Chapter 13: MidOcean Rifts

1
Chapter 13 Mid-Ocean Rifts

• The Mid-Ocean Ridge System

Figure 13-1. After Minster et al. (1974) Geophys.
J. Roy. Astr. Soc., 36, 541-576.
2
2 principal types of basalt in the ocean basins
Tholeiitic Basalt and Alkaline Basalt
Common petrographic differences between
tholeiitic and alkaline basalts
Table 10-1
Tholeiitic Basalt
Alkaline Basalt
Usually fine-grained, intergranular
Usually fairly coarse, intergranular to ophitic
Groundmass
No olivine
Olivine common
Clinopyroxene augite (plus possibly pigeonite)
Titaniferous augite (reddish)
Orthopyroxene (hypersthene) common, may rim ol.
Orthopyroxene absent
No alkali feldspar
Interstitial alkali feldspar or feldspathoid may
occur
Interstitial glass and/or quartz common
Interstitial glass rare, and quartz absent
Olivine rare, unzoned, and may be partially
resorbed
Olivine common and zoned
Phenocrysts
or show reaction rims of orthopyroxene
Orthopyroxene uncommon
Orthopyroxene absent
Early plagioclase common
Plagioclase less common, and later in sequence
Clinopyroxene is pale brown augite
Clinopyroxene is titaniferous augite, reddish rims
after Hughes (1982) and McBirney (1993).
3
Thin section textures of basalts
4
Pressure Effects

• Raises melting point
• Shift eutectic position
• (and thus X of first melt, etc.)

Figure 6-15. The system Fo-SiO2 at atmospheric
pressure and 1.2 GPa. After Bowen and Schairer
(1935), Am. J. Sci., Chen and Presnall (1975)
Am. Min.
5
Ridge Segments and Spreading Rates

• Slow-spreading ridges
• lt 3 cm/a
• Fast-spreading ridges
• gt 4 cm/a are considered
• Temporal variations are also known

6
Oceanic Crust and Upper Mantle Structure
4 layers distinguished via seismic
velocities Deep Sea Drilling Program Dredging Ophi
olites
7
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.
8
Oceanic Crust and Upper Mantle Structure

• Layer 1 A thin layer of pelagic sediment

Figure 13-4. Modified after Brown and Mussett
(1993) The Inaccessible Earth An Integrated View
of Its Structure and Composition. Chapman Hall.
London.
9
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
Figure 13-4. Modified after Brown and Mussett
(1993) The Inaccessible Earth An Integrated View
of Its Structure and Composition. Chapman Hall.
London.
10
Layer 3 more complex and controversialBelieved
to be mostly gabbros, crystallized from a shallow
axial magma chamber (feeds the dikes and basalts)
Layer 3A somewhat foliated (transitional)
gabbros Layer 3B is more layered, may exhibit
cumulate textures
11
Layer 4 ultramafic rocks
layered cumulate wehrlite, dunite, harzburgite
12
Oceanic lithosphere

• Note seds get thicker away from ridge

13

• Heat flow drops away from ridge
• How?
• Convecting seawater through fractured basalt
• Process decreases away from ridge

14

• Seafloor gets deeper away from ridge
• Why?
• Crust cools, contracts, more dense, sinks

15
Petrography

• A typical MORB is an olivine tholeiite
• Only glass is certain to represent liquid
  compositions (found in rinds of pillows)
• Pillow rinds with forsterite, little plagioclase
  and rare cpx

16
The common crystallization sequence is olivine
(? Mg-Cr spinel), olivine plagioclase (? Mg-Cr
spinel), olivine plagioclase clinopyroxene
Figure 7-2. After Bowen (1915), A. J. Sci., and
Morse (1994), Basalts and Phase Diagrams. Krieger
Publishers.
17
Major element chemistry

• Uniform
• SiO2 47-51
• Low concentration of incompatibles (Ti, P)
• Phenocrysts olivine, plag, cpx
• Anything taking up Fe?
• Trend on AFM diagram?

