[meteorite-list] Classification question-Equilibration vs Differentation

From: j.divelbiss_at_att.net <j.divelbiss_at_meteoritecentral.com>
Date: Thu Apr 22 10:14:08 2004
Message-ID: <20030429014032.B8F205376F_at_pairlist.net>

Elton,
great explanation. I'll ask a question I asked earlier this year...at what
point(material texture/composition) does a chondrite body differentiate enough
to become an achondrite...when an iron-nickel core begins to form or before
this point? Is there an in between state? Are they called the primitive
chondrites?

The transition between a chondritic body to an anchondritic body through
differentiation seems to be a bit of mystery...at least to me.

John
> Hello All,
>
> Lars asked: Is the whole petrologic group 3 (and below) what is called
> unequilibrated, and from group 4 we are talking different degrees of
> chemical unequilibration ?
>
> So far as I am aware(<--disclaimer)The Short and Long of it.
>
> Short answer: Level 3 is the intersection point where chondrules are
> unaltered. Above level 3.0 is alteration due to heating and is limited
> to Chondrites other than Carbonaceous (E,H,L,LL,R). Levels 2 and 1 are
> chemical alteration (usually water) and is limited to Carbonaceous
> Chondrites. Level Zero (0) is used to indicate a complete absence of
> chondrules with no inferences as to alteration.
>
> Alteration and equilibration aren't the same processes. Alteration, as
> used here, is a reference to hydration and dissolution by a fluid to
> change the chemistry and form of the meteorite. Equilibration is the
> physical relaxation of boundaries on a micro level although some levels
> of heat and proximity may allow for minor chemistry changes.
>
>
> Long answer:
> Like the fairy tale, this is the soup bowl which was not to hot and not
> to cold. Equilibration is a relative process.
>
> Equilibration in this sense refers to the process of changing
> (metamorphosing) the original conglomeration of objects (chondrules,
> inclusions, etc.) within the parent body. Heating from radioactive decay
> warms up the internal areas of the parent body and slowly starts melting
> the material which has coalesced from the solar nebula.
>
> For common chondrites we must assume they started as level 3.0s. As it
> goes up the stages from 3-6, there are incresing degrees of change. The
> original boundaries between the chondrules and matrix breakdown,
> dissolve,etc. This results in a blend of the molecules (i.e."equalize")
> of various mineral pockets within the meteorite into a locally
> consistent and balanced mixture. When internal radioactive decay ceases
> or heat is lost faster than generated such as in a collision which opens
> the interior of the parent, the metamorphosing stops . To be
> equilibrated in this sense is to be homogenized. Note that this point is
> just short of differentiating, as in forming crystals and distinct
> mineral masses.
>
> If you go beyond level 6 (an undefined stage some call it level 7) you
> have a homogenous melt of which I don't have an approved example but
> look at the sort of things which happen in the acapulcoites-lodrinites
> etc. Additionally an impact melt is assumed to have occurred after the
> equilibration process and isn't a part of that process and scale even if
> it is an extreme form of melting. When the internal temperature can be
> held high enough, long enough--usually limited to larger bodies, it goes
> beyond "level 7" to enter the realm of Achondrites, which come from a
> "differentiated" body. Differentiated means the mineral molecules have
> migrated to zones and sorted themselves into crystals based on a myriad
> of factors such as density, melt temperatures, cooling rates and so on.
>
> Regards,
> Elton
>
>
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Received on Mon 28 Apr 2003 09:40:32 PM PDT