Meteorites 275
they exhibitFractionated andUnidentifiedNuclear isotopic
effects involving not only Kr and Xe but also elements
like Mg, Si, Sr, Ba, Nd, and Sm. Six FUN inclusions con-
tain mass-fractionated oxygen (i.e., follow slope 1/2 lines in
Fig. 11), and the two Type B inclusions of these six exhibit
isotopic anomalies for every element thus far studied.
Although CAI, in general, and FUN inclusions, in par-
ticular, yield much information, we do not yet know why
isotopic anomalies appear in some CAI but not others, and
why some elements in a sample exhibit anomalies but others
do not. The CAI apparently formed from unhomogenized
matter early in the solar system’s history just as or before
chondrules did, by analogous processes.
5.3 Elements other than Noble Gases
Having briefly touched upon some important but compli-
cated meteoritic constituents, let us consider information
conveyed by trace elements in whole-rock samples. Most
elements in the Periodic Table are present in a meteorite at
very low levels—microgram/gram (ppm), nanogram/gram
(ppb), or picogram/gram (ppt) concentrations. Such low
concentrations exist because nucleosynthesis produced sta-
ble isotopes of trace elements in only small amounts, and be-
cause their geochemical and/or physical properties prevent
enrichment—indeed, may cause significant depletion—
during genetic episodes. Their geochemistry may cause
some trace elements to be sited in specific hosts of particular
meteorites [e.g., siderophiles like Ir, Ga, or Ge are enriched
(relative to Cl levels) in iron meteorites (cf. Fig. 9)], whereas
others are dispersed among a variety of minerals. The same
element may be dispersed in one meteorite class but be
sited in a particular host in another. For example, REE are
found in phosphates in achondrites, but some are dispersed
elements in chondrites. They concentrate in whitlockite in
eucrites, and are even more enriched in CAI. Trace ele-
ments convey important information because a small abso-
lute concentration change induced by a genetic process will
result in a large relative effect.
This improvement in “signal-to-noise” is illustrated in
explosive meteorite impact. Whatever the initial composi-
tion of proto-Earth material, much of its initial complement
of refractory siderophiles was extracted into the core, thus
depleting them in the crust. Fall of a massive chondrite
or, even better, an iron meteorite enriched in siderophiles,
followed by an explosion, will spread mixed projectile and
target ejecta widely, redepositing the ejecta in a thin layer.
Subsequent chemical analysis of a vertical slice that in-
cludes the deposition layer will reveal siderophile enrich-
ment in that layer. Siderophile enrichments—especially of
refractory Ir—in the K-T boundary layer around the Earth
suggested that dinosaurs (and many other biota) died off
from sudden environmental changes created by a mete-
oroid/asteroid impact 65 Ma ago. Initially controversial, this
idea is now generally accepted. In many instances, enrich-
ments of several siderophiles in impact breccias at an ex-
plosion crater on Earth or the Moon provide a fingerprint
of the meteoritic type that created the crater.
As discussed in Section 2.4.4.1, volatile elements con-
densable at very low temperatures may not have similar con-
tents in C1 chondrites and the solar photosphere. Meteorite
compositions are referenced to readily condensable mate-
rial by normalization to a refractory lithophile—most com-
monly Si, sometimes Mg or Al—rather than hydrogen as
in the solar photosphere. For meteorites, normalized ra-
tios can be on a weight or atom basis: in the latter, trace
element contents are usually referred to as atomic abun-
dances and are often normalized to C1 contents. In the
most primitive chondrites—EH or UOC—C1-normalized
abundances approach or exceed C1 levels. On this basis,
we say that moderately to highly refractory siderophiles are
enriched in iron meteorites or that refractory lithophiles
are enriched in achondrites. Contents of the more refrac-
tory trace elements are characteristic of, hence can define,
achondrite associations (Fig. 14).
A priori identification of a trace element as refractory or
volatile is impossible because its chemical form in a me-
teorite is usually unknown. For example, indium metal or
gaseous oxygen are each quite volatile as elements, but re-
fractory when chemically bonded in InO. Because In exists
only at ppb levels in even the most volatile-rich meteorites,
neither InO nor any other In compound is identifiable.
Several approaches have been used to obtain at least a
qualitative elemental volatility order: The orders obtained
generally agree, with some minor differences. Criteria used
include calculation of theoretical condensation tempera-
tures in a nebular gas of solar composition at pressures
of 10−^3 –10−^6 atm, determination of C1-normalized atomic
abundances in equilibrated (petrographic types 5 and 6)
ordinary chondrites, and laboratory studies of elemental
mobility (ease of vaporization and loss) during week-long
heating of primitive chondrites under conditions simulating
parent body metamorphism (400–1000◦C, 10−^4 atm H 2 ).
By these criteria, elements considered as moderately
volatile include (in increasing order) Ni, Co, Au, Mn, As, P,
Rb, Cu, K, Na, Ga, and Sb, whereas strongly volatile ones
include Ag, Se, Cs, Te, Zn, Cd, Bi, Tl and In.
Small but real (< 2 ×) differences exist in contents of
the more refractory trace elements in the various chon-
dritic groups. Siderophile contents are higher in EH than
in EL chondrites and decrease in ordinary chondrites as
H>L>LL, in keeping with total iron contents. Naturally,
achondrites are enriched in refractory lithophiles and de-
pleted by orders of magnitude in siderophiles (whether re-
fractory or volatile) and volatile elements of any geochemi-
cal character. In some achondrites (mainly HED meteorites
and at least one lunar meteorite), high levels of volatile con-
tents are evident, reflecting deposition of late volcanic em-
anations on their parents. As expected from our picture of
how iron meteorites formed, these meteorites are rich only