18
Fe-Ti oxides are restricted to the groundmass,
and thus form late in the MORB sequence Plag,
cpx, Fe-Ti oxides, no olivine in groundmass
Figure 8-2. AFM diagram for Crater Lake
volcanics, Oregon Cascades. Data compiled by Rick
Conrey (personal communication).
19

• The major element chemistry of MORBs

20

• MgO and FeO
• Al2O3 and CaO
• SiO2
• Na2O, K2O, TiO2, P2O5

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.
21

• 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
• MORBs cannot be primary magmas, but are
  derivative magmas resulting from fractional
  crystallization ( 60) (too low MgO)

22

• 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

23
Trace Element and Isotope Chemistry

• REE diagram for MORBs

Figure 13-10. Data from Schilling et al. (1983)
Amer. J. Sci., 283, 510-586.
24
Incompatible-rich and incompatible-poor mantle
source regions for MORB magmas N-MORB (normal
MORB) taps the depleted upper mantle
source E-MORB (enriched MORB) taps the (deeper)
fertile mantle
25

• Evidence of extraction of magmas from upper
  mantle peridotite
• More melting episodes, more depleted mantle gets

26

• Conclusions
• MORBs have gt 1 source region
• The mantle beneath the ocean basins is not
  homogeneous
• N-MORBs tap an upper, depleted mantle
• E-MORBs tap a deeper enriched source

27
Mid Atlantic Ridge

• Slow spreading ridge
• Small magma production
• Lower T
• More narrow zone of melt

28
Model for magma chamber beneath a slow-spreading
ridge, such as the Mid-Atlantic Ridge Dike-like
mush zone and a smaller transition zone beneath
well-developed rift valley Most of body well
below the liquidus temperature, so convection and
mixing is far less likely than at fast ridges
Figure 13-16 After Sinton and Detrick (1992) J.
Geophys. Res., 97, 197-216.
29
Nisbit and Fowler (1978) suggested that numerous,
small, ephemeral magma bodies occur at slow
ridges (infinite leek) 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 (hence more primitive), with some
mixing of mush
Figure 13-16 After Sinton and Detrick (1992) J.
Geophys. Res., 97, 197-216.
30
East Pacific Rise

• 1-2 melt in dotted region below ridge
• Fast spreading ridge
• shallow zone of melt
• Broad zone of melt
• 100 kms wide
• 150 km deep

31
Iceland

• On Mid Atlantic Ridge
• Surface version of submarine ridge
• Lots more magma production than most rifts
• Unusual hot mantle below

32
Iceland plume

• 300 km wide
• 410 km deep
• Very deep source
• Ascending plume begins melting deeper
• Generates more melt
• Builds thicker crust

33
Iceland mantle melting

• Hotter rising mantle
• Hits solidus earlier
• More melt generated

34
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 60-80 km depth in
  upper depleted mantle where it inherits depleted
  trace element and isotopic char.

Figure 13-13. After Zindler et al. (1984) Earth
Planet. Sci. Lett., 70, 175-195. and Wilson
(1989) Igneous Petrogenesis, Kluwer.
35
Generation
Region of melting Melt blobs separate at about
25-35 km
Figure 13-13. After Zindler et al. (1984) Earth
Planet. Sci. Lett., 70, 175-195. and Wilson
(1989) Igneous Petrogenesis, Kluwer.
36
Lower enriched mantle reservoir may also be drawn
upward and an E-MORB plume initiated
Figure 13-13. After Zindler et al. (1984) Earth
Planet. Sci. Lett., 70, 175-195. and Wilson
(1989) Igneous Petrogenesis, Kluwer.
37

• Rising magma fills chamber
• Phenocrysts form
• Episodically replenished with new magma, mixes
  with more evolved magma

38
Source of mid-ocean ridge basalts