Transcript Chapter 13

Chapter 13 Mid-ocean Ridge Basalts
The Mid-Ocean Ridge System
Figure 13-1. After Minster et al.
(1974) Geophys. J. Roy. Astr.
Soc., 36, 541-576.
Ridge Segments and Spreading Rates
Table 13-1. Spreading Rates of Some Mid-Ocean
Ridge Segments
• Slow-spreading ridges:
< 3 cm/a
• Fast-spreading ridges:
> 4 cm/a
• Temporal variations are
also known
Category
Ridge
Fast
East Pacific Rise
Slow
Indian Ocean
Mid-Atlantic Ridge
Latitude
21-23oN
13oN
11oN
8-9oN
2oN
20-21oS
33oS
54oS
56oS
SW
SE
Central
85oN
45oN
36oN
23oN
48oS
Rate (cm/a)*
3
5.3
5.6
6
6.3
8
5.5
4
4.6
1
3-3.7
0.9
0.6
1-3
2.2
1.3
1.8
From Wilson (1989). Data from Hekinian (1982), Sclater et al .
(1976), Jackson and Reid (1983).
*half spreading
Oceanic Crust and Upper Mantle Structure

4 layers distinguished via seismic velocities
Sample Sources:
 Deep Sea Drilling Program
 Dredging of fracture zone scarps

Ophiolites with subaerial exposure
Oceanic Crust and
Upper Mantle
Structure
Typical Ophiolite
Wehrlite: a Peridotite mostly
composed of olivine plus
clinopyroxene
Figure 13-3. Lithology and thickness of
a typical ophiolite sequence, based on
the Samial Ophiolite in Oman. After
Boudier and Nicolas (1985) Earth
Planet. Sci. Lett., 76, 84-92.
Oceanic Crust and Upper Mantle
Structure
Layer 1
A thin layer
of pelagic
sediment
Figure 13-4. Modified after
Brown and Mussett (1993) The
Inaccessible Earth: An
Integrated View of Its Structure
and Composition. Chapman &
Hall. London.
Oceanic Crust and Upper Mantle
Structure
Layer 2 is basaltic
Subdivided into
two sub-layers
Layer 2A & B =
pillow basalts
Layer 2C = vertical
sheeted dikes
Figure 13-4. Modified after
Brown and Mussett (1993) The
Inaccessible Earth: An
Integrated View of Its Structure
and Composition. Chapman &
Hall. London.
Oceanic Crust and
Upper Mantle
Structure
Discontinuous diorite
and tonalite
(“plagiogranite”)
bodies = late
differentiated liquids
Tonalite is an igneous, plutonic (intrusive) rock, of felsic
composition, with phaneritic texture. Similar to Granite except
Feldspar is mostly present as plagioclase, with less than 10%
alkali feldspar.
Figure 13-3. Lithology and thickness of
a typical ophiolite sequence, based on
the Samial Ophiolite in Oman. After
Boudier and Nicolas (1985) Earth
Planet. Sci. Lett., 76, 84-92.
Layer 3 more complex and controversial
Believed to be mostly gabbros, crystallized from a shallow
axial magma chamber (feeds the dikes and basalts)
Layer 3A = upper
isotropic and
lower, somewhat
foliated
(“transitional”)
gabbros
Layer 3B is more
layered, & may
exhibit cumulate
textures
Layer 4 = ultramafic rocks
Ophiolites: base of 3B grades into
layered cumulate wehrlite &
gabbro
Wehrlite intruded into layered
gabbros
Below  cumulate dunite with
harzburgite xenoliths
The ultramafic igneous rock, harzburgite, is a variety of
peridotite consisting mostly of the two minerals,
olivine and low-calcium (Ca) pyroxene (enstatite)
Below this is a tectonite harzburgite and dunite:
unmelted fraction of the
partially melted (depleted)
mantle.
MORB Petrography and Major Element
Chemistry
A “typical” MORB is an olivine Tholeiite
with low K2O (< 0.2%)
and low TiO2 (< 2.0%)
 Lab analyses use glass, which is certain to
represent liquid. I’ll explain why below.


