268 Encyclopedia of the Solar System
TABLE 6 Extinct Radionuclides Whose Decay
Products Are Detected in Meteorites
Radio- Half-life
nuclide (Ma) Daughter Initial Ratio(^41) Ca 0.13 (^41) K (^41) Ca/ (^40) Ca=1.4× 10 − 8
(^26) Al 0.7 (^26) Mg (^26) Al/ (^27) Al= 5 × 10 − 5
(^10) Be 1.5 (^10) Be (^10) Be/ (^9) Be=9.5× 10 − 4
(^60) Fe 1.5 (^60) Ni (^60) Fe/ (^56) Fe=∼ 6 × 10 − 8
(^53) Mn 3.7 (^53) Cr (^53) Mn/ (^55) Mn=4.4× 10 − 5
(^107) Pd 6.5 (^107) Ag (^107) Pd/ (^108) Pd= 4 × 10 − 4
(^182) Hf 9 182 W (^182) Hf/ (^180) Hf= 2 × 10 − 4
(^129) I16 (^129) Xe (^129) I/ (^127) I=1.4× 10 − 4
(^244) Pu 82 131 Xe– (^244) Pu/ (^238) U=∼ 5 × 10 − 3
(^136) Xe
and E chondrites exhibiting evidence for unusually strong
shock (i.e., to S5) are a CK5 and an EL3.
The mineralogy (really, metallography) of iron mete-
orites is relatively simple and their shock classification is
easy at<13, 13–75, and>75 GPa. Some iron meteorites
that were shock-loaded at 13–75 GPa were subsequently
annealed at 400–500◦C for days or weeks, presumably by
contact with massive chunks of collisional debris at these
or higher temperatures; they are readily identified. About
half of all iron meteorites were shocked at>13 GPa, nearly
all during collisions that disrupted their parents. Only large
meteoroids that formed explosion craters can generate pres-
sures as high as 13 GPa when they hit Earth.
The best preserved, perhaps only, case of strong shock-
loading during terrestrial impact involves the Canyon
Diablo meteoroid that produced Meteor Crater, Arizona
(Fig. 5b). Some Canyon Diablo fragments contain
millimeter- to centimeter-sized graphite–diamond aggre-
gates, indicating partial transformation of graphite to dia-
mond. [Highly unequilibrated chondrites contain very tiny
(∼0.002μm) vapor-deposited diamond grains that do not
have a high-pressure origin; see Section 5.2.1.] These aggre-
gates contain lonsdaleite, a hexagonal diamond polymorph
produced, so far as is known, only by shock-transformation
of graphite, which also is hexagonal. Diamond-containing
Canyon Diablo specimens always show metallographic ev-
idence for exposure to shock>13 GPa; are mainly on the
crater rim, not in the surrounding plain; and contain low lev-
els of cosmogenic stable nuclides and radionuclides indicat-
ing derivation from the interior near the front of the impact-
ing meteoroid where the greater explosive shock existed.
The mutual correlations between degree of shock-loading,
depth in the impacting meteoroid, and geographic locations
around Meteor Crater, argue that strongly shocked Canyon
Diablo specimens experienced this during terrestrial im-
pact.
The percentages of strongly shocked (i.e.,>13 GPa)
members of iron meteorite chemical groups differ widely.
The IIIAB irons constitute the plurality of all known iron
meteorites, and have virtually all been shocked preterres-
trially in the 13–75 GPa range. Nearly 60% of IVA irons, the
next largest group, show such shock. A similar proportion
of IIB iron meteorites have been shocked at>13 GPa, but
this group is small. No other chemical group of iron mete-
orites shows an especially high proportion of shocked mem-
bers. Shock-loading experiments show that pressures of 13–
75 GPa acting on metallic Fe impart a free-surface veloc-
ity of 1–3 km/s. This shock-impulse was important, maybe
essential, in bringing large numbers of strongly shocked
meteorites to Earth. The parent bodies of the IIIAB and
IVA irons may have been in the Asteroid Belt: the shock-
impulse could produce ejecta with more elliptical, perhaps
Mars-crossing orbits. Mars could, with time, gravitationally
perturb these fragments into Earth-crossing orbits.
Semiquantitative petrographic shock indicators in
basaltic achondrites (i.e., mainly the HED association and
shergottites) suggest a 6-stage shock scale corresponding to
<5, 5–20, 20–45, 45–60, 60–80, and>80 GPa pressures.
The full range is seen in HED samples—primarily in clasts
in howardites, a solar gas-rich group (Section 5.1), that are
mainly polymict breccias containing eucrite and diogen-
ite fragments. These and other data, mainly compositional,
suggest that howardites are a shock-produced, near-surface
mixture of two deeper eucritic and diogenitic igneous layers
in the HED parent body.
Nearly all ureilites show petrographic evidence for very
substantial shock. Most also contain large graphite-diamond
aggregates generally believed to have formed during preter-
restrial impacts.
Lunar meteoroids, which were ejected by impacts some-
where on the 95% of the Moon’s surface not sampled by
the Apollo or Luna programs, are breccias in most cases
(Section 4.1). Otherwise, they show no unusual evidence
for shock greater than that evident in rocks returned by
these programs.
Shergottites have been heavily shocked, in keeping with
their accepted derivation from a massive object, like Mars
with its 5-km/s escape velocity. Nakhlites, linked to sher-
gottites by oxygen isotopic compositions (Fig. 11) and other
properties, are less shocked (Section 4.2).
Metallic portions of few stony-irons indicate strong
shock: No pallasites and only 3 of 18 mesosiderites were
shocked>13 GPa. Somehow, parent bodies of the stony-
irons and half of the iron meteorites were disrupted, and the
meteoroids were excavated from appreciable depth without
subjecting them to major shock-loading. More puzzling is
the fact that silicate portions of mesosiderites contain much
shocked material. Apparently, these stony-irons formed by
intrusion of shock-loaded silicate into or onto preexisting,
generally unshocked metal, possibly after excavation from
parent body interiors.