Geoscience Reference
In-Depth Information
Table 25.1
Effect of eclogite and olivine fractionation on primitive magma
Magma
SiO
2
Al
2
O
3
FeO
MgO
CaO
TiO
2
Na
2
O
2
O
1. Primitive
46.2
11.1
10.8
20.2
9.4
0.77
1.06
0.08
2. Extract
46.2
13.9
9.3
16.3
11.9
0.81
1.29
0.02
3. Picrite
46.2
8.3
12.3
24.1
6.9
0.74
0.83
0.14
Tholeiites
4. Model
50.0
13.8
12.4
8.5
11.5
1.23
1.38
0.23
5. Hawaiian
50.0
14.1
11.4
8.6
10.4
2.53
2.16
0.39
6. Continental
50.6
13.6
10.0
8.5
10.0
1.95
2.90
0.54
7. Average oceanic
50.7
15.6
9.9
7.7
11.4
1.49
2.66
0.17
1. Possible primitive magma. The partial melt product of primitive mantle
differentiation (O'Hara and others, 1975).
2. Eclogite extract (O'Hara and others, 1975).
3. Residual liquid after 50 percent eclogite (2) removal from primitive magma
(1). This is a model picritic primary magma.
4. Residual liquid after a further removal of 40 percent olivine (Fo
8.75
)from
liquid (3).
5. Average Hawaiian parental tholeiite.
6. Continental tholeiite (Tasmania) (Frey and others, 1978).
7. Average oceanic tholeiite glass (Elthon, 1979).
dependent on composition and temperature. If
the mantle is chemically stratified, mixing will
be less vigorous and chemically distinct compo-
nents can survive.
Part of this gravitational stratification will be
irreversible. The coefficient of thermal expansion
is high at low-pressure and high-temperature.
This means that temperature can overcome
intrinsic density differences. However, at high
pressure, this is no longer possible and deep
dense layers may be trapped. At lower mantle con-
ditions, a chemically distinct layer with an intrin-
sic density contrast of
will be selective. It will be uniform in the very
incompatible elements, giving apparently primit-
ive ratios of Rb/Sr, Sm/Nd and such, but will
impart a pattern of depletion in the HREE, yttr-
ium, sodium, manganese and so on since these
are the eclogite-compatible elements. Partial
melts from a shallow enriched reservoir will
therefore appear to have a garnet-residual pat-
tern, even if this reservoir contains no garnet.
This pattern can be transferred to any MORB mag-
mas interacting with this layer.
1% can be stable against
convective over-turn and mixing. Crystallization
of a melt layer or magma ocean leads to a series
of cumulate layers, and fractionation of the LIL.
Cumulate layers originally contain interstitial
fluids that hold most of the incompatible ele-
ments. As crystallization proceeds, these melts
may migrate upward. Melts from an eclogite or
olivine eclogite cumulate have the characteristics
of kimberlites. Removal of late-stage (kimberlite)
intercumulus fluids from an eclogite-rich cumu-
late layer will deplete it and enrich the overly-
ing olivine-rich layer. The enrichment, however,
∼
Access to deep layers
Convection in a chemically stratified system
causes lateral variations in temperature, and
deformation of the interfaces because of the
buoyancy of the uprising currents. If this defor-
mation raises a chemical boundary across the
solidus, or if the temperature is perturbed by,
for example, continental insulation, then partial
melting can generate buoyant diapirs, even in
a dense eclogite-rich layer. Subsolidus reactions
between garnet and clinopyroxene also occur at
