Geoscience Reference
In-Depth Information
Chapter 24
The nature and cause of mantle heterogeneity
The right to search for truth implies
also a duty. One must not conceal any
part of what one has discovered to
be true.
processes internal to the mantle. The fertility
and fertility heterogeneity of the upper mantle
are due to subduction of young plates, aseismic
ridges and seamount chains, and to delami-
nation of the lower continental crust, as dis-
cussed in Part II. These heterogeneities eventually
warm up past the melting point of eclogite and
become buoyant low-seismic-velocity diapirs that
undergo further adiabatic decompression melt-
ing as they encounter thin or spreading regions
of the lithosphere.
The heat required for the melting of cold
subducted and delaminated material is extracted
from the essentially infinite heat reservoir of
the mantle, not the core. Melting in the upper
mantle does not require an instability of a deep
thermal boundary layer or high absolute tem-
peratures. Melts from fertile regions of the
mantle, recycled oceanic crust and subducted
seamounts, can pond beneath the lithosphere,
particularly beneath basins and suture zones,
with locally thin, weak or young lithosphere, or
they can erupt. The stress state of the lithophere
can control whether there is underplating and
sill intrusion, or eruption and dike intrusion.
Absolute mantle temperature has little to do with
this.
The characteristic scale lengths -- 150 km to
600 km -- of variations in bathymetry and magma
chemistry, and the variable productivity of vol-
canic chains, probably reflect compositional het-
erogeneity of the asthenosphere, not the scales
of mantle convection or the spacing of hot
plumes. High-frequency seismic waves, scatter-
ing, coda studies and deep reflection profiles are
Albert Einstein
Overview
The lithosphere clearly controls the location of
volcanism. The nature and volume of the volcan-
ism and the presence of 'melting anomalies' or
'hotspots,' however, reflect the intrinsic chemi-
cal and petrologic heterogeneity of the upper
mantle. Melting anomalies -- shallow regions of
ridges, volcanic chains, flood basalts, radial dike
swarms -- and continental breakup are frequently
attributed to the impingement of deep mantle
thermal plumes on the base of the lithosphere.
The heat required for volcanism in the plume
hypothesis is from the core; plumes from the
deep mantle create upper mantle heterogeneity.
This violates the dictum of good Earth science:
never go for a deep complex explanation if a shallow
simple one will do .
Mantle fertility and melting point variations,
ponding, focusing and edge effects, i.e. plate tec-
tonic and near-surface phenomena, may control
the volumes and rates of magmatism. The magni-
tude of magmatism may reflect the fertility and
homologous temperature, not the absolute tem-
perature, of the asthenosphere. The chemical and
isotopic heterogeneity of the mantle is, in part,
due to recycling and, in part, due to igneous
 
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