Geoscience Reference
In-Depth Information
peridotites, kimberlites and CC can be mixed
together to achieve chondritic ratios of the refrac-
tory trace elements (Chapter 13) or inferred BSE
compositions of LIL, there is no compelling rea-
son to involve the lower mantle in mass bal-
ance calculations for the volatile and very incom-
patible elements, including U, Th and K. The
lower mantle, of course, was involved in the orig-
inal differentiation and is needed to balance the
major elements and the chalcophiles and the
more compatible elements such as Pb. In fact var-
ious geochemical paradoxes may be resolved with
a hidden isolated depleted (but not fertile) reser-
voir, such as parts of the mantle below 1000 km.
depend on which ones are selected to be repre-
sentative. Even tholeiites have been subdivided
into DMORB, TMORB, NMORB, PMORB, EMORB,
OIB and so on. Should one attempt to construct
averages, or pick endmembers, or pick represen-
tative values, or correct observed compositions
for contamination?
The MORB sources
Midocean ridge basalts range in composition
from depleted (DMORB and normal, N-type
MORB) to transitional, enriched and plume-type
basalts (TMORB, EMORB, PMORB); NMORB are
tholeiites from normal ridge segments; DMORB
are particularly depleted basalts that represent
endmembers of this class of component. Even
MORB from normal ridge segments display a
range of compositions, including EMORB; the
definitions of N-type MORB and 'normal ridge
segments' are arbitrary. From a major element
and petrological point of view even OIB tholei-
ites are similar to MORB and imply similar
amounts of melting. Estimates of the composi-
tion of the upper mantle, however, focus on the
most depleted products.
Salters and Stracke (2004) presented an esti-
mate for the composition of
depleted mantle
(DM),
tailored to be a suitable source for midocean-
ridge basalts. The depleted mantle reservoir is
defined as mantle that can generate 'pure' MORB
uncontaminated by enriched or ' plume components'
;
'pure' MORB is then used to estimate upper-
mantle composition. Estimates for some ele-
ments are derived from elemental and isotopic
compositions of peridotites assumed to be com-
plementary to MORB. The concentrations of some
elements are estimated by subtraction of a the-
oretical low-degree melt from an unfractionated
bulk silicate Earth (BSE) composition. The meth-
ods used by Salters and Stracke to select and
filter the input data are typical of this forward
approach to modeling compositions.
A variety of criteria are used to identify
'depleted' (i.e. lacking an enriched component)
MORB and to eliminate enriched or potentially
enriched samples: the choice of criteria influ-
ences the calculated source region -- DM -- com-
position.
Mass balance
In the standard model of mantle geochem-
istry, the continental crust, CC, is considered
to be complementary to the MORB reservoir,
MORB extraction leaves behind a complementary
depleted residue, and the lower mantle remains
primordial (PM);
BSE
=
CC
+
DUM
+
PM
CC
+
DUM
=
PM
DUM
=
MORB
+
UMR
In the
RAdial ZOne Refining
model, RAZOR,
for Earth accretion and differentiation, the man-
tle melts, fractionates and differentiates dur-
ing accretion. The proto-crust and proto-upper-
mantle are the buoyant products of this
accretional differentiation and a perovskite-rich
residue (PV) makes up the deeper mantle.
=
+
+
+
+
BSE
CC
MORB
UMR
PV
Q
There is a large literature on the composition
of each of these components and attempts have
been made to filter and correct values of natu-
ral samples to be representative of uncontami-
nated material. There are several recent attempts
to derive the composition of DUM from literature
values of the composition of MORB, UMR and CC
(Donnelly
et al.
, 2004; Salters and Stracke, 2004;
Workman and Hart, 2005).
Data selection is a problem in this approach.
Basalts and peridotites have a large range in com-
position. Estimates of upper mantle composition
Only
those
MORB
samples
'thought
to
be
representative
of
DM'
are
considered.
