[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: <102420030032.25888.5e37_at_att.net>

Hello all,

As usual I goofed...Norbert said NWA 1950 was closer to a Chassignite and not
a Nakhlite...because it is has over 50% olivine. And after reviewing the
Bulletin it appears it has about 30% pigeonite(a clinopyroxene). So it really
is in between a Chassignite and a Nakhlite. Which got it(1950) generalized as
a Shergottite!!!

Sheesh...no wonder I'm confused,

John

 
> 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 08:32:26 PM PDT