Presentation is loading. Please wait.

Presentation is loading. Please wait.

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

Similar presentations


Presentation on theme: "Mid-Ocean Ridge Basalts (MORB), oceanic crust and ophiolites."— Presentation transcript:

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

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

9 Obduction

10 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

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,

12

13 Layer 1 A thin layer of pelagic sediment Oceanic Crust and Upper Mantle Structure Figure Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.

14 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 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 S Ophiolite

16 Basaltic pillows

17 Pillow Lavas in the Josephine Ophiolite

18

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 S Ophiolite

22 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

23 Layered Gabbros and Moho S

24 Gabbros

25 Discontinuous diorite and tonalite (“plagiogranite”) bodies = late differentiated liquids Oceanic Crust and Upper Mantle Structure 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

27

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)

29 Serpentinites (weathered peridotites)

30

31 Evidence for melting in serpentinites

32

33 65 Ma

34

35 Petrography and Major Element Chemistry l A “typical” MORB is an olivine tholeiite with low K 2 O (< 0.2%) and low TiO 2 (< 2.0%)

36 The major element chemistry of MORBs

37 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

38 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 “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 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.

40 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 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 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 After Sinton and Detrick (1992) J. Geophys. Res., 97,

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

44 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

45 Trace Element and Isotope Chemistry l REE diagram for MORBs Figure Data from Schilling et al. (1983) Amer. J. Sci., 283,

46 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 Data from Schilling et al. (1983) Amer. J. Sci., 283,

47 N-MORBs: 87 Sr/ 86 Sr ,  depleted mantle source N-MORBs: 87 Sr/ 86 Sr ,  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 Data from Ito et al. (1987) Chemical Geology, 62, ; and LeRoex et al. (1983) J. Petrol., 24,

48 l Lower enriched mantle reservoir may also be drawn upward and an E-MORB plume initiated Figure After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, and Wilson (1989) Igneous Petrogenesis, Kluwer.

49 Fast and slow spreading ridges Table Spreading rates of some mid-ocean ridge segments. CategoryRidgeLatitudeRate (cm/a)* FastEast Pacific Rise o N 3 13 o N o N o N 6 2 o N o S 8 33 o S o S 4 56 o S 4.6 SlowIndian OceanSW1 SE3-3.7 Central0.9 Mid-Atlantic Ridge 85 o N o N o N o N 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

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

51

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

60

61

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

63 “HOT” vs. “LOT”

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

65 Thermal modelling: melt fraction under fast and slow ridges

66 Restite composition K2OK2OMgOCaO MORB DM Residues for successive F values: F= MORB DM Residues for increasing F

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

68 Fast-spreading ridge Figure After Perfit et al. (1994) Geology, 22,

69 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 After Sinton and Detrick (1992) J. Geophys. Res., 97,


Download ppt "Mid-Ocean Ridge Basalts (MORB), oceanic crust and ophiolites."

Similar presentations


Ads by Google