Geoscience Reference
In-Depth Information
Table 15.3
Compositions of peridotites and pyroxenites
Lherzolites
Peridotites
Spinel
Garnet
Dunite
Pyroxenite
Oxide
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
SiO
2
44.15
44.40
44.90
41.20
48.60
44.14
46.36
42.1
Al
2
O
3
1.96
2.38
1.40
1.31
4.30
1.57
0.98
FeO
8.28
8.31
7.89
11.0
10.0
8.31
6.56
7.10
MgO
42.25
42.06
42.60
43.44
19.10
43.87
44.58
48.3
CaO
2.08
1.34
0.82
0.80
13.60
1.40
0.92
Na
2
O
0.18
0.27
0.11
0.08
0.71
0.15
0.11
K
2
O
0.05
0.09
0.04
0.016
0.28
—
—
MnO
0.12
0.17
0.11
0.15
0.18
0.11
0.11
TiO
2
0.07
0.13
0.06
0.06
0.83
0.13
0.05
P
2
O
5
0.02
0.06
—
0.10
0.10
—
—
NiO
0.27
0.31
0.26
—
—
—
—
Cr
2
O
3
0.44
0.44
0.32
—
—
0.34
0.33
H
2
O
—
—
—
0.50
0.90
—
—
(1) Average of 301 continental spinel lherzolites (Maal
/
e and Aoki, 1977).
(2) Average of 83 oceanic spinel lherzolites (Maal
/
e and Aoki, 1977).
(3) Average garnet lherzolite (Maal
/
e and Aoki, 1977).
(4) Dunite (Beus, 1976).
(5) Pyroxenite (Beus, 1976).
(6) High-T peridotites, South Africa (Boyd, 1987).
(7) Low-T peridotites, South Africa (Boyd, 1987).
(8) Extrapolated lherzolite trend (
∼
0 percent Al
2
O
3
, CaO, Na
2
O, etc.).
not in agreement with that hypothesis. It is not
even clear that peridotite has the proper seismic
properties for the lower lithosphere. In the depth
interval 200--400 km both eclogite and peridotite
can satisfy the seismic data.
Garnet pyroxenites and eclogites are also
found among the rocks brought up from the
mantle as xenoliths, and they have physical prop-
erties that overlap those of the ultramafic rocks.
Some garnet-rich pyroxenites and eclogites are
denser than some peridotites but densities over-
lap. The extrapolation of the properties of peri-
dotites to the deep upper mantle, much less the
whole mantle, should be done with caution. Not
only do other rock types emerge from the mantle,
but there is reason to believe that peridotites will
be concentrated in the shallow mantle and to be
over-represented in our rock collections (except
that eclogites are exotic-looking rocks and are
preferred by some).
If picrites are the parent for tholeiitic basalts,
then roughly 30% melting is implied for gener-
ation from a shallow peridotitic parent. If the
parent is eclogitic, then similar temperatures
would cause more extensive melting. Genera-
tion of basaltic magmas from an eclogitic par-
ent does not require extensive melting. Melts of
basaltic composition are provided over a large
range of partial melting, and basalts and eclog-
ites come in a variety of flavors. The sources
of basalts may not even be rocks, as conven-
tionally defined. The fertile components may be
km-size eclogitic blobs separated by tens of km
in a refractory mantle. Melting, particularly at
midocean ridges, takes place over large regions;
the various components do not have to be dis-
tributed over grain-scale or hand-specimen size
domains.
Basalts and peridotites are two of the results
of mantle differentiation. They both occur near
