Geoscience Reference
In-Depth Information
Table 15.8
Comparisons of picrites, komatiites and garnet pyroxenite
Group II
1
Komatiite
Oxide
Svartenhuk
Baffin Is.
S. Africa
2
Gorgona
3
66SAL-1
4
SiO
2
44.4
45.1
45.7
45.3
44.8
TiO
2
1.18
0.76
0.91
0.60
0.52
Al
2
O
3
10.2
10.8
7.74
10.6
8.21
Cr
2
O
3
0.22
0.27
—
—
—
FeO
10.92
10.26
—
10.9
9.77
MnO
0.17
0.18
0.23
0.18
MgO
18.6
19.7
19.43
21.9
26.53
CaO
9.7
9.2
8.04
9.25
8.12
Na
2
O
1.37
1.04
0.61
1.04
0.89
K
2
O
0.13
0.08
—
0.02
0.03
P
2
O
5
0.14
0.09
0.12
—
0.04
Fe
2
O
3
—
—
15.44
—
—
NiO
0.13
0.12
—
—
—
H
2
O
2.33
2.14
3.46
—
—
1
Clarke (1970).
3
Echeverria (1980).
2
Jahn and others (1980).
4
Frey (1980).
are not compositionally equivalent to basalts and
do not necessarily require large degrees of par-
tial melting to generate basaltic magmas. How-
ever, if piclogites are an important component in
the midocean-ridge-basalt source rock, the garnet
must be mainly eliminated in order to satisfy the
HREE data and in order to decrease the density
of the material so that it can rise into the shal-
low mantle. Therefore, clinopyroxene probably
exceeds garnet in any component with clinopy-
roxene rather than garnet as a near-liquidus
phase. Piclogites can be regarded as
assemblages
that are sampled by large volume melts; they do
not have to exist on the hand specimen scale.
Peridotitic komatiites (STP, for spinifex-
texture peridotites) have major-element
chemistries about midway between harzburgites
and tholeiites and occupy the compositional
gap between lherzolites and picrites (Figures
15.4 and 15.4). The present rarity of komatiitic
magmas may be partially due to their high
density and the thick lithosphere. Komatiites
evolve to picrites and then to tholeiites as they
crystallize and lose olivine, becoming less dense
all the while.
Compositions of picrites, komatiites and a gar-
net pyroxenite (66 SAL-1) from Salt Lake Cen-
ter, Hawaii are given in Table 15.8. These are
all examples of rocks that fall between basalts
and peridotites in their major-element chemistry.
In terms of mineralogy at modest pressure they
would be olivine eclogites, garnet pyroxenites or
piclogites. These and lower crustal cumulates are
all suitable source rocks for basalts.
