Ocean basins: OIBs and MORBs
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Ocean basins: OIBs and MORBs. Figure 14-1. After Crough (1983) Ann. Rev. Earth Planet. Sci., 11, 165-193. Updates/questions. Next Wednesday review, midterm on April 2nd Lab next week Topics: Systematics for: Mid ocean ridge basalts (oceanic plate) Ocean island basalts (hotspots).

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Ocean basins: OIBs and MORBs

Figure 14-1.After Crough (1983)Ann. Rev. Earth Planet. Sci., 11, 165-193.


Updates/questions

  • Next Wednesday review, midterm on April 2nd

  • Lab next week

  • Topics:

  • Systematics for:

    • Mid ocean ridge basalts (oceanic plate)

    • Ocean island basalts (hotspots)


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.


Oceanic Crust and Upper Mantle Structure

Layer 1: sediment

Layer 2: a&b: pillows

c: sheeted dikes

Layer 3: a: transitional gabbros

b: layered gabbros -- magma chamber

Layer 4: ultramafics

-- cumulates, then mantle

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


Petrography and major element chemistry

12

10

Alkaline

8

%Na2O + K2O

6

4

2

Tholeiitic

55

65

40

60

35

45

50

%SiO2

Petrography and Major Element Chemistry


Crystallization Sequence

Constant

composition

Constant

pressure

Figure 7-2. After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams. Krieger Publishers.



Major elements

  • MgO and FeO  by olivine

  • Al2O3 and CaO  by cpx

  • SiO2  less in crystals

  • Na2O, K2O, TiO2, P2O5 all : not in crystals

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.


Trace element chemistry
Trace Element Chemistry

Figure 13-10. Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586.



Conclusions about MORBs

  • Range in MORB composition due to fractional crystallization

  • Modeling suggests ~ 60% fractional crystallization

  • MORBs have > 1 source type:

    • N-MORB

    • E-MORB

    • Transition between them (T-MORB)



Types of oib magmas

12

Ol

10

Alkaline

%Na2O + K2O

8

Opx

6

4

Tholeiitic field

Alkaline field

2

Tholeiitic

55

65

40

60

35

45

50

%SiO2

Ab

Q

Ne

Dividing line

Types of OIB Magmas


Hawaiian stages eruptive cycle
Hawaiian Stages-eruptive cycle

1. Pre-shield stage: variable alkaline/tholeiitic

2. Shield-building stage:tholeiitic, 98% of the volcano

3.Post-shield stage more alkaline, more differentiated

4. After ~1Ma break, post-erosional stage:highly

alkaline and

silica-undersaturated

magmas


Evolution in the series
Evolution in the Series

Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.


Trace elements rees
Trace Elements: REEs

Enriched

Depleted

Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.


Morb normalized spider diagrams
MORB-normalized Spider Diagrams

Figure 14-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).



Isotope geochemistry
Isotope Geochemistry

  • Isotopes do not fractionate during partial melting of fractional melting processes, so will reflect the characteristics of the source

  • OIBs only cross oceanic plate, limiting contamination (<-> continent); good estimate of mantle


Sr isotope evolution
Sr Isotope Evolution

Figure 9-13. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.


Sm-Nd: Evolution opposite to Rb - Sr

Ctl Crust (enriched) = hi 87Sr/86Sr, lo 143Nd/144Nd

147Sm143Nd

by alpha decay

half life 106 Ga

Daughter more

incompatible

  • 143Nd/144Nd = (143Nd/144Nd)o

    + (147Sm/144Nd)(elt-1)


Morb sr nd isotopes
MORB Sr - Nd Isotopes

Figure 13-12. Data from Ito et al. (1987) Chemical Geology, 62, 157-176; and LeRoex et al. (1983) J. Petrol., 24, 267-318.


MORB+OIB: Sr - Nd Isotopes

Range in compositions = mantle array

Every composition by mixing end-members:

DM -depleted

mantle

EM 1&2 -

enriched mantle

HIMU - high m

= 238U/204Pb

238U -> 206Pb

So high 206Pb/204Pb

DM

MORB

}

OIB (colors)

HIMU

BSE

EM2

EM1


A Model for Oceanic Magmatism

Continental

Reservoirs

DM

OIB

EM and HIMU from crustal sources (subducted OC + CC seds)

Figure 14-10. Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993).


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