Geoscience Reference
In-Depth Information
model involves transport of cold slabs, includ-
ing the crust, to the core--mantle boundary (CMB)
where
tle is polluted by delaminated lower
continental crust.
Enriched MORB (EMORB)
is also common along the global spreading ridge
system and on near-ridge seamounts. There is
a continuum between depleted and enriched
tholeiites (DMORB, NMORB, TMORB, EMORB,
PMORB, OIB). Veins in mantle peridotites and
xenoliths contained in alkali basalts and kim-
berlites are also commonly enriched and, again,
crustal contamination is unlikely unless conti-
nental crust somehow gets back into the man-
tle. In many respects enriched magmas and xeno-
liths are also complementary to MORB (in LIL
contents and isotopic ratios), suggesting that
there is ancient enriched material in the mantle.
Island-arc basalts are also high in LIL,
87
Sr/
86
Sr,
143
Nd/
144
Nd and
206
Pb/
204
Pb, suggesting that
there are shallow -- and global -- enriched com-
ponents. Back-arc-basin basalts (BABB) are similar
to MORB in composition and, if the depth of the
low-velocity zone and the depths of earthquakes
can be used as a guide, tap a source deeper than
150 km. Many BABBs are intermediate in chem-
istry to MORB and OIB. This and other evidence
indicates that enriched components -- or enriched
reservoirs -- may be as shallow or shallower than
the depleted MORB reservoir. On average, because
of recycling and crustal delamination, enriched
and fertile components may be preferentially col-
lected in the shallow mantle. However, they may
be dispersed throughout much of the upper man-
tle. It is quite probable that the upper man-
tle is also lithologically stratified. This would
show
∼
3000 km
deep narrow plumes that come up under oceanic
islands. Enrichment of the lower part of the litho-
sphere by upward migrating metasomatic melts
and fluids, and inefficient extraction of resid-
ual melts could produce extensive fractionation,
and also allow the isotopic anomalies to form
within a region that was not being stirred by
mantle convection. Many oceanic islands and vol-
canic chains -- often called hotspots and hotspot
tracks -- are on pre-existing lithospheric features
such as fracture zones, transform faults, conti-
nental sutures, ridges and former plate bound-
aries. Volcanism is also associated with regions of
lithospheric extension and thinning, and swells.
The cause and effect relations are often not obvi-
ous. Oceanic islands are composed of both alkali
and tholeiitic basalts. Alkali basalts are subor-
dinate, but they appear to dominate the early
and waning stages of volcanism. The newest sub-
marine mountain in the Hawaii chain, Loihi,
is alkalic. Intermediate-age islands are tholeiitic,
and the latest stage of volcanism is again alka-
lic. The volcanism in the Canary Islands in the
Atlantic changes from alkalic to tholeiitic with
time.
Ocean-island basalts, or OIB, are LIL-rich and
have enriched isotopic ratios relative to MORB.
Their source region is therefore enriched, or
depleted parent magmas have suffered contami-
nation en route to the surface. The larger oceanic
islands such as Iceland and Hawaii generally
have less enriched magmas than smaller islands
and seamounts. A notable exception is Kergue-
len, which may be built on a micro-continent.
Other volcanic islands may involve continental
crust in a less obvious way. The islands in the
Indian ocean and the south Atlantic are from
mantle that was recently covered by Gondwana
and may contain fragments of delaminated con-
tinental crust.
Enriched magmas such as alkali-olivine
basalts, OIB and nephelinites occur on oceanic
islands and have similar LIL and isotopic ratios
to continental flood basalts (CFB) and continen-
tal alkali basalts. Continental contamination is
unlikely in these cases
unless the upper man-
they
heat
up
and
generate
up
as
scattered
energy
in
short-period
seismograms.
The trace-element signatures of some ocean-
island basalts and some ridge basalts are con-
sistent with derivation from recycled lower-
continental crust. Others are apparently derived
from a reservoir that has experienced eclogite
fractionation or metasomatism by melts or other
fluids from an eclogite-rich source. Kimberlites
are among the most enriched magmas. Although
they are rare, the identification of a kimberlite-
like component in enriched magmas means that
they may be volumetrically more important, or
more wide-spread, than generally appreciated.
Simple mass balance calculations suggest that
kimberlite-like components may account for up
