Geoscience Reference
In-Depth Information
Chapter 23
The upper mantle
Let's descend to that blind world
below. I'll go first, and you can follow
Dante
mantle
by combining known components of the
upper mantle -- basalts, peridotites, recycled crust
and so on. The MORB source is just part of the
upper mantle and it is not the only LIL-depleted
part of the mantle. It is not necessarily convec-
tively homogenized.
Attempts to establish an average composi-
tion for 'the upper mantle' focus on the most
depleted MORB lavas or abyssal peridotites and
involve major assumptions about melt genera-
tion, melt transport and differentiation processes
that have affected these. The
depleted upper
mantle
, that part of the mantle that is assumed
to provide MORB by partial melting is variously
called DUM, DM, DMM and the
convecting
upper mantle
. Simplified mass balance cal-
culations suggested to early workers that this
depleted mantle constituted only
Overview
The composition of the crust and the upper man-
tle are the results of a series of melting and frac-
tionation events, including the high-temperature
accretion of the planet. Early attempts to esti-
mate upper mantle (UM) chemistry started from
the assumption that UM initially was the same
as
bulk silicate Earth
(BSE) and differed
from it only by the extraction of the continen-
talcrust,orthatthemostdepleted -lowin
LIL, U, Th, K -- midocean-ridge basalts plus their
depleted refractory residues -- unaltered abyssal
peridotites -- constitute the entire upper mantle.
Traditionally, geochemists have assumed that the
lower mantle is
undegassed BSE
or
primitive mantle
(PM)
. Geodynamicists have noted that there are
heat flow problems with this model, which they
fixed up by putting a large amount of U, Th and
Kina
lower mantle stealth layer
.
Large-scale melting and differentiation upon
accretion probably pre-enriched the upper man-
tle with incompatible elements, including the
radioactive elements; the crust and the vari-
ous enriched and depleted components sam-
pled by current melting events were already in
the upper mantle shortly after accretion and
solidification.
Recycling of crust into the upper mantle is an
important current process. It is possible to esti-
mate the composition of the
fertile upper
30% of the
mantle; the 650--670 km discontinuity was there-
fore adopted as the boundary between DUM and
'the primitive undepleted undegassed lower man-
tle.' The starting condition for the upper mantle
(UM) was taken as identical to
primitive man-
tle
(PM) -- or
bulk silicate Earth
(BSE) --
and the present lower mantle (LM). Estimates of
PM and BSE are based on cosmological and petro-
logical considerations. The hypothetical
prim-
itive upper mantle
-- crust plus DUM -- is
labeled PUM. It is further assumed that the
upper mantle -- from the base of the plate to
650 km -- after extraction of the crust, became
well-stirred and chemically homogenous by vig-
orous
∼
convection.
The
non-MORB
basalts
that
occur
at
the
initiation
of
spreading
and
at
various
locations
along
the
global
spreading
