1. Chapter 13 Mid-ocean Ridge Basalts

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

3. Ridge Segments and Spreading Rates
Slow-spreading ridges:
< 3 cm/a
Fast-spreading ridges:
> 4 cm/a
Temporal variations are also known

4. Oceanic Crust and Upper Mantle Structure
4 layers distinguished via seismic velocities
Sample Sources:
Deep Sea Drilling Program
Dredging of fracture zone scarps
Ophiolites with subaerial exposure
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

5. Oceanic Crust and Upper Mantle Structure
Typical Ophiolite
Wehrlite: a Peridotite mostly composed of olivine plus clinopyroxene
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,

6. 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.

7. 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.

8. 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
Tonalite is an igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic texture. Similar to Granite except Feldspar is mostly present as plagioclase, with less than 10% alkali feldspar.
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,

9. 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

10. 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
The ultramafic igneous rock, harzburgite, is a variety of peridotite consisting mostly of the two minerals, olivine and low-calcium (Ca) pyroxene (enstatite)
Below this is a tectonite - harzburgite and dunite: unmelted fraction of the partially melted (depleted) 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

11. MORB Petrography and Major Element Chemistry
A “typical” MORB is an olivine Tholeiite with low K2O (< 0.2%)
and low TiO2 (< 2.0%)
Lab analyses use glass, which is certain to represent liquid. I’ll explain why below.
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

12. The low-P crystallization sequence is: olivine ( Mg-Cr Spinel), olivine + plagioclase ( Mg-Cr Spinel), olivine + plagioclase + clinopyroxene
Why low pressure?
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.

13. In MORBS, Fe-Ti oxides are restricted to the groundmass, and thus form late in the MORB sequence
(h)
Hence the early Fe-enrichment characteristic of the tholeiite trend on an ACF diagram – the iron doesn’t precipitate out until late, so it becomes relatively more abundant in early glass as Mg++ is used up.
Ulvöspinel - TiFe2O4

14. 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

15. The major element chemistry of MORBs
MAR : Mid-Atlantic Ridge
EPR : East-Pacific Rise
IOR: Indian Ocean Ridge
Normative minerals: q Quartz, or Orthoclase, ab Albite, an Anorthite, di Diopside, hy Hyperthene, ol Olivine, mt Magnetite, il Ilmenite, ap Apatite
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)
MORBs vary a little in composition
EPR the most different

16. MORBs cannot all be primary magmas; most are derivative magmas resulting from fractional crystallization
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 of the MAR. Note different ordinate scales. From Stakes et al. (1984) J. Geophys. Res., 89,

17. Recall Mg# = 100 Mg++/ Mg++ + Fe++
Even when we compare for constant Mg# considerable variation is still apparent. Fig shows the variation in K2O with Mg# for the MAR data set of Schilling et al. (1983)
Figure Data from Schilling et al. (1983) Amer. J. Sci., 283,
Recall Mg# = 100 Mg++/ Mg++ + Fe++

18. Conclusions about MORBs, and the processes beneath mid-ocean ridges
MORBs are not the completely uniform magmas that they were once considered to be
They show chemical trends consistent with fractional crystallization of olivine, plagioclase, and perhaps clinopyroxene
As early forming crystals remove elements from the melt, new chemical compositions become frequent.

19. Magma chamber processes may be different at fast-spreading ridges compared to slow ones
Fast ridge segments (EPR) display a broader range of compositions, and produce a larger proportion of evolved liquids than do slow segments
Also magmas erupted slightly off the axis of ridges are more evolved than those at the axis itself
Fast ridge segments (EPR) ® a broader range of compositions and a larger proportion of evolved liquids
Magmas erupted slightly off the axis of ridges are more evolved than those at the axis itself.

20. Depleted mantle is the residue that remains after a given element has been removed from Peridotite to form a basaltic melt. The incompatible elements (e.g. K, Sr, Rb, U, and rare-earth elements) are preferentially partitioned into a melt, and during ocean crustal formation these elements in particular have been removed from the mantle, leaving the mantle depleted in incompatibles.
Incompatibles present in a MORB melt generally solidify in late fractionation minerals derived from the basaltic melt. WE SHOULD USE ANALYSES OF GLASS (no crystal structure) at any stage if we want melt compositions.
The depleted mantle can still partially melt and form MORBs, all you need is low pressure
IDEA later MORBs will have less incompatibles such as LILE K+, as some were already removed by earlier MORB formation.

21. An incompatible element is an element that is unsuitable in size and/or charge to fit in the cation sites of the possible minerals. Elements that have difficulty in entering cation sites of the early high temperature crystallization minerals (Olivine, Ca-Plagioclase, Pyroxenes) are concentrated in the melt phase of magma (liquid phase), and remain there until late in the solidification of the magma.
Another way to classify incompatible elements is by mass: light rare earth elements are La - Sm, and heavy rare earth elements (HREE) are Eu - Lu. Rocks or magmas rich in light rare earth elements (LREE) are referred to as fertile, and those with strong depletions in LREE are referred to as depleted.

