Geoscience Reference
In-Depth Information
needed to detect the kind of chemical het-
erogeneity -- blobs and small-scale layering --
predicted from the recycling and crustal delami-
nation hypotheses.
on are products of these perceived constraints.
Absolute temperature, not lithologic diversity,
is the controlling parameter in these models of
geochemistry and geodynamics, and in the usual
interpretations
of
seismic
images
and
crustal
thickness.
The perception that the mantle is lithologi-
cally homogenous is based on two assumptions:
(1) the bulk of the upper mantle is roughly
isothermal (it has constant potential tempera-
ture) and (2) midocean-ridge basalts are so uni-
form in composition ('the convecting mantle'
is geochemical jargon for what is viewed as
'the homogenous well-stirred upper mantle') that
departures from the basic average composition
of basalts along spreading ridges and within
plates must come from somewhere else. The
only way thought of to do this is for nar-
row jets of hot, isotopically distinct, mantle to
arrive from great depths and impinge on the
plates.
The fact that bathymetry follows the square
root of age relation is an argument that the cool-
ing plate is the main source of density varia-
tion in the upper mantle. The scatter of ocean
depth and heat flow -- and many other param-
eters -- as a function of age, however, indicates
that something else is going on. Plume influence
is the usual, but non-unique, explanation for
this scatter, and for depth and chemical anoma-
lies along the ridge. Lithologic (major elements)
and isotope homogeneity of the upper mantle
are two of the linchpins of the plume hypoth-
esis and of current geochemical reservoir mod-
els. Another is that seismic velocities, crustal
thicknesses, ocean depths and eruption rates are
proxies for
mantle potential tempera-
tures
. The asthenosphere, however, is variable
in melting temperature and fertility (ability to
produce magma) and this is due to recycling
of oceanic crust and delaminated continental
crust and lithosphere. In addition, seismic veloc-
ities are a function of lithology, phase changes
and melting and are not a proxy for tempera-
ture alone. Some lithologies melt at low tem-
perature and have low seismic velocities with-
out being hotter than adjacent mantle. These
can be responsible for melting and tomographic
anomalies.
Mantle homogeneity: the old
paradigm
Global tomography and the geoid characterize
the large-scale features of the mantle. Higher
frequency and higher resolution techniques are
required to understand the smaller-scale fea-
tures, and to integrate geophysics with tecton-
ics and with mantle petrology and geochemistry.
The upper mantle is often regarded as being
extremely homogenous, based on low-resolution
tomographic studies and the chemistry of mido-
cean ridge basalts. Both of these approaches aver-
age out the underlying heterogeneity of the man-
tle. The intrinsic chemical heterogeneity of the
shallow mantle, however, is now being recog-
nized. This heterogeneity contributes to the iso-
tope diversity of magmas and the scattering of
seismic waves. Melting anomalies themselves --
hotspots and swells -- reflect lithologic hetero-
geneity and variations in fertility and melting
point of the underlying mantle. The volume of
basalt is related more to lithology of the shal-
low mantle than to absolute temperature. Thus,
both the locations of volcanism and the volume
of volcanism can be attributed to shallow -- litho-
spheric and asthenospheric -- processes, processes
that are basically athermal and that are intrinsic
to plate tectonics.
Much of mantle geochemistry is based on the
assumption of chemical and mineralogical homo-
geneity of the shallow mantle, with so-called
normal or depleted midocean-ridge basalt
(NMORB and DMORB) representative of the
homogeneity and depletion of the entire upper
mantle source (
the convecting upper mantle
).
The entire upper mantle is perceived to be
a homogenous depleted olivine-rich lithology
approximating pyrolite (pyroxene--olivine-rich
rock) in composition; all basalts are formed by
melting of similar peridotitic lithologies. Venera-
ble concepts such as isolated reservoirs, plumes,
temperature--crustal-thickness
relations
and
so
