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In-Depth Information
of oxygen isotopes are dominated by low-
temperature mass-dependent fractionation pro-
cesses, such as water--rock interactions. Rocks
that react with the atmosphere or hydrosphere
become richer in oxygen-18. Crustal contamina-
tion and flux of crustal material into the mantle
serve to increase the
18
O/
16
O ratio.
Oxygen is not a trace element; it comprises
about 50 wt.% of most common rocks. There
are three stable isotopes of oxygen, mass 16, 17
and 18, which occur roughly in the proportions
of 99.76%, 0.038% and 0.20%. Variations in mea-
sured
18
O/
16
O are expressed as ratios relative to a
standard;
0.514
+
20
143
'MORB'
0.513
DEPLETED
0
Nd
ENRICHED
−
10
0.512
+
40
0.706
+
20
0.704
ENRICHED
DEPLETED
0
Sr
'MORB'
−
20
21
0.702
19.3
19
18
O(0
/
00)
=
1000
×
[(
18
O
/
δ
16
O)
m
−
(
18
O
/
16
O)
smow
]
/
(
18
O
/
16
O)
smow
ENRICHED
'MORB'
18
Primitive Mantle
where
smow
is Standard Mean Ocean Water.
Oxygen isotopic variations in rocks are chiefly
the result of fractionation between water and
minerals; the fractionation between minerals is
small.
DEPLETED
17
0
0.2
0.4
0.6
0.8
1.0
Contaminant fraction
18
O values of minerals decreases in the
order quartz--feldspar--pyroxene--garnet--ilmenite,
with
Fig. 17.3
Isotopic ratios versus contamination. Note that a
small amount of contamination has a large effect on the lead
system. Enriched magmas and slightly contaminated depleted
magmas will both fall in the 'enriched' field relative to
primitive mantle and will both give 'future' ages on a
single-stage Pb--Pb evolution diagram. Slight contamination
has less effect on Nd and Sr isotopes, and contaminated
MORB will still appear depleted. The correlation line cannot
be used to estimate the primitive value for
87
Sr/
86
Sr if basalts
are mixtures (e.g. Figure 17.2).
δ
olivine
and
spinel
showing
large
vari-
18
Oandthe
ations.
Metasomatism
increases
δ
LIL
elements.
At
high-temperature
clinopyrox-
18
O
ene
and
garnet
have
lower
δ
values
than
olivine.
Oxygen isotope ratios constrain the origins
of chemical and isotopic heterogeneity in the
sources of basalts and the role of subducted mate-
rial.
18
O of oceanic basalts correlate with trace
element abundances and radiogenic isotope trac-
ers such as Sr, Nd, Pb and Os isotopes (Eiler
et al.
1997). The oxygen isotope variations directly
trace those parts of the mantle that have inter-
acted with water at or near the Earth's surface.
Osmium and oxygen isotopes are useful tracers
of crustal involvement in the sources of MORB
and OIB.
Large variations in
18
O/
16
O are produced
in the oceanic crust and associated sediments
from interaction with the hydrosphere, either
becoming
δ
of contamination, less than 0.5%, pushes MORB
compositions into the enriched field for lead
but not for neodymium or strontium. In terms
of single-stage 'evolution,' both observed (con-
taminated) MORB and oceanic-island basalt will
appear to have
future ages
on a lead--lead geochron
diagram. The neodymium and strontium isotopic
ratios are not affected as much, and contami-
nated MORB will appear to come from depleted
reservoirs.
Oxygen isotopes
18
O-enriched
by
alteration
and
low-
300
◦
C) in layered dike
sequences, pillow basalts and sediments at the
top of the subducted crustal section, or moder-
ately
temperature exchange (
<
The
18
O/
16
O ratio is a powerful geochemical diag-
nostic because of the large difference between
crustal and mantle rocks. Basalts also exhibit dif-
ferences in this ratio. The relative abundances
18
O-depleted by high-temperature exchange
with
seawater
in
the
gabbroic
lower
crust.
