[meteorite-list] Slickensides vs Shock Veins Revisited

From: Mr EMan <mstreman53_at_meteoritecentral.com>
Date: Mon, 8 Oct 2007 19:33:52 -0700 (PDT)
Message-ID: <499344.3061.qm_at_web51001.mail.re2.yahoo.com>

Hello Graham, Bernd, list

While there is no technical definition of a "shock
vein" so far as I know, it is in wide use and I hold
it to be a version of a "healed" fracture; healed by
the 1) injection or accumulation of adjacent wall melt
where the filling material has an origin in high
pressure, high temperature phase change or 2) formed
by shock front passage: compression melting followed
by decompression solidification. In the later case,
material goes from crystalline solid to non
crystalline liquid to non crystalline solid aka
"glass" within a nano to micro to a second of time.
Feldspars form masklynite glass but since fledspar in
meteorites are usually limited to small blebs it isn't
a major shock vein component.

Other silicates such as olivine go through a phase
change and melt only to recrystallize into the mineral
ringwoodite. In this case the high pressure squeezes
all the extra space out of the crystal matrix on an
atomic level by repacking them but maintaining
molecular bonds There are other minerals that are
formed by other pyroxenes resulting in similar phase
shifts (majorite, akimotoite, NaAlSi3O8<--a felspar,
hollandite and jadeite). Oxides such as chromite also
morph. there presence of these minerals provide
information about the shock history of the parent.

Fractures occur when there is a physical rupture of
adjacent material. It may be "unhealed"-- meaning a
void filled only by liberated gases or it may be
"healed" by any one of a several processes. If that
process is shock melting, it is a high pressure, high
temperature event where a rapid cycling of solid-melt-
solid, expands to cement the adjacent sides and
strengthening the fracture. As such, meteorites almost
never fragment along shock veins but across them, from
personal observation.

When vewing under a hand lens or microscope,
Ringwoodite has a dark almost black yet translucent
appearance, with a slight purple hue. Maskylenite is
transparent and clear.

A fracture can be a slickenside filled with smectite:
a low temperature,low to mid pressure, mechanical
weathering mixture produced as adjacent surfaces mill
and grind each other down. Separation of surfaces
along these is more common as they are natural lines
of weakness.

Slickensides are striated and the striations generally
orientated in the same direction; shock veins are
smooth and may be branched. Slickensides are opaque
and look like dark shale, graphite, or fusion crust in
color and texture.

I believe that formation of slickensides and shock
veins at the same location in the same event are
mutually exclusive.

So as to your question. I believe that all the veins
seen in this photo represent mid to low pressure, low
temperature fractures of which slickensides are a
subset. Without examination under a microscope, one
would not be able to positively determine if any shock
veins are present. Given the clear presence of
slickensides and the appearance of the matrix of this
meteorite to others which have an easily crumbled
consistency (i.e. friable), the occurrence of shock
veins is unlikely--Furthermore, it is extremely remote
that any of these features are from terrestrial
impact.

This meteorite over all appears to have had a really
brutal cosmic history.
EPILOGUE:
That said there is a report that slickensides in
meteorites might be better interpreted as "shock
fractures" as opposed to analogues of terrestrial
slickensides.
<http://adsabs.harvard.edu/abs/1966Metic...3...31D>

Elton
For more reading:
<http://adsabs.harvard.edu/abs/2004AGUFMMR11A0925X>
<http://www.nsls.bnl.gov/newsroom/science/2004/06-Chen.htm>
<http://www3.sympatico.ca/jeff.rowell/Shock.htm>
<http://www.meteorites.com.au/features/sahara99898.html>
<http://adsabs.harvard.edu/abs/1966Metic...3...31D>
<http://books.google.com/books?id=ZnUPAAAAIAAJ&pg=PA182&lpg=PA182&dq=slickensides+in+meteorites&source=web&ots=YfWcqNTll-&sig=kIPuKfITF2U4paiP_RBNB36db9g>

--- ensoramanda <ensoramanda at ntlworld.com> wrote:

> Hi Elton,
>
> I thought these looked like shock veins...still
> having trouble knowing
> the difference between shock veins and slickenslides
> in this one?
>
<http://i117.photobucket.com/albums/o43/LaburnumStudio/DSCN6902.jpg>
>
> Graham
Received on Mon 08 Oct 2007 10:33:52 PM PDT