Chapter 13: Mid-Ocean RiftsChapter 13: Mid-Ocean RiftsThe Mid-Ocean Ridge SystemThe Mid-Ocean Ridge System

Figure 13-1. After Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36, 541-576.

Oceanic Crust and Upper Mantle StructureOceanic Crust and Upper Mantle Structure

4 layers distinguished via 4 layers distinguished via seismic velocitiesseismic velocities Deep Sea Drilling ProgramDeep Sea Drilling Program DredgingDredging of fracture zone scarps of fracture zone scarps OphiolitesOphiolites

Oceanic Crust and Oceanic Crust and Upper Mantle Structure Upper Mantle Structure

Typical OphioliteTypical Ophiolite

Figure 13-3.Figure 13-3. Lithology and thickness of Lithology and thickness of a typical ophiolite sequence, based on a typical ophiolite sequence, based on the Samial Ophiolite in Oman. After the Samial Ophiolite in Oman. After Boudier and Nicolas (1985) Earth Boudier and Nicolas (1985) Earth Planet. Sci. Lett., 76, 84-92. Planet. Sci. Lett., 76, 84-92.

Layer 1Layer 1

A thin layer A thin layer of pelagic of pelagic sediment sediment

Oceanic Crust and Upper Mantle StructureOceanic Crust and Upper Mantle Structure

Figure 13-4.Figure 13-4. Modified after Modified after Brown and Mussett (1993) Brown and Mussett (1993) The The Inaccessible Earth: An Inaccessible Earth: An Integrated View of Its Structure Integrated View of Its Structure and Composition. Chapman & and Composition. Chapman & Hall. London.Hall. London.

Layer 2 Layer 2 isis basaltic basaltic

Subdivided into Subdivided into two sub-layerstwo sub-layers

Layer 2A & B = Layer 2A & B = pillow basaltspillow basalts

Layer 2C = vertical Layer 2C = vertical sheeted dikessheeted dikes

Oceanic Crust and Upper Mantle StructureOceanic Crust and Upper Mantle Structure

Figure 13-4.Figure 13-4. Modified after Modified after Brown and Mussett (1993) Brown and Mussett (1993) The The Inaccessible Earth: An Inaccessible Earth: An Integrated View of Its Structure Integrated View of Its Structure and Composition. Chapman & and Composition. Chapman & Hall. London.Hall. London.

Layer 3Layer 3 more complex and controversial more complex and controversialBelieved to be mostly gabbros, crystallized from a shallow Believed to be mostly gabbros, crystallized from a shallow axial axial magma chambermagma chamber (feeds the dikes and basalts) (feeds the dikes and basalts)

Layer 3A Layer 3A = upper = upper isotropic and isotropic and lower, somewhat lower, somewhat foliated foliated (“transitional”) (“transitional”) gabbrosgabbros

Layer 3BLayer 3B is more is more layered, & may layered, & may exhibit cumulate exhibit cumulate texturestextures

Discontinuous diorite Discontinuous diorite and tonalite and tonalite (“plagiogranite”)(“plagiogranite”) bodies = late bodies = late differentiated liquidsdifferentiated liquids

Oceanic Crust and Oceanic Crust and Upper Mantle Upper Mantle

StructureStructure

Figure 13-3.Figure 13-3. Lithology and thickness of Lithology and thickness of a typical ophiolite sequence, based on a typical ophiolite sequence, based on the Samial Ophiolite in Oman. After the Samial Ophiolite in Oman. After Boudier and Nicolas (1985) Earth Boudier and Nicolas (1985) Earth Planet. Sci. Lett., 76, 84-92. Planet. Sci. Lett., 76, 84-92.

