Geoscience Reference
In-Depth Information
radionuclides,
87
Rb and
147
Sm, which decay to
87
Sr and
143
Nd with half-lives of 49 and 106 bil-
lion years, respectively. These half-lives are signif-
icantly longer than the age of the solar system or
any igneous rocks in it and therefore have little
control on early differentiation processes. Sm
actually decays to Nd via two radioactive decay
schemes:
146
Sm--
142
Nd, half-life about 10
6
years
(My) and
147
Sm--
143
Nd, half-life about 10
9
years
(Gy). Sr, Sm and Nd are refractory lithophile (pre-
fer silicates over metal) elements, whose rela-
tive abundances should not be affected by either
volatile loss or core formation. The long-lived
147
Sm--
143
Nd system has been widely used to trace
planetary-scale processes such as the evolution
of the crust and mantle. Because of the lack of
samples from Earth's first 500 My of existence,
the early epic of Earth differentiation is inves-
tigated with short-lived chronometers, such as
146
Sm--
142
Nd.
Midocean-ridge basalts have
87
Sr/
86
Sr less than
0.703, and 'pure' MORB may have values of 0.702
or less. Ocean-island, island-arc and continental
flood basalts are generally much higher than
0.703, commonly higher than 0.71, and are more
obviously mixtures. Attributing the properties of
MORB to 'normal mantle' and, more recently,
to the whole upper mantle, leaves crustal con-
tamination, recycling or lower mantle sources
as the only alternatives to explain ocean-island
and other so-called 'plume' or 'hotspot' basalts.
The standard model of mantle geochemistry orig-
inally ignored recycling and favored continuous
continental growth from a primordial mantle
reservoir, leaving behind a depleted homogenous
upper mantle (called the 'convecting mantle').
The upper mantle, however, is inhomogenous in
composition and isotopes. There is no evidence
that any magma comes from a primordial or
undegassed reservoir or from the lower mantle.
143
Nd/
144
Nd ratios are expressed in terms of
deviations, in parts per 10
4
,fromthevalueina
reservoir that has had chondritic ratios of Sm/Nd
for all time. This deviation is expressed as
However, Sm and Nd are separated by magmatic
process and thus record the magmatic or frac-
tionation history of the Earth. Samarium has
a higher crystal-melt partition coefficient than
neodymium, and thus the Sm/Nd ratio is smaller
in melts than in the original rock. The
143
Nd/
144
Nd ratio, normalized as above, will therefore
be positive in reservoirs from which a melt has
been extracted and negative in the melts or
regions of the mantle that have been infiltrated
by melts. The Sm/Nd ratio depends on the extent
of melting and the nature of the residual phases,
and
ε
Nd
depends on the Sm/Nd ratio and the age
of the fractionation event.
In spite of their geochemical differences,
there is generally good correlation between
neodymium and strontium isotopes. Positive
ε
Nd
correlates with low
87
Sr/
86
Sr and vice versa.
Midocean-ridge basalts have isotopic ratios indi-
cating time-integrated depletions of Nd/Sm and
Rb/Sr. The isotopic ratios are so extreme that the
depletion must have occurred in the MORB reser-
voir more than 1 Ga ago. The original depletion
may have occurred at the time the continental
crust or the proto-crust started to form but more
likely occurred throughout the accretional pro-
cess of the Earth. The measured Sm/Nd and Rb/Sr
ratios in MORB generally do not support such
ancient ages, but the depletion may have been
progressive, MORB may be mixtures of depleted
and enriched materials, and other melt addition
and extraction events may have occurred.
Incompatible-element ratios such as Rb/Sr and
Nd/Sm are high in small-degree partial melts.
However, for large fractions of partial melting
the ratios are similar to the original rock. Since
elements with partition coefficients much less
than unity (such as Rb, Sr, Nd and Sm) are
not retained effectively by residual crystals, it
is difficult to change their ratio in melts, but
the residual crystals, although low in these ele-
ments, have highly fractionated ratios. Partial
melts representing large degrees of partial melt-
ing from primitive mantle will also have nearly
primitive ratios, as will regions of the mantle
invaded by these melts. If the melt cools and crys-
tallizes, with refractory crystals being removed
and isolated, the Sm/Nd ratio changes. Thus, it
is dangerous to infer that a melt came from a
ε
Nd
.A
chondritic unfractionated reservoir has
0
at all times. Samarium and neodymium are both
refractory rare-earth elements and should be in
the Earth in chondritic ratios unless the Earth
was assembled from some other kind of material.
ε
Nd
=
