[meteorite-list] Lunar origin of tektites

From: Marc Fries <m.fries_at_meteoritecentral.com>
Date: Sat Mar 26 12:37:25 2005
Message-ID: <1370.69.140.192.34.1111858632.squirrel_at_webmail.ciw.edu>

Greetings

   Actually, tektites that do not show signs of devitrification are very
rare. I have a single bediasite myself, and the exterior is dotted with
small, dark pockets of devitrifying glass. SE Asia tektites
routinely show pockmarks and pits that are the result of
devitrification at the surface, such as that seen in these pics:

http://www.mineralminers.com/html/tekmins.stm

   They were definately formed at very high temperatures, which is shown
not only in the degraded zircons you mentioned but again by the lack of
water. Volatiles were driven off during formation.
   The void pockets containing a high vacuum are something of a quandry to
me - I still can't wrap my brain around the notion of forming a pocket of
vacuum! Chondrules also contain glass inclusions that contain less than
10^-9 Torr vacuum. There is a seller on ebay that maintains that some of
these contain a fluid in a particular meteorite, although it was shown
many moons ago that such things are the result of cutting fluid getting
sucked into them through small cracks. But I digress...
   The lack of water in tektites probably allow them a longer terrestrial
life than obsidians. You mentioned 15-25 Ma-old obsidians; I gave
tektites up to 100 Ma. Sounds about right to me, although there's
probably a more accurate number out there somewhere.

   Oh, and I wouldn't expect much in the way of solar wind in tektite
inclusions. They followed a suborbital trajectory so their residence time
above the Earth's atmosphere would be quite short; probably in terms of
minutes. They would not escape the Earth's protective
magnetic field into the "densest" (very relative term) part of the solar
wind either.

Cheers,
MDF


> Hi, Marc, List,
>
> Tektites are glass but comparing them to obsidian is very misleading.
> If
> tektites decayed in terrestrial environs like obsidian, then there would
be a
> small number of tektites found in various stages of decay, especially
bediasites and georgiaites which are the oldest tektites.
> I've never heard of a degraded bediasite or georgiaite being found.
> (We
> should ask Norm Lehrman; he would know.) The reason I'm pretty sure
that they've never been found is that any collector of tektites who
finds any tektite with any unusual characteristic will talk about its
unique attributes... forever. You should hear me go on about why my
half-button australite is not the same as half of a button australite!
> While obsidian is a glass, it is formed at much lower temperatures.
> It
> is a "wet" glass with high water content, lots of dissolved gasses, and
partially melted xenoclasts. Moreover, obsidian is often layered, which
contributes to rapid breakdown. Even so, there's plenty of 15-25
million year old obsidian in pristine condition to be found.
> Tektites are the driest mineral known, with water content far lower
> than
> nuclear bomb glass, the next driest item on the list. That tektites
were formed at extremely high temperatures is evidenced by the fact that
the Martha's Vineyard tektite, a rare and precious one and only outlyer
of the North American field, contains a zircon crystal that is partially
melted. I
> could go and look up the melting point of zircon because I can't
remember it,
> but it's very very high (3400 degrees?).
> Someone else mentioned in a post to the List that there are bubbles in
> tektites, thinking that they had terrestrial atmosphere in them. Not
so. The bubbles in tektites either leaked (have atmosphere in them) or
they didn't, in which case they have a high class vacuum in the bubble
with pressures about equal to 40 km into space. The only recovered gas
is Argon40, presumably from potassium decay, although there is one
report of helium detected in an intact bubble. Helium? Captured solar
wind?
>
> Sterling Webb
> -------------------------------------------

-- 
Marc Fries
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Carnegie Institution of Washington
Geophysical Laboratory
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Received on Sat 26 Mar 2005 12:37:12 PM PST