[meteorite-list] Planetary Meteorites - Part 2 of 2

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:55:45 2004
Message-ID: <3C407538.F5903BF8_at_lehrer1.rz.uni-karlsruhe.de>

Allan wrote:

> There were suggestions in print that the SNCs
> were from Mars at least as early as 1979.

... and mentioned the following reference:

WASSON J.T. et al. (1979) Dynamical, chemica, and isotopic
evidence regarding the formation locations of asteroids and
meteorites (In Asteroids, ed. T. Gehrels, pp. 926-974, Univ.
Arizona Press, Tucson, Arizona).


Hello Again!

Some very interesting assumptions - especially now that we can look back
on these ideas with all the scientific knowledge we have accumulated in
the meantime, but, please bear in mind w h e n this paper was written!


Best wishes,
and enjoy,

Bernd


Planetary surfaces, pp. 963-965:

Although detailed studies have not been made, preliminary work shows
that an appreciable quantity of Mars ejecta will become Earth-crossing
on the short time scale associated with the exposure ages of stony
meteorites. It is not clear if collisions of this ejecta with asteroidal
material will be sufficiently frequent to eliminate unobserved longer
exposure ages.
Ejecta from Mars would be expected to be differentiated and heavily
shocked, perhaps severely enough to reset radiometric clocks. The
differentiated stony meteorite Shergotty (like its sister meteorite
Zagami) has been heavily altered by shock; in particular, the abundant
plagioclase has been converted to glass. A recent Rb-Sr investigation by
Nyquist et al. (1979) yielded a Rb-Sr isochron age of 165 Myr; a
39Ar/40Ar age by Bogard (see his chapter in this book) gave a poorly
defined plateau of ~250 Myr. The Rb-Sr age is interpreted as the time
when the shock alteration occurred, the higher 39Ar/40Ar age presumably
reflecting incomplete Ar outgassing during the shock event. If any
meteorite came from Mars, Shergotty seems the best candidate. The chief
problem with this hypothesis seems to be its short cosmic ray age of 2
Myr. If Shergotty was removed from Mars 165 Myr ago, the ejected mass
must have had dimensions of >/ ~5 m until ~2 Myr ago. Nyquist et al.
infer a diameter of ~100 m in order to retain enough heat to permit
diffusive resetting of the Rb-Sr and argon ages at 165 Myr. Here the
tektite analogy may break down; we have no evidence that such large
masses can be ejected from the surface of a planet as massive as Mars.
As noted by Stolper et al. (1979), Nakhla and two siblings (Lafayette,
Governador Valadares) and Chassigny and a sister (Brachina) may be
related to Shergotty. The O-isotope data of Shergotty and the Nakhla
twin Lafayette are consistent with such a relationship (Clayton et al.
1976), and rare-earth data suggest a genetic link between Nakhla and
Chassigny (Boynton et al. 1976). Radiometric ages of the Nakhla trio are
~1.3 Gyr (Podosek 1973; Papanastassiou and Wasserburg 1974; Gale et al.
1975; Bogard and Husain 1977), and a K-Ar age of 1.39 Gyr for Chassigny
was reported by Lancet and Lancet (1971). These low ages and the
presence of hydrated silicates (Bunch and Reid 1975; Floran et al. 1978)
in these meteorites would be consistent with an origin on Mars.
Chassigny has been shocked to pressures of 150-200 kbar, while evidence
for shock in the Nakhla trio is absent. Papanastassiou and Wasserburg
(1974) attribute some scatter in the Nakhla Rb-Sr mineral isochron to
late metamorphism, whereas Gale et al. (1975) hold that the ~1.3 Gyr
event was of igneous origin. As discussed for Shergotty, the relatively
low cosmic ray ages of the Nakhla trio of 10 Myr (Ganapathy and Anders
1969) and 9 Myr for Chassigny (Lancet and Lancet 1971) may present
problems for an origin on Mars.
Although it is obvious from the foregoing discussion that there are
serious difficulties which must be surmounted before a Martian origin
for these meteorites appears likely, the possibility that we have rocks
from Mars in our meteorite collections is important enough to warrant
giving some attention to this consideration.
Received on Sat 12 Jan 2002 12:41:12 PM PST