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

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

Ophiolites in Himalaya

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

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

Obduction

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

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.

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

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

Pillow lavas in the Semail Ophiolite

Basaltic pillows

Pillow Lavas in the Josephine Ophiolite

Submarine eruptions and pillows

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

Sheeted Dykes in Semail Ophiolite

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

Layered Gabbros and Moho Semail

Gabbros

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 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.

Plagiogranites

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

Serpentinites (weathered peridotites)

Evidence for melting in serpentinites

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

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)

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

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 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.

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.

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 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 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 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 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 N-MORBs and E-MORBs Fast and slow spreading ridges, Harzburgite and Lherzolite ophiolites

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 < 0.10 TiO2 < 1.0 E-MORB (enriched MORB, also called P-MORB for plume) taps the deeper fertile mantle Mg# > 65: K2O > 0.10 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 < 0.10 TiO2 < 1.0 E-MORB (enriched MORB, also called P-MORB for plume) taps the deeper (incompatible-richer) mantle Mg# > 65: K2O > 0.10 TiO2 > 1.0 Major elements are not the best way to make these distinctions, which must be substantiated by trace element and isotopic differences

Trace Element and Isotope Chemistry REE diagram for MORBs Figure 13-10. Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586. Also see two types N-MORB has depleted trend E-MORB has non-depleted trend

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

N-MORBs: 87Sr/86Sr < 0. 7035 and 143Nd/144Nd > 0 N-MORBs: 87Sr/86Sr < 0.7035 and 143Nd/144Nd > 0.5030, ® depleted mantle source 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. Figure 13-11: 143Nd/144Nd vs. 87Sr/86Sr data for MORBs N-MORBs plot as a relatively tight cluster with 87Sr/86Sr < 0.7035 and 143Nd/144Nd > 0.5030, 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

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 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. 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

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

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

OSC = Overlaping Spreading Center

Schematic view of a fast ridge

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

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

Model of a slow ridge

Oceanic crust in a slow ridge

Pillow-lavas: ophiolitic pillows in the French alps Moho

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

“HOT” vs. “LOT”

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

Thermal modelling: melt fraction under fast and slow ridges

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

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