[meteorite-list] Equilibration vs Differentation

From: j.divelbiss_at_att.net <j.divelbiss_at_meteoritecentral.com>
Date: Thu Apr 22 10:25:40 2004
Message-ID: <20030515040105.83A8B5399F_at_pairlist.net>

Hello All,

A couple weeks ago I had a followup question to Elton and others about how do
you/we know when a planetoid/asteroid has gone from being a chondrite body to
being an achondrite body? I received great comments from Elton Jones and John
Curchin. Something to think about...

First(1) listed is my response to the original discussion to Elton...then
responses from John(2) and Elton(3) follow, respectively.
 JD

(1) 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.

(2)hello john:

i think i can help you with your achondrite/chondrite question (which we'll
address in a couple of weeks over a 'scope, as well!). the difference is
basically one of melting. the chondrites are aggregates of individual
grains - the equivalent of sedimentary rocks on earth (sort of). the
achondrites are from asteroids/planets that have melted, and hence are
distinguished by a 'crystalline' texture - minerals that grow from a melt in
an interlocking network. that's the short answer - we'll look at specific
examples when we get together soon.
 
cheers,

john curchin


(3)John,

You posed the question "When does a chondrite become an achondrite?"

When it has no more chondrules it technically becomes an achondrite(
save for the CI-CM 1's which have no chondrites but are chemically
similar to the other carbonaceous chondrites so we stuck them there)

As to the actual transition, we may not yet have continuous samples of
achondrites which formed from an H or L body that fully differentiated/
evolved. That combination is likely to be more resistant to impact
spalding or lie much deeper inside the parent body separated by many
kilometers. There are some chemical similarities which suggest a common
parent body-Type may have been excavated at depths sufficient to sample
most of the remelting products. We may have common chondrites and
achondrites which may have come from the same immediate parent but we
aren't sure. Perhaps chondrite to achondrite evolution is likened to the
fossil record. We have fossils which show evolution, but we don't have
a sequential record for any species.

I think the reality is there is no single achondrite product. The
process of differentiation carried away components that crystallize on
many different levels of the parent body. The primitive and ungrouped
meteorites that we have do have samples of tend to follow trends. The
Acapulcoite-Lodranite favor H Chondrite chemistry. The Aubrites tend to
follow bulk chemistry of the E Chondrites and so on.

These are the major classes of achondrites and represent the end or near
end stage of differentiation/ evolution. Studies suggest that some of
these may have been disrupted in the process of differentiation and are
"works in suspension". Achondrites which are non-planetary(SNC,L)
generally fall into four or so classes with ungrouped samples mixed
around and in between:
1)Basaltic: Eucrites Dioginites (Howardites being the
combination)Angrites. They exhibit crystals from any of several
pyroxenes(i.e. pigeonite, aegrine, hyperstene etc.)plus P.felspar
2) Magmatics: Urelites olivine-augite-pigeonite+ others indicating a
deep origin within a parent.
3) Enstatite: Aubrites which tend to be homogenous masses of the
pyroxene enstatite showing a bulk composition of this single mineral in
an oxygen depleted environment. The jury is out as to whether they are
magmatic or eruptive as in lava-like.
4) The Winonaties(Primative?) appear to come from the "near core
boundary" of iron in its parent and the type specimen may have come from
the Canyon Diablo impactor. These are so far removed from the chondrite
level it is unclear what the parent was.


So looking for transitional examples that we can clearly conclude are
from a chondrite parent, we come to the primitive chondrites. They have
chondrite chemistry but relic or no chondrules, metal depletion etc. and
represent step two of the conversion to achondrite. These aren't all
inclusive examples accounting for what we would expect to find but we
assume that all the chondrite conversions went through a state like
these have.

They have lost most, if not all, chondrule content, show depletion of
trace elements and/or removal of sulfides and metal. Or the metal may
be concentrated if you include the bencubinites (even if a carbonaceous
chondrite). They may exhibit some silicate melting or crystal growth or
develop an equiangular texture. I think the Acapulcoites are likely
chondrites which have started the process of differentiation. They and
the Lodranites share a common parent based on isotopic studies. Another
"equigranular-textured" meteorite of which only one example exists is
Itqiy. It appears uniformly textured.

As to when does an achondrite parent become one. Is it at the point an
iron core forms? Possibly, but there are some theories that a true
center core doesn't always form. Perhaps because of centrifugal forces
disruption, size, tempo, etc., there may be several large pockets of
pure metal throughout the asteroid.

Finally, as to the whole concept of differentiation, it can be fairly
simply described. If I take a mixture of mineral pellets or different
mineral types and mix them together I get a common chondrite cake. If I
melt them completely (magma) keep them hot enough for long enough and
the molecules with sort themselves out and form crystals the layered
Achondrite cake forms. If I interrupt the process by cracking open the
batter before it is done or turning off the oven then I'll get a
partially cooked combination.

Elton
 
     
Note: Other comments are welcome on this subject. JD
Received on Thu 15 May 2003 12:01:05 AM PDT