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Re: Tektite Formation/Tektite Ejection

 Full content for this article includes illustration, photograph, table and
    graph.
                                                                           
  
   Source:  Science, Feb 1, 1985 v227 p515(2).
                                                                           
  
    Title:  Lunar sample 14425: characterization and resemblance to
            high-magnesium microtektites.
   Author:  J.A. O'Keefe and B.P. Glass
                                                                           
  
 Subjects:  Lunar mineralogy - Research
            Tektite - Analysis
                                                                           
  
Electronic Collection:  A3624510
                   RN:  A3624510
                                                                           
  

Full Text COPYRIGHT American Association for the Advancement of Science
1985

Tektites, unlike stony or iron meteorites, cannot originate outside the
earthmoon system because they lack the isotopic indications--that is,
adequate levels of sup.10.Be, sup.26.Al, and so on--of exposure to primary
cosmic rays over periods of 10.sup.6 to 10.sup.7 years (1, 2). From their
distribution on the earth, it is clear that, whether terrestrial or lunar,
they were launched by a powerful mechanism, presumably either volcanism or
some kind of impact event. Earth volcanism is too feeble to produce the
observed strewn fields of tektites, up to halfway around the earth (3), and
impact on the moon would yield objects with much the same composition
(anorthositic gabbro or basalt) as most of the lunar crust. We are thus left
with two alternatives for their origin: namely, meterorite impact on the
earth or volcanic ejection from the moon.

The two alternatives lead to different explanations of the extinctions
that
appear to be associated with at least some tektite falls (3). If tektites
are
terrestrial, the extinctions may have been due to dust clouds in the
atmosphere, thrown up by an impact event, as implicity suggested by Alvarez
et al. (4) and Ganapathy (5). If, however, they are of lunar origin, then it
would be expected that the ash particles that missed the earth would
organize themselves into Saturn-like rings around the earth and that the
rings would be quantitatively sufficient to make significant climatic
changes (6).

Geochemists (7) generally favor origin from the earth because tektites are
closer to earth rocks than to most returned lunar samples in several
respects, particularly in age, potassium-uranium ratios, rare-earth
abundances, and oxygen isotopes. Some geophysicists, on the other hand, find
that a terrestrial origin appears to conflict with basic aerodynamic
principles (8) and with the physical principles that underlie the practice
of glass-making (9). Clearly this paradox would be illuminated if a tektite
should be found among the returned lunar samples. We describe the close
chemical resemblance of lunar sample 14425 to the high-magnesium, low-silica
microtektites of the Australasian strewn field (10).

The sample's diameter of 8.006 [plus or minus] 0.006 mm is constant within
about 0.1 percent over the surface. Its mass, 0.78410 [plus or minus]
0.00002 g, implies a specific gravity of 2.917 [plus or minus] 0.002. The
glass is black in some parts and brown in others. The surface is decorated
with mounds that have a metallic luster, the largest being about 0.9 mm in
diameter. By x-ray (Sperry model SPX, 200 kV, working at 60 kV, 4 mA) it is
found that the interior contains spherules up to 0.7 mm in diameter; the
mounds result when these spheres protrude through the glass surface (Fig.
1). The total volume of the spherical inclusions that are larger than 0.1 mm
in diameter is estimated at 0.6 percent of the volume of the sample.

The surface composition of the glass was examined with a Cambridge 150
scanning electron microscope together with energy-dispersive x-ray analysis
at the University of Delaware and by similar analysis with the Philips PSEM
500 electron microscope and an EDAX 9100 at Goddard Space Flight Center. In
order to preserve the surface, the sample was not ground and polished, nor
was it coated (for example, with carbon); it was used as found, and we
relied on the smoothness of much of the surface. At both laboratories the
sample was held in place with a brass holder, which produced spurious lines
of copper and zinc at both places and lead at Delaware. These lines
disappeared when an aluminum holder was substituted at Goddard, but this
produced anomalous aluminum values
that could not be corrected because of the aluminum in the sample. All of
these effects, in addition to some spurious lines of gold in the Goddard
spectra, resulted from the fact that without a conducting coat, the
surface
charges up and repels the electron beam, which then wanders. It is believed
that by comparison of the three holders, it has been possible to eliminate
these effects.

In both analyses, the small peak due to sodium was difficult to separate
from
the adjoining magnesium peak, which was about 100 times stronger. With the
use of a wavelength-dispersive Microspec system at Goddard, a value of 0.1
percent was found for Na.sub.2.O.

The means of the analyses on the glass are shown together with the standard
deviations in Table 1. In Fig. 2, measurements from both laboratories are
plotted against silica content, as in the Harker diagram. Measurements made
with the aluminum holder failed to show any substantial difference between
the light and the dark regions of the surface.

The composition of sample 14425 is different from the other known lunar
samples, so far as we can find. We have examined 1200 analyses of lunar
glasses (11), and 100 papers on lunar samples, including 50 described as
referring to lunar norites. The analysis of the ground-mass of lunar
sample
14068 (12), which is one of the closest fits, is plotted in Fig. 2; it is
clearly not the same material.

The small dots in Fig. 2 are plotted from a report (13) on bottle-green
(high-magnesium) microtektites from the Australasian strewn field. The
resemblance of the Australasian glasses to sample 14425 is conspicuous.
Microtektites from the Ivory Coast strewn field (not shown) do not fit as
well--for example, FeO averages 10 percent.

Two of the mounds with metallic luster were analyzed. The largest mound
showed iron and sulfur in a 5 to 2 ratio with a few percentages each of
nickel and phosphorus; a smaller mound gave iron and nickel in a 4 to 1
ratio, with small quantities of sulfur and phosphorus. A possible
interpretation would be nickel-iron spherules containing schreibersite and
troilite, such as have been reported for tektites (14) and impactites (15).
Reid et al. (16) did not find schreibersite in nickel-iron blebs from lunar
basalts. The presence of these spherules could be consistent either with
impact or lunar volcanism (17).

In conclusion, we find a close chemical resemblance between lunar sample
14425 and the high-magnesium microtektites. However, to establish that this
sample is a tektite, more data on age, isotopic composition, and trace
element abundances are required. In particular, the rare earth element
pattern should be studied for comparison with the data of Frey (18) on
high-magnesium microtektites. The siderophile element abundance ratios could
tell us whether the metal was derived from a meteorite or was produced by
reduction of the matrix glass.
                                                                           
  
                                -- End --



LOUIS VARRICCHIO
 Environmental Information Specialist &
 Producer/Writer, "Our Changing Planet"
  (Visit OCP-TV on the Web at: www.umac.org/ocp)
  Upper Midwest Aerospace Consortium
  Odegard School of Aerospace Sciences
  University of North Dakota
  Grand Forks, N.D. 58202-9007
    Phone: 701-777-2482
    Fax: 701-777-2940
    E-mail: varricch@umac.org (in N.D.); morbius@together.net (in Vt.)

"Behind every man alive stand thirty ghosts, for that is the ratio by
which the dead outnumber the living. Since the dawn of time, a hundred
billion human beings have walked the planet Earth." -- Arthur C. Clarke

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