Geoscience Reference
In-Depth Information
1.4
1.2
Geochemical
fractionation
1.0
C2
C1
H
+
L
CV3
0.8
E
Earth's
composition
0.6
Cosmochemical
fractionation
0.4
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Al/Si
Figure 12.10
Evaluation of the composition of the primitive Earth by intersection of a trend for mantle rock and
a trend for chondritic meteorites. The choice of refractory lithophile elements allows the share of
the core and the crust to be ignored (after Jagoutz
et al.
,
1979
). E: enstatite chondrites; H, L:
high-Fe and low-Fe chondrites, respectively; C: carbonaceous chondrites, with 1-3 defining the
degree of metamorphism.
only by their contents of highly volatile elements such as hydrogen and nitrogen. The most
common ordinary chondrites are depleted in gaseous elements and are more reduced than
carbonaceous chondrites. Since chondrites come from the asteroid belt, well beyond the
orbit of Mars, they did not suffer the same heating as the Earth, which was substantially
closer to the radiation inferno of the nascent Sun. In addition, the dynamical simulations of
planetary accretion show that the planets from the Inner Solar System must have accreted
material from between Jupiter and Mercury. There is therefore no reason why the Earth
should be identical in its composition to any particular type of chondrite, with the probable
exception of the refractory elements. Chondrites represent an unlikely parent material for
the Earth. The oxygen isotope composition of most chondrite classes, either carbonaceous
or ordinary chondrites, is different from that of the Earth-Moon system. A special category
of meteorites, enstatite chondrites, could satisfy this criterion, but then we can infer from
the Si content of the Earth's mantle that the core should contain 8-14 wt% Si, for which
there is little supporting experimental evidence. The broadly chondritic composition of
the Earth is often challenged today but, at least for the refractory elements, nevertheless
remains a reasonable reference particularly in the face of no better alternative.
The composition of the primitive mantle (Bulk Silicate Earth, BSE), i.e. the mantle prior
to crust extraction, whatever the misgivings about its very existence as an extant geody-
namic entity, can be determined through simple mass balance. It can be considered that the
separation of the core had as its only effect to enrich the non-siderophile or chalcophile
elements by a constant factor (ignoring the crust) in the silicate fraction. The composi-
tions of meteorites and of the Earth's mantle are plotted on the graph of Mg/Si vs. Al/Si
(
Fig. 12.10
), along with that of fertile peridotites. The composition of these mantle samples
