Petrology Lecture 7 MidOcean Ridge Volcanism GLY 4310

Petrology Lecture 7 Mid-Ocean Ridge Volcanism GLY 4310 - Spring, 2012 1

MOR System 2

MOR Spreading Rates 3

Oceanic Crust Cross-Section Figure 13 -5 Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London. 4

Oceanic Crust & Upper Mantle Structure Layer 1 A thin layer of pelagic sediment

Oceanic Crust & Upper Mantle Structure Layer 2 is basaltic Subdivided into two sub-layers Layer 2 A & B = pillow basalts Layer 2 C = vertical sheeted dikes

Layer 3 more complex and controversial Believed to be mostly gabbros, crystallized from a shallow axial magma chamber (feeds the dikes and basalts) Layer 3 A = upper isotropic and lower, somewhat foliated (“transitional”) gabbros Layer 3 B is more layered, & may exhibit cumulate textures

Oceanic Crust & Upper Mantle Structure Discontinuous diorite and tonalite (“plagiogranite”) bodies = late differentiated liquids Figure 13. 4. 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.

Layer 4 = ultramafic rocks Ophiolites: base of 3 B grades into layered cumulate wehrlite & gabbro Wehrlite intruded into layered gabbros Below cumulate dunite with harzburgite xenoliths Below this is a tectonite harzburgite and dunite (unmelted residuum of the original mantle)

Chemical Analyses of MORB 10

Fenner Diagrams for MORB Figure 13 -6. “Fenner-type” variation diagrams for basaltic glasses from the Amar region of the MAR. Note different ordinate scales. From Stakes et al. (1984) J. Geophys. Res. , 89, 6995 -7028. 11

Ca. O/Al 2 O 3 vs. Mg. Figure 13 -7. From Stakes et al. (1984) J. Geophys. Res. , 89, 6995 -7028. 12

MORB Variation Diagrams Figure 13 -8. Data from Schilling et al. (1983) Amer. J. Sci. , 283, 510 -586. 13

Glass Composition: Slow vs. Fast Spreading Ridges Figure 13 -9. Histograms of over 1600 glass compositions from slow and fast mid-ocean ridges. After Sinton and Detrick (1992) J. Geophys. Res. , 97, 197 -216. 14

K 2 O vs. Mg for MAR MORB Fig. 13 -10 shows the variation in K 2 O with Mg# for the MAR data set of Schilling et al. (1983) 15

REE Patterns for MAR MORBS Figure 13 -11. Data from Schilling et al. (1983) Amer. J. Sci. , 283, 510 -586. 16

LREE vs. Mg# • Blue = E-Morb • Red = N-Morb • Green = T-Morb Figure 13 -12. Data from Schilling et al. (1983) Amer. J. Sci. , 283, 510 -586. 17

143 Nd/ 144 Nd vs. 87 Sr/ 86 Sr Figure 13 -13. Data from Ito et al. (1987) Chemical Geology, 62, 157 -176; and Le. Roex et al. (1983) J. Petrol. , 24, 267 -318. 18

Generation of N-MORB and EMORB Figure 13 -14. After Zindler et al. (1984) Earth Planet. Sci. Lett. , 70, 175 -195. and Wilson (1989) Igneous Petrogenesis, 19 Kluwer.

The Axial Magma Chamber Original Model • Semi-permanent • Fractional crystallization derivative MORB magmas • Periodic reinjection of fresh, primitive MORB • Dikes upward through extending/faulting roof Figure 13. 16. From Byran and Moore (1977) Geol. Soc. Amer. Bull. , 88, 556 -570. Hekinian et al. (1976) Contr. Min. Pet. 58, 107.

Semi-Permanent Axial Magma Chamber • Infinite onion model, since it resembled an infinite number of onion shells created from within and added to the walls 21

Axial Magma Chamber, Fast-Spreading Ridge Figure 1317. After Perfit et al. (1994) Geology, 22, 375 -379. 22

Crystal Mush Zone The crystal mush zone contains perhaps 30% melt and constitutes an excellent boundary layer for the in situ crystallization process proposed by Langmuir Figure 11. 12 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall

Discontinuous Axial Magma Chamber Figure 13 -21 After Sinton and Detrick (1992) J. Geophys. Res. , 97, 197 -216. 24

Axial Magma Chamber, Slow-Spreading Ridge Depth (km) 2 Rift Valley 4 6 Moho Transition zone Gabbro Mush 8 10 5 0 Distance (km) 5 10 Figure 13. 22 After Sinton and Detrick (1992) J. Geophys. Res. , 97, 197 -216 25

Oceanic Basalt • Figure 10 -16 (a) Initial 143 Nd/144 Nd vs. 87 Sr/86 Sr for oceanic basalts. From Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler et al. (1982) and Menzies 26 (1983).

Ultramafic Xenoliths • Figure 10 -16 (b) Initial 143 Nd/144 Nd vs. 87 Sr/86 Sr for mantle xenoliths. From Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler et 27 al. (1982) and Menzies (1983).