Geoscience Reference
In-Depth Information
Table 13.3
(
cont
.)
Morgan
Normalized to
and Anders
Upper
Lower
Mantle
Morgan and
Cl
(1980)
UDS
Mantle
Mantle
Crust
Anders (1980)
Cl
+
Ta
0.03
0.023
0.28
0.11
0.004
0.04
1.10
1.24
Re
†
60
60
0.89
0.45
0.10
0.21
0.0023
0.0034
Os
†
945
880
1.08
2.43
3.10
2.90
0.0022
0.0031
Ir
†
975
840
0.88
2.43
3.20
2.97
0.0024
0.0031
Au
†
255
257
0.62
0.53
0.49
0.50
0.0013
0.002
Tl
†
0.22
0.0039
0.03
0.02
0.00
0.01
1.28
0.033
Pb
3.6
0.068
0.60
0.33
0.03
0.12
1.19
0.033
Bi
0.17
0.0029
0.02
0.009
0.0007
0.0033
0.75
0.019
Th
0.051
0.0541
0.48
0.224
0.014
0.0765
0.96
1.50
U
0.014
0.0135
0.12
0.057
0.004
0.0196
0.98
1.40
∗
Results in percent.
†
Results in ppb.
Anderson (1983a).
in the oxyphile elements by a factor of 1.48. This
factor matches the value for
k
determined from
the inversions.
The Earth's mantle can be chondritic in
major and refractory-element chemistry if an
appreciable amount of oxygen has entered the
core. H
2
OandCO
2
are not included in most mass
balance calculations and unknown amounts of
these may be in the upper mantle or escaped
from the Earth.
Table 13.3 compares these results with cosmo-
chemically based models. Both the volatile ele-
ments and the siderophile elements are strongly
depleted in the crust--mantle system relative to
cosmic abundances.
In pyrolite-type models, it is assumed that
primitive mantle (PM) is a mix of basalt and peri-
dotite and that one knows the average composi-
tions of the basaltic (MORB or OIB) and ultramafic
(UMR) components. These are mixed in some-
what arbitrary, predetermined, proportions; the
results are comparable with volatile-free chondri-
tic abundances for some of the major elements.
Tholeiitic basalts are thought to represent
the largest degree of partial melting among
common basalt types and to nearly reflect the
trace-element chemistry of their mantle source.
Tholeiites, however, range in composition from
depleted midocean-ridge basalts to enriched
ocean-island basalts (OIB) and continental flood
basalts (CFB). Enriched basalts (alkali-olivine, OIB,
CFB) can be modeled as mixtures of MORB and an
enriched (Q) component that experience varying
degrees of crystal fractionation prior to eruption.
Ultramafic rocks from the mantle, likewise, have
a large compositional range. Some appear to be
crystalline residues after basalt extraction, some
appear to be cumulates, and others appear to
have been secondarily enriched in incompatible
elements (metasomatized). This enriched compo-
nent is similar to the Q component of basalts.
Some ultramafic rocks are relatively 'fertile' (they
can yield basalts upon partial melting), but they
do not have chondritic ratios of all the refractory
elements.
The above calculation, in essence, decom-
poses the basaltic component of the mantle into
a depleted (MORB) and LIL-enriched (Q) com-
ponent. These can be combined and compared
with the basaltic component of other two-
component models, such as pyrolite, by com-
paring 'undepleted basalt' (MORB
1.5% KIMB)
with the basalts (Hawaiian tholeiite or 'primi-
tive' oceanic tholeiite) used in the construction
of pyrolite. The present model has an undepleted
basaltic
+
component
(MORB
+
Q)
of
914
ppm
