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TEKTITE QUESTION

>From the editors => Tektites: Some unresolved problems (Meteoritics
31-1, Jan 1996, p. 4)

These small glassy bodies continue to arouse the interest of the
scientific world. One might have thought, following the lunar sample
return that demonstrated that "t e k t i t e s  w e r e  t e r r e s t r
i a l," that the subject would have become of interest mainly to the
historians of science. In the words of Schnetzler (1970), "t h e  l u n
a r  o r i g i n  of tektites, a controversial and stimulating theory on
the scientific scene for almost 75 years, d i e d on  J u l y 20, 1969.
The cause of death has been diagnosed as a massive overdose of lunar
data." However in this issue,there are no less than four papers on
various aspects of tektites, one coauthored by the indomitable C.
Schnetzler himself. For despite the resolution of the debate over a
lunar vs. terrestrial origin more than a quarter of a century ago,
interesting scientific problems remain. High on the list of unresolved
questions is the source region for the Australasian strewn field, that
covers >10% of the surface of the Earth. The mass and areal distribution
of these tektites have long pointed to a source somewhere in eastern
Indochina (Taylor, 1968). The  e v e n t  is  d a t e d  rather
precisely at  7 7 0,0 0 0 years ago, but the crater remains elusive. The
paper by Schnetzler and McHone specifically addresses the question of
whether a site in Laos is a possibility. They found only "flat-lying,
undisturbed Mesozoic sedimentary rocks" in the area, with no sign of a
candidate crater. Of course, the c h e m i c a l and i s o t o p i c   
e v i d e n c e in tektites have long identified such rocks as having
the r i g h t   a g e and c o m p o s i t i o n  to be potential source
material (e.g.,Blum et al., 1991). So the search continues but it is
narrowing.
Two papers address the problems of the distribution and origin of the
large layered, or Muong Nong-type tektites (e.g., Koeberl, 1992) that
are found in an area that extends for over 1000 km from Hainan Island to
the southern regions of Indochina. The Muong Nong tektites were helpful
in establishing the sedimentary nature of the source material since they
contained relict grains of both chromite and zircon, which are uncommon
bedfellows in igneous rocks. The largest reported piece weighs 24 kg,
and everyone agrees that their present distribution is close to the
source.
There are currently two hypotheses to explain the origin of these
layered tektites. Wasson (1995), following earlier ideas of Barnes and
Pitakpaivan (1962), has postulated that they formed as puddles of melt
when the surface was heated (to 2300 K according to Wasson) by a comet
that broke up, in a manner analogous to Shoemaker-Levy 9, and exploded
over the region. Thus, according to this hypothesis, the layered
tektites are in situ. In the other model, they were ejected from the
parent crater and are analogues of volcanic bombs. The first of two
papers on this topic by Fiske, Puttapiban and Wasson deals with the
results of a field expedition to northeast Thailand to test this
hypothesis. Among other benefits, the c a r e f u l  f i e l d  w o r k,
with excavations conducted in the manner of archaelogical digs, d i s p
o s e s  o f the "age paradox" (e.g., lzokh, 1994) that suggested a
recent (ca. 1 0, 0 0 0 years ago) event and confirms the assessment of
Fudali (1993) that there was  o n l y  o n e  A u s t r a l a s i a n
tektite  e v e n t 770,000 years ago.
In a companion paper, Fiske advances arguments in support of the more
conventional hypothesis that the layered Muong Nong-type tektites
represent the local ejecta from the tektite-forming impact, while the
splash forms of which the button-type australite form the most dramatic
example of atmospheric reentry at a few km/s were scattered over several
thousand kilometers. His careful interpretation of the excavated 6 kg of
fragments that were derived from a "single homogeneous mass" certainly
lends little support to the melted puddle hypothesis, all features being
equally well explained by deformation during flight or landing.

Meanwhile, Prasad and Sudhakar report on a well-preserved tektite
recovered from the  I n d i a n  O c e a n  that shows  e v i d e n c e
of  i m p a c t  WITH OTHER MICROTEKTITES DURING FLIGHT AT ELEVATED
TEMPERATURES. This remarkable object is the first of its kind to be
reported and clearly provides clues to conditions early in the
tektite-forming event. Like other oceanic finds, this microtektite is
located at the Brunhes-Matuyama magnetic reversal boundary, a
coincidence of doubtful genetic significance but serving once again to
tie down the age of the event. Microcraters of the sort observed are   
c o m m o n  on  l u n a r  g l a s s  spherules on the lunar surface,
but hopefully this new observation will not encourage a Lazarus-like
reappearance of the lunar origin hypothesis.

S.R. Taylor Associate Editor



Meteoritics & Planetary Science 31, 46-49 (1996)
ã Metcoritical Society, 1996.  Printed in USA.

Impact microcraters on an Australasian microtektite

M. SHYAM PRASAD* AND M. SUDHAKAR
Geological Oceanography Division, National Institute of Oceanography,
Goa 403004, India
*Correspondence author's e-mail: shyam@bcgoa.ernet.in

(Received 1994 January10; accepted in revised form 1995 August 24)


Abstract - Microcraters attributable to impact have been discovered on
an Australasian microtektite from a CORE in the Central Indian Basin.
The craters resemble lunar microcraters and those generated during
impact experiments. The largest crater here, which has a welded
promontory, is unique. The projectiles that produced the impacts defined
varying trajectories and velocities, ranging from hypervelocity to low
velocity (a few 10 m/s). The impacts took place while the microtektite
was in flight at an elevated target temperature. This is the first
observation of the microimpact phenomenon on a microtektite.

INTRODUCTION

Impact microcraters, generated by cosmic dust colliding with the Moon,
have been found ubiquitously on lunar samples (LSPET, 1969; Horz et al.,
1971; Morrison et al., 1972).  Microimpacts have also been simulated in
the lab by particle accelerators (Vedder, 1971; Vedder and Mandeville,
1974) and at other facilities (Cintala, 1987). The purpose of these and
other related studies has been primarily to understand the formation of
lunar regolith (Cintala and Horz, 1988) and the micrometeoroid complex
at 1 AU (Hörz et al., 1975). Also, these studies provide data for the
design of sensitive parts of spacecraft that are likely to be impacted
while in space (Christiansen, 1993).
On tektites, the  o n l y  o b s e r v a t i o n  of this phenomenon has
been by Margolis et al.(1971), who reported POSSIBLE IMPACT SPLASHES
BENEATH THE FLANGES OF AUSTRALITE BUTTONS. Gault and Wedekind (1969)
carried out comminution experiments on tektite simulant glasses, which
has been the only such study to date. Extensive experiments into
SiO2-glass targets were reported by Schneider et al. (1990).

Regards, Bernd