1. Mid-Ocean Ridge Basalts (MORB), oceanic crust and ophiolites

2. The Mid-Ocean Ridge System
Figure After Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36,

3. Rifting of continental crust to form a new ocean basin

4. Collision – welding together of continental crust
Subducting oceanic lithosphere deforms sediment at edge of continental plate
Collision – welding together of continental crust
Post-collision: two continental plates are welded together, mountain stands where once was ocean

5. Ophiolites in Himalaya

6. World’s distribution of ophiolites

7. Distribution of European Ophiolites
European ophiolites are related to the collision of Europe with Africa.
They represent remnants of the Jurassic Tethyan Ocean

8. Oman (Semail) Ophiolite
Greenschist facies
shear zones
Layered … massive
gabbros
Dykes
Pillows

9. Obduction

10. Oceanic Crust and Upper Mantle Structure
4 layers distinguished via seismic velocities
Deep Sea Drilling Program
Dredging of fracture zone scarps
Ophiolites
4 layers of the oceanic crust and mantle distinguished on the basis of discontinuities in seismic velocities
Deep Sea Drilling Program rarely penetrates the volcanics, and then only to a maximum depth of 1500 m
Dredging of fracture zone scarps  samples from deeper sources, but no reliable stratigraphic control
Ophiolites = masses of oceanic crust and upper mantle thrust onto the edge of a continent or incorporated in mountain belts, now exposed by erosion

11. Oceanic Crust and Upper Mantle Structure
Typical Ophiolite
Figure 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,

13. Oceanic Crust and Upper Mantle Structure
Layer 1 A thin layer of pelagic sediment
Absent on newly generated crust at ridge axes, and thickens away from it
Figure Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.

14. 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
Layer 2 A  B with fracture in-filling by mineral deposition (some call both A, and then call 2B sheeted dikes)
Layer 2C = vertical sheeted dikes emplaced in the shallow brittle extensional environment at the ridge axis
Many dikes have only a single chill margin  later dikes split and intruded earlier ones
Figure Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.

15. Pillow lavas in the Semail Ophiolite

16. Basaltic pillows

17. Pillow Lavas in the Josephine Ophiolite

19. Submarine eruptions and pillows

20. Sheeted Dyke / Lava Transition
The vertical slabs of rock are dikes intruding into lavas that erupted on the seafloor. This section represents the transition from lavas to sheeted dikes and is thought to correspond to seismic Layer 2B

21. Sheeted Dykes in Semail Ophiolite

22. Layer 3B is more layered, & may exhibit cumulate textures
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
The layering may be horizontal, but more commonly dips at angles locally up to 90o

23. Layered Gabbros and Moho Semail

24. Gabbros

25. Oceanic Crust and Upper Mantle Structure
Discontinuous diorite and tonalite (“plagiogranite”) bodies = late differentiated liquids
At the top of the gabbros in the Oman are small discontinuous diorite and tonalite (“plagiogranite”) bodies = late differentiated liquids filter pressed and mobilized along the gabbro-sheeted dike contact, and may extend up into the pillow layer
Figure 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,

26. Plagiogranites

28. 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
Below this is a tectonite harzburgite and dunite (unmelted residuum of the original mantle)
The boundary between layers 3 and 4 is, broadly speaking, the Moho
Upper portion of layer 4 is thought to be layered, and of cumulate origin (olivine and pyroxenes sink to the bottom of the axial magma chamber)
Below this is the original, unlayered, residual mantle material
Exactly what is the crust/mantle boundary?
1) Top of the original mantle
2) Mafic/ultramafic transition (top of the added ultramafic cumulates)
Is the mantle defined by petrogenesis or by composition?
A number of authors distinguish a seismic Moho from a petrological Moho

29. Serpentinites
(weathered peridotites)

31. Evidence for melting in serpentinites

33. 65 Ma

35. Petrography and Major Element Chemistry
Quartz-rich
Granitoid 90 60 20
Alkali Fs.
Quartz Syenite
Quartz
Syenite
Monzonite
Monzodiorite
(Foid)-bearing 5 10 35 65
(Foid)
Monzosyenite
(Foid) Syenite
(Foid) Gabbro
Qtz. Diorite/
Qtz. Gabbro
Diorite/Gabbro/
Anorthosite
Diorite/Gabbro
(Foid)olites
Quartzolite
Granite
Grano-
diorite
Tonalite
Alkali Feldspar Granite Q A P F
A “typical” MORB is an olivine tholeiite with low K2O (< 0.2%) and low TiO2 (< 2.0%)
MORBs are chemically distinct from basalts of other petrogenetic associations
Glass samples are very important chemically, because they represent liquid compositions, whereas the chemistry of phyric samples can be modified by crystal accumulation

36. The major element chemistry of MORBs
All analyses are of glasses, so that only liquid compositions are represented
Note the very low content of K2O and that all analyses are quartz-hypersthene normative (although olivine is common in the mode)

