[meteorite-list] Vs: Researcher Says... Tektite Events

From: Jarmo Moilanen <jarmom_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:43 2004
Message-ID: <004a01c0b699$7fe2b7b0$2e9bedc3_at_impact>

Hi Kelly and List,

> Hi, Jarmo and List,
>=20
> Jarmo Moilanen wrote:
>=20
> > Melosh doesn't mention anything about deeper excavation.
>=20
> "Evidently, tektites originate from DEEPER WITHIN THE TARGET ROCKS..."
> -- quote from H.J.Melosh, "Impact physics constraints on the =
origin of
> tektites", Meteoritics & Planetary Science Vol. 33, No 4 (Supplement),
> 1998. (Reprinted in Heinen, Tektites, 1998)

Yes, you quoted him right, but he does not mean that the impact has =
deeper excavation than normally. As far as I understand this, I think =
that Melosh is saing than instead of forming tektites by jetting which =
happens only from interface of the projectile and target (which =
contaminate the ejecting material from the projectile) the tektite =
forming event happens not in this interface zone of the projectile and =
target but in a zone which is deeper inside compressed target.=20

We have to keep in mind that pressure which silicates need for melting =
is at least 100 GPa and in the contact point (interface zone) of the =
projectile and target pressure may be as high as 400 GPa. The region =
where this 100 GPa of pressures do occurs is way inside the trancient =
cavity which the impact will produce. Pressure which occurs in bottom of =
trancient cavity of impact is only few GPa (1-2 GPa). Since most of =
rocks need only about 50 GPa for complete melting this tektite producing =
zone is inside that zone between 100 GPa pressure contour and interface =
of the projectile and target.

> The problem I have with this is, after the decompression by
> rarefication waves and the silicate vapor and fluid phase is formed, =
that
> this tektite source material, to get out of the bottom of the forming
> crater, has to fight its way through the vaporized upper layers which =
are
> already mixed with and contaminated by impactor material, then fight =
its
> way through the even more thoroughly vaporized impactor which contains =
more energy (kinetic and thermal) than any of the target materials.

I think that you are right that the silicate vapor and fluid will =
contaminate if this material is in the bottom of the forming crater. In =
matter of fact, that material should mix totally with melts and will be =
part of impact melt rocks. But I think (Melosh doesn't say it directly) =
that tektites are formed closer to the surface around the impact point. =
Material which is almost on the surface will escape from the ground as a =
vapor and they should produce those silicate spherules. From a little =
bit deeper, where this liquid silicate material occurs, material is =
ejected out and up into atmosphere without going through any other =
material. Deeper in target this material will become heavily contaminate =
and form typical glass bombs like those which can be found from Ries and =
some of that material will become part of suevite. Somewhere between =
tektite forming zone and glass bomb forming zone there are zone where =
accelerated tektite material start to turn more solid before leaving the =
ground and there you got layered tektites which doesn't get so high =
velocities as tektites does, thus they are not ejected so far from the =
impact site than non-layered tektites. Also some tektites with breccia =
like structures may form when collisions of cooling droplets will take =
place.

> If the tektite source vapor is expanding more slowly than the =
other
> phases, it will never get out of the atmosphere. If the tektite source
> vapor is moving at the same speed as the layers of vapor and liquid =
phases
> above it, it will be in contact and they will mix. If the tektite =
source
> vapor is moving faster than the other material, it will mix even =
better. So how does it escape without contact with the rest of the =
impact material
> above it?

Because tektites are formed close to the surface and they have free way =
out from the impact point. Vaporizing and melting does happen also very =
close to surface of the target since that rafefaction waves spread all =
directions inside the projectile and target.

> Melosh doesn't say how much deeper his source layer has to be, but =
it
> has to be below the lower limit of terrestrial material which gets
> contaminated by impactor material (since it's not supposed to get
> contaminated). I don't know how deep this would have to be (and Melosh
> doesn't say) , but I do know that chemically resolvable traces of =
impactor
> material can be found in the breccia lens left behind at the bottom of
> recent craters. Where is this uncontaminated layer?

That is true what you say here. Material which is deep inside forming =
crater have to become contaminate. I think that if this Melosh theory is =
correct or at least close to it, those tektites are formed in a =
relatively narrow area near the surface, so they may escape without =
contamination.
=20
> > He also doesn't mention rebound decompression but rarefaction waves =
(=3D
> > release waves) which has a major role in tektite forming in his =
theory.
> > Rebound decompression and rarefaction waves are, as far as I know,
> > totally different things during cratering process.
>=20
> Whoops! Got me there. I was working from memory of the abstract. =
What
> we have is a sudden drop in pressure which allows the superheated =
fluid to
> transition to the vapor phase more or less instantly. Still, we have a =
few billion tons of silicate and iron vapor and another few billion tons =
of vaporized country rock churning around between the tektite =
vapor/liquid and the escape hatch.
> I can't help it; I see mixing as the likely outcome.

However, I don't know why all impact events has not form (or we haven't =
found them yet) widely spread tektites layers. With small craters this =
is not a problem but how about those 69 impact structures without =
tektites which are bigger than Bosumtwi (diameter 10.5 km)?

One thing which came into my mind is that tektite producing impact may =
be more vertical impacts than impacts which doesn't produce tektites (of =
cource, this is hard to prove by looking those craters). In vertical =
impact this tektite producing zone should be larger and more material is =
able to escape from the impact site without contamination. Statistically =
calculated average impact angle is 45=B0 and vertical and almost =
horizontal impacts are way more rarer events. Also movement of surface =
layers of the target around impact point during the impact may play some =
role in this question.

> Kelly

Jarmo Moilanen
Received on Tue 27 Mar 2001 03:39:04 AM PST