Geoscience Reference
In-Depth Information
20
PLAG
20
CaO
Na
2
O
CPX
16
8
OCEANIC
CRUST
12
6
ICELAND
PICRITES
66SAL1
MORB
CPX
8
4
10
Gorgona
ECLOGITES
ICELAND
PICRITES
PERIDO
4
2
66 SAL1
OPX
TITES
MORB
S
PINEL
0
0
60
10
20
30
40
0
20
40
PLAG
30
AI
2
O
3
TiO
2
0
0
10
20
Al
2
O
3
(weight percent)
G
20
4
Fig. 15.4
CaO versus Al
2
O
3
for igneous rocks and
minerals. Note that peridotites and STP are CaO-rich
compared to Cl chondrites, and meteorites in general, while
the continental crust in Al
2
O
3
-rich. MORB falls on the Cl
trend, possibly suggesting that it is an extensive melt of
primitive mantle. Gorgona is an island in the Pacific that has
komatiitic magmas. The solar CaO/Al
2
O
3
ratio is slightly
higher than peridotites (Chapter 1).
OCEANIC
CRUST
ICELAND
PICRITES
66SAL1
3
MORB
10
2
OCEANIC
CRUST
66SAL1
PERIDO
TITES
1
SP
OPX
0
0
0 0 0 0 0
MgO (weight percent)
0 0 0 0
orthopyroxene. Olivine is generally more than
60% by volume, and both minerals together typ-
ically constitute more than 80% of the rock.
Clinopyroxene and aluminous phases such as
spinel, garnet and jadeite -- the basalt assem-
blage -- are minor constituents. At the other
extreme are rocks such as pyroxenites, clinopy-
roxenites and eclogites, which are low in orthopy-
roxene and olivine. Pyroxenites, by definition,
contain
Fig. 15.3
Major-element oxides of igneous rocks and
rock-forming minerals. If peridotites represent residues after
basalt extraction and MORB and picrites represent melts, and
if these are genetically related, the MgO content of the basalt
source is probably between about 20 and 30%, much less than
most lherzolites or peridotites. 66 SAL-1 is a garnet
pyroxenite from Salt Lake Crater, Hawaii, and falls in the
composition gap between basalts and peridotites. It is a
piclogite.
40% olivine. Intermediate are rocks
such as picrites, olivine eclogites and komati-
ites. Rocks that have less than 50% olivine
plus orthopyroxene have been given the gen-
eral name piclogites. In major elements they
fall between dunites and basalts and between
lherzolites and picrites. They contain a larger
basaltic fraction than peridotites, although they
contain the same minerals: olivine, orthopyrox-
ene, diopside-jadeite and garnet. At high pressure
they are denser than lherzolite, but at high tem-
perature they can become less dense. Piclogites
can represent garnet-rich cumulates or frozen
high-pressure melts and can generate basaltic
melts over a wide range of temperatures. They
<
oceanic crust and delaminated continental crust
place large blobs of eclogite, of variable density,
into the mantle; 10--20% melting of eclogite can
cause it to become buoyant and this happens
well below the solidus temperature of mantle
peridotites. I refer to a basalt- or eclogite-rich
assemblage as a piclogite. Large eclogite blobs
surrounded by peridotite can be a piclogite, even
if hand-specimen-size samples from the region
are either eclogite or peridotite.
Piclogite
Peridotites and pyrolite are rock types composed
primarily of the refractory crystals olivine and
