Geoscience Reference
In-Depth Information
single assumption has created many geochemi-
cal paradoxes.
Mass-balance calculations make no statement
about the origins, locations or shapes of the
enriched domains. They are usually assumed to
be large isolated regions of the mantle, such
asthelowermantle,orthesizesofgrain
boundaries and veins. Fertile, or mafic, material
gets into the mantle via subduction of oceanic
crust and delamination of continental crust,
and fertile blobs can have dimensions typical of
these tectonic elements. Delaminated lower crust
may have dimensions of tens of km. Subducted
seamountchainsmayhavedimensionsoftensby
hundreds of km. If oceanic plateaus can subduct
they would generate fertile regions having lateral
dimensions of thousands of km.
The present crust is 0.566% of the mass of
the mantle and contains more than 50% of the
Earth's inventory of some elements (Chapter 13).
Therefore, there cannot be more than two crustal
masses in the BSE, or one in the mantle. The max-
imum thickness of the crust, except in actively
converging areas, is about the depth to the
basalt--eclogite phase change, suggesting that
the volume of the present crust may not be
controlled by the volume of potential crust but by
the depth of a phase change. The potential crust
in the mantle may be about equal to the present
continental crust. Some of this may appear as
enriched components -- EM2, say -- in midocean-
ridge and ocean-island basalts
EMORB is found far from any melting
anomalies and bathymetric highs, showing that
enriched components are widely available in the
shallow mantle; it is chemically similar to OIB.
Donnelly
et al.
(2004) estimate that about 2--10%
of ridge samples are EMORB in which incom-
patible elements such as Th and Ba are a fac-
tor of 10 more abundant than in typical NMORB
sources. The continental crust and kimberlites
contain enrichment factors of up to 100 and
1000, respectively -- relative to BSE -- for the
more incompatible elements. End-member kim-
berlites can account for no more than 0.1% of
the mass of BSE, by simple mass balance. This
is about one-sixth of the present mass of the
continental crust and about one-tenth of the
upper bound on the allowable amount of CC
in BSE.
Estimates of the degree of melting required to
generate MORB and EMORB from a fertile peri-
dotite -- ultramafic source -- range from 6--20%.
Estimates of the degree of melting obviously
depend on the source composition. Magmas can
also be derived from mafic sources such as recy-
cled ocean crust, delminated lower continental
crust and eclogitic cumulates. Eclogite is a type
of rock and also a metamorphic facies; it can
have an original basaltic extrusive or gabbroic
cumulate protolith; or it can be a residue left
over from partial melting and melt extraction.
These 'eclogites' are not expected to have the
same compositions as unprocessed ocean crust
unless the metamorphic transformation to eclog-
ite is isochemical. Eclogites are cpx- and ga-rich
by definition but may also contain trace to abun-
dant oxide minerals (e.g. rutile) that replace orig-
inal magmatic ilmenite and magnetite. Phases
such as rutile, zircon, amphibole and micas (phlo-
gopite in particular) can dominate some trace-
element patterns.
