[meteorite-list] Fossil, Relict, or Paleo- was "Fossil" NWA 2828

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Sun, 07 Jan 2007 09:52:01 -0500
Message-ID: <7.0.1.0.2.20070107072738.038a18d0_at_usgs.gov>

As I was heavily involved in developing the NomCom rules, let me give
my take on this whole discussion:

Our actions were stimulated by the Osterplana "fossil" meteorites, as
they are commonly called in the literature. For those who don't know
these, they are a group of several dozen objects embedded in
Ordovician age carbonate rocks in Sweden. Many of these have
chondritic structures, with chondrule textures clearly
visible. Others are simply dark clasts in the limestone. The only
primary meteoritic mineral left, if anything, is some chromite, and
everything else has been replaced by secondary, terrestrial
minerals. Thus, the chondrules are pseudomorphs and the chromite is
relict. Brunflo is a similar occurrence found in another quarry.

We needed a guideline to cover the naming of these objects, but the
usual guidelines for meteorites were inadequate. These meteorites
present a continuum of cases, from ones we were comfortable calling
"fossil" meteorites (they preserve the original structure and can be
classified, mostly as L chondrites) to ones that nobody can or will
ever classify, or even prove beyond a reasonable doubt to be
meteoritic in origin.

Now combine these occurrences with other things in the
literature. There are meteoritic clasts in some terrestrial rocks
that survive with most of their primary minerals and structures
intact. The Morokweng meteorite described in Nature by Maier et al
last year was also called a "fossil" meteorite, but only the metal
and sulfide were replaced by secondary minerals; silicates are little
altered. For the purpose of meteorite nomenclature, we would want to
treat this case like any other meteorite found on Earth... it's a
meteorite by any reckoning. I don't know the details, but perhaps
NWA 2828 is a similar case.

We also have things around like iron shale, which I don't think
should be distinguished from cases like the Osterplana objects, as
Sterling Webb does. Both types can contain some of the chemical
signature or structure of the original meteorite and are largely
composed of secondary minerals. However, these are not fossils, as
they are not preserved in the geologic record. They are the products
of weathering meteorites at the earth's surface. But from a
nomenclature perspective, they need to be treated the same as the
Osterplana objects.

This is why we came up with the operational term "relict
meteorite." It covers all objects that are largely altered, but
which have relict signatures indicating a meteoritic origin,
including composition, texture, isotopes, or whatever. Relict does
not mean fossil. Some relict meteorites are fossil meteorites, like
the classifiable Osterplana specimens and Brunflo, and some are not,
like a highly weathered, barely recognizable stone from the Sahara, a
piece of iron shale from Arizona, or a chip of fusion crust collected
on the Antarctic ice (like LAP 04531). Conversely, some fossil
meteorites are not relict meteorites, like the Morokweng stone. The
terms relict and fossil are independent of each other. (I have no
opinion on this term "paleometeorite" which I have not seen defined.)

One last comment. The Wlotzka weathering scale (W0-W6) is not
applicable to anything but ordinary chondrites. This is why we can't
just extend it to W7 and use it to cover all highly weathered meteorites.

jeff

At 12:46 AM 1/7/2007, Mr EMan wrote:
>Under this NomCom guideline NWA2828 isn't "relict" as
>it is hardly altered and should be referred to as a
>paleo meteorite. (Note:If this gets too drawn out all
>meteoritic material is paleo as most is 4.5 billion
>years old). However, paleo is a best choice of the
>three proposed terms.
>
>My take on the three options:
>
>Relict: in petrology and geology is used to describe
>the occurrence of traces of original material after
>alteration. e.g. Serpentine is the hydrated
>alteration product of olivine and the presence of
>olivine or peridot within serpentine would be referred
>to as "relict olivine" etc. Lignite within a coal seam
>is relict lignite. NWA2828 is hardly relict under this
>definition and the NomCom guidelines. However, Relict
>is a valid incorporation of the concept into
>meteorites. Note that Relict is consistent with the
>almost complete alteration to secondary minerals.
>Where "Fossil" may include replacement of the original
>mineral. This is a subtle but important distinction.
>
>Fossil: (Greek Dug or to Dig) Obviously evolved this
>term is in wide use but rarely specified. It is
>usually descriptive of any ancient "organically"
>produced artifact; Trace, imprint, hard or soft
>tissue, premineralized, mineralized segment,
>mummified-- in some fashion altered from its original
>composition or state. By convention and to which
>source one subscribes,a fossil must be older than
>20,000 OR 2 million years, cannot be derived from a
>living species, nor produced artificially .
>(AFAIRecall). Charcoal from the wildfire caused by
>Canyon Diablo can't be fossil but is paleo. Omitting
>the organically derived stipulation "Fossil" has
>been applied for example, to describe meteorites which
>were found in Ordovician aged sediments where the
>meteorites had been completely altered. It is also
>loosely used to describe ancient geological processes
>NWA2828 doesn't meet the criteria for being fossil.
>
>Paleo: denotes "ancient" and is used to describe
>events or things that are prehistoric--prehistory as
>in what that was not recorded: also to describe a
>process, condition or state occurring before the
>present Paleosoil, paleoatmosphere, paleoclimate, etc.
>The Winona Meteorite has been called paleo and was
>associated with a paleo settlement. Under strict
>reference Wolf Creek, Canyon Diablo, Winona, Lake
>Murray -- all would be paleo meteorites, as would
>anything derived from a prehistoric event as would
>tektites however that distinction need not always be
>applied.
>
>By elimination and like it or not PALEO is a best fit
>for NWA2828. As with any science, this represents the
>discovery of yet another distinction that we need a
>new category for. Both "fossilized" and "relict" seem
>to be subsets of the term "paleo".
>
>Eman
>
>
>--- David Weir <dgweir at earthlink.net> wrote:
>
> >
> > Yea, "fossil" may be accurate or maybe not, but why
> > not use the broader
> > terminology as designated by NomCom in their latest
> > revision in which
> > this new category is proposed?
> >
> > Read it here in section 1.2(c) Relict meteorites:
> >
> >
>http://www.meteoriticalsociety.org/bulletin/nc-guidelines.htm#s12c
> >
> > This section is copied here for your convenience:
> >
> > c) Special provisions are made in these Guidelines
> > for highly altered
> > materials that may have a meteoritic origin,
> > designated relict
> > meteorites, which are dominantly (>95%) composed of
> > secondary minerals
> > formed on the body on which the object was found.
> > Examples of such
> > material may include some types of "meteorite
> > shale," "fossil
> > meteorites," and fusion crust.
> >
> > David
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Dr. Jeffrey N. Grossman phone: (703) 648-6184
US Geological Survey fax: (703) 648-6383
954 National Center
Reston, VA 20192, USA
Received on Sun 07 Jan 2007 09:52:01 AM PST