Geoscience Reference
In-Depth Information
Table 14.3
Trace-element chemistry of MORB, kimberlite and intermediate (alkali) basalts (ppm)
Alkali Basalts
Kimberlite
Element
MORB
EPR
∗
Australia
Hawaii
BCR
†
Theoretical
∗∗
Average
Max
Sc
37
27
25
33
34--37
35
30
—
V
210
297
260
170
399
197--214
120
250
Co
53
41
60
56
38
57--61
77
130
Ni
152
113
220
364
15
224--297
1050
1600
Rb
0.36
13
7
24
47
10--45
65
444
Sr
110
354
590
543
330
200--289
707
1900
Y
23
37
20
27
37
23--28
22
75
Zr
70
316
111
152
90
97--133
250
700
Nb
3.3
—
24
24
14
19--48
110
450
Ba
5
303
390
350
700
150--580
1000
5740
La
1.38
21
24
23
26
16--31
150
302
Ce
5.2
46
47
49
54
31--57
200
522
Nd
5.6
28
24
23
29
16--26
85
208
Sm
2.1
6.7
6.8
5.5
6.7
3.1--4.8
13
29
Eu
0.8
2.1
2.1
2.0
2.0
1.1--1.4
3
6.5
Tb
0.5
1.2
0.92
0.87
1.0
0.6--0.7
1
2.1
Yb
2.1
3.4
1.6
1.8
3.4
2.0--2.1
1.2
2.0
Lu
0.34
0.49
0.28
0.23
0.55
0.32--0.34
0.16
0.26
Hf
1.4
5.3
2.8
3.9
4.7
2.2--4.3
7
30
Ta
0.1
9
1.8
2.1
0.9
1.3--2.5
9
24
Pb
0.08
—
4.0
5
18
1.6--5.1
10
50
Th
0.35
2.7
2.0
2.9
6.8
2.4--5.4
16
54
K
660
8900
4700
8800
1200
3200--4770
17 600
41 800
U
0.014
0.51
0.4
0.6
1.8
0.48--1.84
3.1
18.3
∗
EPR
=
East Pacific Rise.
†
BCR
Basalt, Columbia River (USGS Standard Rock).
∗∗
Mixture ranging from 85 percent MORB plus 15 percent Average Kimberlite to 90 percent
MORB plus 20 percent Max Kimberlite.
=
relatively cold rocks to have very low seismic
velocities. The low-velocity zone in the upper
mantle and the low-velocity regions under some
so-called hotspots may be due to this rather than
to high absolute temperatures.
There is no particular reason for supposing
that carbonatites come from deep in the mantle,
although their isotopic similarity with some OIB
has led to suggestions that they come from the
lower mantle. This is circular reasoning and is
based on the assumption that only MORB origi-
nates in the upper mantle.
The isotopic components HIMU, EM1, FOZO
and DMM (depleted MORB mantle) have been
identified in carbonatites. HIMU may represent
recycled crust, or lower continental crust, dehy-
drated and converted to eclogite. HIMU sources
are enriched in U, Nb and Ta relative to MORB.
HIMU islands include Mangaia, and St. Helena.
EM1 has extremely low
143
Nd/
144
Nd and very low
lead isotopic ratios compared to other OIB compo-
nents. Pitcairn and the Walvis Ridge are the type
locations for EM1 basalts. EM1 sources may be
recycled crust plus pelagic sediment or metasom-
atized continental lithosphere. The EM1 compo-
nent has
18
O similar to MORB sources and peri-
dotite xenoliths, ruling out a large contribution
of subducted sediment.
δ
