262 Encyclopedia of the Solar System
groups. The total iron in some enstatite (E) chondrites ex-
ceeds that in the H group of ordinary chondrites, denoting
them as EH chondrites; the EL chondrite designation is
self-evident.
Achondrites, formed at high-temperatures, contain es-
sentially no metal or sulfide and are enriched in refrac-
tory lithophiles (cf. Table 3), which, with their constituent
minerals, allow classification into specific groups (Fig. 7a).
Most groups are named for a specific prototypical mete-
orite; others—howardites, eucrites, and diogenites (HED
meteorites)—were named nonsystematically. At least
10 achondrite groups can be distinguished from their ox-
idized iron and calcium contents (FeO and CaO). Some
apparently were associated in the same parent body
but derive from different regions: the HED and the
SNC (Shergottites–Nakhlites–Chassigny) associations. The
HED meteorites are thought to come from 4 Vesta, and/or
other V class asteroids produced from it. The consensus that
the 32 SNC meteorites come from Mars is so strong, that
these are often called martian meteorites, not SNCs.
2.3 Oxygen Isotopics and Interpretation
Meteorites “map” the solar system by isotopic composition
of oxygen (Fig. 11), a major element in all but the irons.
Because its high chemical reactivity causes oxygen to form
numerous compounds, it exists in many meteoritic miner-
als, even in silicate inclusions in iron meteorites. In stan-
dard references, such as the Chart of the Nuclides, the
terrestrial composition of its three stable (i.e., nonradioac-
tive) isotopes is given as 99.756%^16 O, 0.039%^17 O, and
0.205%^18 O. In fact, any physical or chemical reaction al-
ters its isotopic composition slightly by mass-fractionation.
Since the mass difference between^16 O and^18 O is twice
that existing between^16 O and^17 O, a mass-dependent re-
action (e.g., physical changes and most chemical reactions)
increases or decreases the^18 O/^16 O ratio by some amount
and will alter the^17 O/^16 O ratio in the same direction, but by
half as much. Accordingly, in a plot of^17 O/^16 O vs.^18 O/^16 O
or units derived from these ratios (i.e.,δ^17 O andδ^18 O; cf.
Fig. 11 caption), all mass-fractionated samples derived by
chemical or physical processes from an oxygen reservoir
with a fixed initial isotopic composition will lie along a line
of slope∼1/2.
Data from terrestrial samples define the Terrestrial Frac-
tionation Line (TFL) in Fig. 11, whose axes are like those
described earlier, but normalized to a terrestrial reference
material, Standard Mean Ocean Water (SMOW). Not only
do all terrestrial data lie along the TFL line, but so too do
the oxygen isotopic compositions of lunar samples, which
occupy a small part of it. The single Earth–Moon line (de-
fined by data covering the solid line’s full length) suggests
that both bodies sampled a common oxygen isotopic reser-
voir, thus supporting the idea that the Moon’s matter spun
FIGURE 11 Relation between oxygen isotopic compositions in
whole-rock and separated mineral samples from the Earth,
Moon, and various meteorite classes. Units,δ^17 O (‰) andδ^18 O
(‰), are those used by mass spectrometrists and are, in effect,(^17) O/ (^16) O and (^18) O/ (^16) O ratios, respectively. Bothδ (^17) O (‰) and
δ^18 O (‰) are referenced to SMOW. Oxygen isotopic
compositions for carbonaceous chondrites are much more
variable than for other meteorite classes (dashed box in the
upper part expanded in the lower one).
off during the massive impact of a Mars-sized projectile
with a proto-Earth (see relevant chapters).
One important feature of Fig. 11 is that many chondrite
and achondrite groups defined by major element compo-
sition and mineralogy (e.g., Figs. 7a and 7b) occupy their
own regions in oxygen isotope space. These data suggest
that at least eight major chondritic groups (H, L, LL, CH,
CI, CM, CR, and E) and a minor one (R), acapulcoites
and brachinites, the two achondrite associations (SNC and
HED), ureilites (U) and the silicate inclusions in group IAB
iron meteorites derive from different “batches” of nebular
material. The HED region also includes data for most pal-
lasites and many mesosiderites suggesting derivation from
a common parent body. Extension of the HED region by
a line with slope 1/2 passes through the isotopic region of