Geoscience Reference
In-Depth Information
Table 13.4
Elemental ratios.
Normalized
Morgan and
Upper
Lower
Mantle
C1
Anders
∗
UDS
Mantle
Mantle
Crust
M & A
∗
Cl
+
Rb/Sr
0.3026
0.0395
0.0304
0.0281
0.0101
0.024
0.61
0.08
K/U
63 571
10 000
11 429
8356
3552
7693
0.77
0.12
Sm/Nd
0.3249
0.3297
0.315
0.319
0.318
0.319
0.968
0.982
Th/U
3.64
4.01
4.08
3.94
3.62
3.98
0.97
1.07
U/Pb
0.0039
0.20
0.20
0.17
0.13
0.16
0.82
42
La/Yb
1.48
1.66
2.18
1.85
1.43
1.77
1.07
1.20
K/Na
0.11
0.11
0.10
0.08
0.04
0.07
0.69
0.66
Mg/Si
0.90
0.92
0.43
0.84
0.95
0.92
1.00
1.01
Ca/Al
1.08
1.09
0.99
1.07
1.12
1.09
1.00
1.01
Yb/Sc
0.032
0.024
0.054
0.04
0.007
0.021
0.87
0.65
Ce/Nd
1.35
1.46
1.37
1.35
1.49
1.37
0.94
1.02
Eu/Nd
0.12
0.11
0.12
0.13
0.13
0.13
1.10
1.03
Yb/Lu
6.53
5.87
6.26
6.10
4.77
5.80
0.99
0.89
Sr/Ba
3.17
2.90
3.17
3.14
2.99
3.11
1.07
0.98
U/La
0.038
0.036
0.03
0.035
0.032
0.034
0.97
0.90
∗
Model of Morgan and Anders (1980).
rubidium, 0.53 ppm, strontium, 25 ppm and
cesium, 0.02 ppm. The alkalis are generally
within 50% of the concentrations determined
previously. The K/U ratio is 44% lower.
A four-component (crust, basalt, peridotite
and Q) model for the upper mantle gives
close to chondritic ratios for the refractory
trace elements. The model gives predictions for
volatile/refractory ratios such as K/U and Rb/Sr. It
predicts that EMORB in the upper mantle can be
up to 10% of NMORB.
For BSE a pyroxene-rich component is
required in order to match chondritic ratios of
the major elements. Such a component is found
in the upper mantle and is implied by the seismic
data for the lower mantle. The abundances in the
mantle-plus-crust system (BSE) are 151 ppm potas-
sium, 0.0197 ppm uranium and 0.0766 ppm tho-
rium, giving a steady-state heat flow that implies
that slightly more than half of the terrestrial heat
flow is due to cooling of the Earth, consistent
with convection calculations in a stratified Earth.
In summary, primitive mantle (PM) can be
viewed as a five-component system; crust, MORB,
peridotite, pyroxenite and Q (quintessence, the
fifth essence) or, alternatively, as olivine, orthopy-
roxene, garnet plus clinopyroxene (or basalt) and
incompatible element- and alkali-rich material
(crust and kimberlite or LIL-rich magmas). Advan-
tages of the inverse method are that the mixing
proportions do not have to be fixed in advance
and all potential components can be included;
the inversion will decide if they are needed.
The upper mantle
The mass-balance method gives the average com-
position of the mantle but makes no state-
ment about how the components are distributed
between regions of the mantle. We can, however,
estimate the composition of the MORB source
region prior to extraction of crust and Q. The
bulk of the mantle is depleted and refractory,
UMR plus orthopyroxene (OPX). Pyroxenite, the
most uncertain and to some extent arbitrary of
the components, plays a minor role in the mass-
balance calculations for the trace refractories and
is required mainly to obtain chondritic ratios
of major elements. Detailed discussion of the
composition of the upper mantle is postponed
until Chapter 26.
