[meteorite-list] Paper on chondrule formation and synthetic chondrules

From: starsandscopes at aol.com <starsandscopes_at_meteoritecentral.com>
Date: Tue, 19 Jan 2010 18:54:48 EST
Message-ID: <153f0.5e7ebe00.3887a048_at_aol.com>

Hi List, I thought some of you might enjoy this portion of a science
paper on meteorite chondrules. It is part of a paper on microscopes posted in
Molecular Expressions (An online microscope site) The first half of the
paper is on microscopes so many of you will want to skip that part.
Tom Phillips

PHOTOMICROGRAPHY IN THE
GEOLOGICAL SCIENCES
Michael W. Davidson
Institute of Molecular Biophysics
Center for Materials Research and Technology (MARTECH)
National High Magnetic Field Laboratory (NHMFL)
Supercomputer Computations Research Institute (SCRI)
Florida State University, Tallahassee, Florida 32306
Telephone: 850-644-0542 Fax: 850-644-8920

Gary E. Lofgren
Planetary Materials Branch
Solar System Exploration Division
Code SN2
NASA Johnson Space Center
Houston, Texas 77058
Telephone: 713-483-6187 Fax: 713-483-2696

The whole article is at
http://micro.magnet.fsu.edu/publications/pages/journal.html



Chondrules are small spheres (.1 to 10mm in diameter) which are the major
constituent of chondritic meteorites. Chondrites are considered samples of
primitive solar system materials. If we can understand how chondrules form,
we will have an important clue to the early history of our solar system.
Most chondrules have an igneous texture which forms by crystal growth
(usually rapid) from a supercooled melt. Such textures are commonly described as
porphyritic (large, equant crystals in a fine grained matrix), barred
(dendrites comprised of parallel thin blades or plates), or radiating (sprays of
fine fibers).
The models proposed for formation of chondrules can be divided into two
groups (McSween, 1977). In one group of models, chondrules form by melting
and subsequent crystallization of preexisting, largely crystalline material
from the solar nebula. The primary differences between these models are the
kinds of materials which are melted and the nature of the sources of heat
for the melting. In the other group of models, chondrules form by
condensation of liquids from the solar nebula gas which then crystallize upon cooling.
 Variations between these models result from differences in the
condensation sequence of the minerals and melts and the temperatures of nucleation.
One means of testing models of chondrule formation is to determine the
conditions necessary to duplicate these textures by experimentally
crystallizing chondrule melts in the laboratory. Efforts to reproduce the textures of
chondrules experimentally have been successful only when we began to
understand the important role that heterogeneous nucleation plays in the
development of igneous rock textures. Unless heterogeneous nuclei are present in
the chondrule melt, porphyritic textures will not be produced. The dendritic
or radiating textures will form instead.
The treatment of heterogeneous nucleation follows the model developed by
Turnbull (1950) to explain many of the characteristics of heterogeneous
nucleation. This model was applied to heterogeneous nucleation in basaltic
systems by Lofgren (1983). Simply stated, the model says that in any
steady-state melt at a given temperature there is a characteristic distribution of
embryos. The embryo is crystalline material which is smaller than the
critical size necessary to be a stable nucleus and cause nucleation. It is a
subcritical-sized potential heterogeneous nucleus. Embryos exist whether stable,
supercritically-sized nuclei are present or not. If a melt is sufficiently
superheated, embryos can be eliminated. Nucleation would then require a
surface, presumably the container and the barrier to nucleation would be much
higher than in the case where embryos were present. Qualitatively, such
nucleation would resemble homogeneous nucleation; but, if a surface is
available, the energy barrier would be much lower than for homogeneous nucleation.
Glasses would form from chondrule melts most readily if they are
superheated, thus destroying the embryos and increasing the barrier to nucleation.
Lower melting temperatures would allow embryos to be retained. These can
then grow upon cooling and become nuclei. Embryos also can become nuclei
without changing size, because the size at which an embryo becomes a nucleus
depends upon the degree of supercooling in the melt. Thus, an increase in the
degree of supercooling can cause an embryo to become a nucleus and
nucleation to occur.
If relict crystals are present in the melt at the initiation of cooling,
the more equilibrium-like crystals typical of porphyritic textures are
formed. When such experiments are quenched, the final product contains glass or
fine grained material, often dendritic, enclosing the equilibrium
phenocrysts. An example of this texture produced in experiments is shown in Figure
