[meteorite-list] Shergotty a eucrite ... not: Then and Now

From: bernd.pauli at paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: 01 Jun 2007 19:25:33 UT
Message-ID: <DIIE.000000B900001A69_at_paulinet.de>

Hello List,

We all know that the acronym SNC stands for the three "typical" samples of the group:
Shergotty, Nakhla, and Chassigny. But 135 years ago, G. Tschermak was still unaware
of the exotic nature of these meteorites and grouped them with the eucrites. In 1872,
he wrote: "Shergotty, which I described several years ago, should also be included with
the eucrites. It is a conspicuously granular rock consisting essentially of dull yellow-gray
grains and prisms, which were found to be augite, and water-clear glassy grains and laths.
The latter resemble no known mineral; I named this material maskelynite."

The SNC's must have come from a different parent body - different from Earth, from our Moon,
and from the parent body or bodies of the HED suite. This is reflected in the displacement of
their oxygen isotopes when plotted in a diagram (see O.R. Norton's Encyclopedia of Meteorites,
p. 157, Fig. 8,15). Another telling feature of the SNC group is their young crystallization ages
- in other words they must have come from a celestial neighbor where volcanic activity was an
ongoing process not too long ago.

According to Susanne Schwenzer et al., the following observations are
considered highly indicative of a Martian origin of the SNC meteorites:

1. SNC meteorites belong to one group, as proven by many chemical and isotopic
investigations, pointing to a common parent body.

2. All SNC meteorites are differentiated magmatic rocks, with some of them showing
volcanic textures.

3. The young crystallization ages of 1.3 Gyr, requiring a parent body that still shows
igneous activity at such a late time in solar system history.

4. Direct evidence from Viking 1, when the concentrations and compositions of noble gases
in the Martian atmosphere were measured and compared to those found in SNC meteorites
in our collections.

5. The rovers Spirit and Opportunity found rocks on Mars with chemical signatures identical
to those of some of the Martian meteorites in our collections (Bounce Rock, for example, has
geochemical characteristics, such as the Fe/Mg ratio, consistent with that of shergottites).

Reference:

Schwenzer S. et al. (2007) Noble gases in mineral separates from three shergottites:
Shergotty, Zagami, and EETA79001 (MAPS 42-3, pp. 387-412, Introduction).
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Best wishes,

Bernd
Received on Fri 01 Jun 2007 03:25:33 PM PDT