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

[starsandscopes 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  1650ºC. A minimum melting temperataure of 1550ºC 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.