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Re: Equilibrated vs. Unequilibrated

At 11:53 PM 6/24/98 EDT, you wrote:
>Hello Everyone,
>
>The May, 1998 issue of Meteorite! defines the above terms in the Ask the
>Geologist section but I must admit, after (re)reading it, I still have a hard
>grasping the concept as applied to meteorites (sorry Joel).  Would someone
>more enlightened than myself please explain this using meteoritical examples?
>
>-Walter
>

Here's how these terms apply to chondrites.  They refer to the effects
of thermal metamorphism (regardless of what causes it).

* Equilibrated chondrites generally have olivine, (Fe,Mg)2SiO4, that
is all one composition.  Operationally, we define this as the standard
deviation of the Fe/(Fe+Mg) ratios of many olivine grains being less
than 5% relative.  We believe that most chondrites started out with
more heteorogeneous olivine compositions, and that thermal metamorphism
caused diffusional Fe-Mg exchange between the grains until an
"equilibrium" value was attained.  Chondrites with larger sigmas are
called "unequilibrated."  H chondrites equilibrate to a different
Fe/(Fe+Mg) ratio in olivine and pyroxene than do L (and LL) chondrites,
which led to the old terms "bronzite-olivine chondrite" and "olivine-
hypersthene chondrite" for H and L.  Highly unequilibrated ordinary
chondrites like Sharps (H3.4) and Chainpur (LL3.4) are hard to tell
apart by their mineralogy, since both have really wide ranges of
mineral compositions.

* Pyroxene does the same sort of thing as olivine, except that diffusion
is slower in this mineral, and equilibration may be incomplete
even in meteorites with fairly homogeneous olivine.

* Thermally metamorphosed meteorites may also have more "equilibrated"
textures.  Extensive thermal annealing tends to coarsen grain-sizes, leading
to a blurring of chondrule outlines and the disappearance of fine-grained
matrix in high-petrologic type chondrites.  A fully equilibrated
texture would have equant olivine and pyroxene grains meeting at 120-degree
triple-junctions (like the way tiled hexagons do).  Textures like that
are generally only seen in primitive achondrites (closely related
to chondrites).

Most ordinary, enstatite, and R chondrites are "equilibrated" with respect
to olivine and pyroxene compositions.  ~5% are unequilibrated.

Most carbonaceous chondrites are "unequilibrated."  Only the rare
CK group has a significant number of metamorphosed members.

jeff