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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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slide1
Ocean basins: OIBs and MORBs

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

slide2
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)
slide3
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.
slide4
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
slide6
Crystallization Sequence

Constant

composition

Constant

pressure

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

slide8
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.

slide11
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.

slide21
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.

slide23
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

slide24
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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