Geoscience Reference
In-Depth Information
Xenoliths
Kimberlite
=
15
2
ISLAND
ARCS
t
=
1.5
Ga
I
0
t
=
2
Ga
St. H.
C
RE
KI
R
T
+
K
15.5
I
1.0
OCEANIC
ISLANDS
2
H
OCEAN
RIDGE
BASALTS
H
=
8
R
I
2.5
=
9
−
20
1
T
KI
C
2
=
7.0
15.0
Depleted
t
=
l
−
2.5
Ga
<7.9
K
2
Enriched or contaminated
2
RE
7
1
=
>
7.9
14.5
13
15
17
19
21
206
Pb /
204
Pb
have come from ancient enriched reservoirs or
contain, as a component, ancient enriched mate-
rial. MORBs are thought to come from an ancient
depleted reservoir but they also have ratios in
excess of the geochron. This suggests that either
the mantle (or upper mantle) has lost lead, rela-
tive to uranium, or that ocean-ridge basalts have
been contaminated by material with high iso-
topic ratios, prior to eruption.
In
Fig. 17.1
Lead isotope diagram. Age of Earth is taken as
4.57 Ga. Straight lines labeled with letters are values for
oceanic islands. Black dots are the inferred primary isotopic
ratios if the island data are interpreted as secondary
isochrons. Growth curves for
μ
values of 7.0 and 8.0 and
primary isochrons at 1 Ga intervals are shown. The primary
mantle reservoir appears to have a
μ
of 7.9. Oceanic-island
basalts appear to have evolved in enriched reservoirs ranging
in age from 1 to 2.5 Ga with the second-stage
μ
values
ranging from 9 to 20. A point is shown for a two-stage model
with
μ
=
7
.
9 before 1.5 Ga and
=
15 subsequently. The black
bar represents the range of values for depleted reservoirs
with
μ
=
7
.
0 and a range of depletion ages from 1 to 2.5 Ga.
The range for midocean-ridge basalts could be due to growth
in an enriched reservoir or due to contamination by enriched
magmas. Isotopic ratios for xenoliths and kimberlites are
shown along the axes. Xenoliths are primarily from the
shallow mantle and many are enriched. KI is kimberlite.
Diagram is modified from Chase (1981).
μ
of the residual melt will increase with time,
assuming that solid silicates and sulfides retain
lead more effectively than they retain uranium.
Pb isotopes show that most of the mantle had
solidified prior to 3.8 Ga, close to the ages of
the oldest known rocks, measured by a vari-
ety of techniques. Basalts from oceanic islands
have apparently experienced secondary growth
in reservoirs with
a
cooling,
crystallizing
mantle
the
from about 10 to 20, after
a long period of growth in a more 'primitive'
reservoir (
μ
events. A few other systems also involve efficient
separation of parents and daughters and ancient
isotopic ratios can be
frozen-in
. These include
U--He and Re--Os. U and He are fractionated and
separated both by melting and degassing, so
ancient high
3
He/
4
He ratios can be frozen into
gas-filled inclusions in peridotites, for example.
The
7.9).
Leads from basaltic suites in many oceanic
islands form linear areas on
206
Pb/
204
Pb vs.
207
Pb/
204
Pb diagrams (Figure 17.1). These could
represent either mixing lines or secondary
isochrons. Two-stage histories indicate that the
leads
μ
∼
values for basaltic magmas are usually
quite high, 15--45, compared to primitive man-
tle. Their lead-isotopic ratios will therefore grow
more rapidly with time than the primitive man-
tle, and the
206
Pb/
204
Pb and
207
Pb/
204
Pb ratios of
such magmas are high. Some oceanic islands
have such high lead-isotopic ratios that they must
μ
from
each
island
were
derived
from
a
common primary reservoir (
9) at different
times from 2.5 to 1.0 Ga. Alternatively, the mag-
mas from each island could represent mixtures
between
enriched, less-enriched
or
depleted
compo-
nents. In either case basalts involve a source
region
μ
=
7
.
with
ancient
U/Pb
enrichment,
or
are
