Lecture 4: MORB petrogenesis

Outline1) Overview of igneous petrogenesis

2) Mid-Ocean Ridges – how are they characterized?

3) MORB – where and how do they form?

4) Geochemical variations in MORB (major elements, trace elements and isotopic characteristics)

Igneous Petrogenesis

1. Mid-ocean ridges

2. Continental rifts

3. Island Arcs

4. Active continental margins

5. Back-arc basins

6. Ocean Islands

7. Intraplate hotspot activity, carbonatites, or kimberlites

Mid-ocean ridgesMid-ocean ridges produce ~ 21 km3 of lava per year

~60% of the earth’s surface is covered with oceanic crust

Mid-ocean Ridges

Spreading rate influences thermal structure, physical structure, crustal thickness and amount of melting

-2

0

2

4

6

8

10

0 2 4 6 8 10Vp (km/s)

Water

Moho

Mantle = altered peridotite

Layer 3 = gabbro

Layer 2 = extrusives

Layer 2a = dikes

Layer 1 = sediment

Spreading rate and structure

Slow-spreading Mid-Atlantic Ridge

Fast-spreading East Pacific Rise• Thermal structure is warmer

• Crust is thicker, lithosphere is thinner

• Higher degrees of melting

• Sustained magma chambers and volcanism

• Less compositional diversity

• Thermal structure is cooler

• Crust is thinner, lithosphere is thicker

• lower degrees of melting

• Episodic volcanism

• Higher compositional diversity

The Axial Magma Chamber: original model

• Semi-permanent

• MORB magmas are produced by fractional crystallization within the chamber

• Periodic reinjection of fresh, primitive MORB

• Dikes upward through extending/faulting roof

• Crystallization at top and sides successive layers of gabbro (layer 3) “infinite onion”

• Dense olivine and pyroxene crystals ultramafic cumulates (layer 4)

• Moho?? Seismic vs. Petrologic

Figure 13.16. From Byran and Moore (1977) Geol. Soc. Amer. Bull., 88, 556-570.

Hekinian et al. (1976)Contr. Min. Pet. 58, 107.

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

A modern concept of the axial magma chamber beneath a

fast-spreading ridge

Model for magma chamber beneath a slow-spreading ridge, such as the Mid-Atlantic Ridge

• Most of body well below the liquidus temperature, so convection and mixing is far less likely than at fast ridges

• numerous, small, ephemeral magma bodies occur at slow ridges• Slow ridges are generally less differentiated than fast ridges - no continuous

liquid lenses, so magmas entering the axial area are more likely to erupt directly to the surface

Distance (km)10 105 50

2

4

6

8

Dep

th (

km)

Moho

Transitionzone

Mush

Gabbro

Rift Valley

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

Oceanic Crust and Upper Mantle Structure

1) Geophysical studies

2) Mantle xenoliths

3) Ophiolites: uplifted oceanic crust + upper mantle

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.

Rock types in the mantlePeridotite is the dominant rock type of the Earth’s upper mantle• Lherzolite: fertile unaltered mantle; mostly composed of olivine,

orthopyroxene (commonly enstatite), and clinopyroxene (diopside), and have relatively high proportions of basaltic ingredients (garnet and clinopyroxene).

• Dunite (mostly olivine) and Harzburgite (olivine + orthopyroxene) are refractory residuum after basalt has been extracted by partial melting

• Wehrlite: mostly composed of olivine plus clinopyroxene.

wehrlite lherzolite

Ocean Crust Geology

Modern and ancient pillow basalts

Glassy pillow rinds are used to infer original melt compositions

P. Asimow

Magma: mixture of molten rock, gases and mineral phases, produced by mantle melting

Mantle melts between ~800-1250ºC due to:

