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Tektite Formation/Tektite Ejection

Hello dormant list!

As the list is still relatively quiet, I'd like to pour some more oil on
the fiery tektite debate :-)

SCOTT E.R.D. (1999) How were tektites formed and ejected? (MAPS 34-4,
1999, A 103):

Four tektite strewn fields and many kinds of tektite-like glasses were
formed by impact melting of sediments and ejection of glasses over
distances of up to 10^4 km [1,2]. Three of the tektite-producing impacts
formed craters 10-90 km in diameter; a source crater for the
Australasian tektites has not been identified.
Two major constraints on tektite formation have not been addressed: the
remarkable absence of target fragments and the lack of projectile
contamination, which are characteristic features of most other impact
melts. Traces of shocked grains are found in Muong Nong types but are
absent in other tektites [1]. Iridium concentrations indicate the
presence of <0.01-0.001% projectile, whereas most other terrestrial and
lunar impact melts contain much higher inferred concentrations [3,4].
The lack of siderophiles in impact melts from one crater might be
attributed to an achondritic or low-lr iron projectile [4], but it is
unlikely that all four projectiles were Ir-poor. However, normal
Ir-bearing impact melts have not yet been identified at any
tektite-forming sites. High-velocity projectiles, such as comets, can
also form low-Ir impact melts.
Nearly all authors favoring an impact origin for tektites infer that
melting was due to shock compression of the target by the projectile.
But the absence in tektites of unmelted target grains and projectile
material is very difficult to reconcile with this origin. If tektites
had formed by jetting at the projectile-target interface as many argue
[5], they should be especially rich in siderophiles from the projectile
[6,7]. Near-surface materials that are rapidly ejected by spallation are
relatively lightly shocked. The bulk of the impact melts, which form
subsequently, are intimate mixtures of melts and shocked and unshocked
clasts from diverse depths. Thus, alternative impact processes for
melting and ejecting tektites should be explored.
On Earth, the atmosphere plays a major role in heating and transporting
target materials in impacts of subkilometer and kilometer projectiles.
On Venus, craters <30 km in diameter are rare, unusually shallow, and
multiple or complex and irregular in shape [8,9] due to projectile
flattening by the atmosphere and catastrophic fragmentation and
explosion [9-11]. On Earth, a high proportion of craters in the size
range 0.1-10 km were formed by iron meteoroids [4], and some small
fresh, terrestrial craters like the 14-km Zhamanshin structure are
unusually shallow and irregularly shaped, rather than bowl-shaped [12].
Both features are consistent with atmospheric fragmentation, which
enhances atmospheric heating.
Interactions between atmosphere and projectile are complex and difficult
to model [13], and several possible processes might have been involved
in forming and ejecting tektites. Tektites may have formed by radiative
heating from the atmospheric plume [14,15], or surface material may have
been injected by impact-generated winds into hot ionized trailing wakes.
Catastrophic atmospheric disruption of stony asteroids generates peak
shock pressures > 1 GPa in surface rock on Venus [10], but melting would
require much higher pressures.
The absence of a well-defined crater for the Australasian tektites was
attributed by Wasson [14] to a comet that fragmented before reaching
Earth, forming numerous small craters. However, an asteroid that
catastrophically fragmented close to the surface may have been
responsible. Even if a crater did form, it might be shallow and complex
like Zhamanshin [12], where tektite-like irghizites deficient in Ir [4]
are found.

Conclusion:

The lack of projectile material and target fragments implies that
tektites did not form by shock compression. A more plausible
explanation is that surface sediments were melted prior to
ejection by an impact-heated atmosphere or after ejection inside
the hot trailing wake of the projectile.

References:

[1] Glass B.P. (1982) Introduction to Planetary Geology, pp. 145-172,
     Cambridge Univ.
[2] Koeberl C. (1994) GSA Spec. Paper 293, 133-151.
[3] Koeberl C. (1998) Geol. Soc. Lond. Spec. Pub. 140, 133-153.
[4] Palme H. (1982) GSA Spec. Paper 190, 223-233.
[5] Vickery A.M. (1989) LPS XX, 1154-1155.
[6] Melosh H.J. (1989) Impact Cratering, Oxford.
[7] Melosh H.J. (1998) MAPS, 33, A104.
[8] Schaber G.G. et al. (1992) JGR 97, 13257-13301.
[9] Herrick R. and Phillips R.J. (1994) Icarus 112, 253-281.
[10] Zahnle K.J. (1992) JGR 97, 10243-10255.
[11] Lyne J.E. et al. (1996) JGR 101, 23207-23212.
[12] Garvin J.B., Schnetzler C.C. (1994) GSA Spec. Paper 293, 249-257.
[13] Schultz P.H. (1992) JGR 97, 975-1005.
[14] Wasson J.T. (1995) LPS XXVI, 1469-1470.
[15] Boslough M.B. and Crawford D.A. (1996) LPS XXVII, 135-136.


Best wishes,

Bernd

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