[meteorite-list] Ordinary chondrites - rarest to the most common classes

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Thu, 17 Dec 2009 17:10:37 -0500
Message-ID: <4B2AAC5D.3050703_at_usgs.gov>

First off, be careful of the words "clan" and "class." The way most of
us use it, carbonaceous chondrites are a "class", comprising many
groups. Some C chondrite groups are associated into "clans", like the
CV-CK clan or the CM-CO clan. But not all people use the term clan, and
those who do sometimes differ in which groups are in which clans. In
any case, a "clan" is an association of groups that are thought to be
related in some way, not necessarily originating on the same parent body.

There are two major characteristics that separate ordinary from
carbonaceous chondrites. 1) OCs are above the terrestrial fractionation
line (TFL) for oxygen isotopes and CCs are below it (except for highly
altered ones). 2) OCs are depleted in refractory lithophile elements
and CCs either have solar abundances or above. (E chondrites are on the
TFL and depleted in refractories.)

R chondrites share these major properties with ordinary chondrites, and
therefore are better lumped with the OCs. In fact, bulk composition and
O isotopes are the key properties used to assign all the C chondrite
groups to the carbonaceous class. I think it is reasonable to do the
same with the R, H, L, and LL groups, that is, to assign them to a
common class. We don't really have a name for this class, as "ordinary
chondrites" have come to be synonymous with H-L-LL, which I would
consider to be a clan of this unnamed class. I would NOT put R
chondrites in the H-L-LL clan; they are in the same class.

I guess part of the confusion is whether "ordinary chondrites" has to
apply only to H-L-LL chondrites, or whether we can use this phrase as
the name of an entire class. My initial preference was to do the
latter. I said the ordinary chondrite class has two major clans, the
H-L-LL clan and the R clan (which has but one member). On reflection, a
more palatable solution would be to find a new name for this class, and
then we could refer to the ordinary chondrite clan and the R chondrite
clan within it.

So, what do we call this class? You can't use "noncarbonaceous
chondrites" because we also have the enstatite chondrites in their own
class. I have no idea.

Jeff


Carl 's wrote:
> Hi Jeff,
>
> I've been puzzled about what you said and perhaps I've misread or missed your comments. Why do you think the R chondrites should be included in the oc clan (rather than the carbonaceous)? I thought this was a very unique idea.
>
>
> Thank you all for this interesting topic.
>
> Carl
>
>
>
> Jeff Grossman wrote:
>
>
>> I didn't say they ARE included in the OCs... I
>>
> said that I thought they should be. As far as I
> know, I am alone in this opinion...
>
> and
>
>
>> ...If we take a more expansive definition of "ordinary chondrite" than most of my rather
>>
> conservative colleagues are normally willing to accept, I would say that
> the rarest group of OCs is the R chondrites (only ~100 are known and
> many of those are paired).In addition, a number of unique ungrouped
> meteorites are OC-like.But again, I don't know of any colleagues who
> agree with me that R chondrites are in the OC class. [I would say that
> the OC class has two clans, the H-L-LL clan and the R clan].
>
>
>
>
> _________________________________________________________________
> Hotmail: Powerful Free email with security by Microsoft.
> http://clk.atdmt.com/GBL/go/171222986/direct/01/
> ______________________________________________
> http://www.meteoritecentral.com
> Meteorite-list mailing list
> Meteorite-list at meteoritecentral.com
> http://six.pairlist.net/mailman/listinfo/meteorite-list
>
>

-- 
Dr. Jeffrey N. Grossman       phone: (703) 648-6184
US Geological Survey          fax:   (703) 648-6383
954 National Center
Reston, VA 20192, USA

Received on Thu 17 Dec 2009 05:10:37 PM PST