1) Increase in temperature

2) Decrease in pressure

3) Addition of volatile phases

Adiabatic rise of mantle material with no heat loss – decompression melting

Mid-Ocean Ridges

Partial melting

A model for mantle melting• Several models are possible of how and where the melt is extracted

and what happens to it during transport• This average melt is primary mid-ocean ridge basalt (MORB).• Hot mantle starts melting at deeper depths, thus has a larger melt

triangle or area over which melting occurs than a cooler mantle • Mantle rising nearer axis of plume traverses greater portion of

triangle and thus melts more extensively

Hot mantle cool mantle

Asimow et al., 2004

Igneous rock classification by composition• There are several classifications, of individual rocks or rock suites.• By silica percentage:%SiO2 Designation % Dark Minerals Designation Example rocks>66 Acid <40 Felsic Granite, rhyolite52-66 Intermediate 40-70 Intermediate Diorite, andesite45-52 Basic 70-90 Mafic Gabbro, basalt<45 Ultrabasic >90 Ultramafic Dunite, komatiite

The common crystallization sequence at mid-ocean ridges is: olivine ( Mg-Cr spinel), olivine + plagioclase ( Mg-Cr spinel), olivine + plagioclase + clinopyroxene

After Bowen (1915), A. J. Sci., and Morse (1994)

(plagioclase)

(olivine)(clinopyroxene)

“Fenner-type” variation diagrams for basaltic glasses from the Afar region of the MAR. From Stakes et al. (1984)

The major element chemistry of MORBs

• MORBs are the product of fractional crystallization, melt aggregation, seawater interaction and crustal contamination

• MgO contents are a good index for fractional crystallization (typically, more primitive melts have higher MgO)

• Data is often “corrected” back to 8 wt% MgO to estimate primary melt compositions and to compare data sets

Increased fractional crystallization

Global systematics• The values of regionally-averaged Na8 (i.e., Na2O concentration

corrected to 8% MgO), Fe8, water depth above the ridge axis, and crustal thickness show significant global correlations.

– Where Na8 is high, Fe8 is low

– Where Na8 is high, the ridges are deep

– Where Na8 is high, the crust is thin

1.5

2.5

3.5

2.0

3.0

6 7 8 9 10 11 12Fe8.0

Na8.0

Deep ridges

Shallow ridges

Na8 is an incompatible element, thus an indicator of mean extent of melting.

Fe8 is an indicator of mean pressure of melting.

Axial depth is an indicator of mantle temperature, extent of melting, and crustal thickness combined – see slide #5

Synthesis of global systematics• The global correlation implies that extent of melting and pressure of

melting are positively correlated, on a global scale. This relates to the mantle potential temperature.

• If melting continues under the axis to the base of the crust everywhere, then high potential temperature means: long melting column high mean extent of melting low Na8 and high crustal thickness shallow axial depth; high mean pressure of melting high Fe8. Cold mantle yields the opposite.

sea level

crust

axial depth

solidus 1.5 GPa

solidus 4.5 GPa

5%10%15%20%25%

Fmean P

mean F

5%

10%15%20%25%

F30%

35%

40%

mean Pmean F

Cold mantle Hot mantle

P. Asimow

Spider diagram of crust vs mantle

Workman and Hart, 2005

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

A modern concept of the axial magma chamber beneath a fast-

spreading ridge

Generating enriched signatures in MORB1) Low degrees of melting

2) Mantle source enrichment

• N-MORB: normal MORB• T-MORB: transitional MORB• E-MORB: enriched MORB

Isotope systematics of MORB

Radiogenic isotope systems (Sr, Nd, Pb) are used to see mantle enrichments due to relative compatibilities of radiogenic parents and daughters

e.g., 87Rb 87Sr, Rb is more incompatible than Sr so high 87Sr/86Sr ratios indicate an enriched source

Compared to ocean islands and subduction zones, MORBs are relatively homogeneous

Stable isotopes

• Like radiogenic isotopes, stable isotope can be used to trace source enrichments and are not influenced by degrees of melting

• Oxygen, boron, helium and nitrogen isotopes show very little variability in MORB, and are distinct from enriched OIB and subduction related lavas

Macpherson et al., 2000

Manus

Craig and Lupton (1981)Craig and Lupton (1981)

He isotopes:3He : key tracer of a primordial component4He : representing a radiogenic component (U+Th decay)

3He anomalies at ridges is evidence for degassing of primordial gases from the earth

Typical 3He/4He ratios:

Crust : 0.01-0.05 RA

MORB : 8 ± 1 RA

Arcs: 5 - 8 RA

Hotspots: up to 37RA