Meteorites 267
to chondrule-containing meteorites prompt this classifica-
tion. Compositionally, C1 (or CI) chondrites closely re-
semble the solar photosphere, the correlation between
abundances in the solar photosphere and C1 chondrites
exist over 10 orders of magnitude (10 billion). A few
differences exist: Elements depleted in C1 chondrites rel-
ative to the Sun’s surface (e.g., hydrogen, helium, or car-
bon) are gaseous or easily form volatile compounds that
largely remained as vapor in the nebular region where C1
chondrite parent material condensed and accreted. Other
elements (e.g., lithium, beryllium, and boron) are eas-
ily destroyed by low-temperature nuclear reactions dur-
ing pre-main-sequence stellar evolution; consequently, they
are depleted in the solar photosphere relative to C1
chondrites.
Because chemical analysis of C1 chondrites (or any plan-
etary material) on Earth is more precise and accurate than
is spectral analysis of the solar photosphere, “cosmic abun-
dance” tables of chemical and isotopic data for most ele-
ments mainly derive from C1 chondrite analyses. Generally
these data are used to estimate our solar system’s compo-
sition. Only where such processes, as incomplete nebular
condensation, are suspected do such compilations adopt
solar photospheric values. Recall, however, that earlier we
inferred chemical heterogeneity of the pre-solar nebula ex-
isted (Fig. 2), so cosmic abundances may not have been the
same in all nebular regions.
2.4.4.2 Organic Constituents
Although chondrites are depleted in carbon, hydrogen, and
nitrogen relative to the solar photosphere, C1 and, to a lesser
extent, C2 chondrites contain large amounts of organic mat-
ter (Table 3). They are visible in situ (as globules) only in
the unique carbonaceous chondrite Tagish Lake (Fig. 3).
Over 400 different organic (C-based) molecules of very dif-
ferent types are known mainly in C1 and C2 chondrites, but
their concentrations are very low. Molecular characteristics
demonstrate that many are preterrestrial, but the problem
of terrestrial contamination is ever-present.
Polycyclic aromatic hydrocarbons (PAH) were found in-
side 2 martian meteorites, but not near their surfaces, sug-
gesting that the PAH are not terrestrial contaminants but,
rather, originated on Mars. Particles identified as microfos-
sils were reported in at least one martian meteorite and,
decades earlier, in CI and CM chondrites. Some advocate
biogenic formation of these, but their arguments fail to alter
the consensus view that meteoritic organics formed abio-
genically.
Since many organic compounds in meteorites can be
altered or destroyed by even brief exposure to tempera-
tures of 200–300◦C, their presence in meteorites constitute
a thermometer for postaccretionary heating during meta-
morphism, shock, or atmospheric transit.
2.4.5 SHOCK
A meteorite parent body cannot be disrupted by internal
processes, but only by collision with another similarly sized
object. Accordingly, many meteorites evidence exposure
to significant shock. A few decades ago, chondrites were
qualitatively classed “shocked” if the hand-specimen inte-
rior exhibited blackening, veining, or brecciation. Now, pet-
rographic and mineralogic characteristics provide a semi-
quantitative estimate of the shock-exposure level. Such
characteristics reflect changes induced directly, by the peak
pressure wave, or indirectly, by the shock-associated, high
residual temperature. Specific shock-pressure indicators
(“shock barometers”) have been calibrated against charac-
teristics produced by laboratory shock-loading experiments.
Using these criteria, the degree of shock-loading is known
for almost 4300 ordinary chondrites (Table 5).
The current scheme to estimate shock histories of equi-
librated ordinary chondrites involves the addition of S1,
S2,... , S6 to its chemical-petrographic classification. The
peak shock pressures at the transitions are<5 GPa, S1/S2;
5–10 GPa, S2/S3; 15–20 GPa, S3/S4; 30–35 GPa, S4/S5;
45–55 GPa, S5/S6. Whole-rock melting and formation of
impact melt rocks or melt breccias occur at 75–90 GPa.
Thus, the Noblesville H4 regolith breccia (Fig. 1a) is S1
as a whole with some H6 clasts being S2. Other chondritic
compositional data (radiogenic gases and thermally mobile
trace elements, i.e., easily volatilized and lost in an open
system) also give information on shock histories (cf.
Sections 5.3 and 6.3). Equilibrated L chondrites exhibit
the highest proportion of heavily shocked chondrites,
almost half having been shocked above 20 GPa. Lesser,
but significant, proportions of H and LL chondrites show
substantial degrees of shock loading (Table 5). The only CTABLE 5 Degrees of Shock Loading in Ordinary
Chondrites as a Function of Specific
Chemical-Petrologic Type
Type Total S1 S2 S3 S4 S5 S6H3 229 53 119 41 14 2 —
H4 511 91 257 125 33 4 1
H5 1021 94 444 403 75 3 2
H6 549 77 159 234 65 12 2
L3 53 9 23 12 9 — —
L4 206 18 71 69 37 3 8
L5 412 23 108 151 99 19 12
L6 972 28 121 319 333 104 67
L7 5—— 122—
LL3 50 6 27 10 6 1 —
LL4 49 4 27 16 1 1 —
LL5 83 7 42 21 9 4 —
LL6 143 2 44 55 33 6 3