Presentation on theme: "Mid-Ocean Ridge Basalts (MORB), oceanic crust and ophiolites."— Presentation transcript:
Mid-Ocean Ridge Basalts (MORB), oceanic crust and ophiolites
The Mid-Ocean Ridge System Figure 13-1. After Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36, 541-576.
Rifting of continental crust to form a new ocean basin
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
Oceanic Crust and Upper Mantle Structure l 4 layers distinguished via seismic velocities l Deep Sea Drilling Program l Dredging of fracture zone scarps l Ophiolites
Oceanic Crust and Upper Mantle Structure Typical Ophiolite 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 1 A thin layer of pelagic sediment Oceanic Crust and Upper Mantle Structure Figure 13-4. Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.
Layer 2 is basaltic Subdivided into two sub-layers Subdivided into two sub-layers Layer 2A & B = pillow basalts Layer 2C = vertical sheeted dikes Oceanic Crust and Upper Mantle Structure Figure 13-4. Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.
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
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
Discontinuous diorite and tonalite (“plagiogranite”) bodies = late differentiated liquids Oceanic Crust and Upper Mantle Structure 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 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)
l Originally considered to be extremely uniform, interpreted as a simple petrogenesis l More extensive sampling has shown that they display a (restricted) range of compositions
l MgO and FeO l Al 2 O 3 and CaO l SiO2 l Na 2 O, K 2 O, TiO 2, P 2 O 5 Figure 13-5. “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, 6995-7028.
l The common crystallization sequence is: olivine ( Mg-Cr spinel), olivine + plagioclase ( Mg-Cr spinel), olivine + plagioclase + clinopyroxene Figure 7-2. After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams. Krieger Publishers.
Figure 13-15. After Perfit et al. (1994) Geology, 22, 375-379.
The crystal mush zone contains perhaps 30% melt and constitutes an excellent boundary layer for the in situ crystallization process proposed by Langmuir Figure 11-12 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall
Melt body continuous reflector up to several kilometers along the ridge crest, with gaps at fracture zones, devals and OSCs Melt body continuous reflector up to several kilometers along the ridge crest, with gaps at fracture zones, devals and OSCs l 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.
Some complications l N-MORBs and E-MORBs l Fast and slow spreading ridges, Harzburgite and Lherzolite ophiolites
There must be incompatible-rich and incompatible-poor source regions for MORB magmas in the mantle beneath the ridges F N-MORB (normal MORB) taps the depleted upper mantle source s Mg# > 65: K 2 O 65: K 2 O < 0.10 TiO 2 < 1.0 F E-MORB (enriched MORB, also called P-MORB for plume) taps the deeper fertile mantle s Mg# > 65: K 2 O > 0.10 TiO 2 > 1.0
Trace Element and Isotope Chemistry l REE diagram for MORBs Figure 13-10. Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586.
E-MORBs (squares) enriched over N-MORBs (red triangles): regardless of Mg# l Lack of distinct break suggests three MORB types F E-MORBs La/Sm > 1.8 F N-MORBs La/Sm < 0.7 F T-MORBs (transitional) intermediate values Figure 13-11. Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586.
N-MORBs: 87 Sr/ 86 Sr 0.5030, depleted mantle source N-MORBs: 87 Sr/ 86 Sr 0.5030, depleted mantle source E-MORBs extend to more enriched values stronger support distinct mantle reservoirs for N- type and E-type MORBs 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.
l 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.
Fast and slow spreading ridges Table 13-1. Spreading rates of some mid-ocean ridge segments. CategoryRidgeLatitudeRate (cm/a)* FastEast Pacific Rise 21-23 o N 3 13 o N 5.3 11 o N 5.6 8-9 o N 6 2 o N 6.3 20-21 o S 8 33 o S 5.5 54 o S 4 56 o S 4.6 SlowIndian OceanSW1 SE3-3.7 Central0.9 Mid-Atlantic Ridge 85 o N 0.6 45 o N 1-3 36 o N 2.2 23 o N 1.3 48 o S 1.8 From Wilson (1989). Data from Hekinian (1982), Sclater et al. (1976), Jackson and Reid (1983). *half spreading Slow-spreading ridges:Slow-spreading ridges: < 3 cm/a Fast-spreading ridges:Fast-spreading ridges: > 4 cm/a are considered Temporal variations are also knownTemporal variations are also known
Two extension models on ridges l High magma flux, magmatism > tectonic l Lower magma influx, tectonic > magmatism
l Abundant basalts => thick crust => fast ridge = HOT l Moderate amounts of basalts => finer crust => slow ridge = LOT
Thermal modelling: melt fraction under fast and slow ridges
Restite composition K2OK2OMgOCaO MORB0.167.511.5 DM0.1315 Residues for successive F values: F= 0.010.1031.244.93 0.020.1031.484.87 0.050.1032.244.66 0.10.0933.614.28 0.20.0936.883.38 0.250.0838.832.83 0.30.0741.072.21 0.40.0646.670.67 0.430.0548.730.10 MORB DM Residues for increasing F
l Melt abundant = fast ridge = thick crust = depleted mantle, HOT l Melt moderate = slow ridge = fine crust = less depleted mantle, LOT
Fast-spreading ridge Figure 13-15. After Perfit et al. (1994) Geology, 22, 375-379.
Model for magma chamber beneath a slow-spreading ridge, such as the Mid-Atlantic Ridge F Dike-like mush zone and a smaller transition zone beneath the well-developed rift valley F The bulk of the body is well below the liquidus temperature, so convection and mixing is far less likely than at fast ridges Figure 13-16 After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.