of 9
Chapter
IS
The
hard
rock
cafe
There
are
three
kinds
of
rocks:
Ingenious
,
Metaphoric,
and
Sedentary,
named
after
the
three
kinds
of
geologists.
Anon.
Ultramafic
rocks
Ultramafic
rocks
(UMR)
are
composed
chiefly
of
ferromagnesian
minerals
and
have
a
low
silicon
content
compared
with
the
crust,
mafic
rocks
and
basalts.
The
term
is
often
used
interchange-
ably
with
ultrabasic;
pyroxene-rich
rocks
are
ultra-
mafic
but
not
ultrabasic
because
of
their
high
Si0
2
content.
Peridotites,
lherzolite
,
dunite
and
harzburgite
are
specific
names
applied
to
ultra-
mafic
rocks
that
are
chiefly
composed
of
olivine,
orthopyroxene,
clinopyroxene
and
an
aluminous
phase
such
as
plagioclase,
spinel
or
garnet.
Ultra-
mafic
rocks
are
dense
and
mainly
composed
of
refractory
minerals
with
high
seismic
velocities.
Basic
rocks,
such
as
basalts,
become
dense
at
high
pressure
(for
example,
eclogite)
and
can
have
properties
comparable
to
the
more
refrac-
tory
peridotites.
Some
eclogites
overlap
basalts
in
their
bulk
chemistry.
The
relationships
between
these
rocks
are
shown
in
Figure
15.1.
Peridotites
can
represent
(1)
The
refractory
residue
left
after
basalt
extrac-
tion.
(2)
Cumulates
formed
by
the
crystallization
of
a
magma.
(3)
Primitive
mantle
that
can
yield
basalts
by
par-
tial
melting.
(4)
Cumulates
or
residues
that
have
been
intruded
by
basalt.
(5)
High-pressure
or
high-temperature
melts
.
Peridotites
contain
more
than
40%
olivine
.
They
are
divided
into
fertile
or
infertile
(or
barren).
Fertile
peridotites
can
be
viewed
as
having
an
appreciable
basaltic
component.
The
terms
'enriched'
and
'dep
leted'
are
often
used
inter-
changeably
with
'fertile'
and
'infertile'
but
have
trace-element
and
isotopic
connotations
that
are
often
inconsistent
with
the
major-element
chem-
istry
. Table
15.1
gives
compositions
for
represen-
tative
ultramafic
rocks.
Garnet
lherzolites
are
composed
mainly
of
olivine
and
orthopyroxene
(Table
15.2).
Olivine
is
generally
in
the
range
of
60-70
vol.
%
and
orthopyroxene
30-50%.
The
average
clinopyrox-
ene
and
garnet
proportions
are
about
5%
and
2
%,
respectively.
The
major
oxides
in
peridotites
and
lherzo-
lites
generally
correlate
well
(Figure
15.2
and
Table
15
.3).
The
lherzolite
trend
can
be
explained
by
variable
amounts
of
clinopyroxene
and
gar-
net.
Olivine-
and
orthopyroxene-rich
rocks,
pre-
sumably
from
the
mantle
,
are
found
in
foldbelts,
ophiolite
sections,
oceanic
fracture
zones
and,
as
xenoliths,
in
kimberlites
and
alkali-rich
magmas
.
They
are
rare
in
less
viscous
magmas
such
as
tholeiites
.
Olivine
and
orthopyroxene
in
varying
proportion
are
the
most
abundant
minerals
in
peridotites.
These
are
dense
refractory
minerals,
and
peridotites
are
therefore
genera
lly
thought
190
I
THE
HARD
ROCK
CAFE
Table
15.1
I
Compositions
of
spinel
and
gar-
net
lherzolites
Sp
i
nel
Lherzolite
Continental
Ocea
n
ic
Garnet
Oxide
(avg.
of
30
I)
(avg.
of
83)
Lh
erzo
lite
Si0
2
44.
