Geoscience Reference
In-Depth Information
Table 3.2
Table 3.3
Element ratios (by weight) in four
subtypes of chondritic meteorites
Normative
mineralogy
of
ordi-
nary chondrites (Mason, 1962)
Species
High Iron
Low Iron
Ratio
C1
H
L
E6
Olivine
36.2
47.0
Al/Si
0.080
0.063
0.063
0.044
Hypersthene
24.5
22.7
Mg/Si
0.91
0.80
0.79
0.71
Diopside
4.0
4.6
Ca/Al
1.10
1.11
1.08
1.06
Feldspar
10.0
10.7
Cr/Mg
0.025
0.025
0.026
0.024
Apatite
0.6
0.6
Chromite
0.6
0.6
Ilmenite
0.2
0.2
and one subclass (the SNC group) have apparently
come from Mars. Many of the achondrites crystal-
lized between 4.4 and 4.6 billion years ago. They
range from almost monomineralic olivine and
pyroxene rocks to objects that resemble lunar
and terrestrial basalts. Two important subgroups,
classified as basaltic achondrites, are the
eucrites
and the
shergottites
. Two groups of meteoritic
breccias, the
howardites
and the
mesosiderites,
also contain basaltic material. The eucrites,
howardites, mesosiderites and diogenites appear
to be related and may come from different depths
of a common parent body. They comprise the
eucritic association.
The shergottites, nakhlites and
chassignites form another association and are
collectively called the SNC meteorites.
Eucrites are plagioclase--pyroxene rocks simi-
lar to basalts and have textures similar to basalts.
However, terrestrial basalts have higher abundan-
ces of sodium, potassium, rubidium and other
volatile elements and have more calcium-rich
pyroxenes. Eucrite plagioclase is richer in cal-
cium and poorer in sodium than terrestrial basal-
tic feldspar. The presence of free iron in eucrites
demonstrates that they are more reduced than
terrestrial basalts.
Studies of basalts from the Moon and the
eucrite parent body have several important impli-
cations for the early history of the Earth and
the other terrestrial planets. They show that even
very small bodies can melt and differentiate. The
energy source must be due to impact, rapid accre-
tion, short-lived radioactive isotopes or formation
in a hot nebula. The widespread occurrence of
chondrules in chondritic meteorites also is evi-
dence for high temperatures and melting in the
early solar system.
The depletion of volatiles in eucrites and
lunar material suggests that small planet and
Troilite
5.3
6.1
Ni--Fe
18.6
7.5
Mason (1962).
and other siderophiles, and in the ratio of oxi-
dized to metallic iron. As the amount of oxi-
dized iron decreases, the amount of reduced iron
increases. Olivine is the most abundant mineral
in chondrites, followed by hypersthene, feldspar,
nickel-iron, troilite and diopside with minor
apatite, chromite and ilmenite (Table 3.2). The
composition of the olivine varies widely, from
0to30mole%Fe
2
SiO
4
(Fa). Enstatite chondrites
are distinguished from ordinary chondrites by
lower Mg/Si ratios (Table 3.3), giving rise to a min-
eralogy dominated by MgSiO
3
andhavinglittleor
no olivine. They formed in a uniquely reducing
environment and contain silicon-bearing metal
and very low FeO silicates. They contain several
minerals not found elsewhere (CaS, TiN, Si
2
N
2
O).
In spite of these unusual properties, enstatite
chondrites are within 20% of solar composition
for most elements. They are extremely old and
have not been involved in major planetary pro-
cessing. They have been suggested as possible con-
stituents of the Earth because of their high free-
iron content, their oxidation state and oxygen iso-
topic ratios. If Earth is to be made out of a single
meterorite class, the enstatite chondrites are the
closest match.
Achondrites
The achondrites are meteorites of igneous ori-
gin that are thought to have been dislodged by
impact from small bodies in the solar system.
Some of these may have come from the asteroid
belt, others are almost certainly from the Moon,