37. 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

38. MgO and FeO Al2O3 and CaO SiO2 Na2O, K2O, TiO2, P2O5
Decrease in MgO and relative increase in FeO  early differentiation trend of tholeiites
Patterns are compatible with crystal fractionation of the observed phenocryst phases
Removal of olivine can raise the FeO/MgO ratio, and the separation of a calcic plagioclase can cause Al2O3 and CaO to decrease
SiO2 is a ~ poor fractionation index (as we’d suspected)
Na2O K2O TiO2 and P2O5 are all conserved and the concentration of each triples over FX range
This implies that the parental magma undergoes 67% fractionation in a magma chamber somewhere beneath the ridge to reduce the original mass by 1/3
Figure “Fenner-type” variation diagrams for basaltic glasses from the Afar region of the MAR. Note different ordinate scales. From Stakes et al. (1984) J. Geophys. Res., 89,

39. The common crystallization sequence is: olivine ( Mg-Cr spinel), olivine + plagioclase ( Mg-Cr spinel), olivine + plagioclase + clinopyroxene
From textures and experiments on natural samples at low pressure the common crystallization sequence is:
Figure 7-2. After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams. Krieger Publishers.

40. Figure 13-15. After Perfit et al. (1994) Geology, 22, 375-379.
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 (transmits shear waves, but may still have a minor amount of melt)
“Magma chamber” = melt + mush zone (the liquid portion is continuous through them)
As liquid  mush the boundary moves progressively toward the liquid lens as crystallization proceeds
Lens maintained by reinjection, much like the “infinite onion”
Figure After Perfit et al. (1994) Geology, 22,

41. The crystal mush zone contains perhaps 30% melt and constitutes an excellent boundary layer for the in situ crystallization process proposed by Langmuir
Seismic velocities are still low beyond the mush (transition zone where the partially molten material grades to cooler solid gabbro)
The small sill-like liquid chamber seems difficult to reconcile with the layered gabbros and cumulates, which appear to be more compatible with a large liquid chamber
In situ crystallization in the mush zone, however may be a viable alternative for gabbro formation
Much of the layering of ophiolite gabbros may be secondary, imposed during deformation of the spreading seafloor, and not by crystal settling
Figure From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall

42. Melt body ® continuous reflector up to several kilometers along the ridge crest, with gaps at fracture zones, devals and OSCs
Large-scale chemical variations indicate poor mixing along axis, and/or intermittent liquid magma lenses, each fed by a source conduit
Figure After Sinton and Detrick (1992) J. Geophys. Res., 97,

43. Some complications N-MORBs and E-MORBs
Fast and slow spreading ridges, Harzburgite and Lherzolite ophiolites

44. N-MORB (normal MORB) taps the depleted upper mantle source
There must be incompatible-rich and incompatible-poor source regions for MORB magmas in the mantle beneath the ridges
N-MORB (normal MORB) taps the depleted upper mantle source
Mg# > 65: K2O < TiO2 < 1.0
E-MORB (enriched MORB, also called P-MORB for plume) taps the deeper fertile mantle
Mg# > 65: K2O > TiO2 > 1.0
There must be incompatible-rich and incompatible-poor source regions for MORB magmas in the mantle beneath the ridges (related to the lower and upper mantle reservoirs?)
N-MORB (normal MORB) taps the depleted (incompatible-poor) upper mantle source
Mg# > 65: K2O < TiO2 < 1.0
E-MORB (enriched MORB, also called P-MORB for plume) taps the deeper (incompatible-richer) mantle
Mg# > 65: K2O > TiO2 > 1.0
Major elements are not the best way to make these distinctions, which must be substantiated by trace element and isotopic differences

45. Trace Element and Isotope Chemistry
REE diagram for MORBs
Figure Data from Schilling et al. (1983) Amer. J. Sci., 283,
Also see two types
N-MORB has depleted trend
E-MORB has non-depleted
trend

46. Lack of distinct break suggests three MORB types
E-MORBs (squares) enriched 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 Data from Schilling et al. (1983) Amer. J. Sci., 283,
Ratios La/Sm vs. Mg#  slope for many REE analyses at once
Note that E-MORBs (squares) always have a higher La/Sm ratio than N-MORBs (open triangles): enriched regardless of Mg#
The lack of any distinct break between the enriched and depleted lavas suggests three MORB types
E-MORBs have La/Sm > 1.8
N-MORBs have La/Sm < 0.7
T-MORBs (for “transitional”) have intermediate values
Because T-MORBs form a continuous spectrum between N- and E-MORBs they may be the result of simple binary mixing of the two magma types
T-MORBs do not necessarily imply a third distinct source

