Geology Reference
In-Depth Information
10
Mantle chemical evolution
Trace element heterogeneity of the mantle; apparent ages. Global bud-
gets and a mildly depleted MORB source. Distinct OIB source, no
primitive mantle. Major element heterogeneity, sources and survival.
Melting reconsidered; physical partitioning, disequilibrium, melt trap-
ping. Recycling oceanic crust and hybrid pyroxenites. Critique of pre-
vious abundance estimates. Geochemical modelling using tracers in
convection models. Density differences. Residence times. Ages due to
remelting, not homogenising. Incorporating noble gases.
The chemistry of the mantle has already entered this presentation implicitly and
explicitly. It is explicit in the discussions of compositional differences and radioac-
tive heating, and it is implicit in that the material properties we have called upon
depend of course on the composition of the relevant materials. The geochemistry
of the mantle is divided for convenience into major elements, trace elements and
isotopes. Trace elements and their isotopes give us some key information, but in
order to interpret them properly we need to consider the major elements as well.
Trace elements and isotopes give us some important, though indirect, informa-
tion about the structure of the mantle, and the isotopes give us some time informa-
tion as well. These are very important kinds of information that are not available
from other sources, so we should take advantage of them if we possibly can. It
is turning out that the interpretation of these geochemical observations requires a
careful consideration of both geochemical and geophysical processes. This should
not be too surprising, but there have been many proposed interpretations using
only one side or the other of the available information, with the result that two
quite incompatible pictures of the mantle were built up, one layered and the other
not. This incompatibility of interpretations obviously has implied that there were
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