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Glass bombs - part 1 of 3
Gaetan Cormier wrote:
> I have a couple of Glass Bombs from Ries Crater in Germany, I always
> heard that they were a type of impactite, but apparently somme say
> that they are tektites.
Hello Gaetan, Darryl, and List!
GRIEVE R.A.F. (1995) From the Editors: The Ries: Still giving up its
secrets (Meteoritics 30-3, 1995, p. 241, excerpts):
One of the advantages of Ries over other complex impact structures is
its relative youth, ca. 15 Ma. It is in a relatively good state of
preservation with portions of its exterior ejecta blanket intact. This
blanket, which consists of a lower unit, known as Bunte breccia, is made
up largely of unshocked to weakly shocked sedimentary material, overlain
by suevite that contains mostly crystalline material, showing a wide
range of shock states, up to various types of glass bombs.
Basically, the impact melt glasses in the suevite fall into two types:
vitreous light colored glasses, which occur at the top and bottom; and
devitrified dark brown glasses, which occur in the interior of the
suevite breccias. They are chemically similar, with the coloration due
to the oxidation of the slower-cooled glasses in the interior.
Devitrification products include crystallites and trichites of high
Al-pyroxene, plagioclase and magnetite. High-Al pyroxenes appear to be
characteristic of glassy impact melt rocks, with a high degree of
undercooling. We recently described them in glassy melt rocks from the
Wanapitei impact structure (Grieve and Ber, 1994).
Petrographic indicators and the results of experimental modelling of
devitrification suggest that the Ries suevite melts were generated from
the crystalline basement at initial temperatures in excess of 2000°C.
During the ejection process, these melts, in the form of bombs, cooled
rapidly, deformed viscously in flight and were deposited as part of the
suevite at temperatures below the glass transition temperature (i.e., as
solid glass bodies, at temperatures ca. 750°C). Those at the top and
bottom of the suevite deposit cooled below 650°C in less than an hour
and failed to devitrify. In order to cool this rapidly, conductive heat
losses are insufficient and convective heat losses, presumably through
the vertical degassing pipes that occur in the suevite, are required.
Those in the interior cooled more slowly and devitrified and oxidised in
the temperature range 750-650°C in presence of about 20-30 bars water
pressure. The presence of water is required to account for the
crystallization of plagioclase. In this respect, these glasses differ
from the moldavite tektites associated with the Ries, which are
essentially water free and perhaps, as suggested, the products of impact
vaporization and condensation rather than melting.
Best wishes,
Bernd
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