1. Ocean basins: OIBs and MORBs
Figure After Crough (1983) Ann. Rev. Earth Planet. Sci., 11,

2. 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)

3. Figure 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,

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

5. Petrography and Major Element Chemistry12 10 8 6 4 2 35 40 45 50 55 60 65
%SiO2
%Na2O + K2O
Alkaline
Tholeiitic

6. Crystallization Sequence
Constant
composition
Constant
pressure
Figure 7-2. After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams. Krieger Publishers.

7. The major element chemistry of MORBs

8. 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 “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,

9. Trace Element Chemistry
Figure Data from Schilling et al. (1983) Amer. J. Sci., 283,

10. Trace Element Chemistry

11. 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)

12. OIBs: ocean islands and seamounts

13. Types of OIB Magmas Alkaline Tholeiitic Ol Opx Ne Ab Q
Alkaline field
Tholeiitic field
Dividing line 12 10 8 6 4 2 35 40 45 50 55 60 65
%SiO2
%Na2O + K2O
Alkaline
Tholeiitic

14. 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

15. Evolution in the Series
Figure After Wilson (1989) Igneous Petrogenesis. Kluwer.

16. Trace Elements: REEs Enriched Depleted
Figure After Wilson (1989) Igneous Petrogenesis. Kluwer.

17. MORB-normalized Spider Diagrams
Figure Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

18. Trace Elements

19. 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

20. Sr Isotope Evolution
Figure After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.

21. 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)

22. MORB Sr - Nd Isotopes
Figure Data from Ito et al. (1987) Chemical Geology, 62, ; and LeRoex et al. (1983) J. Petrol., 24,

23. } 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

24. A Model for Oceanic Magmatism
Continental
Reservoirs DM
OIB
EM and HIMU from crustal sources (subducted OC + CC seds)
Figure Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993).