Geoscience Reference
In-Depth Information
18
Oof
fractionation
and
contamination,
a
δ
in Sr and Nd isotope ratios and low (for Hawaii)
3
He/
4
He ratios (the 'Koolau' end-member). This
component of Hawaiian basalts was originally
thought to be a pure deep-mantle plume end-
member but was later attributed to sediments.
δ
about
17 per mil is implied for the enriched
or recycled component.
Contamination of mantle-derived magmas by
the shallow mantle, lithosphere or crust may
be caused by bulk assimilation of solid rock,
by isotopic and trace-element exchange between
magma and wallrock, or by magma mixing
between the original melt and melts from the
wallrock. Isotopic and trace-element exchange
between magma and solid rock are likely to
be too inefficient to be important because of the
very low diffusivities. Diffusion distances, and
therefore equilibration distances, are only a few
centimeters per million years. Bulk assimilation
or isotope exchange during partial melting are
probably the most efficient means of magma con-
tamination. This is not a simple two-component
mixing process. It involves three end-members,
the magma, the contaminant and a cumulate
phase, which crystallizes to provide the heat
required to partially melt the wallrock or dissolve
the assimilated material.
Eclogitic
+
18
O values similar to MORB and peridotite xeno-
liths rules out a large contribution of subducted
sediment. With high radiogenic osmium, heavy
oxygen and low
3
He/
4
He (comparable to some
values found along spreading ridges), the Koolau
end-member may represent upper oceanic crust
and/or mixing of EM2 and HIMU.
Some of the components in ocean-island
basalts (e.g. EM1, EM2, HIMU, high- and low-
3
He/
4
He) identified with radiogenic isotopes also
have distinctive oxygen-isotope ratios. There
are correlations between oxygen and radiogenic
isotopes.
EM2 basalts are enriched in oxy-
gen isotopes relative to MORB
, consis-
tent with the presence of subducted sediments
in
87
Sr/
86
Sr
their
sources;
is
positively
corre-
lated
18
O among all OIBs; HIMU lavas
and lavas with low
3
He/
4
He are often depleted
in
18
O relative to normal upper mantle, con-
sistent
with
δ
18
O
garnets
have
higher
δ
values
18
O
than
peridotite
garnets.
δ
values
in
some
with
the
presence
of
recycled
lower
18
O
values found in low-temperature altered oceanic
basalts. Diamond inclusions tend to have higher
δ
18
O values are associated with radio-
genic Pb isotope ratios and with
depleted
Sr and
Nd
eclogites
are
equivalent
to
the
extreme
δ
crust. Low
δ
3
He/
4
He
isotope
ratios
and
relatively
low
18
O than eclogite xenoliths, suggesting that
the latter may have exchanged with 'normal'
mantle. The anomalous oxygen-isotope ratios of
some eclogites might be the result of processes
other than subduction and metamorphism of
altered
ratios.
The various recycled components -- sediments,
lower crust, upper crust, lithosphere -- that have
been identified in OIB by
18
O variations imply
a heterogenous dynamic upper mantle. The vari-
ous heterogeneities introduced by plate tecton-
ics and surface processes have different melt-
ing points and densities. These components all
coexist in the shallow mantle. The isotope evi-
dence does not imply that these components can
only be returned to the shallow mantle by deep
plumes.
δ
ocean-floor
basalt,
such
as
melting
of
18
O
values, 3
‰
--4
‰
, appear to require remelting
of hydrothermally altered oceanic crust or
meteoric--hydrothermal alteration after magma
crystallization.
Hawaiian basalts have significantly lower
delaminated lower continental crust. Low
δ
18
O
than typical upper mantle olivines (Eiler
et al.
,
1996a,b); low values are associated with radio-
genic Pb-isotope ratios and with
depleted
Sr-
and Nd-isotope ratios and relatively low
3
He/
4
He
ratios. The low-
18
O-end member may repre-
sent Pacific ocean crust that underlies Hawaii,
or
δ
Reservoirs
The standard model
At one time it was routinely assumed that
3
He/
4
He and other isotopic ratios in mantle-
derived materials represented two distinct popu-
lations corresponding to two distinct reservoirs.
a
recycled
upper-mantle
lithosphere
com-
ponent.
Hawaiian
lavas
also
contain
a
high
18
O end member, enriched
(i.e.
>
5.2 per mil)
δ
