Geoscience Reference
In-Depth Information
Clair Patterson of Caltech determined the age
of the Earth to be 4.550 billion years (or 4.55
Gyr)
rently, the upper mantle is oxidized and the main
'metallic' phases are Fe-Ni sulfides.
146
Sm--
142
Nd
and
182
Hf--
182
W chronometry indicate that
core
formation and mantle differentiation
took place during accretion
, producing a
chemically differentiated and depleted mantle.
The decay of
182
Hf into
182
W occurs in the silicate
mantle and crust. Tungsten is then partitioned
into the metal. The hafnium--tungsten pair shows
that most of Earth formed within
±
70 million years from long-lived Pb iso-
topes (Patterson, 1956). This age was based on
isotopic dating of meteorites and samples of
modern Earth lead, all of which plot along a
linear isochron on a plot of
207
Pb/
204
Pb versus
206
Pb/
204
Pb.
Numerous other isotopic systems are used to
determine the ages of solar system materials and
ages of significant events, such as Moon forma-
tion. The isotopes include both those with long
half lives, such as rubidium-87 -- written 87Rb or
87
Rb (half life of 48.8 billion years), which decays
to strontium-87 -- written 87Sr or
87
Sr -- and those
that have half lives that are so short that the
radioactive isotope no longer exists in measur-
able quantities. Meteorites contain evidence for
decay of short-lived extinct natural radioactivities
that were present when solids condensed from
the primitive solar nebula. Three such short-
lived radioactivities,
53
Mn,
182
Hf and
146
Sm, have
half-lives of 3.7, 9 and 103 million years, respec-
tively. The evidence indicates rapid accretion of
solid bodies in the solar nebula, and early chem-
ical differentiation. A hot origin of the Earth is
indicated. The energetics of terrestrial accretion
imply that the Earth was extensively molten in
its early history; giant impacts would have raised
temperatures in the Earth to about 5000--10 000 K.
The rates and timing of the early processes
of Earth accretion and differentiation are stud-
ied using isotopes such as
129
I--
129
Xe,
182
Hf--
182
W,
146
Sm--
142
Nd,
235/238
U--
207/206
Pb and
244
Pu--
136
Xe
and simple assumptions about how parent and
daughter isotopes distribute themselves between
components or geochemical reservoirs. The Hf--W
and U--Pb chronometers are thought to yield the
time of formation of the core, assuming that
it is a unique event. The parent elements (Hf
and U) are assumed to be retained in silicates
during accretion and the daughters (W and Pb)
to be partitioned into the core. The partition-
ing of W and Pb between metals and silicates --
mantle and core -- also depends on the oxygen
fugacity and the sulfur content of the metal
and the mantle. Under some conditions, W is a
siderophile element while Pb is a chalcophile par-
titioning only slightly into the metal phase. Cur-
10 million
years after the formation of the first solid grains
in the solar nebula.
A plausible model for the origin of the Moon
is that a Mars-sized object collided with the
Earth at the end of its accretion, generating the
observed angular momentum and an Fe-depleted
Moon from the resulting debris disc. This may
have occurred 40--50 Myr after the beginning of
the solar system. The
Moon-forming impact
[Google images] contributed the final 10% of
the Earth's mass, causing complete melting and
major degassing.
Core formation occurred
before, during and after the giant
Moon-forming impact
, within tens of millions
of years after the formation of the solar system.
Some of the terrestrial core was probably from
the impactor. Giant impacts melt a large fraction
of the Earth and reset or partially reset isotopic
clocks.
Mass balance calculations show that
∼
70%
of the mantle was processed in order to form
the crust and upper mantle. Parts of the upper
mantle are enriched but most of the mantle
is either depleted and fertile (the MORB reser-
voir) or depleted and infertile or barren. Enriched
regions (crust) or components (kimberlites, car-
bonatites . . .) typically are so enriched that a
small volume can balance the depleted regions.
Short-lived radioactivities can, in principle, deter-
mine when this fractionation occurred. Some
was contemporaneous with accretion and some
may have happened during Moon formation.
>
Elementary isotopology
Pb
Isotopes are usually expressed as ratios involving
a parent and a daughter, or a decay product and