The low-P crystallization sequence is: olivine (
Mg-Cr Spinel), olivine + plagioclase ( Mg-Cr
Spinel), olivine + plagioclase + clinopyroxene
Why low
pressure?
Figure 7-2. After Bowen
(1915), A. J. Sci., and
Morse (1994), Basalts and
Phase Diagrams. Krieger
Publishers.

In MORBS, Fe-Ti
oxides are restricted
to the groundmass,
and thus form late in
the MORB sequence
http://wwwodp.tamu.edu/publications/176_SR/chap_08/
c8_f4.htm
Hence the early Fe-enrichment
characteristic of the tholeiite trend
on an ACF diagram – the iron
doesn’t precipitate out until late, so
it becomes relatively more
abundant in early glass as Mg++ is
used up.
Ulvöspinel - TiFe2O4
The major element chemistry of
MORBs
Originally considered to be extremely
uniform, interpreted as a simple
petrogenesis
 More extensive sampling has shown that
they display a (restricted) range of
compositions

The major element
chemistry of MORBs
MAR : Mid-Atlantic Ridge
EPR : East-Pacific Rise
IOR: Indian Ocean Ridge
Normative minerals: q Quartz,
or Orthoclase, ab Albite, an
Anorthite, di Diopside, hy
Hyperthene, ol Olivine, mt
Magnetite, il Ilmenite, ap
Apatite
MORBs vary a little in composition
Table 13-2. Average Analyses and CIPW Norms of MORBs
(BVTP Table 1.2.5.2)
Oxide (wt%)
SiO2
TiO2
Al2O3
FeO*
MgO
CaO
Na2O
K2O
P2O5
Total
All
50.5
1.56
15.3
10.5
7.47
11.5
2.62
0.16
0.13
99.74
MAR
50.7
1.49
15.6
9.85
7.69
11.4
2.66
0.17
0.12
99.68
EPR
50.2
1.77
14.9
11.3
7.10
11.4
2.66
0.16
0.14
99.63
IOR
50.9
1.19
15.2
10.3
7.69
11.8
2.32
0.14
0.10
99.64
Norm
q
or
ab
an
di
hy
ol
mt
il
ap
0.94
0.95
22.17
29.44
21.62
17.19
0.0
4.44
2.96
0.30
0.76
1.0
22.51
30.13
20.84
17.32
0.0
4.34
2.83
0.28
0.93
0.95
22.51
28.14
22.5
16.53
0.0
4.74
3.36
0.32
1.60
0.83
19.64
30.53
22.38
18.62
0.0
3.90
2.26
0.23
All: Ave of glasses from Atlantic, Pacific and Indian Ocean ridges.
EPR the most different
MAR: Ave. of MAR glasses. EPR: Ave. of EPR glasses.
IOR: Ave. of Indian Ocean ridge glasses.
MORBs cannot all
be primary
magmas; most are
derivative magmas
resulting from
fractional
crystallization
Figure 13-5. “Fenner-type” variation
diagrams for basaltic glasses of the
MAR. Note different ordinate scales.
From Stakes et al. (1984) J. Geophys.
Res., 89, 6995-7028.
Even when we compare for constant Mg#
considerable variation is still apparent. Fig.
13-9 shows the variation in K2O with Mg#
for the MAR data set of Schilling et al.
(1983)
Recall Mg# = 100 Mg++/ Mg++ + Fe++
Figure 13-9. Data from Schilling et
al. (1983) Amer. J. Sci., 283, 510-586.
Conclusions about MORBs, and the processes
beneath mid-ocean ridges
– MORBs are not the completely uniform
magmas that they were once considered to
be
 They show chemical trends consistent with
fractional crystallization of olivine,
plagioclase, and perhaps clinopyroxene
 As early forming crystals remove elements
from the melt, new chemical compositions
become frequent.
Magma chamber processes
maybe
different
at fast- (EPR)  a broader range of
Fast
ridge segments
spreading
ridges compared
compositions
and a larger proportion of evolved
to slow ones
liquids
Magmas
erupted
slightly off the axis of ridges are
Fastridge
segments
(EPR)
display
a broader
range
more
evolved
thanofthose at the axis itself.
compositions, and produce a
larger proportion of evolved
liquids than do slow
segments
Also magmas erupted
slightly off the axis of ridges
are more evolved than those
at the axis itself
Depleted mantle is the residue that remains after a given element has been
removed from Peridotite to form a basaltic melt. The incompatible elements (e.g.
K, Sr, Rb, U, and rare-earth elements) are preferentially partitioned into a melt,
and during ocean crustal formation these elements in particular have been
removed from the mantle, leaving the mantle depleted in incompatibles.
Incompatibles present in a MORB melt generally solidify in late fractionation
minerals derived from the basaltic melt. WE SHOULD USE ANALYSES OF GLASS
(no crystal structure) at any stage if we want melt compositions.
The depleted mantle can still partially melt and form MORBs, all you need is low
pressure
IDEA later MORBs will have less incompatibles such as LILE K+, as some were
already removed by earlier MORB formation.
An incompatible element is an element that is
unsuitable in size and/or charge to fit in the cation
sites of the possible minerals. Elements that have
difficulty in entering cation sites of the early high
temperature crystallization minerals (Olivine, CaPlagioclase, Pyroxenes) are concentrated in the melt
phase of magma (liquid phase), and remain there
until late in the solidification of the magma.
Another way to classify incompatible elements is by
mass: light rare earth elements are La - Sm, and
heavy rare earth elements (HREE) are Eu - Lu.
Rocks or magmas rich in light rare earth elements
(LREE) are referred to as fertile, and those with
strong depletions in LREE are referred to as
depleted.
An incompatible element is an element that is unsuitable in size and/or charge to fit in the cation sites of the possible minerals.
Elements that have difficulty in entering cation sites of the minerals are concentrated in the melt phase of magma (liquid phase).
Another way to classify incompatible elements is by mass: light rare earth elements are La - Sm, and heavy rare earth elements
(HREE) are Eu - Lu. Rocks or magmas rich, or only slightly depleted in light rare earth elements (LREE) are referred to as fertile,
and those with strong depletions in LREE are referred to as depleted.

