Geoscience Reference
In-Depth Information
Table 15.4
Representative compositions of pyrolites and peridotites
(wt.%)
Pyrolite
Garnet Peridotite
Oxide
(1)
(2)
(3)
(4)
(5)
(6)
SiO
2
45.1
42.7
46.1
45.0
42.5
46.8
TiO
2
0.2
0.5
0.2
0.2
0.1
0.0
Al
2
O
3
3.3
3.3
4.3
4.4
0.8
1.5
Cr
2
O
3
0.4
0.5
—
0.5
—
—
MgO
38.1
41.4
37.6
38.8
44.4
42.0
FeO
8.0
6.5
8.2
7.6
3.8
4.3
MnO
0.15
—
—
0.11
0.10
0.11
CaO
3.1
2.1
3.1
3.4
0.5
0.7
Na
2
O
0.4
0.5
0.4
0.4
0.1
0.1
K
2
O
0.03
0.18
0.03
0.003
0.22
0.02
(1) Ringwood (1979), p. 7.
(4) Green and others (1979).
(2) Green and Ringwood (1963).
(5) Boyd and Mertzman (1987).
(3) Ringwood (1975).
(6) Boyd and Mertzman (1987).
the surface of the Earth and may not represent
the whole story. They are also not necessarily the
result of a single-stage differentiation process.
melting in the source region. They do not satisfy
trace-element or isotopic data and they violate
chondritic abundances and evidence for mantle
heterogeneity. On the other hand, mantle com-
positions based on isotopic constraints alone are
equally arbitrary and do not satisfy elementary
petrological considerations.
Source rocks
Pyrolite
Pyrolite (pyroxene--olivine-rock) is a hypothet-
ical primitive mantle material that on frac-
tional melting yields a typical basaltic magma
and leaves behind a residual refractory dunite-
peridotite. It is approximately one part basalt and
3--4 parts dunite, assuming that 20--40% melt-
ing is necessary before liquid segregates and
begins an independent existence. Garnet pyro-
lite is essentially identical with garnet peridotite
but is more fertile than most natural samples.
Pyrolite compositions have been based on three
partsduniteplusonepartoftheaveragesof
tholeiitic and alkali olivine basalt and a three-to-
one mix of Alpine-type peridotite and a Hawaiian
olivine--tholeiite. Table 15.4 gives compositions of
some of these pyrolite models. Pyrolite compo-
sitions are arbitrary and are based entirely on
major elements and on several arbitrary assump-
tions regarding allowable amounts of basalt and
Eclogites
The most abundant material coming out of the
mantle is basalt and eclogites are the high-
pressure forms of basalts. The term 'eclogite'
refers to rocks composed of omphacite (diopside
plus jadeite) and garnet, occasionally accompa-
nied by kyanite, zoisite, amphibole, quartz and
pyrrhotite. Natural eclogites have a variety of
associations, chemistries, mineralogies and ori-
gins, and many names have been introduced to
categorize these subtleties. Garnet pyroxenites
are essentially eclogites that have less omphacite,
or sodium.
'Eclogite' implies different things to different
workers. To some eclogites mean metamorphic
crustal rocks, and to others the term implies
bimineralic kimberlite xenoliths. The chemical
similarity of some eclogites to basalts promp-
ted
early
investigators
to
consider
eclogite
as
