[meteorite-list] Classification question-Equilibration vs Differentation

From: E.L. Jones <jonee_at_meteoritecentral.com>
Date: Thu Apr 22 10:14:08 2004
Message-ID: <3EAD96C0.7070002_at_epix.net>

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
Received on Mon 28 Apr 2003 05:01:52 PM PDT