REE diagram for MORBs
We see two
types of
MORBs with
Rare Earths:
Figure 13-10.
Data from
Schilling et al.
(1983) Amer. J.
Sci., 283, 510-586.
There are incompatible-rich and incompatible-poor
mantle source regions for MORB magmas
– N-MORB (normal MORB) taps the depleted
upper mantle source
 Mg# > 65: K2O < 0.10 TiO2 < 1.0
 Depleted in LREE, Low LILE e.g. K+
– E-MORB (enriched MORB, also called P-MORB
for plume) taps the (deeper) fertile mantle
 Mg# > 65: K2O > 0.10 TiO2 > 1.0
 Rich in LREE, higher in LILE e.g. K+
An incompatible element is an element that is unsuitable in size and/or charge to fit in the cation sites of the possible minerals.
Elements that have difficulty in entering cation sites of the minerals are concentrated in the melt phase of magma (liquid phase).
Another way to classify incompatible elements is by mass: light rare earth elements are La - Sm, and heavy rare earth elements
(HREE) are Eu - Lu. Rocks or magmas rich, or only slightly depleted in light rare earth elements (LREE) are referred to as fertile,
and those with strong depletions in LREE are referred to as depleted.
E-MORBs (squares) enriched in LREE over N-MORBs
(red triangles): regardless of Mg#
 Lack of distinct break suggests three MORB types
– E-MORBs La/Sm > 1.8
– N-MORBs La/Sm < 0.7
– T-MORBs (transitional) intermediate values
Figure 13-11. Data from
Schilling et al. (1983) Amer.
J. Sci., 283, 510-586.
N-MORBs: 87Sr/86Sr < 0.7035 and
143Nd/144Nd > 0.5030,  depleted mantle
source
 E-MORBs extend to more enriched values 
stronger support distinct mantle reservoirs
for N-type and E-type MORBs

Figure 13-12. Data from Ito
et al. (1987) Chemical
Geology, 62, 157-176; and
LeRoex et al. (1983) J.
Petrol., 24, 267-318.
Conclusions:
 MORBs have multiple source regions
 The mantle beneath the ocean basins
is not homogeneous
– N-MORBs tap an upper, depleted mantle
– E-MORBs tap a deeper enriched source
Idea:
– T-MORBs = mixing of N- and E- magmas
during ascent and/or in shallow
chambers
MORB Petrogenesis
Generation




