Geoscience Reference
In-Depth Information
1.4
Earth fractionation line
1.2
Solar
photosphere
Earth-PUM
R
J
1.0
P
CV
0.8
H
รท
L
CM
+
CO
CI
E
0.6
Mars mantle-crust
fractionation line
0.4
Terrestrial peridotite
Shergottites and Chassigny
Martian soil and rock
0.2
0 0
0.05
0.10
0.15
0.20
Al/Si (weight ratio)
Fig. 3.2 The major-element composition of material in the
inner Solar System is not of uniform composition, but exhibits
trends in Mg/Si versus Al/Si ratios in chondritic, terrestrial
and martian materials. If the average composition of the
Earth's mantle is similar to chondrites then the lower mantle
is perovskite-rich and komatiites and basalts represent only a
small fraction of the mantle. The Earth fractionation trend
then only represents the final stages of mantle differentiation.
Abbreviations: enstatite (E), ordinary (H, L) and carbonaceous
(CI, CM, CO and CV) chondrites. Circles are terrestrial
peridotites. R, J and P are estimates of the bulk silicate Earth
composition. The martian trend is defined by Chassigny,
shergottites and martian samples. (Drake & Righter, 2002).
Olivine
Lherzolite
Upper
Mantle
Refractory
Cumulates
Primitive
Mantle
Crystals
Magma
Ocean
Dense Fluids
and Cumulates
(komatiite
piclogite)
Transition
Region
usually only the refractory parts that are meant.
The exception is the so-called standard model
of noble gas geochemistry ;inthismodel
the high 3 He/ 4 He ratios of some ocean island
basalts is assumed to imply a He-rich reservoir,
deep in the mantle, with abundances of helium
similar to undegassed carbonaceous chondrites.
Javoy (1995) presented a model in which the
Earth is built from essentially pure EH (high-iron
enstatite achondrites) material. This is justified
by the fact that most elements in these mete-
orites exist in very refractory phases, more so
than in most other types of meteorites. Such a
model implies a chemically stratified mantle, lay-
ered convection, and limitations on the chemical
interchanges between lower and upper mantle.
Formation of Earth from EH material involves a
Picrite
Eclogite
Pyroxenites
Orthopyroxene
Basalt
Fig. 3.3 Representation of mantle components in terms of
olivine and orthopyroxene (the high-melting-point minerals)
and basalt (the most easily fusible component). Primitive
mantle is based on cosmic abundances. Melting of chondritic
material at high temperature gives an MgO-rich melt
(basalt + olivine) and a dense refractory residual (olivine +
orthopyroxene). Crystallization of a magma ocean separates
clinopyroxene and garnet from olivine. Melting during
accretion tends to separate components according to density
and melting temperature, giving a chemically zoned planet. If
melting and melt-crystal separation occur primarily at low
pressure, the upper mantle will be enriched in basalt, olivine
and the incompatible elements relative to the lower mantle
and relative to the chondritic starting material.
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