Layer 4Layer 4 = = ultramafic rocksultramafic rocks

Ophiolites: base of 3B Ophiolites: base of 3B grades into layered grades into layered cumulate wehrlite & cumulate wehrlite & gabbro gabbro

WehrliteWehrlite intruded into intruded into layered gabbroslayered gabbros

Below Below cumulate cumulate dunitedunite with harzburgite xenolithswith harzburgite xenoliths

Below this is a Below this is a tectonitetectonite harzburgite and dunite harzburgite and dunite (unmelted residuum of the (unmelted residuum of the original mantle)original mantle)

MgO and FeOMgO and FeO

AlAl22OO33 and CaO and CaO

SiO2SiO2

NaNa22O, KO, K22O, TiOO, TiO22, ,

PP22OO55

Figure 13-5.Figure 13-5. “Fenner-type” variation “Fenner-type” variation diagrams for basaltic glasses from the diagrams for basaltic glasses from the Afar region of the MAR. Note different Afar region of the MAR. Note different ordinate scales. From Stakes et al. ordinate scales. From Stakes et al. (1984) (1984) J. Geophys. Res., 89, 6995-7028.J. Geophys. Res., 89, 6995-7028.

Ternary Variation Diagrams Ternary Variation Diagrams Example: AFM diagramExample: AFM diagram

(alkalis-FeO*-MgO)(alkalis-FeO*-MgO)

Figure 8-2. AFM diagram for Crater Lake volcanics, Oregon Cascades. Data compiled by Rick Conrey (personal communication).

Conclusions about MORBs, and the processes Conclusions about MORBs, and the processes beneath mid-ocean ridgesbeneath mid-ocean ridges

MORBs are not the completely uniform MORBs are not the completely uniform magmas that they were once considered to magmas that they were once considered to bebe

They show chemical trends consistent They show chemical trends consistent with fractional crystallization of olivine, with fractional crystallization of olivine, plagioclase, and perhaps clinopyroxeneplagioclase, and perhaps clinopyroxene

MORBs MORBs cannot be primary magmascannot be primary magmas, but , but are derivative magmas resulting from are derivative magmas resulting from fractional crystallization (~ 60%)fractional crystallization (~ 60%)

FastFast ridge segments ridge segments (EPR) (EPR) a a broader rangebroader range of compositions and a of compositions and a larger proportion of larger proportion of evolvedevolved liquids liquids

(magmas erupted slightly (magmas erupted slightly off the axis of ridges are off the axis of ridges are more evolved than those more evolved than those at the axis itself)at the axis itself)

Figure 13-8.Figure 13-8. Histograms of over 1600 glass Histograms of over 1600 glass compositions from slow and fast mid-compositions from slow and fast mid-ocean ridges. After Sinton and Detrick ocean ridges. After Sinton and Detrick (1992) (1992) J. Geophys. Res., 97, 197-216.J. Geophys. Res., 97, 197-216.

For constant Mg#For constant Mg# considerable variation is still apparent. considerable variation is still apparent.

Figure 13-9. Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586.

Incompatible-rich and incompatible-poor mantle source Incompatible-rich and incompatible-poor mantle source regions for MORB magmasregions for MORB magmas

N-MORBN-MORB ( (normalnormal MORB) taps the depleted upper MORB) taps the depleted upper mantle sourcemantle source

Mg# > 65: KMg# > 65: K22O < 0.10 TiOO < 0.10 TiO22 < 1.0 < 1.0 E-MORBE-MORB ( (enrichedenriched MORB, also called MORB, also called P-MORBP-MORB for for

plumeplume) taps the (deeper) fertile mantle) taps the (deeper) fertile mantle Mg# > 65: KMg# > 65: K22O > 0.10 TiOO > 0.10 TiO22 > 1.0 > 1.0

Trace Element and Isotope ChemistryTrace Element and Isotope Chemistry REE diagram for MORBsREE diagram for MORBs

Figure 13-10.Figure 13-10. Data from Data from Schilling et al. Schilling et al. (1983) (1983) Amer. J. Amer. J. Sci., 283, 510-586.Sci., 283, 510-586.

