[meteorite-list] AW: New Mars Meteorite Found In Morocco (NWA 1950)

From: j.divelbiss_at_att.net <j.divelbiss_at_meteoritecentral.com>
Date: Thu Apr 22 10:28:35 2004
Message-ID: <102420030000.1591.20aa_at_att.net>

David, Norbert and others:

Statement of fact: I am a novice and I don't know exactly what I'm talking
about it.

Comment: With that said I find this whole subject of naming martian rocks to
be somewhat difficult to sort out, or understand completely. Here goes...

1. Chassignites - very, very rare "martian" meteorite that is almost entirely
composed of iron rich olivine, with a small amount of chromite,
orthopyroxene, feldpars, and a tiny bit of hornblende. This rock is the
equivalent of Dunite here on earth. On the gabroic triangle figure/chart,
Dunite would be at the bottom center below the Peridotites that are split
into Wehrlite(clinopyroxene based), Lherzolite(a residual melt/mixture of
Wehrlite and Harzburgite), and Harzburgite(orthopyroxene based).

2. Nakhlites - a rare, but becoming less rare "martian" meteorite that is
mostly made up of augite clinopyroxene, and has iron rich olivine, and small
amounts of feldspars and other clinopyroxenes. This rock is the equivalent to
earthly clinopyroxenite and olivine clinopyroxenite which both can be
classified as a type of gabbro on earth.

3. Shergottites - a more typical "martian" meteorite that is mostly made up
of clinopyroxene, and has several varieties based on the amount of olivine
and feldspars to go along with the clinopyroxene. Now these are usually
called basalts because of the mineral makeup that is similar for extrusive
(volcanic) basalt and intrusive (dike/sill magma) basalt here on earth which
is also know as gabbro.

4. Orthopyroxenite - ALH 84001 is not even close to these others from a
mineral standpoint and it stands alone in the "martian" category. On earth it
is also called an orthopyroxenite which is also known as a norite instead of
a gabbro or basalt.

A martian or earthly Lherzolite is really not a basalt/gabbro (shergottite),
but a mixture or melt of a clinopyroxenite (Nakhlite) and an orthopyroxenite
(ALH 84001). And depending on the level of one pyroxene versus another it is
closer to one or the other. As Norbert suggested it must have more
clinopyroxene than orthopyroxene, so that is why he said it is closer to a
Nakhlite.

Is anyone confused yet??? Bottom line is that I agree that NWA 1950 maybe
misrepresented by calling it a shergottite...or the boundaries for being a
shergottite are a lot broader than the other categories. Does anyone have a
clear understanding of this criteria?

Also, is there any known criteria for the shergottites that differentiates
them from being extrusive or intrusive rocks...and if so, would it not be
clearer to call them either a basalt or a gabbro?

Going to deep I suppose,

John


     
> Hello Norbert,
>
> I wonder why it is still the convention to name this Martian group
> "lherzolitic shergottites". Although this geochemical group was
> historically included as a subgroup within the shergottite class, and
> therefore its members were called lherzolitic shergottites (or
> shergottitic peridotites), there is no genetic relationship between the
> basaltic and lherzolitic subgroups. The term "lherzolites" was proposed
> by Eugster and Polnau in 1997 to represent this unique group of Martian
> meteorites. In fact, they showed that the chemical composition of the
> basaltic shergottite group is closer to the nakhlites than to the
> lherzolites. Furthermore, these groups are resolved from each other on
> an O-isotope plot.
>
> I look forward to my next meeting with Bruno and Carine to add a small
> sample of this rare lherzolite to my collection (but not as big as yours
> Norbert!).
>
> David
>
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Received on Thu 23 Oct 2003 08:00:39 PM PDT