Magmas: windows into the mantle - New Theory of the Earth - page 173

to 0.05% of the mantle, or about 10% of the
crustal volume. A small amount of such enriched
magma

plume head. Suitable stress conditions of the
plate and fertility of the mantle may be more
important than the absolute temperature of the
mantle. The final stages of ocean closure at a
convergent margin or suture and the presence
of batholiths with cumulate roots serve to fertil-
ize the mantle and lower the melting point of
the mantle and the hydrated crust.

can turn a depleted MORB into an

enriched melt.
Trace-element and isotopic patterns of OIB
overlap continental flood basalts (CFB), and com-
mon sources and fractionation patterns can be
inferred. Island-arc basalts (IAB) also share many
of the same geochemical characteristics, suggest-
ing that the enriched character of these basalts
may be derived from the shallow mantle. This is
not the usual interpretation. The geochemistry of
CFB is usually attributed to continental contami-
nation or to a plume head from the lower mantle,
that of IAB to sediment and hydrous melting of
the mantle wedge, and that of OIB to 'primitive'
lower mantle. In all three cases the basalts have
evolved beneath thick crust or lithosphere, giving
more opportunity for crystal fractionation and
contamination by crust, lithosphere and shallow
mantle prior to eruption. CFB and continental
island arcs are on continental or craton bound-
aries, or on old sutures, and may therefore tap
over-thickened continental or arc crust.

Apparently hot magmas

Magmas that are associated with so-called
hotspots differ in various ways from what are
thought to be normal mid-ocean ridge basalts (i.e.
along normal portions of the ridge system). There
is little evidence, however, that they are hotter
than normal basalts. High MgO contents are usu-
ally considered to be a diagnostic of high man-
tle temperatures but such magmas are rare at
hotspots. These magmas include the following.

Boninite: A high-MgO, low-alkali, andesitic
rock with textures characteristic of rapid
crystal growth. Boninites are largely restric-
ted to Pacific island arcs, and are more
abundant in the early stages of magmatism.
Picrite: A high-MgO basalt extremely rich in
olivine and pyroxene. In some petrological
schemes, picrite is the parent magma of
other basalts, prior to olivine separation.
Picrites are common at the base of conti-
nental flood basalt sequences. They occur in
Hawaii, and, possibly, in Iceland. Sometimes
they have had olivine added by crystal set-
tling and are not true high-MgO melts.
Komatiite: An ultramafic lava with 18--32%
MgO. The more highly magnesian varieties
are termed peridotitic komatiite .
Meimechite: A high-MgO, TiO 2 -rich ultramafic
volcanic rock composed of olivine, clinopy-
roxene, magnetite and glass occurs in
association with flood basalts, Ni--Cu--PGE-
bearing plutons, and diamond-bearing kim-
berlite and carbonatite.

Rhyolites and silicic LIPs
Silicic volcanism is a component of large igneous
provinces (LIPs) such as continental flood basalt
provinces related to continental breakup and
assembly. A crustal source is the main differ-
ence in silicic and mafic LIP formation. Large
degrees of crustal partial melting, implied by
the large volumes of rhyolitic magma, are con-
trolled by the water content and composition of
the crust, and the thermal input from the mantle
via basaltic magmas. Rhyolites are more common
and voluminous on the continents and continen-
tal margins than in the ocean basins. Sili-
cic volcanism is associated with most
CFB provinces , and some oceanic plateaus (e.g.
Kerguelen, Wallaby and Exmouth).
Silicic LIPs -- SLIPs -- have erupted volumes
similar to CFB, but are produced over time peri-
ods extending up to 40 Myr. The Kerguelen
and Ontong--Java oceanic plateaus were erupted,
episodically, over similar time-spans (30--40 Myr).
The generation of SLIPs and oceanic plateaus
requires the sustained input of basaltic mantle
melts rather than the transient impact of a hot

All of these, plus kimberlites and carbon-
atites, have been taken by many authors as
diagnostics of deep mantle plumes. Ocean-island
basalts, almost by definition, and certainly by