15
44.40
44
.90
AI
20 3
1.96
2.38
1.40
FeO
8.28
8.3
1
7.89
M
gO
42
.
25
4206
42
.60
CaO
2.08
1.34
0.82
Na20
0.
18
0.27
0.
11
K20
0.05
O.Q9
0.04
MnO
0.12
0.17
0.
11
Ti0
2
O.Q7
0.
13
0.06
P20 s
O.Q2
0.06
N iO
0.27
0.31
0.26
Cr
20 3
0.44
0.44
0.32
M
aa
l0e
and
Aoki
(1977
).
to
be
the
residue
af
ter
melt
extractions.
Some
peridotites
are
shallow
cumul
a tes
deposited
from
cooling
basalts
and
are
therefore
not
direct
sam-
ples
of
the
mantle
.
Alumina
in
peridotites
is
distributed
among
th
e
pyroxen
es a
nd
accessory
minerals
such
as
pla
giocl
as
e,
s
pinel
and
garnet
.
At
hi
g
her
pressure
most
of
th
e
A1
2
0
3
would
be
in
garnet.
Garnet-ri
ch
peridotite
,
or
pyrolite,
is
the
commonly
assumed
par
e
nt
of
mantle
basalts.
This
variety
is
fertile
peridotit
e
since
it
can
pro-
vide
basalt
by
partial
meltin
g . Most
peridotites
,
Table
15.2
1
Mineralogy
of
Lherzolites
Olivin
e
Garnet
--~
Ol
0
0
w
Clinopyo
xen
e
GARNET
yo.\
\~
~
Lherzolite
DUNITE
Harzburgit
e
Orthopyro
xe
ne
Nomenclature
tetr
a
hedron
for
as
semblages
of
olivine,
clinopyroxene,
orthopyroxene
and
garnet.
Dunite
s
and
garnet
peridotites
lie
within
the
tetrahedron.
how
ever,
have
relatively
low
Al
2
0
3
and
ca n
be
termed
ba
rren
.
These
ar
e c
ommonly
thought
to
be
residual
after
melt
extraction
. Al
2
0
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
d e n-
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-krn
depth
c
onverts
to
eclogite
and
this
is
similar
to
UMR
assemblages
in
physical
properti
es.
Spine
l
Lherzo
lite
Garnet
Lherzol
i
te
Mineral
Average
(vvt
.
pet.)
Range
(vol.
pet.)
Average
(vol.
pet.)
Range
(vol.
pet
.)
Olivine
Orthopyroxene
Clinopyroxene
Spinel
Garnet
Phlogopite
66.7
23.7
7.8
1
.7
Maa
l0e
and
Aok i
(1
9 77
).
65-90
5-20
3-14
0.2-3
62.6
30
2
5
0.4
60-80
20-40
0-5
3-
10
0-0.5
14
c
Q)
~
Q)
Q.
E
4
Ol
·
a;
16
§2]
~
Q)
"0
·
;;:
12
0
c
Q)
c
0
8
Q.
E
0
(.)
4
0
50
40
30
20
10
0
50
40
MgO
(weight
percent)
Oxides
versus
MgO
for
igneous
rocks.
The
basalt
source
region
probably
has
a
composition
intermediate
between
basalt
(MORB)
and
harzburgite.
Most
petrological
models
(PM)
of
the
major-element
chemistry
of
the
source
region
favor
a
small
basalt
fraction
. STP
(spinifex
textured
peridotites)
are
high-temperature
MgO-rich
magmas
.
Picrites
(PICR)
are
intermediate
in
composition
between
STP
and
MORB
and
may
evolve
to
MORB
by
olivine
separation.
Picrites
and
eclogites
(ECL)
overlap
in
composition.
Lherzolites
(LZ)
contain
an
orthopyroxene
component
,
but
the
other
rock
types
are
mainly
clinopyroxene
+
garnet
and
olivine.