7. Equant, well formed crystals of olivine are set in a glassy matrix with a
few dendrites present. In the natural prophyritic chondrule the glass has
usually crystallized to a very fine grained material. In general, the size
of the phenocrysts decreases and their number increases as the temperature
at which the crystalline starting material melted is lowered and thus the
number of nuclei increases. The range of conditions that control the
development of porphyritic texture has been studied as a function of variations in
the number, distribution, and kinds of heterogeneous nuclei (Lofgren and
Russell, 1986; Lofgren, 1989). The transition from porphyritic texture to
radial or barred (dendritic) texture for melts of constant composition has
been studied as a function of the presence or absence of heterogeneous nuclei
and cooling rate. Such variations in texture within a single melt have
already been demonstrated for melts of lunar and terrestrial basalt composition
(Lofgren, 1980, 1983; Grove and Beatty, 1980).
The "classic" barred olivine texture is a single plate dendrite
(Donaldson, 1976) which shares the entire chondrule with the remaining glass or
subsequent crystallization products. Olivine rimming the chondrule is often in
optical continuity with the dendrite and thus is part of the plate dendrite.
Because this texture is so striking, barred olivine (BO) chondrules are
well known even to people outside the field of meteorites. When chondrules
are discussed, a photomicrograph of a barred olivine texture is usually
chosen as one of a few or even the only example. It is not surprising that
considerable effort has been expended understanding its origin. Barred olivine
textures comprise only a few percent of melt-textured chondrules, usually
less than 5% (Gooding and Keil, 1981). The "classic" barred texture
represents only 10% of the type 3 ordinary chondrite BO chondrules. By careful
study, Weisberg (1987) determined that the multiple plate dendrite is a much
more common that the single dendrite. Most investigators propose that BO
chondrules form from melt droplets that crystallize rapidly upon cooling.
Attempts to duplicate BO textures experimentally showed that it is
difficult to produce the "classic" single dendrite chondrule; conversely, multiple
plate dendrites are observed commonly in experimental charges (Lofgren and
Lanier, 1990). It turns out to be very difficult to restrict nucleation to
a single event. An example of a barred dendrite is shown in Figure 8. Each
dendrite is a series of parallel plates connected in the third dimension
with respect to the plane of the thin section. The dendrite forms when nuclei
are eliminated from the melt and only embryos remain. If the melt is
cooled rapidly and does not crystallize, it becomes supercooled and embryos
eventually become stable nuclei. When an olivine nucleus begins to grow, it
will become a dendrite if the supercooling is sufficiently high.
These experiments clearly demonstrate the crystalline material must be
present in the solar nebula when the chondrules form and suggests that they
did not form by direct condensation from vapors in the solar nebula.
Individual crystals most likely formed first and these were remelted in clusters to
form the chondrules. An interesting fact that has come out of these
studies is that the rate at which the melt droplets cool is not critical. They
can cool at nearly the same rate and produce either the porphyritic texture
if nuclei are present when cooling is initiated, or form dendrites (barred)
chondrules if only embryos are present. The important factor is how hot the
droplets become before they begin to cool and thus whether they retain any
crystalline precursor material to act as nuclei or whether nuclei have to
form from embryos. If the melt droplets are heated hot enough that even the
embryos are eliminated, the droplets usually do not crystallize when cooled
 and form glass chondrules. Glass chondrules are rare and this places an
upper temperature limit to which the melt droplets are heated which is
approximately 1650?C. A minimum melting temperataure of 1550?C is dictated by the
minimum amount of melting required to produce the observed textures. It is
still not clear, however, what heat source provides such conditions (Wood,
1988). A popular model is heating due to viscous drag on particles as they
move through dense parts of the solar nebula as proposed by Wood (1984
Chemical analysis of chondrites (Wasson, 1974) indicates that there is a
variety in their composition leading us to believe that they are not all
derived from a common source. Most chondrites are composed primarily of
olivine, feldspar, orthopyroxene, with several metals including kamacite and
taenite. Continuing studies on the chemical and physical nature of chondrites
and their formation is providing insight into the history of the solar
system.
 
Received on Tue 19 Jan 2010 06:54:48 PM PST