Geoscience Reference
In-Depth Information
Chapter 3
The building blocks of planets
For as the sun draws into himself
the parts of which he has been
composed, so earth receives the
stone as belonging to her, and draws
it toward herself...
gesting that distinct oxygen reservoirs were avail-
able in the early solar system over relatively small
annuli of heliocentric distance.
Most workers assume that the Earth accreted
from some sort of
primitive material
delivered to
the Earth as meteorites and probably originat-
ing in the asteroid belt. But it is understood that
when the mantle is referred to as having the com-
position of
CI chondrites
,or
chondritic
,itis
usually only the refractory parts that are meant.
In the cosmological context
refractory
and
volatile
refer to the condensation temperature in a cool-
ing nebula of solar composition. The Earth is
clearly deficient in elements more volatile than
about Si, and this includes Na and K. In some
models constructed by noble gas geochemists,
however, the lower mantle is actually assumed to
be
primordial
or undegassed and to approach CI
in overall composition. The actual material form-
ing the Earth is unlikely to be represented by a
single meteorite class. It may be a mixture of vari-
ous kinds of meteorites and the composition may
have changed with time. The oxidation state of
accreting material may also have changed with
time.
Plutarch
The Earth is part of the solar system and the com-
position of the Sun, meteorites, comets, inter-
planetary dust particles and other planets pro-
vide information that may be useful in deducing
the overall composition of our planet, most of
which is inaccessible to direct observation.
Car-
bonaceous chondrites
(CI) appear to be the
most primitive and low-temperature extraterres-
trial (ET) objects available to us. Even if these
weretheonlybuildingblocksofplanetsthefinal
planet would differ in composition from them
because of vaporization during accretion, and
loss of low-molecular-weight material. But the
ratios of refractory elements in ET materials may
provide a useful constraint. Since most of the vol-
ume of a terrestrial planet is oxygen, the oxygen
isotopes of candidate materials play a key role in
deciding how to assemble a planet. Oxygen iso-
topes require that the Earth either be made of
enstatite meteorites
or a mixture of meteorites that
bracket the isotopic composition of the Earth
or enstatite meteorites. The bulk oxygen-isotopic
composition of the Earth precludes more than
a few percent of carbonaceous chondritic mate-
rial accreting to the Earth. Mars has a different
oxygen-isotopic composition from the Earth, sug-
Meteorites
Using terrestrial samples, we cannot see very far
back in time or very deep into a planet's interior.
Meteorites offer us the opportunity to extend
both of these dimensions. Some meteorites, the
chondrites, are chemically
primitive,
having com-
positions
--
volatile
elements
excluded
--
very