Squares
represent
estimates
of
primitive
mantle
composition
based
on
a chondritic
model.
If
the
upper
mantle
is
primarily
lherzolite,
basalt
and
harzburgite,
the
lower
mantle
(open
dot)
will
be
primarily
orthopyroxene
.
The
composition
of
the
MORB
source
(piclogite
model)
probably
falls
between
PICR
and
PM
or
STP
.
Lherzolites
typically
contain
60-80
%
olivine,
20-40
%
orthopyroxene,
less
than
14
%
clinopyrox-
ene
and
1-10
%
of
an
aluminous
phase
such
as
spinel
or
garnet.
Spinel
lherzolites
,
the
lower-
pressure
assemblages,
dredged
from
the
ocean
bottom
are
similar
in
composition
to
those
found
in
alkali
basalts
and
kimberlites
on
oceanic
islands
and
continents.
Garnet
lherzolites
are
denser
than
spinel
lherzolites
only
when
they
cont
a
in
appreciable
garnet.
They
would
become
less
dense
at
higher
temperature,
lower
pressure
or
if
partially
molten.
The
major-element
chemistries
of
lherzolites
vary
in
a
systematic
fashion.
Most
of
the
oxides
vary
linearly
with
MgO
content
.
These
trends
are
ULTRAMAFIC
ROCKS
191
(5J
j
At2o
3J
30
20
10
0
generally
consistent
with
variable
amounts
of
a
basaltic
component
. However,
the
basaltic
com-
ponent
is
not
tholeiitic
or
MORE.
If
lherzolites
represent
olivine-orthopyroxene-rich
rocks
with
variable
amounts
of
melt
extraction
or
addition
,
this
melt
component
is
andesitic
in
major
ele-
ments
.
The
major-element
trends
of
lherzolites
may
also
be
controlled
by
melt-crystal
equilibration
at
various
depths
in
the
mantle.
Lherzolites,
and
most
other
ultramafic
rocks,
are
generally
thought
to
be
the
refractory
residue
comple-
mentary
to
melts
presently
being
extracted
from
the
mantle.
They
differ
, however,
from
primitive
mantle
compositions
.
In
particular
they
contain
more
olivine
and
less
orthopyroxene
than
would
be
appropriate
for
a
chondritic
or
'cosmic'
man-
tle.
Upper-mantle
lherzolites
and
basalts
may
be
complementary
to
the
lower
mantle,
represent-
ing
melts
from
the
original,
accretional
differ-
entiation
of
the
mantle.
The
MgO
content
of
melts
increases
with
temperature
and
with
depth
of
melting
. At
great
depth
(> 200
km)
relati-
vely
low
-MgO
phases,
such
as
orthopyroxene
and
garnet-majorite
may
remain
behind
, giv-
ing
olivine-rich
melts.
The
major-element
trends
in
lherzolites
may
therefore
represent
trends
in
high-pressure
melts
.
Unserpentinized
peridotites
have
seismic
velocities
and
anisotropies
appro-
priate
for
the
shallow
mantle.
This
situation
is
often
generalized
to
the
whole
mantle,
but
seis-
mic
data
for
depths
greater
than
400
km
are
192
THE
HARD
ROCK
CAFE
Table
I
5.3
I
Compositions
of
peridotites
and
pyroxenites
Lherzolites
Spinel
Garnet
Dunite
Pyroxenite
Peridotites
Oxide
(I)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
Si0
2
44.15
44.40
44.90
41.20
48.60
44
.1
4
46.36
42.1
AI 20 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
8.3
1
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
20
0.18
0.27
0.11
0.08
0.71
0.15
0.11
K20
0.05
O.Q9
O.G4
0.016
0.28
MnO
0.12
0.17
0.1
1
0.15
0.18
0.
