This document provides an overview of mid-ocean ridge basalt (MORB) petrogenesis. It discusses how MORBs form at mid-ocean ridges through decompression melting of the mantle. Spreading rate influences the structure and thermal regime of the ridges, which controls the degree of melting and crustal thickness. MORBs show some geochemical variability related to the extent and pressure of mantle melting, which correlates with mantle potential temperature on a global scale. However, compared to other rock types, MORBs have relatively homogeneous major element, trace element, and radiogenic and stable isotope compositions indicative of a depleted mantle source.

1. Lecture 4: MORB petrogenesis

2. Outline
1) 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)

3. 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

4. Mid-ocean ridges
Mid-ocean ridges produce ~ 21 km3 of lava per year
~60% of the earth’s surface is covered with oceanic crust

5. Mid-ocean Ridges
Spreading rate influences thermal structure, physical
structure, crustal thickness and amount of melting

6. 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

7. 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.

8. After Perfit et al. (1994)
Geology, 22, 375-379.
A modern concept of the axial
magma chamber beneath a fast-
spreading ridge

9. 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 10
5 5
0
2
4
6
8
Depth
(km)
Moho
Transition
zone
Mush
Gabbro
Rift Valley
After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.

10. 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.

11. Rock types in the mantle
Peridotite 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

12. Ocean Crust Geology
Modern and ancient pillow basalts
Glassy pillow rinds are used to infer
original melt compositions
P. Asimow

13. 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

14. 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

15. 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, rhyolite
52-66 Intermediate 40-70 Intermediate Diorite, andesite
45-52 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)

16. “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

17. 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 12
Fe8.0
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

18. 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%
F
me
an P
me
an F
5%
10%
15%
20%
25%
F
30%
35%
40%
me
an P
me
an F
Cold mantle Hot mantle
P. Asimow

19. Spider diagram of crust vs mantle
Workman and Hart, 2005

20. 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

21. Generating enriched signatures in MORB
1) Low degrees of melting
2) Mantle source enrichment
• N-MORB: normal MORB
• T-MORB: transitional MORB
• E-MORB: enriched MORB

22. 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

23. 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

24. Craig and Lupton (1981)
He isotopes:
3He : key tracer of a primordial component
4He : 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