Geoscience Reference
In-Depth Information
Table 13.5
Elemental ratios in upper-mantle components
Morgan
Ultramafic
and Anders
MORB
Rocks
KIMB
Crust
Picrite
OPX
(1980)
Rb/Sr
0.0033
0.0135
0.0919
0.105
0.0035
0.0089
0.039
K/U
47 143
1346
5677
10 000
29 632
—
10 000
Sm/Nd
0.371
0.328
0.153
0.231
0.368
0.25
0.3297
Th/U
2.50
3.62
5.16
3.84
2.94
—
4.01
U/Pb
0.18
0.13
0.31
0.18
0.15
—
0.20
La/Yb
0.65
1.24
125
8.64
0.69
2.22
1.66
K/Na
0.84
0.01
8.67
0.48
0.04
—
0.11
Mg/Si
0.25
1.06
1.09
0.08
0.45
0.93
0.92
Ca/Al
1.00
1.20
3.72
0.56
1.01
1.09
1.09
Yb/Sc
0.057
0.027
0.08
0.073
0.053
0.0017
0.0239
Ce/Nd
0.93
1.31
2.35
2.38
0.95
2.67
1.46
Eu/Nd
0.14
0.15
0.04
0.07
0.14
0.05
0.11
Yb/Lu
6.21
5.82
7.50
7.33
6.18
2.75
5.87
Sr/Ba
22.00
2.97
1.18
1.14
18.83
3.00
2.90
U/La
0.01
0.46
0.021
0.066
0.014
—
0.036
We first find the composition of a possible
picritic parent to MORB. The relation PICRITE
VOLATILES
200
=
100
0
.
75 MORB
+
0
.
25 UMR
gives
the
results
tabu-
lated under 'picrite' in Table 13.2 and 13.4.
The mixing ratios that are required to give
a chondritic pattern for the refractory elements
yield
CRUST
KIMBERLITE
10
UDS
=
0
.
935 PICRITE
+
0
.
016 Q
+
0
.
00559 CRUST
The composition of this reconstructed
Undepleted
Source Region
is tabulated under UDS. This has
also been called primitive upper mantle (PUM).
UDS accounts for about 10% of the mantle. The
remainder of the mantle above 670-km depth is
assumed to be ultramafic rocks (UMR). This gives
the composition tabulated under 'upper man-
tle' in Tables 13.3 and 13.4. This region contains
23.4% basalt (MORB). The resulting composition
has element ratios (Table 13.5), which, in gen-
eral, are in agreement with chondritic ratios.
The La/Yb, Al/Ca and Si/Mg ratios, however, are
too high. These are balanced by the deeper man-
tle in the full calculation. The solution for the
mantle below 670-km depth, is 0.145 UMR and
0.855 OPX. This gives chondritic ratios for Mg/Si
and Ca/Al for the Earth as a whole. The mixing
UPPER MANTLE
1.0
ULTRAMAFIC
ROCK
MORB
Cs K Rb Pb Tl Ba U Th La Ce Sr Nd Sm Eu Tb Zr Hf Ti
Li
Y Yb Lu
0.1
Fig. 13.3
Shows the normalized concentrations of the
lithophile elements in the various components, upper mantle
and mantle-plus-crust. The refractory elements in the upper
mantle have normalized concentrations of about 3; this
includes the crustal contribution. Since the upper mantle is
about one-third of the whole mantle, a strongly depleted
lower mantle is implied. Note that the upper mantle is
depleted in lithium and titanium. These elements may be in
the inaccessible lower mantle (below 1000 km). The
composition of the upper mantle is discussed in more detail
in Chapter 23. Dots are derived PM.