11
0.1
1
Ti0
2
O.G7
0.13
0.06
0.06
0.83
0.13
0.05
P20 s
0.02
0.06
0.10
0
.1
0
NiO
0.27
0.31
0.26
Cr
20 3
0.44
0.44
0.32
0.34
0.33
H 20
0.50
0.90
(1)
Average
of
301
continental
spi
nel
lherzolites
(Maal0e
and
Aoki,
1977).
(2)
Average
of
83
oceanic
spinellherzolites
(Maal0e
and
Aoki,
1977).
(3)
Average
garnet
lherzolite
(Maal0e
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)
Extrapo
l
ated
lh
erzolite
trend
(~0
percent
Al
2
0
3
,
CaO,
Na
2
0,
etc.).
not
in
agreement
with
that
hypothesi
s.
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
fi·om
the
mantle
as
xenoliths,
and
they
have
physical
prop-
erties
that
overlap
those
of
the
ultram
.afic
rocks.
Some
gar
n
et
-rich
pyroxenites
and
eclogites
are
denser
than
some
peridotites
but
densities
over-
lap.
The
extrapo
lation
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
sha
llow
mantle
and
to
be
over-represented
in
our
rock
co
lle
ctions
(except
that
eclogites
are
exotic-
lookin
g
rocks
and
are
preferred
by
some).
If
picrites
are
the
parent
for
tholeiitic
basalts,
then
roughly
30%
melting
is
implied
for
gener
-
ation
fi-om
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
bas
alts
and
eclog-
ites
come
in
a
variety
of
flavors.
The
so
urc
es
of
bas
a
lt
s
may
not
even
be
rocks,
as
conven
-
tionally
defined.
The
fertile
compo
n
ents
may
be
lm1-size
eclogitic
blobs
separated
by
tens
of
km
in
a
refractory
mantle.
Melting,
particularly
at
midocean
ridges,
takes
place
over
large
regions;
the
various
co
mpon
ents
do
not
have
to
be
dis-
tributed
over
grain-scale
or
hand-specimen
size
domains.
Basalts
a
nd
peridotites
are
two
of
the
results
of
mantle
differentiation.
They
both
occur
near
SOURCE
ROCKS
193
Table
15.4
1
Repr
ese
ntativ
e
compositions
ofpyrolites
and
peridotit
es
(wt
.
%)
Pyrolite
Garnet
Peridotite
Oxide
(I)
(2)
(3)
(4)
(5)
(6)
Si0
2
45.1
42
.7
46
.1
45.0
42.5
46.8
Ti0
2
0.2
0.5
0.2
0.2
0.1
0.0
AI
20 3
3.3
3.3
4.3
4.4
0.8
1.5
Cr
20 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
20
0.4
0.5
0.4
0.4
0.1
0.1
K20
O.Q3
0.18
O.Q3
0 .
003
0.22
0.02
(1)
Ringwood
(1979),
p.
7.
(4)
Green
a
nd
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
who
le story.
They
are
also
not
necessarily
the
result
of
a single-stage
differentiation
process.
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
residua
l
refra
ctory
dunite-
peridotite.
lt
is
approximately
one
part
basalt
and
3-4
parts
dunite,
assuming
that
20-40%
melt-
in
g
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
samp
les.
Pyrolite
compositions
have
been
based
on
three
parts
dunite
plus
one
part
of
the
averages
of
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
ar
bitrar
y
and
are
based
entire
ly
on
major
elements
and
on
severa
l
arbitrary
ass
ump-
tions
regarding
allowable
a
mounts
of
basalt
and
melting
in
the
source
region.
They
do
not
satisfy
trace-element
or
isotopic
data
and
they
viol
ate
chond
ri
tic
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
cons
id
erations.
Eclogites
The
most
abundant
material
co
min
g
out
of
the
mantle
is
basalt
and
eclog
ites
are
the
hi
gh-
pressure
forms
of
basalts
.