47. N-MORBs: 87Sr/86Sr < 0. 7035 and 143Nd/144Nd > 0
N-MORBs: 87Sr/86Sr < and 143Nd/144Nd > , ® depleted mantle source
E-MORBs extend to more enriched values ® stronger support distinct mantle reservoirs for N- type and E-type MORBs
Figure Data from Ito et al. (1987) Chemical Geology, 62, ; and LeRoex et al. (1983) J. Petrol., 24,
Figure 13-11: 143Nd/144Nd vs. 87Sr/86Sr data for MORBs
N-MORBs plot as a relatively tight cluster with 87Sr/86Sr < and 143Nd/144Nd > , both of which indicate a depleted mantle source
E-MORBs extend the MORB array to more enriched values (higher 87Sr/86Sr and lower 143Nd/144Nd), providing even stronger support for the distinct mantle reservoirs for N-type and E-type MORBs
T-MORBs (not shown) exhibit intermediate mixed values

48. Lower enriched mantle reservoir may also be drawn upward and an E-MORB plume initiated
The plume may be of independent (but geograph-ically coincidental) origin
The enriched plume undergoes decompression melting to form E- MORB
As with N-MORB, the melt will not segregate until shallower depths, where the major element and mineralogical character is determined
The E-MORB and N-MORB melt blobs may mix to varying degrees as funnel to the ridge (T-MORB)
Figure After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, and Wilson (1989) Igneous Petrogenesis, Kluwer.

49. Fast and slow spreading ridges
Table 13-1
. Spreading rates of some mid-ocean
ridge segments.
Category
Ridge
Latitude
Rate (cm/a)*
Fast
East Pacific Rise
21-23 o N 3 13
5.3 11
5.6
8-9 6 2
6.3
20-21 S 8 33
5.5 54 4 56
4.6
Slow
Indian Ocean SW 1 SE
3-3.7
Central
0.9
Mid-Atlantic Ridge 85
0.6 45
1-3 36
2.2 23
1.3 48
1.8
From Wilson (1989). Data from Hekinian (1982), Sclater
et al .
(1976), Jackson and Reid (1983) *half spreading
Slow-spreading ridges:
< 3 cm/a
Fast-spreading ridges:
> 4 cm/a are considered
Temporal variations are also known

50. Two extension models on ridges
High magma flux, magmatism > tectonic
Lower magma influx, tectonic > magmatism

52. The Futuna Ridge (W. Pacific), a fast-spreading ridge

53. OSC = Overlaping Spreading Center

54. Schematic view of a fast ridge

55. Oceanic crust of a fast ridge
The Vema Fracture Zone (N. Atlantic)

56. A slow ridge
The “FAMOUS” area,
N. Atlantic

57. Model of a slow ridge

58. Oceanic crust in a slow ridge

59. Pillow-lavas: ophiolitic pillows in the French alps
Moho

62. Fast vs. slow ridges No axial valley Important magmatism
“complete” sequence (peridotite-gabbros-basalts)
Deep axial valley
Moderate magmatism
Incomplete sequence

63. “HOT” vs. “LOT”

64. Abundant basalts => thick crust => fast ridge = HOT
Moderate amounts of basalts => finer crust => slow ridge = LOT

65. Thermal modelling: melt fraction under fast and slow ridges

66. Residues for successive F values:
K2O
MgO
CaO
MORB
0.16
7.5
11.5 DM
0.1 31 5
Residues for successive F values: F=
0.01
0.10
31.24
4.93
0.02
31.48
4.87
0.05
32.24
4.66
0.09
33.61
4.28
0.2
36.88
3.38
0.25
0.08
38.83
2.83
0.3
0.07
41.07
2.21
0.4
0.06
46.67
0.67
0.43
48.73
Restite composition
MORB DM
Residues for
increasing F

67. Melt abundant = fast ridge = thick crust = depleted mantle, HOT
Melt moderate = slow ridge = fine crust = less depleted mantle, LOT

68. Fast-spreading ridge
Recent seismic work has failed to detect any chambers of this size at ridges, thus causing a fundamental shift away from this traditional view of axial magma chambers as large, steady-state, predominantly molten bodies of extended duration
Combines the magma chamber geometry proposed by Sinton and Detrick (1992) with the broad zone of volcanic activity noted by Perfit et al. (1994)
Figure After Perfit et al. (1994) Geology, 22,

69. Model for magma chamber beneath a slow-spreading ridge, such as the Mid-Atlantic Ridge
Dike-like mush zone and a smaller transition zone beneath the well-developed rift valley
The bulk of the body is well below the liquidus temperature, so convection and mixing is far less likely than at fast ridges
Distance (km) 10 5 2 4 6 8
Depth (km)
Moho
Transition
zone
Mush
Gabbro
Rift Valley
With a reduced heat and magma supply, a steady-state eruptable magma lens is relinquished in favor of a dike-like mush zone and a smaller transition zone beneath the well-developed rift valley. With the bulk of the body well below the liquidus temperature, convection and mixing is far less likely than at fast ridges
Figure After Sinton and Detrick (1992) J. Geophys. Res., 97,