Separation of the plates
Upward motion of mantle
material into extended zone
Decompression partial
melting associated with nearadiabatic rise
N-MORB melting initiated ~
60-80 km depth in upper
depleted mantle where it
inherits depleted trace
element and isotopic char.
Figure 13-13. After Zindler et al. (1984) Earth
Planet. Sci. Lett., 70, 175-195. and Wilson (1989)
Igneous Petrogenesis, Kluwer.
Generation
Region of melting
 Melt blobs separate at
about 25-35 km

Figure 13-13. After Zindler et al. (1984) Earth
Planet. Sci. Lett., 70, 175-195. and Wilson (1989)
Igneous Petrogenesis, Kluwer.
Idea:
Convective flow
that caused the
divergence at
MOR runs to
Boundary Layer.
• Lower enriched
mantle reservoir
may also be
drawn upward
and an E-MORB
plume initiated
•
Figure 13-13. After Zindler et al.
(1984) Earth Planet. Sci. Lett., 70,
175-195. and Wilson (1989) Igneous
Petrogenesis, Kluwer.
The Axial Magma Chamber
Original Model





Semi-permanent, large
Fractional crystallization 
derivative MORB magmas
Periodic reinjection of
fresh, primitive MORB from
below
Dikes upward through the
extending and faulting roof
Differentiated melts on
sides
The infinite onion
Figure 13-14. From Byran and Moore (1977)
Geol. Soc. Amer. Bull., 88, 556-570.
The crystal mush zone
contains perhaps 30%
melt and constitutes an
excellent boundary layer
for the in situ
crystallization process
proposed by Langmuir
Langmuir’s idea:
crystallization is nearly
complete along the cold
wall rock, so the liquid
there is more evolved than
in the interior of the
chamber.
Figure 11-12 From Winter
(2001) An Introduction to
Igneous and Metamorphic
Petrology. Prentice Hall
•Recent seismic work has failed to detect any chambers of this size at
ridges.
•Modern View: completely liquid body is a thin (tens to hundreds of
meters thick) and narrow (< 2 km wide) sill-like lens 1-2 km beneath
the seafloor
•Provides reflector noticed in detailed seismic profiles shot along
and across sections of the EPR
•Melt surrounded by a wider mush and transition zone of low seismic
velocity
•Transition zone transmits shear waves, but may still have a minor
amount of melt)
•“Magma chamber” = melt + mush zone (the liquid portion is continuous
through them)
•Lens maintained by reinjection.
•Completely liquid body is a thin (tens to hundreds of meters thick) and narrow (< 2
km wide) sill-like lens 1-2 km beneath the seafloor
•Provides reflector noticed in detailed seismic profiles shot along and
across sections of the EPR
•Melt surrounded by a wider mush and transition zone of low seismic velocity
•Transition zone transmits shear waves, but may still have a minor amount of melt)
•“Magma chamber” = melt + mush zone (the liquid portion is continuous through them)
•Lens maintained by reinjection.
A modern concept of the
axial magma chamber
beneath a FAST ridge
Figure 13-15. After Perfit et al.
(1994) Geology, 22, 375-379.
Melt body  continuous reflector up to several
kilometers along the ridge crest, with gaps at
fracture zones, small deviations in alignment
(devals) and offset spreading centers ( OSCs ).
 Large-scale chemical variations indicate poor
mixing along axis, and/or intermittent liquid
magma lenses, each fed by a source conduit

Figure 13-16 After Sinton
and Detrick (1992) J.
Geophys. Res., 97, 197-216.
Devals: subtle bends or tiny offsets less
than 500 meters in size.
Sinton and Detrick (1992) Model for magma
chamber beneath a slow-spreading ridge,
such as the Mid-Atlantic Ridge
Depth (km)
– Model: With a reduced heat and magma supply, a steady-state
eruptible melt lens is absent. Instead a dike-like mush zone and a
smaller transition zone are beneath a well-developed rift valley
– Model assumption: Most of body well below the liquidus
temperature.
– Prediction: convection and mixing is far less likely than at fast
ridges.
2
Rift Valley
4
Figure 13-16 After
Sinton and
Detrick (1992) J.
Geophys. Res., 97,
197-216.
6
Moho
Transition
zone
Gabbro
Mush
8
10
5
0
5
Distance (km)
10