Geoscience Reference
In-Depth Information
The Hawaiian tholeiites can be modeled with
variable degrees of deep crystal fractionation
(0--95%), mixed with 1--5% of an enriched compo-
nent. Alkali basalts represent greater extents of
crystal fractionation and contamination. All of
this is consistent with magma evolution beneath
thick crust or lithosphere, the main tectonic dif-
ferences between midocean ridges and oceanic
islands.
0.724
Continental basalts
Ocean Islands
EM
0.720
Upper
crust
0.716
Lower
crust
0.712
Deep Sea
Sediments
Kerguelen
0.5 EM
0.708
0.1
0.2
BCR-1
0.704
St.Helena
MORB
0.700
17
18
19
20
21
206
Pb
22
23
24
25
26
Lead and Sr isotopes
Rubidium, strontium and the light REEs are
classic incompatible elements, and the effects
of partial melting, fractionation and mixing
can be explored with some confidence. Rela-
tions between these elements and their isotopes
should be fairly coherent. Some enriched mag-
mas also have high
3
He/
4
He,
204
Pb
/
Fig. 17.7
Sr versus Pb isotopes; trajectories for mixtures of
a fractionating depleted MORB-like magma and an enriched
component. Mixing hyperbolas are solid lines. Hawaiian
basalts fall in the region of 50--99% crystal fractionation
(garnet plus clinopyroxene) and 10--30% contamination by EM
(dashed lines). The enriched component may be recycled
mafic crust or cumulates.
18
Oand
206
Pb/
204
Pb.
These isotopes provide important constraints on
mantle evolution, but they may be decoupled
from the LIL variations and are usually assumed
to be so.
3
He/
4
He depends on the uranium and
thorium content and age of the enriched compo-
nent.
206
Pb/
204
Pb and
3
He/
4
He are sensitive to the
age of various events affecting a reservoir because
of the short half-life of uranium. The U/Pb ratio
may be controlled by sulfides and metals as well
as silicates.
A minimum of three components is required
to satisfy the combined strontium and lead iso-
topicsystemswhenonlysimplemixingiscon-
sidered. A mixing--fractionation curve for Sr and
δ
Pb isotopes (Figure 17.7) shows that the data can
be explained with only two isotopically distinct
components. Enriched components can have vari-
able isotope ratios since these are sensitive to the
U/Pb ratio and the age of enrichment events. The
various contributions to mantle variability (sub-
duction, trapped magmas, trapped bubbles) make
it likely that the OIB 'reservoir' is in the shallow
mantle and is laterally inhomogenous. In this
sense the mantle contains multiple 'reservoirs';
their dimensions are likely to be of the order of
tens of kilometers.
