Meteorites 263
the oxygen-containing silicate inclusions from IIIAB irons,
suggesting that they, too, may be related to the HED as-
sociation. Perhaps these irons come from deeper in the
HED parent body, but this would imply more complete dis-
ruption than V-class asteroids (e.g., 4 Vesta) exhibit. Even
though oxygen isotopic compositions of the rare angrites
and brachinites resemble those of the HED association, dif-
ferences in other properties weaken the connection. Other
possible links indicating common nebular reservoirs (based
upon limited oxygen isotopic data) are silicate inclusions
in IIE irons with H chondrites, silicates in IVA irons with
L or LL chondrites, aubrites with E chondrites, winon-
aites (primitive meteorites modified at high-temperatures)
with silicates from IAB and IIICD irons, and the very rare,
highly-metamorphosed—even melted—primitive acapul-
coites and lodranites.
One interpretation of Fig. 11 is that the solar system was
isotopically inhomogeneous because each batch of nebular
matter seems to have its characteristic oxygen isotopic com-
position. Isotopic homogenization of gases is more facile
than is chemical homogenization so that the isotopic inho-
mogeneity demonstrated by Fig. 11 implies that the solar
system condensed and accreted from a chemically inhomo-
geneous presolar nebula (Fig. 2).
The other important feature to be noted from Fig. 11
is the “carbonaceous chondrite anhydrous minerals line,”
with slope near 1. A feature distinguishing C1 and C2 chon-
drites (Section 2.4.4.1) from all others (cf. Fig. 7b) is ev-
idence for preterrestrial aqueous alteration or hydrolysis
of some phases in them. (Evidence for hydrous alteration
of minerals is also observed in some unequilibrated ordi-
nary chondrites.) Anhydrous minerals (including CAI) in
carbonaceous chondrites were seemingly never exposed to
water so that these chondrites are regarded as a mixture
of materials with different histories. As seen from Fig. 11,
oxygen isotopic compositions of anhydrous minerals in CM,
CV, and CO chondrites are consistent with a line defined
by CAI whose slope cannot reflect the mass-fractionation
process indicated by a slope 1/2 line like TFL. Instead, the
anhydrous minerals line seems to represent a mixture of
two end members (batches of nebular material), which,
at the^16 O-rich (i.e., low^17 O,^18 O) end lie at or beyond
the CO region. Ureilite oxygen isotopic compositions lie
on an anhydrous minerals line near CM, suggesting a link.
These achondrites contain carbon (as graphite-diamond
mixtures) in amounts intermediate to those of CV or CO
chondrites and CM. Ureilite data do not indicate forma-
tion by differentiation of material with uniform oxygen iso-
topic composition. Rather, ureilite formation may reflect
carbonaceous chondrite-like components mixed in various
proportions.
As originally interpreted, the anhydrous minerals line
represented a mixture of nebular material containing pure
(^16) O with others higher in (^17) O and (^18) O. If so, the former
reflected a unique nucleosynthetic history, perhaps ma-
terial condensed from an expanding, He- and C-burning
supernova shell. Subsequently, photochemical reactions of
molecular oxygen with a given isotopic composition were
shown to yield oxygen molecules with isotopic composition
defining a slope 1 line as in Fig. 11.
Which process—nebular or photochemical—produced
the trends in Fig. 11 is unknown. Even so, Fig. 11 still
serves to link meteorites or groups of them produced from
one batch of solar system matter. Moreover, the position
of any sample(s) could reflect some combination of the
mass-fractionated and mixing (slope 1) lines. For exam-
ple, primary matter that ultimately yielded L chondrites (or
any ordinary chondrite group) and HED meteorites could
have had a single initial composition, subsequently mass-
fractionated and/or mixed or reacted photochemically to
produce meteorite groups with very different oxygen iso-
topic compositions. However, suitable meteorites with in-
termediate oxygen isotopic compositions are unknown.
2.4 Chondrites
The available data suggest that heat sources for melting
primitive bodies (presumably compositionally chondritic)
that formed differentiated meteorites were within rather
than external to parent bodies. Important sources no doubt
include radioactive heating from radionuclides—both ex-
tant (^40 K,^232 Th,^235 U, and^238 U) and extinct (e.g.,^26 Al)—
which were more abundant in the early solar system, and
impact heating. Calculations show that^26 Al was important
in heating small (a few kilometers) primitive parents; other
heat sources were effective in differentiating larger ones.
Electrical inductive heating driven by dense plasma outflow
along strong magnetic lines of force associated with the very
early, pre-main-sequence (T-Tauri stage) Sun is possible but
not proven.
2.4.1 PETROGRAPHIC PROPERTIES
Major element and/or oxygen isotope data demonstrate that
differences between parent materials of chondrites of the
various chemical groups (e.g., H, CM or EH) are of primary
nebular—preaccretionary—origin. Parent body differenti-
ation, on the other hand is secondary (postaccretionary).
Such heating does not necessarily melt the entire parent
body, and it is thus reasonable to expect an intermediate
region between the primitive surface and the molten dif-
ferentiated interior. Properties of many chondrites support
this expectation and suggest that solid-state alteration of
primary chondritic parent material (similar to type 3 chon-
drites) occurred during secondary heating. Eight charac-
teristics observed during petrographic study of optically
thin sections (Fig. 12) serve to estimate the degree of
thermal metamorphism experienced by a chondrite and to
categorize it into the major 3–6 types (Table 4). The ab-
sence of chondrules and the presence of abnormally large