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Petrology Lecture 7

Mid-Ocean Ridge Volcanism

GLY 4310 - Spring, 2015

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MOR System

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MOR Spreading

Rates

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

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Oceanic Crust Cross-Section

Figure 13-5 Figure 13-5 Modified Modified after Brown and after Brown and Mussett (1993) Mussett (1993) The The Inaccessible Earth: An Inaccessible Earth: An Integrated View of Its Integrated View of Its Structure and Structure and Composition. Composition. Chapman & Hall. Chapman & Hall. London.London.

Layer 1

A thin layer of pelagic sediment

Oceanic Crust & Upper Mantle StructureOceanic Crust & Upper Mantle Structure

Layer 2 is basaltic

Subdivided into two sub-layers

Layer 2A & B = pillow basalts

Layer 2C = vertical sheeted dikes

Oceanic Crust & Upper Mantle StructureOceanic Crust & Upper Mantle Structure

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

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

Oceanic Crust & Oceanic Crust & Upper Mantle Upper Mantle

StructureStructure

Figure 13.4. 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 4Layer 4 = = ultramafic rocksultramafic rocks

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

WehrliteWehrlite intruded into intruded into layered gabbroslayered gabbros

Below Below cumulate cumulate dunitedunite with harzburgite xenolithswith harzburgite xenoliths

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

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Chemical Analyses of

MORB

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.

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Fenner Diagrams for

MORB Figure 13-6. Figure 13-6. “Fenner-type” “Fenner-type” variation diagrams for variation diagrams for basaltic glasses from the basaltic glasses from the Amar region of the MAR. Amar region of the MAR. Note different ordinate Note different ordinate scales. From Stakes et al. scales. From Stakes et al. (1984) J. Geophys. Res., (1984) J. Geophys. Res., 89, 6995-7028.89, 6995-7028.

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CaO/Al2O3

vs. Mg.

Figure 13-7.Figure 13-7. From Stakes et al. (1984) J. Geophys. From Stakes et al. (1984) J. Geophys. Res., 89, 6995-7028.Res., 89, 6995-7028.

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MORB Variation Diagrams

Figure 13-8. Figure 13-8. Data Data from Schilling from Schilling et al. et al. (1983) Amer. J. Sci., (1983) Amer. J. Sci., 283, 510-586.283, 510-586.

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Glass Composition:Slow vs. Fast

Spreading Ridges

Figure 13-9. Figure 13-9. Histograms of over Histograms of over 1600 glass compositions from 1600 glass compositions from slow and fast mid-ocean ridges. slow and fast mid-ocean ridges. After Sinton and Detrick (1992) After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.J. Geophys. Res., 97, 197-216.

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K2O vs. Mg for MAR MORB

Fig. 13-10 shows the variation in K2O with Mg# for the MAR data set of Schilling et al. (1983)

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REE Patterns for

MAR MORBS

Figure 13-11. Figure 13-11. Data from Schilling et al. (1983) Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586.Amer. J. Sci., 283, 510-586.

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LREE vs. Mg#

Figure 13-12. Figure 13-12. Data from Schilling et al. (1983) Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586.Amer. J. Sci., 283, 510-586.

• Blue = E-Morb

• Red = N-Morb

• Green = T-Morb

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143Nd/ 144Nd vs.

87Sr/ 86Sr

Figure 13-13. Figure 13-13. Data from Ito et al. (1987) Chemical Data from Ito et al. (1987) Chemical Geology, 62, 157-176; and LeRoex et al. (1983) J. Geology, 62, 157-176; and LeRoex et al. (1983) J. Petrol., 24, 267-318.Petrol., 24, 267-318.

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Generation of N-MORB

and E-MORB

Figure 13-14. Figure 13-14. After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, 175-195. and Wilson (1989) Igneous Petrogenesis, Kluwer.

The Axial Magma Chamber

Original Model• Semi-permanent • Fractional crystallization

derivative MORB magmas

• Periodic reinjection of fresh, primitive MORB

• Dikes upward through extending/faulting roof

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

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

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Semi-Permanent Axial Magma

Chamber

• Infinite onion model, since it resembled an infinite number of onion shells created from within and added to the walls

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Axial Magma Chamber, Fast-Spreading Ridge

Figure 13-17. After Perfit et al. (1994) Geology, 22, 375-379.

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

Figure 11.12 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall

Crystal Mush Zone

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Discontinuous Axial Magma Chamber

Figure 13-21 After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.

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Axial Magma Chamber,Slow-Spreading Ridge

Distance (km)10 105 50

2

4

6

8

De

pth

(km

)

Moho

Transitionzone

Mush

Gabbro

Rift Valley

Figure 13.22 After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216

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Oceanic Basalt

• Figure 10-16 (a) Initial 143Nd/144Nd vs. 87Sr/86Sr for oceanic basalts. From Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler et al. (1982) and Menzies (1983).

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Ultramafic Xenoliths

• Figure 10-16 (b) Initial 143Nd/144Nd vs. 87Sr/86Sr for mantle xenoliths. From Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler et al. (1982) and Menzies (1983).