The
term
'eclog
ite'
refers
to
rocks
composed
of
omphacite
(diopside
plus
jadeite)
and
ga
rnet,
occasionally
accompa-
ni
ed
by
kyanite,
zo
i
site,
amp
hibol
e,
quart
z
an
d
pyrrhotite.
Natural
eclogites
have
a
variety
of
associations,
c
hemistries
,
mineralo
gies
and
ori-
g
ins
,
and
many
names
have
been
introduced
to
categorize
these
su
btleti
es.
Garnet
pyroxenites
are
esse
ntially
eclog
ites
that
h ave
less
omphacite,
or
sodium.
'E
clo
gite'
implies
diff
erent
things
to
different
workers.
To
some
eclogites
m
.ean
metamorphic
crustal
rock
s,
and
to
others
the
term.
implies
bimineralic
kimberlite
xenoliths.
The
chemical
simi
larity
of
some
eclogites
to
ba
sa
lts
promp-
ted
early
inv
est
i
gato
rs
to
co
nsider
eclogite
as
194
THE
HARD
ROCK
CAFE
Table
15
.5
I
Typical
trace
element
concentrations
(ppm)
Eclogites
Synthetic
Peridotites
Omphacite
Garnet
Eclogite
Diopside
K
1164
337
820
6 15
Rb
0.565
1.14
0.7
2.
I
Sr
249
8.25
95
337
Na
52244
74
12700
1332
Ti
4856
899
2500
659
Zn
106
IS
55
28
Rb/Sr
0.002
0.14
0.006
Sm/Nd
0.206
0.522
0.2
11
Rb/K
4.
9*
33.8*
8.5*
34
.
I*
Basu
and
Tatsumoto
(1982),
Wedepohl
and
Muramatsu
(1979).
*
X
10
-
4
.
Table
15.6
I
Comparison
of
eclogites
and
other
mafic
rocks
Garnet
296
1.45
5.50
2420
959
69
0.26
0.590
49.0*
Oxide
Eclo
gite
Pic
rite
MORB
Ocean
Crust
(percent)
(I)
(2)
(3)
(4)
Si0
2
45.2
47.2
44.4
47
.2
Ti0
2
0.5
0.6
I.
18
0.7
AI
20 3
17.8
13.9
10.2
15.0
Cr
20 3
0.4
0.22
Fe
20 3
3.4
FeO
11.2
11.0
10.92
6.6
MgO
13.
I
14.3
18.6
I 0.5
MnO
0.3
0.17
0
.1
CaO
9.6
10.1
9.7
I
1.4
Na
20
1.6
I
.6
I
.37
2.3
K20
om
0.8
0
.1
3
0.1
(1)
Bobbejaan
eclogite
(Smyth
and
Caporuscio,
1984).
(2)
Roberts
Victor
eclogite
(Smyth
an
d Capo
ruscio
,
1984)
.
(3)
Picrite,
Svartenhuk
(Clarke,
1970).
(4)
Oceanic
tholeiite
(MORB).
(5)
Average
oceanic
crust
(Elthon,
1979).
(5)
47.8
0.6
12
.1
9.
I
17.8
11.2
I
.3
om
MORB
700
0.4
110
17
300
5500
80
0.004
0.335
5.7*
a
possible
source
of
basalts
but
more
recently
has
been
taken
as
evidence
that
these
eclog-
ites
are
simply
subducted
oceanic
crust
or
basaltic
melts
that
have
crystallized
at
high
pres-
sure.
Some
eclogites
are
demonstrably
metamor-
phosed
basalts,
while
others
appear
to
be
igneous
rocks
ranging
from
melts
to
cumulates.
Eclogite
cumulates
may
accompany
the
formation
of
island
arcs
and
batholiths
and
delamination
of
thick
arc
crust
is
another
way
to
introduce
eclogite
into
the
mantle.
The
trend
in
petrology
has
been
toward
the
splitters
rather
than
the
lumpers
,
and
toward
explanations
that
empha-
size
the
derivative
and
secondary
nature
of