274 Encyclopedia of the Solar System
FIGURE 16 Stable Ba isotopes in a SiC separate from the
Murchison CM chondrite normalized to those in normal
terrestrial Ba. Letters indicate nucleosynthetic processes by
which individual isotopes are produced. The presolar
neutron-capture isotopes (on slow, s, and rapid, r, timescales that
formed in presupernova and supernova stages, respectively) are
anomalously high, by up to 4×.
derive from outside our solar system. These grains were
incorporated into the solar nebula with intact memories
of their individual nucleosynthetic sources, accreted into
meteoritic matter and obviously survived all later episodes
in their parent bodies’ histories. The isotopic anomalies
identified thus far point at specific genetic processes. Most
SiC grains probably formed in stars on the asymptotic gi-
ant branch (i.e., AGB stars) in the Hertzsprung-Russell
diagram. This is the source of isotopes produced by neu-
tron capture on a slow timescale (or so-called s-process)
nuclides, with rapid neutron capture (r-process) nuclides
forming immediately prior to the supernova stage. Super-
novae also seem required to explain the isotopic anomalies
in tiny diamonds.
The isotopic anomalies of many trace elements in these
presolar grains provide a wealth of unique information re-
garding the evolution of stars and nucleosynthesis. This in-
formation is only obtainable by exhaustive, detailed, highly
sensitive, and highly accurate analyses of rare interstellar
grains from primitive samples in terrestrial laboratories and
requires both inspiration and perspiration. Undoubtedly,
isotopic anomalies in these rare meteoritic constituents will
tell us more about stellar formation and evolution, as well
as the formation and early history of the Solar System.
5.2.2 CAI
In addition to low-temperature materials, like the matrix
of C1 chondrites and presolar grains, refractory grains like
CAI also record early solar history. The CAI are millimeter-
to centimeter-sized refractory inclusions, especially recog-
nizable in C2 and C3 chondrites but also identifiable in
some UOC and in R and E3 chondrites. Typically, CAI
consist of refractory silicate and oxide mineral assemblages
rimmed by thin multilayered bands of minerals. Major-
element compositions of CAI agree with calculations by
equilibrium vapor-deposition evaporation models to repre-
sent the first 5% of condensable nebular matter solidify-
ing at≥1400 K from a gas of cosmic (solar photospheric)
composition at a pressure of 10−^3 atm or at 0.3 atm, if the
dust/gas ratio is 40-fold enriched. Most individual CAI con-
tain tiny particles (usually< 50 μm) very rich in refractory
siderophiles (Re, W, Mo, Pt, Pd, Os, Ir, and Rh) and, oc-
casionally, refractory lithophiles like Zr and Sc. Sometimes,
even smaller (micrometer-sized) refractory metal nuggets
are found consisting of single-phase pure noble metals or
their alloys.
The textural and mineralogic complexities of CAI in-
dicate a variety of formation and alteration processes in
their history. Undoubtedly, CAI formed at high temper-
atures; properties of some suggest vapor condensation as
crystalline solids, whereas others seemingly reflect liquid
or amorphous intermediates. Volatilization, melting, solid-
state metamorphism, and/or alteration in the nebula or af-
ter accretion may also have affected some to many CAI.
Clearly, CAI had complicated histories that obscured their
primary textural properties but left their chemical and iso-
topic properties relatively unaltered.
Volumetrically, fine-grained CAI are encountered more
often than coarse-grained ones, but the latter are more
easily studied. Coarse-grained CAI are grouped into four
types, defined mainly by mineralogy, formed at progres-
sively lower temperatures: Type A, dominated by melilite,
compositionallyAkermanite ( ̊ Ak) 0–70; Type B, a mixture ̊
of melilite, fassaitic pyroxene, spinel, and minor anorthite;
anorthite-dominated Type C; and forsterite-bearing inclu-
sions. Type A CAI seem the most diverse, having apparently
condensed as solid from vapor with many heavily altered;
thus, reconstruction of their original composition is diffi-
cult. The other three types formed from partly molten mix-
tures to melt droplets, respectively. Type B CAI are min-
eralogically the most complex and host a much wider ar-
ray of isotopic anomalies. Compositionally, CAI reflect a
high-temperature origin and are refractory-rich: refractory
lithophiles like REE are generally enriched 20×or more
relative to C1 compositions, although considerable variabil-
ity occurs in individual CAI due to thermal history and oxy-
gen fugacity variations. The oxygen isotopic compositions of
CAIs help define the anhydrous minerals line (with slope 1)
in Fig. 11.
The centimeter-sized Type B CAI in C3V chondrites at-
tract the most interest, and their individual minerals have
been probed by an array of very sophisticated instruments
that put to shame conventional chemical microanalytical
techniques. Many to all these CAI exhibit isotopic anoma-
lies (both in positive and/or negative directions) for O, Ca,
Ti, and Cr. A few CAI, mineralogically and textually indistin-
guishable from others, are called FUN inclusions because