Volatility fractionation -- This is a process whereby
the composition of an object is changed as a result of the differing tendency
for different elements to partition into a gas phase. The
volatility
scale for elements can be quantified by something known as the 50% condensation
temperature (Tc,50),
which represents the temperature at which half of the element is in the
gaseous state and half is in a condensed (solid or liquid) state.
These Tc,50 values
can be calculated from thermodynamic data if one specifies for the system
the total pressure (for the solar nebula usually 0.001-0.000001 bars, or
atmospheres, corresponding to an estimate for the midplane of the solar
nebula) and bulk composition (usually solar proportions of elements).
Elements that have high condensation temperature are considered to be refractory
elements, those that have low condensation temperatures are considered
to be volatile elements. The sequence of condensates
that appear as temperature is decreased is known as the condensation
sequence. The condensation diagram from the last (chondrule)
glossary is reproduced below. Note that minerals (such as melilite,
spinel, perovskite, diopsidic pyroxene, anorthite) found in CAIs lie near
the top of this diagram.
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Equilibrium mineral assemblages in a system of solar
composition at low pressure.
The arrows indicate reaction paths in a cooling system.
The same assemblages would be
produced in reverse for a system in equilibrium with
increasing temperature.
From http://wapi.isu.edu/Geo_Pgt/images/solarmins.gif
SIMS (secondary ionization mass spectrometry)--
This technique involves sputtering ions off of a sample and putting these
ions through a mass spectrometer to measure trace element compositions.
An accelerated "primary" ion such as Cs or O is used to do the sputtering
of the "secondary" ions that are emitted from the sample.
Initial 87Sr/86Sr and 26Al/27Al
ratios-- These are related to an age of a sample,
with older objects having lower 87Sr/86Sr and higher
26Al/27Al.
87Sr
and 26Al are examples or radiogenic isotopes, those involved
in a radioactive decay scheme, whereas 86Sr and
27Al
are examples of non-radiogenic isotopes, those not involved in any decay
scheme. The rate of decay varies drastically for different isotopes, and
this rate can be specified by half life, which is the time needed
for half of the decaying or parent isotope to decay to a product
or daughter isotope. The reason that less 87Sr
and more 26Al coreeponds to an older object is that in one case
the isotope is a decay product-- 87Rb decays to 87Sr
with a long half-life, whereas in the other case the isotope itself is
decaying-- 26Al decays to 26Mg with a short (~0.72
Ma) half-life. As we will learn later, short-lived nuclides provide
a relative age, whereas long-lived nuclides provide a fixed or absolute
age.
Pre-solar grain-- This is a grain that is inferred
on the basis of anomalous isotopic composition to have originated from
outside our solar system. We wll say more about pre-solar grains
later, but for now it suffices to note that there is no compelling evidence
that CAIs or the grains within them originated outside of our solar system.
That is, none are pre-solar objects.
Nucleosynthesis-- This is the process by which
elements are created inside stars as a result of fusion reactions.
We will be saying more about this later.
Fractional condensation - This refers to
the removal of solids from a system that is undergoing condensation.
It is analogous to the igneous process of fractional crystallization, in
which crystals are separated from magma. Fractional condensation
is strongly implicated for the formation of a particular kind (Group II)
of CAI based chiefly on the fractionated rare earth element (REE) abundance
pattern (Fig. 3). If fractional condensation occurs, complete equilibrium
cannot be maintained because solids are no longer in chemical communication
with the gas.
Fractional vaporization (distillation) -
This refers to the removal of melts from a system that is undergoing vaporization.
It is opposite to fractional condensation in the sense that temperatures
are increasing. If fractional vaporization occurs, complete equilibrium
cannot be maintained because liquids are no longer in chemical communication
with the gas.
Type A, B, C CAIs - These are CAIs that have been
classified according to their primary mineral proportions. Type A
inclusions are rich in melilite, and come in two textural varieties ("compact"
and "fluffy"). Type B inclusions contain a Ti-Al-rich diopsidic clinopyroxene
(known as fassaite) and anorthite in addition to melilite, and also come
in two textural varieties (compact "B1" and "B2"). Type C inclusions
are rich in anorthite. Other CAIs, notably those containing nodules
of spinel, have been called fine-grained CAIs. The latter are examples
of unmelted CAIs.
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"Fluffy" Type A CAI in the Allende CV3 chondrite.
From http://www.meteorlab.com/METEORLAB2001dev/labphoto/CAI.htm
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Fine-grained CAI (~2 x 1 mm across) in a thin section
of Allende
observed with cross-polarized transmitted light.
From http://flood.nhm.ac.uk/cgi-bin/earth/metcat/image.dsml?picName=cai
FUN inclusion - This is a type of CAI that has many isotopic anomalies of many elements, including anomalies produced by what were originally recognized as mass-fractionation (F) and unknown nucleosynthethic (UN) processes. This acronym was developed in the 1970s by the "Lunatic Asylum" folks in CalTech-- and we've had it ever since. I guess it's too FUN to let go.
Figure 4, 6-- These figures show the bulk compositions of objects expressed as equivalent chemical proportions of corundum (Al2O3), forsterite (Mg2SiO4), larnite (Ca2SiO4), and spinel (MgAl2O4), with the latter implicitly present. The proximity of the data points to the triangle corners signifies an increasing amount of that particular component. The compositions are said to be "projected" from spinel or MgAl2O4 because this component is above the plane of the diagram, forming an apex of an imaginary tetrahedron that has a corundum-forsterite-larnite base.
