Geoscience Reference
In-Depth Information
temperatures are not required. Thus, there are a
varietyofwaysofgenerating
melting anomalies
.
In a stratified mantle, without transfer of
material between layers, convection in a lower
layer can control the location of melting in the
overlying layer. In a homogenous mantle, the
high temperature gradient in the thermal bound-
ary layer between layers is the preferred source
of magma genesis since the melting gradient is
larger than the adiabatic gradient in a homoge-
nous convecting fluid (Figure 14.5). In a homoge-
nous mantle, if the melting point is not exceeded
in the shallow mantle, then it is unlikely to
be exceeded at greater depth since the melt-
ing point and the geotherm diverge with depth.
Material can leave a deep source region by several
mechanisms.
the only complement -- to the depleted MORB
source. Early mass-balance calculations based on
this premise, and on just a few elements and
isotopes, suggested that most of the mantle
remains undepleted or
primitive
. The depleted
reservoir was assumed to occupy most or all of
the upper mantle. Since the continental crust is
the only enriched reservoir in this three-reservoir
model, magmas that have high LIL-contents and
87
Sr/
86
Sr ratios are assumed to be contaminated
by the continental crust, or to contain a recycled
ancient crustal component.
A depleted reservoir (low in LIL, low in Rb/Sr,
87
Sr/
86
Sr and so on) can still be
fertile
, i.e. it can
provide basalts by partial melting. A clinopyrox-
enite or gabbro cumulate, for example, can be
depleted but fertile. Similarly, an enriched reser-
voir can be infertile, being low in Ca, Al, Na and
so on. The fact that most of the mantle must
be depleted as implied by mass-balance calcula-
tions, does not mean that it is fertile or similar
to the MORB reservoir. Most of the volume of the
mantle is depleted infertile- or barren-refractory
residue of terrestrial accretion, and most of it is
magnesium
perovskite
in the lower mantle.
(1) Melting in the thermal boundary at the bot-
tom or the top of a region because of the high
thermal gradient.
(2) Melting, or phase changes, due to adiabatic
ascent
of
hotter
or
lower
melting
point
regions.
(3) Entrainment of material by adjacent convect-
ing layers.
(4) Heating of fertile blobs by internal radioac-
tivity or by conduction of heat from the sur-
rounding mantle.
Ocean-island basalts
The trace element and isotope ratios of basalts
from ocean islands (OIB) differ from otherwise
similar MORB. There is still no general agreement
on how such variations are produced; chemical
and isotope variability of the mantle is likely to
be present everywhere, not just in the source
regions of OIB. Both geochemical and geophys-
ical observations, and plate-tectonic processes,
require the upper mantle to be inhomogenous,
and a variety of mechanisms have been suggested
to produce such inhomogeneities. Perhaps the
most obvious model for generating a variably fer-
tile and inhomogeneous mantle is subduction,
delamination and incomplete melt extraction.
Variability is generated by recycling of oceanic
and continental crust, and of seamounts, aseis-
mic ridges, oceanic islands and sediments, all
of which are being transported into subduction
zones. Recycling of the oceanic crust involves
the greatest mass flux of these various compo-
nents but this does not require that such mate-
rial is the dominant source of OIB. A popular
Some of these mechanisms are illustrated in Fig-
ure 7.1.
Midocean ridge basalts
The most voluminous magma type, MORB, has
low LIL-content and depleted isotopic ratios, with
very low variance, that is, they are homogeneous.
The Rb/Sr, Nd/Sm and U/Pb ratios have been low
since
1 Ga. Since these ratios are high in melts
and low in residual crystals, the implication is
that the MORB source is a cumulate or a crys-
talline residue remaining after the removal of
a melt fraction or residual fluid. Extraction of
very enriched, very small-degree melts (
>
1%) is
implied. In some geochemical models the LIL-
enriched melt fraction is assumed to efficiently
leave the upper mantle (the depleted MORB-
source in these models) and enter, and become,
the continental crust. The continental crust is
therefore regarded as the complement -- often
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