E-MORBs (squares) E-MORBs (squares) enrichedenriched over N-MORBs (red over N-MORBs (red triangles): triangles): regardless of Mg# regardless of Mg#

Lack of distinct break suggests Lack of distinct break suggests threethree MORB types MORB types E-MORBs La/Sm > 1.8E-MORBs La/Sm > 1.8 N-MORBs La/Sm < 0.7N-MORBs La/Sm < 0.7 T-MORBsT-MORBs (transitional) intermediate values (transitional) intermediate values

Figure 13-11.Figure 13-11. Data from Data from Schilling et al. (1983) Schilling et al. (1983) Amer. Amer. J. Sci., 283, 510-586.J. Sci., 283, 510-586.

N-MORBs: N-MORBs: 8787Sr/Sr/8686Sr < 0.7035 and Sr < 0.7035 and 143143Nd/Nd/144144Nd > Nd > 0.5030, 0.5030, depleted mantle sourcedepleted mantle source

E-MORBs extend to more enriched values E-MORBs extend to more enriched values stronger support distinct mantle reservoirs for N-stronger support distinct mantle reservoirs for N-type and E-type MORBstype and E-type MORBs

Figure 13-12.Figure 13-12. Data from Ito Data from Ito et al. (1987) et al. (1987) Chemical Chemical Geology, 62, 157-176; Geology, 62, 157-176; and and LeRoex et al. (1983) LeRoex et al. (1983) J. J. Petrol., 24, 267-318.Petrol., 24, 267-318.

Conclusions:Conclusions: MORBs have > 1 source regionMORBs have > 1 source region The mantle beneath the ocean basins is not The mantle beneath the ocean basins is not

homogeneoushomogeneous N-MORBs tap an upper, depleted mantleN-MORBs tap an upper, depleted mantle E-MORBs tap a deeper enriched sourceE-MORBs tap a deeper enriched source T-MORBs = mixing of N- and E- magmas T-MORBs = mixing of N- and E- magmas

during ascent and/or in shallow chambersduring ascent and/or in shallow chambers

Experimental data: parent was multiply saturated with Experimental data: parent was multiply saturated with olivine, cpx, and opx olivine, cpx, and opx P range = 0.8 - 1.2 GPa (25-35 km) P range = 0.8 - 1.2 GPa (25-35 km)

Figure 13-10.Figure 13-10. Data from Data from Schilling et al. Schilling et al. (1983) (1983) Amer. J. Amer. J. Sci., 283, 510-586.Sci., 283, 510-586.

MORB PetrogenesisMORB Petrogenesis

Separation of the platesSeparation of the plates Upward motion of mantle Upward motion of mantle

material into extended zonematerial into extended zone Decompression partial Decompression partial

meltingmelting associated with associated with near-adiabatic rise near-adiabatic rise

N-MORBN-MORB melting initiated melting initiated ~ 60-80 km depth in upper ~ 60-80 km depth in upper depleted mantle where it depleted mantle where it inherits depleted trace inherits depleted trace element and isotopic char.element and isotopic char.

GenerationGeneration

Figure 13-13.Figure 13-13. After Zindler et al. (1984) After Zindler et al. (1984) Earth Earth Planet. Sci. Lett., 70, 175-195.Planet. Sci. Lett., 70, 175-195. and Wilson (1989) and Wilson (1989) Igneous Petrogenesis, Kluwer. Igneous Petrogenesis, Kluwer.

Lower enriched Lower enriched mantle reservoir mantle reservoir may also be drawn may also be drawn

upward and an upward and an E-E-MORBMORB plumeplume initiatedinitiated

Figure 13-13.Figure 13-13. After Zindler et al. After Zindler et al. (1984) (1984) Earth Planet. Sci. Lett., 70, 175-Earth Planet. Sci. Lett., 70, 175-195.195. and Wilson (1989) Igneous and Wilson (1989) Igneous Petrogenesis, Kluwer. Petrogenesis, Kluwer.