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crust. Stable isotopes can be used to test these
hypotheses. Early models of mantle geochemistry
assumed that all potential crust-forming mate-
rial had not been removed from the mantle or
that the crust formation process was not 100%
efficient in removing the incompatible elements
from the mantle. Recycling was ignored. Later
models assumed that crustal elements were very
efficiently removed from the upper mantle -- and,
importantly, only the upper mantle -- leaving it
depleted. Vigorous convection then homogenized
the source of midocean-ridge basalts, which was
assumed to extend to the major mantle disconti-
nuity near 650 km depth. A parallel geochemical
hypothesis at the time was that some magmas
represented melts from a 'primitive' mantle
reservoir that had survived from the accretion
of the Earth without any degassing, melting or
melt extraction. The assumption underlying this
model was that the part of the mantle that
provided the present crust did so with 100%
efficiency, and the rest of the mantle was iso-
lated, albeit leaky. In this scenario, 'depleted'
magmas were derived from a homogenized reser-
voir, complementary to the continental crust
that had experienced a multi-stage history (stage
oneinvolvedanancientremovalofasmallmelt
fraction, the crust; stage two involved vigorous
convection and mixing of the upper mantle;
stage three involved a recent extensive melt-
ing process, which generated MORB). Non-MORB
magmas (also called 'primitive,' 'less depleted,'
'hotspot' or 'plume' magmas) were assumed to
be single-stage melts from a 'primitive' reservoir.
There is no room in these models for ancient
enriched mantle components. These early 'box
models' contained three boxes: the present con-
tinental crust, the 'depleted mantle' (which is
equated to the upper mantle or MORB reser-
voir) and 'primitive mantle' (which is equated
to the lower mantle) with the constraint that
primitive mantle is the sum of continental crust
and depleted mantle. With these simple rules
many games were played with crustal recycling
rates and mean age of the crust. When contra-
dictions appeared they were, and are, tradition-
ally explained by hiding material in the lower
crust, the continental lithosphere or the core, or
by storing material somewhere in the mantle for
long periods of time. The products of mantle dif-
ferentiation are viewed as readily and efficiently
separable but, at the same time, storable for long
periods of time in a hot, convecting mantle and
accessible when needed.
A large body of isotope and trace-element
analyses of midocean-ridge basalts demonstrates
that the upper mantle is not homogenous; it con-
tains several distinct geochemical domains on
a variety of length scales. However, the physi-
cal properties of these domains, including their
exact location, size, temperature and dynamics,
remain largely unconstrained. Seismic data indi-
cate that the upper mantle is heterogenous in
physical properties. Plate tectonic processes cre-
ate and remove heterogeneities in the mantle,
and create thermal anomalies.
Global tomography and the use of long-lived
isotopes are very broad brushes with which to
paint the story of Earth structure, origin and evo-
lution. Simple models such as the one- and two-
reservoir models, undegassed undifferentiated
lower-mantle models, and whole-mantle convec-
tion models are the results of these broad-brush
paintings, as are ideas about delayed and contin-
uous formation of the crust and core. Short-lived
isotopes and high-resolution and quantitative
seismic techniques paint a completely different
story.
Isotopes as chronometers
Some of the great scientists, carefully ciphering the
evidences furnished by geology, have arrived at the
conviction that our world is prodigiously old, and
they may be right, but Lord Kelvin is not of their
opinion.
Mark Twain
Earliest history of the Earth
The current best estimate for the age of the Earth
Moon meteorite system is 4.51 to 4.55 billion
years (Dalrymple, 2001). The solar nebula cooled
to the point at which solid matter could con-
dense by
4.566 billion years, after which the
Earth grew through accretion of these solid
particles; the Earth's outer core and the Moon
were in place by
∼
∼
4.51 billion years.
