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Lecture 21 Mid Ocean Ridge Basalts

Friday, April 8th, 2005

The World-Wide Ocean Ridge System(65,000 km in length)

Chapter 13: Mid-Ocean Rifts

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The Mid-Ocean Ridge System

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

Ridge Segments and Spreading Rates

• Slow-spreading ridges: < 3 cm/a

• Fast-spreading ridges:> 4 cm/a

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

Category Ridge Latitude Rate (cm/a)*Fast East Pacific Rise 21-23oN 3

13oN 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 0.9Mid-Atlantic Ridge 85oN 0.6

45oN 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

Note – these are half-spreading rates

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Mid-Ocean Ridges

Echo Sounding

Mid-Atlantic Ridge

The entire Mid-Atlantic Ridge

Sampling Mid-Ocean Ridges

The basic idea behinddredging.

Launching a dredge Launching a camera

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Basalts on Mid-Ocean RidgesThese types of basaltic lavaare called “pillow lavas”

Video of pillow lavas

Basalts on Mid-Ocean RidgesThese types of basaltic lavaare called “pillow lavas”

Video of pillow lavas

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Exploring Mid-Ocean Ridgeswith the Alvin Submarine

The Glomar Challenger

View of the drill rig

Glomar Challenger

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More Glomar Challenger

View from rig

Drill bits

Drill core

Guess who!

Replacement for the Glomar Challenger

The Joides Resolution

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Age of the Sea-Floor

The age of the sea-floor estimated from magnetic anomaliesand drilling.

Oceanic Crust and Upper Mantle Structure

● 4 layers distinguished via seismic velocities● Deep Sea Drilling Program● Dredging of fracture zone scarps● Ophiolites

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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.

Layer 1

A thin layer of pelagic sediment

Oceanic Crust and Upper Mantle Structure

Figure 13-4. Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.

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Layer 2 is basaltic

Subdivided into two sub-layers

Layer 2A & B = pillow basalts

Layer 2C = vertical sheeted dikes

Oceanic Crust and Upper Mantle Structure

Figure 13-4. Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.

Layer 3 more complex and controversialBelieved 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

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

Oceanic Crust and Upper Mantle

Structure

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.

Hypothetical view of a shallow magma chamber beneath the central axis (rift valley) of the Mid-Atlantic Ridge illustrates how the layers might have been formed.

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Layer 4 = ultramafic rocks

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

Wehrlite intruded into layered gabbros

Below → cumulate dunitewith harzburgite xenoliths

Below this is harzburgite and dunite (residuum of the original mantle)

Petrography and Major Element Chemistry● A “typical” MORB is an olivine tholeiite with

characteristically low K2O (< 0.2%) and low TiO2 (< 2.0%)

● Because of its ubiquitous nature and vast erupted volumes (together with low incompatible element abundances) early studies suggested that MORB represented PRIMARY MAGMAS

● Eruptive volume about 5 to 19 km3/year (depending on assumptions)

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● 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.

Melting experiments show that there is a narrow temperature range between olivine crystallization and that of plagioclase andclinopyroxene. This means the magmas are close to multiple-saturation (cotectic or eutectic) and therefore unlikely to be primary.

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● Fe-Ti oxides are restricted to the groundmass, and thus form late in the MORB sequence

Figure 8-2. AFM diagram for Crater Lake volcanics, Oregon Cascades. Data compiled by Rick Conrey(personal communication).

MORB’s follow the typical tholeiitic Fe-enrichment trend

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

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The major element chemistry of MORBs

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 50.9TiO2 1.56 1.49 1.77 1.19Al2O3 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 2.32K2O 0.16 0.17 0.16 0.14P2O5 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.

Note the following:-

● Restricted range in MgO

● Low K2O, TiO2, P2O5

● Increase in FeO

● Decrease in CaO and Al2O3 consistent with plagioclase fractionation as well as olivine.

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.

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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 are unlikely to be primary magmas, but are derivative magmas resulting from fractional crystallization and magma mixing.

Arguments for magma mixing

• Olivine and plagioclase phenocrysts often show reverse zoning and corroded cores• Evidence of plagioclase fractionation (Eu anomalies) in lavas that are crystallizing only olivine• Evidence for clinopyroxene fractionation (lower CaO/Al2O3 ) ratios in lavas crystallizing olivine and plagioclase (the “phantom” pyroxene problem)• Elevated K2O and TiO2 beyond what could be reasonably expected from crystal fractionation

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REE patterns for glassy aphyricbasalts crystallizing ONLYolivine show distinct negative Eu anomalies implying plagioclase fractionation

Evidence of Magma Mixing

Evidence of Magma Mixing

Vectors for olivine, plagioclase and clinopyroxene fractionation.Note that only clinopyroxenefractionation can reduce the CaO/Al2O3 ratio

Glass and rock compositions clearly show chemical effects of clinopyroxene fractionation, even though cpx rarely occurs as a phenocryst. What is the explanation?

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Evidence of Magma Mixing

Curved lines show the expected increase in TiO2with fractionation of olivine, plagioclase and clinopyroxene from a hypothetical primary magma

Note that the field for MORB glasses (liquids) mimic these curves, or plot above the curves, as does the average MORB composition. Explanation – mixing of a primary magma with a more evolved magma!

Explanation of Magma Mixing1. Primary magma (P) crystallizes olivine

until it hits the Ol-Plag cotectic2. Crystallization continues to the

eutectic (E) where Cpx joins Ol and Plag

3. Recharge of a new pulse of primary magma (P) mixes with the evolved magma (E) to produce mixed magma (M)

4. Magma M crystallizes olivine moving to the Ol- Plag cotectic

5. The process is repeated

Periodic replenishment and mixing of primary magmas with more evolved magmas is an important process at mid-ocean ridges and accounts for much of the chemical characteristics of MORB