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contaminated by a U/Pb-rich component. One
mechanism for such enrichment is removal of
a melt from a primitive reservoir to another part
of the mantle that subsequently provides melts
to the oceanic islands or contaminates MORB.
Another mechanism is subduction of continen-
tal sediments. The logical default storage place
is the shallow mantle, although this possibil-
ity is usually ignored in favor of deep-mantle
storage.
Delamination of lower continental crust is
another mechanism for contaminating or pollut-
ing the shallow mantle. In the standard model
of mantle geochemistry the shallow mantle is
assumed to be homogenous and any non-MORB
magmas are considered to be from regions of
the mantle that are polluted by plumes from
the deep mantle .
To explain the various trends of the individ-
ual islands by mixing, the enriched end-members
must come from parts of the crust or mantle
that were enriched at different times or that
have different time-integrated U/Pb ratios. In a
crystallizing cumulate or magma ocean, the U/Pb
ratio of the remaining fluid increases with time,
and regions of the mantle that were enriched
by this melt would have variable
Os
Os is one of the platinum group elements (PGE)
or metals. It is also a siderophile and a compati-
ble element. It should be mainly in the core but
there is enough in the mantle to make it a useful
tracer, particularly of recycled oceanic and conti-
nental crust. It is also a useful tracer of cosmic
dust, particularly in deep-sea sediments. The PGE
in the crust and mantle may primarily be due
to a late veneer. Because Os is compatible (jargon
for silicate-crystal-loving rather than melt-loving)
and occurs in sulfides, it tells us things that most
of the other geochemically useful isotopes can-
not. Peridotites, ultramafic massifs and ophiolites
provide the bulk of the data. This means that the
true isotope heterogeneity of the mantle can be
sampled, rather than the gross averages provided
by magmas. Large isotopic heterogeneities in Nd,
Sr, Pb and Os have been documented in peridotite
massifs related to spreading ridges on a variety of
length scales, ranging from centimeters to tens of
kilometers (Reisberg and Zindler, 1986), indicat-
ing a high degree of geochemical heterogeneity
in the upper mantle, despite the observed homo-
geneity of MORB.
During partial melting, Re is mildly incom-
patible. This results in high Re/Os in basalts, and
low Re/Os in the refractory, depleted solid residue
left behind in the mantle. Thus, 187 Os/ 188 Os ratios
in basalts and the residue rapidly diverge after
melting and separation. Radiogenic and unra-
diogenic (depleted mantle residue) end-members
are
depending
on when and how often they were enriched. If
the enriched reservoir is global, as indicated by
the global distribution of enriched magmas, it is
plausible that different parts of it were enriched,
or contaminated, at different times.
Pb-isotopes have painted a completely differ-
ent story for mantle evolution and recycling
than the early models based on Sr and Nd iso-
topes and the very incompatible elements. The
major mantle reservoirs are enriched (high and
increasing time-integrated U/Pb ratios) and there
is no place for an accessible primordial unfrac-
tionated reservoir or evidence for a depleted
reservoir, relative to primitive mantle. There are
multiple stages of enrichment evident in the
Pb-isotope record. Oxygen and osmium isotopes
are not LIL elements and they imply recycling
and a heterogeneous mantle. The importance of
plate tectonics and recycling are discussed in
early -- but still accessible and rewarding -- papers
on plumbotectonics and the persistent
myth of continental growth .
μ
constantly
produced
by
partial
melting
events.
Variations in osmium isotopic composition
result from the alpha decay of 190 Pt to 186 Os and
the beta decay of 187 Re to 187 Os. Suprachondritic
187 Os/ 188 Os ratios in intraplate volcanic rocks
have been used to support models for generation
of this type of volcanism from recycled oceanic
crust. Suprachondritic 186 Os/ 188 Os ratios in intra-
plate volcanic rocks have been interpreted as
indicating incorporation of outer core material
into plumes. Such signatures may, however, also
be generated in pyroxenites precipitated from
MgO-rich melts by the preferential incorporation
of Pt relative to Os in pyroxenes. High Pt/Os
and Re/Os ratios, which should lead to gener-
ation of suprachondritic
186 Os/ 188 Os- 187 Os/ 188 Os,
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