22. We see two types of MORBs with Rare Earths:
An incompatible element is an element that is unsuitable in size and/or charge to fit in the cation sites of the possible minerals. Elements that have difficulty in entering cation sites of the minerals are concentrated in the melt phase of magma (liquid phase). Another way to classify incompatible elements is by mass: light rare earth elements are La - Sm, and heavy rare earth elements (HREE) are Eu - Lu. Rocks or magmas rich, or only slightly depleted in light rare earth elements (LREE) are referred to as fertile, and those with strong depletions in LREE are referred to as depleted.
REE diagram for MORBs
We see two types of MORBs with Rare Earths:
Figure Data from Schilling et al. (1983) Amer. J. Sci., 283,

23. N-MORB (normal MORB) taps the depleted upper mantle source
There are incompatible-rich and incompatible-poor mantle source regions for MORB magmas
N-MORB (normal MORB) taps the depleted upper mantle source
Mg# > 65: K2O < TiO2 < 1.0
Depleted in LREE, Low LILE e.g. K+
E-MORB (enriched MORB, also called P-MORB for plume) taps the (deeper) fertile mantle
Mg# > 65: K2O > TiO2 > 1.0
Rich in LREE, higher in LILE e.g. K+
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
An incompatible element is an element that is unsuitable in size and/or charge to fit in the cation sites of the possible minerals. Elements that have difficulty in entering cation sites of the minerals are concentrated in the melt phase of magma (liquid phase). Another way to classify incompatible elements is by mass: light rare earth elements are La - Sm, and heavy rare earth elements (HREE) are Eu - Lu. Rocks or magmas rich, or only slightly depleted in light rare earth elements (LREE) are referred to as fertile, and those with strong depletions in LREE are referred to as depleted.

24. Lack of distinct break suggests three MORB types
E-MORBs (squares) enriched in LREE 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

25. 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

26. MORBs have multiple source regions
Conclusions:
MORBs have multiple source regions
The mantle beneath the ocean basins is not homogeneous
N-MORBs tap an upper, depleted mantle
E-MORBs tap a deeper enriched source
Idea:
T-MORBs = mixing of N- and E- magmas during ascent and/or in shallow chambers
The time required for the isotopic systems to develop suggests that these reservoirs have been distinct for a very long time

27. MORB Petrogenesis Generation Separation of the plates
Upward motion of mantle material into extended zone
Decompression partial melting associated with near- adiabatic rise
N-MORB melting initiated ~ km depth in upper depleted mantle where it inherits depleted trace element and isotopic char.
Continue with: % partial melting increases to ~ 15-40% as diapirs of melting mantle rise toward the surface
% melted when reaches top depends on the source depth, temperature, and the rate of rise (and spreading)
Melting is terminated by conductive heat loss to the surface near the top of the column, perhaps aided by the consumption of clinopyroxene, which, when gone, will create a discontinuous temperature jump in melting
Figure After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, and Wilson (1989) Igneous Petrogenesis, Kluwer.

28. Generation Region of melting Melt blobs separate at about 25-35 km
The region of melting is probably ~ 100 km wide, but is focused into the 3-8 km wide zone beneath the ridges
Melt blobs separate at about km where they are last in equilibrium with harzburgite residuum, and migrate to a depth of 1-2 km immediately beneath the ridge axis -> axial magma chamber
Figure After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, and Wilson (1989) Igneous Petrogenesis, Kluwer.

29. Idea:
Convective flow that caused the divergence at MOR runs to Boundary Layer.
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.

30. The Axial Magma Chamber
Original Model
Semi-permanent, large
Fractional crystallization ® derivative MORB magmas
Periodic reinjection of fresh, primitive MORB from below
Dikes upward through the extending and faulting roof
Differentiated melts on sides
Relatively large (~ 5 km wide and 9 km deep)
The infinite onion
Figure From Byran and Moore (1977) Geol. Soc. Amer. Bull., 88,

31. The crystal mush zone contains perhaps 30% melt and constitutes an excellent boundary layer for the in situ crystallization process proposed by Langmuir
Langmuir’s idea: crystallization is nearly complete along the cold wall rock, so the liquid there is more evolved than in the interior of the chamber.
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 formationc
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

32. Lens maintained by reinjection.
Recent seismic work has failed to detect any chambers of this size at ridges.
Modern View: 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
Transition zone transmits shear waves, but may still have a minor amount of melt)
“Magma chamber” = melt + mush zone (the liquid portion is continuous through them)
Lens maintained by reinjection.

33. A modern concept of the axial magma chamber beneath a FAST ridge
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
Transition zone transmits shear waves, but may still have a minor amount of melt)
“Magma chamber” = melt + mush zone (the liquid portion is continuous through them)
Lens maintained by reinjection.
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)
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”
A modern concept of the axial magma chamber beneath a FAST ridge
Figure After Perfit et al. (1994) Geology, 22,

34. Melt body ® continuous reflector up to several kilometers along the ridge crest, with gaps at fracture zones, small deviations in alignment (devals) and offset spreading centers ( 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,
Devals: subtle bends or tiny offsets less than 500 meters in size.

35. Sinton and Detrick (1992) Model for magma chamber beneath a slow-spreading ridge, such as the Mid-Atlantic Ridge
Model: With a reduced heat and magma supply, a steady-state eruptible melt lens is absent. Instead a dike-like mush zone and a smaller transition zone are beneath a well-developed rift valley
Model assumption: Most of body well below the liquidus temperature.
Prediction: 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,