Chapter 13 Mid-ocean Ridge Chapter 13 Mid-ocean Ridge BasaltsBasalts

The Mid-Ocean Ridge SystemThe Mid-Ocean Ridge System

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

Ridge Segments and Spreading Ridge Segments and Spreading RatesRates

• Slow-spreading ridges: Slow-spreading ridges:

< 3 cm/a < 3 cm/a • Fast-spreading ridges:Fast-spreading ridges:

> 4 cm/a > 4 cm/a • TemporalTemporal variations are variations are

also knownalso known

Table 13-1. Spreading Rates of Some Mid-Ocean

Ridge Segments

Category Ridge Latitude Rate (cm/a)*

Fast East Pacific Rise 21-23oN 313oN 5.311oN 5.68-9oN 62oN 6.3

20-21oS 833oS 5.554oS 456oS 4.6

Slow Indian Ocean SW 1SE 3-3.7

Central 0.9

Mid-Atlantic Ridge 85oN 0.645oN 1-336oN 2.223oN 1.348oS 1.8

From Wilson (1989). Data from Hekinian (1982), Sclater et al .

(1976), Jackson and Reid (1983). *half spreading

Oceanic Crust and Upper Mantle Oceanic Crust and Upper Mantle StructureStructure

4 layers distinguished via seismic 4 layers distinguished via seismic velocitiesvelocities

Sample Sources:Sample Sources: Deep Sea Drilling ProgramDeep Sea Drilling Program Dredging of fracture zone scarpsDredging of fracture zone scarps

Ophiolites with subaerial exposureOphiolites with subaerial exposure

Oceanic Crust and Oceanic Crust and Upper Mantle Upper Mantle

Structure Structure

Typical Typical OphioliteOphiolite

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.

Wehrlite: a Peridotite mostly composed of olivine plus clinopyroxene

Layer 1Layer 1

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

Oceanic Crust and Upper Mantle Oceanic Crust and Upper Mantle StructureStructure

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 basalticbasaltic

Subdivided Subdivided into two sub-into two sub-layerslayers Layer 2A & B = Layer 2A & B =

pillow basaltspillow basalts

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

Oceanic Crust and Upper Mantle Oceanic Crust and Upper Mantle StructureStructure

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.

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

Oceanic Crust Oceanic Crust and Upper and Upper

Mantle 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.

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.

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

Layer 4Layer 4 = = ultramafic ultramafic rocksrocks

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

WehrliteWehrlite intruded into layered intruded into layered gabbrosgabbros

Below Below cumulate cumulate dunitedunite with with harzburgite xenolithsharzburgite xenoliths

The ultramafic igneous rock, harzburgite, is a variety of The ultramafic igneous rock, harzburgite, is a variety of peridotite consisting mostly of the two minerals, peridotite consisting mostly of the two minerals, olivine and low-calcium (Ca) pyroxene (enstatite)olivine and low-calcium (Ca) pyroxene (enstatite)

Below this is a Below this is a tectonite -tectonite - harzburgite and dunite: harzburgite and dunite: unmelted fraction of the unmelted fraction of the partially melted (depleted) partially melted (depleted) mantle.mantle.

MORB Petrography and Major MORB Petrography and Major Element ChemistryElement Chemistry

A “typical” MORB is an olivine A “typical” MORB is an olivine Tholeiite with low KTholeiite with low K22O (< 0.2%) O (< 0.2%)

and low TiOand low TiO22 (< 2.0%) (< 2.0%) Lab analyses use glass, which is Lab analyses use glass, which is

certain to represent certain to represent liquid. I’ll explain liquid. I’ll explain why below.why below.

The low-P crystallization sequence is: The low-P crystallization sequence is: olivine olivine (( Mg-Cr Spinel), Mg-Cr Spinel), olivine + olivine + plagioclase plagioclase (( Mg-Cr Spinel), Mg-Cr Spinel), olivine + olivine + plagioclase + clinopyroxeneplagioclase + clinopyroxene

Figure 7-2.Figure 7-2. After Bowen After Bowen (1915), A. J. Sci., and (1915), A. J. Sci., and Morse Morse (1994)(1994), Basalts and , Basalts and Phase Diagrams. Krieger Phase Diagrams. Krieger Publishers.Publishers.

Why low pressure?

