Geoscience Reference
In-Depth Information
These large variations in
18
O in subducted mate-
rial translate to measurable
18
O variations in
ocean island basalts that sample subducted crust.
Oxygen isotopes can be used to quantify crustal
contributions to mantle sources. Unlike other iso-
tope tracers the concentration of oxygen is sim-
ilar in all rocks and mass balance calculations
can be attempted; oxygen isotopes are comple-
mentary to the LIL as tracers of crustal recycling.
Typical
100
15
Sediments
80
Islands
Hawaii
50
10
Italy
20
10
0
5
Crust
Tasmania
CFB
M
O
RB
0.702
0.710
0.720
upper-mantle
olivines
have
oxygen
87
Sr
86
Sr
/
isotope ratios in the range
+
5.0 to
+
5.4 per mil
18
Onear
and most unaltered MORB has
5.7 per
mil. Mantle peridotites and fresh basaltic rocks
have oxygen isotope ratios primarily from
δ
+
18
O versus
87
Sr/
86
Sr for magmas, oceanic
sediments, kimberlites and continental crust. Mixing curves
are shown for various degrees of crystal removal from the
depleted end-member. Oxygen fractionation is ignored in this
calculation.
Fig. 17.4
δ
5.5
‰
+
to
5.9
‰
, which is taken to be the isotopic
composition of common mantle oxygen or as a
typical upper mantle value. Alkali basalts and
mantle-derived
+
eclogite
xenoliths
depart
from
may have more olivine in their sources. In addi-
tion, olivine fractionation at low temperature
increases the
18
O values of eclogite garnets range
these values.
δ
18
O values
of lower crustal granulites range from
18
O of residual melts.
from
9.3
‰
. Such oxygen isotope sig-
natures were initially interpreted as the products
of fractional crystallization of mantle magmas,
although oxygen isotope fractionation effects are
now known to be small under mantle conditions.
The isotope and trace-element geochemistry of
mantle eclogite and pyroxenite xenoliths and the
anomalous oxygen isotope ratios are now taken
as evidence that these rocks are derived from
subducted altered ocean-floor basalts and gab-
bros, and possibly from delaminated continen-
tal crust. The changes in oxygen-isotope ratios
and other geochemical characteristics of oceanic
lithosphere that occur through exchange with
seawater and hydrothermal fluids near ocean
ridges have been documented in studies of ocean-
floor rocks and obducted ophiolite equivalents.
Metamorphic dehydration reactions do not cause
large shifts in oxygen isotope ratios.
Most ocean-ridge tholeiites have
+
1.5
‰
to
+
δ
δ
+
4.8
‰
to
13.5
‰
. If these values are unchanged by
eclogitization then this will be the range expec-
ted for delaminated lower-crustal eclogites, a
potential mantle component. In fact, this range
encompasses the entire OIB range, from HIMU to
EM2.
The
+
maximum
variations
in
rocks
known
to
be
derived
from
the
mantle
are
those
recorded in eclogite,
+
1.5 to
+
9.0 per mil. The
18
O
range of oxygen isotope (
8.5)
from
Sierran pyroxenites show consis-
tently higher values than measured in
spinel
δ
∼+
6.5 to
+
+
garnet and garnet peridotites
(
6.4) from the same locations. Peridotites are
generally
<
7.0. The range in pyroxenes
are very much smaller,
+
5.2 to
+
6.5 per mil.
Garnet in peridotite is on the low end of this
range.
87
Sr/
86
Sr
+
5.3 to
+
18
O between
δ
18
O
among OIBs and other enriched magmas. Pb,
Os and He isotopes also show some correla-
tions with oxygen isotopes; HIMU lavas and other
lavas with low
3
He/
4
He are often depleted in
18
O, consistent with recycled or assimilated lower
oceanic crust or lithosphere component. A sam-
ple mixing calculation is shown in Figure 17.4. In
order to match the higher
+
5and
+
5.7 per mil. Some pillow lavas in ophi-
is
positively
correlated
with
δ
18
O
olites
have
δ
values
of
over
+
12
per
mil.
Potassiclavashavevaluesof
+
6.0 to
+
8.5, conti-
nental tholeiites range up to
+
7.0. Oceanic alkalic
basaltsgoashighas
10.7. Kimberlites and car-
bonatites have values up to 26. EM2 basalts range
from
+
6.1 per mil.
It is possible that some tholeiites originate
from garnet--clinopyroxenite or eclogite while
some involve garnet peridotite; alkalic basalts
+
5.4 to
+
18
O values found for
some oceanic islands with reasonable amounts of
δ
