G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 13 Mid-Ocean Ridge Basalts (MORB) February 22, 2016

P RESENT -D AY M ID -O CEAN R IDGES B IRTHPLACES OF MORB

S PREADING R ATES

O CEANIC C RUST AND U PPER M ANTLE S TRUCTURE EVIDENCE l Seismic Velocities l Deep Sea Drilling Program l Ophiolites l Dredging of Fracture Zone Scarps

O CEANIC C RUST AND U PPER M ANTLE S TRUCTURE Layer 3A = upper isotropic and lower, somewhat foliated (“transitional”) gabbros Layer 3B is more layered, & may exhibit cumulate textures

O CEANIC C RUST AND U PPER M ANTLE FORMATION

C OMPOSITION OF MORB Primitive Magma Fractional Crystallization

C OMPOSITIONAL V ARIABILITY IN MORB NOT RELATED TO F RACTIONAL C RYSTALLIZAITON Incompatible-rich and incompatible- poor mantle source regions for MORB magmas  N-MORB (normal MORB) taps the depleted upper mantle source  Mg# > 65: K 2 O < 0.10 TiO 2 < 1.0  E-MORB (enriched MORB) taps undepleted (deeper?) mantle  Mg# > 65: K 2 O > 0.10 TiO 2 > 1.0 FC

O RIGIN OF NMORB AND EMORB

O RIGINAL S TEADY S TATE A XIAL M AGMA C HAMBER M ODEL Hekinian et al. (1976) Contr. Min. Pet. 58, 107. Periodic replenishment from below Cooling/Crystallization from the walls inward (the infinite onion model) Diking variably fractionated magmas to surface Ultramafic cumulates at floor; layered gabbros on walls Problem – geophysics indicate no large magma body beneath the ridges

X-Sectional View Longitudinal View M ORE R EALISTIC M ODEL S MALL MELT L ENSES ABOVE M USH Z ONES WITH > 30% CRYSTAL L ARGER MELT LENSES IN F AST S PREADING CENTERS PROMOTES MORE FRACTIONATED BASALT COMPOSITIONS Winter (2001) Figure 13-15, After Perfit et al. (1994) Geology, 22, Winter (2001) Figure ; After Sinton and Detrick (1992) J. Geophys. Res., 97,

S LOW - SPREADING R IDGES Winter (2001) Figure After Sinton and Detrick (1992) J. Geophys. Res., 97, Smaller, crystal-rich, dike-like magma bodies results is less fractionation  less evolved MORB

G LOBAL V ARIABILITY OF MORB T HERMAL S TRUCTURE OF THE M ANTLE

H OT VS. C OOL M ANTLE