GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 13
Mid-Ocean Ridge Basalts (MORB)
February 22, 2016
PRESENT-DAY MID-OCEAN RIDGES
BIRTHPLACES OF MORB
SPREADING RATES
Table 13-1. Spreading Rates of Some Mid-Ocean
Ridge Segments
Category
Ridge
Fast
East Pacific Rise
Slow
Indian Ocean
Mid-Atlantic Ridge
Latitude
21-23oN
13oN
11oN
8-9oN
2o N
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
EVIDENCE

Seismic Velocities

Deep Sea Drilling Program

Ophiolites

Dredging of Fracture Zone
Scarps
OCEANIC CRUST AND UPPER MANTLE STRUCTURE
Layer 3A = upper
isotropic and lower,
somewhat foliated
(“transitional”)
gabbros
Layer 3B is more
layered, & may
exhibit cumulate
textures
OCEANIC CRUST AND UPPER MANTLE FORMATION
COMPOSITION OF MORB
Primitive
Magma
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.
MAR: Ave. of MAR glasses. EPR: Ave. of EPR glasses.
IOR: Ave. of Indian Ocean ridge glasses.
Fractional
Crystallization
COMPOSITIONAL VARIABILITY IN MORB
NOT RELATED TO FRACTIONAL CRYSTALLIZAITON
Incompatible-rich and incompatiblepoor mantle source regions for MORB
magmas

N-MORB (normal MORB) taps
the depleted upper mantle
source


Mg# > 65: K2O < 0.10
TiO2 < 1.0
E-MORB (enriched MORB)
taps undepleted (deeper?)
mantle

Mg# > 65: K2O > 0.10
TiO2 > 1.0
ORIGIN OF NMORB AND EMORB
ORIGINAL STEADY STATE
AXIAL MAGMA CHAMBER MODEL
Hekinian et al. (1976)
Contr. Min. Pet. 58,
107.
• Periodic replenishment from below
• Cooling/Crystallization from the walls
inward (the infinite onion model)
• Diking variably fractionated magmas to
surface
• Ultramafic cumulates at floor; layered
gabbros on walls
• Problem – geophysics indicate no large
magma body beneath the ridges
MORE REALISTIC
MODEL
X-Sectional View
SMALL MELT LENSES
ABOVE MUSH ZONES
WITH > 30% CRYSTAL
LARGER MELT LENSES IN
FAST SPREADING CENTERS
PROMOTES MORE
FRACTIONATED BASALT
COMPOSITIONS
Winter (2001) Figure 13-15,
After Perfit et al. (1994)
Geology, 22, 375-379.
Longitudinal View
Winter (2001) Figure 13-16 ; After
Sinton and Detrick (1992) J.
Geophys. Res., 97, 197-216.
SLOW-SPREADING RIDGES
Smaller, crystal-rich, dike-like
magma bodies results is less
fractionation  less evolved MORB
Depth (km)
2
Rift Valley
4
6
Moho
Transition
zone
Gabbro
Mush
8
10
5
0
5
Distance (km)
Winter (2001) Figure 13-16 After Sinton and Detrick (1992) J.
Geophys. Res., 97, 197-216.
10
GLOBAL VARIABILITY OF
MORB
THERMAL STRUCTURE
OF THE MANTLE
HOT VS. COOL MANTLE