In MORBS, Fe-Ti In MORBS, Fe-Ti oxidesoxides are are restricted to the restricted to the groundmass, and groundmass, and thus form thus form latelate in in the MORB the MORB sequencesequence

http://www-odp.tamu.edu/publications/176_SR/chap_08/c8_f4.htm

Ulvöspinel - TiFe2O4

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.

The major element chemistry of The major element chemistry of MORBsMORBs

Originally considered to be Originally considered to be extremely uniform, interpreted as a extremely uniform, interpreted as a simple petrogenesissimple petrogenesis

More extensive sampling has shown More extensive sampling has shown that they display a (restricted) that they display a (restricted) rangerange of compositionsof compositions

The major The major element element

chemistry of chemistry of MORBsMORBs

Table 13-2. Average Analyses and CIPW Norms of MORBs (BVTP Table 1.2.5.2)

Oxide (wt%) All MAR EPR IORSiO2 50.5 50.7 50.2 50.9

TiO2 1.56 1.49 1.77 1.19

Al2O3 15.3 15.6 14.9 15.2FeO* 10.5 9.85 11.3 10.3MgO 7.47 7.69 7.10 7.69CaO 11.5 11.4 11.4 11.8Na2O 2.62 2.66 2.66 2.32

K2O 0.16 0.17 0.16 0.14

P2O5 0.13 0.12 0.14 0.10Total 99.74 99.68 99.63 99.64

Normq 0.94 0.76 0.93 1.60or 0.95 1.0 0.95 0.83ab 22.17 22.51 22.51 19.64an 29.44 30.13 28.14 30.53di 21.62 20.84 22.5 22.38hy 17.19 17.32 16.53 18.62ol 0.0 0.0 0.0 0.0mt 4.44 4.34 4.74 3.90il 2.96 2.83 3.36 2.26ap 0.30 0.28 0.32 0.23All: Ave of glasses from Atlantic, Pacific and Indian Ocean ridges.

MAR: Ave. of MAR glasses. EPR: Ave. of EPR glasses.

IOR: Ave. of Indian Ocean ridge glasses.

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

MORBs vary a little in composition

EPR the most different

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

MORBs cannot all be primary magmas; most are derivative magmas resulting from fractional crystallization

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

Even when we compare for constant Mg# considerable variation is still apparent. Fig. 13-9 shows the variation in K2O with Mg# for the MAR data set of Schilling et al. (1983)

Recall Mg# = 100 Mg++/ Mg++ + Fe++

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 magmas that they were once considered to beto beThey 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

As early forming crystals remove As early forming crystals remove elements from the melt, new chemical elements from the melt, new chemical compositions become frequent.compositions become frequent.

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

Magmas erupted slightly off the axis of Magmas erupted slightly off the axis of ridges are more evolved than those at the ridges are more evolved than those at the axis itself.axis itself.

Magma chamber Magma chamber processes may be processes may be different at fast-different at fast-spreading ridges spreading ridges compared to slow onescompared to slow ones

Fast ridge segments Fast ridge segments (EPR) display a broader (EPR) display a broader range of compositions, range of compositions, and produce a larger and produce a larger proportion of evolved proportion of evolved liquids than do slow liquids than do slow segmentssegments

Also magmas erupted Also magmas erupted slightly off the axis of slightly off the axis of ridges are more evolved ridges are more evolved than those at the axis than those at the axis itselfitself

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

IDEAIDEA later MORBs will have less incompatibles such as LILE K+, as some were already removed by earlier MORB formation.

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.

We see We see two types two types of MORBs of MORBs with Rare with Rare Earths:Earths:

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.

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.

There are incompatible-rich and incompatible-There are incompatible-rich and incompatible-poor mantle source regions for MORB magmaspoor mantle source regions for MORB magmas

– N-MORBN-MORB ( (normalnormal MORB) taps the depleted MORB) taps the depleted upper mantle sourceupper mantle source Mg# > 65: KMg# > 65: K22O < 0.10 TiOO < 0.10 TiO22 < 1.0 < 1.0 Depleted in LREE, Low LILE e.g. K+Depleted in LREE, Low LILE e.g. K+

– E-MORBE-MORB ( (enrichedenriched MORB, also called MORB, also called P-P-MORBMORB for for plumeplume) taps the (deeper) fertile ) taps the (deeper) fertile mantlemantle Mg# > 65: KMg# > 65: K22O > 0.10 TiOO > 0.10 TiO22 > 1.0 > 1.0 Rich in LREE, higher in LILE e.g. K+Rich in LREE, higher in LILE e.g. K+

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.

