Geoscience Reference
In-Depth Information
the early planetesimal stage of planet forma-
tion, may be characterized by volatile loss. These
extraterrestrial basalts also contain evidence that
free iron was removed from their source region.
Alternatively, these objects are fragments of giant
impacts that caused melting and silicate/iron sep-
aration. Nevertheless, the process of core forma-
tion must start very early and is probably con-
temporaneous with accretion.
Shergottites are remarkably similar to ter-
restrial basalts. They are unusual, among mete-
orites, for having very low crystallization ages,
about 10
9
years, and, among basalts, for having
abundant shocked plagioclase. The shergottites
are so similar to terrestrial basalts that their
source regions must be similar to the upper
mantle of the Earth. The similarities extend to
the trace elements, be they refractory, volatile
or siderophile, suggesting a similar evolution
for both bodies. The young crystallization ages
imply that the shergottites are from a large body,
one that could maintain igneous processes for
3 billion years. Cosmic-ray-exposure ages show
that they were in space for several million years
after ejection from their parent body.
Shergottites are slightly richer in iron and
manganese than terrestrial basalts, and, in this
respect, they are similar to the eucrites. They con-
tain no water and have different oxygen isotopic
compositions than terrestrial basalts. The major-
element chemistry is similar to that inferred for
the martian soil. The rare-gas contents of sher-
gottites are similar to the martian atmosphere,
giving strong circumstantial support to the idea
that these meteorites may have come from the
surface of Mars. In any case, these meteorites
provide evidence that other objects in the solar
system have similar chemistries and undergo sim-
ilar processes as the Earth's upper mantle.
The growing Earth probably always had basalt
at the surface and, consequently, was continu-
ously zone-refining the incompatible elements
toward the surface. The corollary is that the deep
interior of a planet is refractory and depleted
in volatile and incompatible elements. The main
difference between the Earth and the other ter-
restrial planets, including any meteorite parent
body, is that the Earth can recycle material back
into the interior. Present-day basalts on Earth
may be recycled basaltic material that formed
during accretion and in early Earth history rather
than initial melts from a previously unprocessed
peridotitic parent. Indeed, no terrestrial basalt
shows evidence, if all the isotopic and geochem-
ical properties are taken into account, of being
from a primitive, undifferentiated reservoir.
Cosmic abundances
The Sun and planets probably formed more or
less contemporaneously from a common mass of
interstellar dust and gas. There is a close sim-
ilarity in the relative abundances of the
con-
densable elements
in the atmosphere of the Sun,
in chondritic meteorites and in the Earth. To
a first approximation one can assume that the
planets incorporated the condensable elements
in the proportions observed in the Sun and the
chondrites. On the other hand, the differences
in the mean densities of the planets, corrected
for differences in pressure, show that they can-
not all be composed of materials having exactly
the same composition. Variations in iron con-
tent and oxidation state of iron can cause large
density variations among the terrestrial plan-
ets. The giant, or Jovian planets, must contain
much larger proportions of low-atomic-weight
elements than Mercury, Venus, Earth, Moon and
Mars.
With the exception of a few elements such
as Li, Be and B, the composition of the solar
atmosphere is essentially equal to the composi-
tion of the material out of which the solar system
formed. The planets are assumed to accrete from
material that condensed from a cooling prim-
itive solar nebula. Various attempts have been
made to compile tables of 'cosmic' abundances.
The Sun contains most of the mass of the solar
system; therefore, when we speak of the elemen-
tal abundances in the solar system, we really
refer to those in the Sun. The spectroscopic anal-
yses of elemental abundances in the solar photo-
sphere do not have as great an accuracy as chem-
ical analyses of solid materials. Carbonaceous
chondrite meteorites, which appear to be the
most representative samples of the relatively
nonvolatile constituents of the solar system, are
used for compilations of the abundances of most
of the elements (Tables 3.4 to 3.6). For the very
