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 8 6 4 2 35 40 45 50 55 60 65 %SiO2 %Na2O + K2O Alkaline Tholeiitic

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

The major element chemistry of MORBs

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

Trace Element Chemistry

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)

OIBs: ocean islands and seamounts

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

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

Trace Elements: REEs Enriched Depleted Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.

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

Trace Elements

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 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 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).