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

Lack of distinct break suggests Lack of distinct break suggests threethree MORB MORB typestypes

– E-MORBsE-MORBs La/Sm > 1.8 La/Sm > 1.8– N-MORBsN-MORBs La/Sm < 0.7 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 > 0.5030, Nd > 0.5030, depleted depleted mantle sourcemantle source

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

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 multiple source regionsMORBs have multiple source regions The mantle beneath the ocean basins The mantle beneath the ocean basins

is not homogeneousis not homogeneous– 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

Idea:Idea:– T-MORBs = mixing of N- and E- magmas T-MORBs = mixing of N- and E- magmas

during ascent and/or in shallow during ascent and/or in shallow chamberschambers

MORB PetrogenesisMORB Petrogenesis Separation of the platesSeparation of the plates Upward motion of mantle Upward motion of mantle

material into extended material into extended zonezone

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 ~ 60-80 km depth in upper depleted mantle upper depleted mantle where it inherits depleted where it inherits depleted trace element and trace element and isotopic char.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.

Region of melting Region of melting Melt blobs Melt blobs separateseparate at at

about 25-35 kmabout 25-35 km

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.

• Idea: Idea:

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

• Lower Lower enriched enriched mantle mantle reservoir may reservoir may also be drawn also be drawn upward and an upward and an E-MORBE-MORB 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.

The Axial Magma Chamber The Axial Magma Chamber Original ModelOriginal Model

Semi-permanent, large Semi-permanent, large Fractional crystallizationFractional crystallization

derivative MORB derivative MORB magmas magmas

Periodic reinjectionPeriodic reinjection of of fresh, primitive MORB fresh, primitive MORB from belowfrom below

Dikes upward through Dikes upward through the extending and the extending and faulting rooffaulting roof

Differentiated melts on Differentiated melts on sidessidesFigure 13-14.Figure 13-14. From Byran and Moore (1977) From Byran and Moore (1977)

Geol. Soc. Amer. Bull., 88, 556-570.Geol. Soc. Amer. Bull., 88, 556-570.

The infinite onion

The The crystal mush zonecrystal mush zone contains perhaps 30% contains perhaps 30% melt and constitutes melt and constitutes an excellent boundary an excellent boundary layer for the layer for the in situin situ crystallization process crystallization process proposed by Langmuirproposed by Langmuir

Langmuir’s idea: Langmuir’s idea: crystallization is nearly crystallization is nearly complete along the complete along the cold wall rock, so the cold wall rock, so the liquid there is more liquid there is more evolved than in the evolved than in the interior of the interior of the chamber.chamber.

Figure 11-12Figure 11-12 From Winter From Winter (2001) An Introduction to (2001) An Introduction to Igneous and Metamorphic Igneous and Metamorphic Petrology. Prentice HallPetrology. Prentice Hall

•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.

Figure 13-15. After Perfit et al. (1994) Geology, 22, 375-379.

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.

Melt body Melt body continuous reflector up to continuous reflector up to several kilometers several kilometers alongalong the ridge crest, the ridge crest, with gaps at fracture zones, small with gaps at fracture zones, small deviations in alignment (devals) and deviations in alignment (devals) and offset spreading centers ( offset spreading centers ( OSCs ). ).

Large-scale chemical variations indicate Large-scale chemical variations indicate poor mixing along axis, and/or poor mixing along axis, and/or intermittent liquid magma lenses, each intermittent liquid magma lenses, each fed by a source conduitfed by a source conduit

Figure 13-16 After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.

Devals: subtle bends or tiny offsets less than 500 meters in size.

Sinton and Detrick (1992) Model for magma Model for magma chamber beneath a chamber beneath a slowslow-spreading -spreading ridgeridge, such as the Mid-Atlantic 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 dike-like mush zone and a smaller transition zone are beneath a mush zone and a smaller transition zone are beneath a well-developed rift valleywell-developed rift valley

– Model assumption: Most of body well below the liquidus Model assumption: Most of body well below the liquidus temperature.temperature.

– Prediction: convection and mixing is far less likely than at Prediction: convection and mixing is far less likely than at fast ridges.fast ridges.

Distance (km)10 105 50

2

4

6

8

Dep

th (

km)

Moho

Transitionzone

Mush

Gabbro

Rift Valley

Figure 13-16 After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.