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are properties of pyroxenites (Smith, 2003). The
isotope signatures do not give any indication
of depth of origin, and are consistent with
shallow-source models for intraplate volcanism.
Pt
--
Os isotope systematics do not prove
an ultra-deep origin for intraplate
volcanism
.
A characteristic of the Re--Os system is the
large Re/Os ratio acquired by magmas and the
complementary low Re/Os ratio retained by
the peridotite residues. Re/Os ratios correlate
with rock type, a characteristic not shared by LIL-
isotope systems. With sufficient time, differences
in Re/Os ratio translate into large differences in
the
187
Os/
188
Os ratio. This makes the
187
Os/
188
Os
system a tracer of the addition of mafic crust or
melt to the mantle, and Os isotopes have con-
sequentlybeenusedtotracetheinvolvement
of recycled mafic crust in the sources of OIB
(Shirey and Walker 1998).
Initial
(corrected for age)
187
Os/
188
Os values for
abyssal peridotites worldwide lie between 0.117
and 0.167. Modern MORB samples have
187
Os/
188
Os
ratios clustering between 0.127 and 0.131, with
some samples as high as 0.15 (Roy-Barman and
Allègre, 1994).
OIB have variable and elevated
187
Os/
188
Os
ratios between 0.13 and 0.15, somewhat higher
than primitive mantle values of 0.126 to 0.130
estimated from meteorites. Osmium and oxygen
isotope correlate, indicating mixing of peridotitic
and crustal components. Enriched mantle com-
ponents (EM1, EM2, HIMU) are characterized by
elevated
are more likely to have compositions close to an
end-member or a pure component.
Meibom
et al
. (2002) attributed the Gaussian
187
Os/
188
Os distribution of peridotite samples to
melt-rock reactions during partial melting events
in the upper mantle. During adiabatic upwelling
of an upper-mantle assemblage (e.g. at a mid-
ocean ridge) domains with relatively low solidus
temperature and radiogenic Os isotopic composi-
tions will melt first at depth. These melts mix
with other melts and react with solid mantle
material at shallower depths. Ancient unradio-
genic Os is released from sulfide nuggets encap-
sulated in silicate and chromite host phases and
mix with the more radiogenic Os in the melt. The
Gaussian distribution represents random mixing
between unradiogenic and radiogenic Os isotopic
components of variable age.
Sr and Nd
Isotopes of Sr and Nd were used to set up
the
two-reservoir model of mantle geo-
chemistry
, a model that is inconsistent with
much of petrology and geophysics, including
Pb-isotopes and most other isotope systems.
The idea of a two-reservoir mantle, a primor-
dial reservoir and slow extraction of the crust
from it (
the persistent myth of conti-
nental growth
) to form a depleted upper
mantle were based on these systems. This so-
called
standard model of mantle geo-
chemistry and convection
conflicts with
Pb-isotope data and with geophysical data and
theoretical models of planetary accretion and dif-
ferentiation. It also conflicts with a broader base
of geochemical data that suggests very early and
rapid differentiation of the silicate Earth. Unfor-
tunately, current
hybrid mantle convection
models
of mantle chemistry and dynamics are
complex but retain the essence of the origi-
nal model, including ready access to primitive
or enriched material in the deepest mantle,
and a well-stirred homogenous upper mantle.
Most current models have many paradoxes
associated with them [see
mantleplumes
],
suggesting that the underlying assumptions are
wrong.
The use of Sr and Nd isotopes usually relies on
natural radioactive decay of two very long-lived
187
Os/
188
Os. Often, these enriched com-
18
O values that are different
from MORB or abyssal peridotites that have been
defined as upper mantle values (Eiler
et al.
1997),
indicating several source for recycled mafic mate-
rial in the mantle.
Histograms of isotope ratios commonly dis-
play nearly Gaussian distributions. Such distribu-
tions can result from shallow mantle processes
involving the mixing of different proportions of
recycled, radiogenic and unradiogenic materials.
Considering the large volume of mantle that is
sampled by ocean ridge, ocean island and con-
tinental basalts, it is unlikely that pure end-
members will be sampled; all basalts are blends
to some extent. Small seamounts and xenoliths
ponents
have
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