Geoscience Reference
In-Depth Information
system are attributed to plumes from the chem-
ically distinct lower mantle.
However, it can be shown that most or all of
the mantle needs to be depleted and degassed to
form the crust and upper mantle and to explain
the amount of
40
Ar in the atmosphere; this
wasamajorthemeinthefirsteditionof
The-
ory of the Earth
(Anderson, 1989). Deple-
tion and degassing of the upper mantle alone
cannot explain the observations. In addition,
the MORB reservoir and the CC are not exactly
complementary; another enriched component is
needed. When this (Q-component) is added in,
MORB
Observed isotopic arrays and mixing curves of
basalts, including ocean-island basalts (OIB), can
be generated by various stages of melting, mix-
ing, melt extraction, depletion and enrichment
and do not require the involvement of unfrac-
tionated, primitive or lower-mantle reservoirs.
However, the first stage of Earth formation -- the
accretional stage -- does involve large degrees of
melting that essentially imparts an unfraction-
ated -- but enriched -- chondritic REE pattern to
the upper mantle. Small-degree melts from this
then serve to fractionate LIL.
Q require that most of the mantle
must be processed and depleted. There must be
other components and processes beyond single
stage small-degree melt removal from part of the
primordial mantle to form CC. There are other
enriched components in the mantle, probably in
the shallow mantle, and other depleted residues
over and above the MORB-source. The upper man-
tle (UM) is still generally treated as if its composi-
tion can be uniquely determined from the prop-
erties of depleted MORB -- NMORB or DMORB --
and depleted peridotites, continental crust, and
an undifferentiated starting condition.
The traditional hotspot and plume models of
OIB and enriched magma genesis and mantle
heterogeneity are unsatisfactory [
mantle plu-
mes, plume paradoxes
]. Traditional models
for both OIB and MORB genesis involving
only peridotitic protoliths are also being re-
evaluated [
olivine-free mafic sources
].
Although recycling has long been used as a mech-
anism for modifying the isotopic character of
OIB, it is now becoming evident that it can also
create
melting anomalies
. The roles of eclog-
ite and garnet pyroxenite in petrogenesis and in
the formation of melting anomalies are becom-
ing evident (Escrig
et al.
, 2004, 2005, Gao
et al.
,
2004, Sobolev
et al.
, 2005) [
delamination man-
tle fertility
]. Midocean-ridge basalts rep-
resent large degrees of melting of a large
source volume, and involve blending of magmas
having different melting histories. The Central
Limit Theorem explains many of the differences
between MORB and other kinds of melts that
sample
+
CC
+
What is the upper mantle?
On the basis of seismic data Bullen divided the
mantle into regions labeled B, C and D. Region B
is the upper mantle and C is the Transition Zone
(410 to 1000 km). D, the lower mantle, starts at
1000 km depth. The upper mantle (Region B) was
subsequently found to contain a high-velocity
lid and a low-velocity zone (LVZ), generally asso-
ciated with the asthenosphere. Region C, the
mantle transition
region (TR), was subse-
quently found to contain two abrupt seismic
discontinuities near the depths of 410 and 650
km, and a region of high and variable gradi-
ent below 650 km depth. A depth of 670 km
was found for the deeper discontinuity in west-
ern North America and this was adopted for the
PREM model
. However, the average depth of the
discontinuity, globally, is 650 km, with a varia-
tion of about 30 km. Some authors have referred
to the 650 (or 670) km discontinuity as the base
of the upper mantle and have suggested that
this represents a profound chemical and isotopic
boundary between depleted upper mantle and
primitive lower mantle, rather than primarily an
isochemical phase change, as originally inferred
(Anderson, 1967). Others have suggested that the
1000 km level is a chemical boundary and should
be retained as the definition of the top of the
lower mantle. Sometimes the TR is included as
part of the upper mantle; sometimes it is defined
as a separate region, Bullen's Region C. This con-
fusion
in
terminology
about
what
constitutes
smaller
volumes
of
the
heterogenous
the
upper
mantle
and
the
lower
mantle
has
mantle.
led
to
the
widespread
view
that
there
are
