Geoscience Reference
In-Depth Information
Garnet
Table 15.1
Compositions of spinel and gar-
net lherzolites
Spinel Lherzolite
Continental
Oceanic
Garnet
Oxide
(avg. of 301)
(avg. of 83)
Lherzolite
SiO
2
44.15
44.40
44.90
Clinopyoxene
Al
2
O
3
1.96
2.38
1.40
GARNET PERIDOTITE
FeO
8.28
8.31
7.89
MgO
42.25
42.06
42.60
Lherzolite
CaO
2.08
1.34
0.82
DUNITE
Na
2
O
0.18
0.27
0.11
K
2
O
0.05
0.09
0.04
Olivine
Harzburgite
Orthopyroxene
MnO
0.12
0.17
0.11
Fig. 15.1
Nomenclature tetrahedron for assemblages of
olivine, clinopyroxene, orthopyroxene and garnet. Dunites
and garnet peridotites lie within the tetrahedron.
TiO
2
0.07
0.13
0.06
P
2
O
5
0.02
0.06
—
NiO
0.27
0.31
0.26
Cr
2
O
3
0.44
0.44
0.32
however, have relatively low Al
2
O
3
and can be
termed barren. These are commonly thought to
be residual after melt extraction. Al
2
O
3
-poor peri-
dotites are less dense than the fertile variety and
should concentrate in the shallow mantle. Given
sufficient water at crustal and shallow man-
tle temperatures, peridotite may be converted
to serpentinite with a large reduction in den-
sity and seismic velocity. Hydrated upper mantle
may therefore be seismically indistinguishable
from lower crustal minerals. Similiarly, basaltic
crust at depths greater than some 50-km depth
converts to eclogite and this is similar to UMR
assemblages in physical properties.
Maal
/
e and Aoki (1977).
to be the residue after melt extractions. Some
peridotites are shallow cumulates deposited from
cooling basalts and are therefore not direct sam-
ples of the mantle. Alumina in peridotites is
distributed among the pyroxenes and accessory
minerals such as plagioclase, spinel and garnet.
At higher pressure most of the A1
2
O
3
would be
in garnet. Garnet-rich peridotite, or pyrolite, is
the commonly assumed parent of mantle basalts.
This variety is fertile peridotite since it can pro-
vide basalt by partial melting. Most peridotites,
Table 15.2
Mineralogy of Lherzolites
Spinel Lherzolite
Garnet Lherzolite
Average
Range
Average
Range
Mineral
(wt. pct.)
(vol. pct.)
(vol. pct.)
(vol. pct.)
Olivine
66.7
65--90
62.6
60--80
Orthopyroxene
23.7
5--20
30
20--40
Clinopyroxene
7.8
3--14
2
0--5
Spinel
1.7
0.2--3
—
—
Garnet
—
—
5
3--10
Phlogopite
—
—
0.4
0--0.5
Maal
/
e and Aoki (1977).
