review, there seems to be
some conflicts/inconsistencies.

Regards,
Elton

Thetoprok_at_aol.com wrote:
>
> Greetings List,
>
> I have a technical question / observation I'd like to discuss with
> some of you knowledgeabe in the area of isotopic counting,
> particularly 26-AL. It seems to me that there is some conflicting
> information out there concerning the true accuracy and / or validity
> of this test when determining ages and origins, (terr. or non terr.)
> through cosmic exposure.
>
> The recent issue with Shirokovsky brought this subject back to the
> surface for me. Don't get me wrong, this is not about Shirokovskys'
> pseudo-meteorite status, that rock is apparently not a meteorite for
> many reasons. It is about the comments made in response to this
> stones cosmic exposure test, that it has not experienced any time in
> space, based on 26-AL counting, asserting that this test is an
> absolute fool proof determinator.
>
> I have read in several publications, information that seems to
> contradict that reasoning. I have also spoken with several semi
> experts, (one author and a couple meteorcists) on the subject and
> opinions are not unanimous. Just recently a post from Ron Baalke
> discusses this, here is a snipet, pay particular attention to the
> last sentence;
>
>
> <Like all radioactive isotopes, short-lived ones decay to another
> <isotope. It is the distinctive nature of the daughter isotopes that
> record <the presence of the short-lived, extinct isotope. For
> example, 26Al decays <into magnesium-26, 26Mg. If present in a
> mineral grain that contains a small amount of magnesium (most of
> which is in the form of non-radioactive 24Mg), <its decay leads to an
> anomalously high ratio of 26Mg to 24Mg. (In cases <where there is a
> lot of magnesium, the presence of 26Al cannot be <determined.
>
> And yet another reference;
>
> A quote from Harry Y. McSweens book, "Meteorites and Their Parent
> Planets",
>
> "The partitioning of 26 Al into eucrite magmas during melting caused
> the crust to become very hot, effectively insulating the mantle and
> preventing heat from escaping from the deep interior. Vesta lost heat
> so slowly that parts of its mantle remained molten long after the
> short lived 26 Al was exhausted, accounting for the general absence
> in eucrites of detectable 26 Mg, the decay product of 26 Al".
>
> What am I missing?
>
> I think I read in O. Richard Nortons first book that the Allende
> meteorites do not have any detectable 26 Al, is this true? If it is,
> and had Allende not been widely wittnessed, would it still be a
> meteorite?
>
> Is there any 'one' test that proves beyond a shadow of doubt if a
> stone is earthly or not?
>
> Thanks, Larry Atkins
>
Received on Wed 11 Jun 2003 02:56:25 AM PDT