276 Encyclopedia of the Solar System
in the more refractory siderophiles: They contain essentially
no volatiles, or lithophiles except in silicate inclusions.
More volatile elements exhibit much greater variability
in stony meteorites. Concentrations of the three or four
most volatile elements are several orders of magnitude
higher in UOC than in their equilibrated analogues and de-
crease by one or two orders of magnitude with increasing
UOC homogenization of ferromagnesian silicates. Contents
of most strongly volatile elements in H and L chondrites of
petrographic types 4–6 are highly variable and do not corre-
late with the petrographic types. However, in H chondrites,
concentrations of many moderately volatile elements vary
as H4>H5>H6, consistent with loss at progressively
higher metamorphic temperatures in stratified parent(s).
As discussed in Section 6.4, chronometric data also are con-
sistent with this theory for H chondrite parent(s). Such a
model cannot be established for the L chondrites because
late shock (evident in the petrographic properties of many
of them) affected other thermometric characteristics, thus
obscuring earlier histories. In addition to the petrographic
evidence, strongly shocked L4–L6 chondrites exhibit loss of
some noble gases, highly mobile elements and siderophiles,
and lithophile enrichments.
Mean contents of Ag, Te, Zn, Cd, Bi, Tl, and In de-
crease in L4–6 chondrites with increasing shock-loading
(and, therefore, residual temperature) estimated from pet-
rographic shock indicators. Trace element contents of H
chondrites do not vary with shock. In unshocked chon-
drites, volatile contents are significantly lower in H than
in L chondrites, suggesting that L chondrite parent mate-
rial formed from the nebula at lower temperatures than did
H. Apparently, nebular temperatures during H chondrite
parent material formation were so high (∼700 K) that only
a very small complement of volatile trace elements could
condense. Hence, essentially none was present to be lost
later at high, shock-induced residual temperatures.
The H chondrite regolith breccias, like Noblesville
(Fig. 1a), differ from “normal” H chondrites in that the
dark, gas-rich portions of the breccias are quite rich in
volatile trace elements, sometimes exceeding C1 levels.
These volatiles, distributed very heterogeneously in the
dark matrix, were apparently not implanted by the solar
wind but rather occur in black clasts. These black clasts
represent either volatile-rich nebular condensate or a sink
for material degassed from the parent body interior. Dur-
ing exposure on the asteroidal surface, these dark clasts
and light ones (containing “normal” levels of volatiles) were
apparently gardened by repeated impacts, ultimately form-
ing the regolith breccia matrix. Less is known about equi-
librated LL chondrites: They may have a unique thermal
history or one like that of H or L chondrites.
In contrast to ordinary chondrites, volatile trace ele-
ments in carbonaceous chondrites are very homogeneously
distributed. These elements are unfractionated from each
other in almost all carbonaceous chondrites, implying that
their parent material incorporated greater or lesser amounts
of C1-like matter during accretion.
The proportions define a continuum from 100% C1
down to about 20% in C5 or C6. As in enstatite chon-
drites, volatile-rich samples have higher proportions of
more siderophile trace elements. These trends accord with
oxygen isotope data, implying a continuum of formation
conditions for parent materials of carbonaceous chondrites.
Contents of mobile trace elements and noble gases,
and the petrography of 15 C1–C3 chondrites (14 from
Antarctica and 1 from a hot desert) provide unambigu-
ous evidence for open-system thermal metamorphism in
their parent bodies. These properties permit a semiquan-
titative metamorphic temperature in the 400–900◦C range
to be estimated for each of the 15. Each was dehydrated
during metamorphism and none (including the 14 Antarc-
tic chondrites) was rehydrated during terrestrial residence.
As noted in Section 3.1, spectral reflection properties of
these 15 thermally metamorphosed carbonaceous chon-
drites (and none of the more numerous “normal” ones) link
them to C, G, B, and F asteroids. Petrographic properties
of C1–C6 chondrites were established during nebular con-
densation and accretion. If C4–C6 or CK chondrites expe-
rienced thermal metamorphism, it occurred under closed-
system conditions.
Enstatite chondrites present a special problem because
nonvolatile siderophiles in them define high (EH) and low
(EL) groups established during primary nebular and accre-
tion. Prior to discoveries of desert meteorites, volatile ele-
ment contents in E3,4 chondrites were known to be orders
of magnitude higher than in E6. Whether this difference
reflected primary or secondary processes was unclear since
E3–E5 chondrites were EH and E6 were EL.
Fortunately, Antarctic collections include previously un-
known EL3 chondrites among others, and new data show
that EL3 and EH3–EH4 chondrites contain comparable
levels of the most volatile elements. These data suggest
source regions of E3 and E4 chondrites, whether EL or EH,
essentially reflect primary nebular condensation and/or ac-
cretion. Volatiles in E5 and especially E6 (whether EH or
EL) are greatly depleted from E3 and E4 levels in a man-
ner suggesting open-system loss during thermal metamor-
phism of their primitive parent(s). Data for these elements
suggest further that enstatite achondrites derived from E6
chondrite-like material that previously experienced FeS–
Fe eutectic loss (formation temperature, 980◦C).
Oxygen isotopic data for all enstatite meteorites (i.e.,
chondrites and achondrites) are similar (Fig. 11) with
δ^18 O increasing systematically in E3–E6, independent of
their being EH or EL. The oxygen isotopic compositions
in EL chondrites lie along the terrestrial fractionation
line (Fig. 11), but the distribution in EH chondrites falls
along a line of slope 0.66, neither purely mass-dependent