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petrology lecture 7

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petrology lecture 7

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    1: 1 Petrology Lecture 7 Mid-Ocean Ridge Volcanism GLY 4310 - Spring, 2010

    2: 2

    3: 3 MOR Spreading Rates

    4: 4 Oceanic Crust Cross-Section

    5: Layer 1 A thin layer of pelagic sediment Absent on newly generated crust at ridge axes, and thickens away from itAbsent on newly generated crust at ridge axes, and thickens away from it

    Slide 6:Layer 2 A ? B with fracture in-filling by mineral deposition (some call both A, and then call 2B sheeted dikes) Layer 2C = vertical sheeted dikes emplaced in the shallow brittle extensional environment at the ridge axis Many dikes have only a single chill margin ? later dikes split and intruded earlier ones Layer 2 A ? B with fracture in-filling by mineral deposition (some call both A, and then call 2B sheeted dikes) Layer 2C = vertical sheeted dikes emplaced in the shallow brittle extensional environment at the ridge axis Many dikes have only a single chill margin ? later dikes split and intruded earlier ones

    Slide 7:The layering may be horizontal, but more commonly dips at angles locally up to 90oThe layering may be horizontal, but more commonly dips at angles locally up to 90o

    Slide 8:At the top of the gabbros in the Oman are small discontinuous diorite and tonalite (“plagiogranite”) bodies = late differentiated liquids filter pressed and mobilized along the gabbro-sheeted dike contact, and may extend up into the pillow layer At the top of the gabbros in the Oman are small discontinuous diorite and tonalite (“plagiogranite”) bodies = late differentiated liquids filter pressed and mobilized along the gabbro-sheeted dike contact, and may extend up into the pillow layer

    Slide 9:The boundary between layers 3 and 4 is, broadly speaking, the Moho Upper portion of layer 4 is thought to be layered, and of cumulate origin (olivine and pyroxenes sink to the bottom of the axial magma chamber) Below this is the original, unlayered, residual mantle material Exactly what is the crust/mantle boundary? 1) Top of the original mantle 2) Mafic/ultramafic transition (top of the added ultramafic cumulates) Is the mantle defined by petrogenesis or by composition? A number of authors distinguish a seismic Moho from a petrological Moho The boundary between layers 3 and 4 is, broadly speaking, the Moho Upper portion of layer 4 is thought to be layered, and of cumulate origin (olivine and pyroxenes sink to the bottom of the axial magma chamber) Below this is the original, unlayered, residual mantle material Exactly what is the crust/mantle boundary? 1) Top of the original mantle 2) Mafic/ultramafic transition (top of the added ultramafic cumulates) Is the mantle defined by petrogenesis or by composition? A number of authors distinguish a seismic Moho from a petrological Moho

    10: 10 Chemical Analyses of MORB

    11: 11 Fenner Diagrams for MORB

    12: 12 CaO/Al2O3 vs. Mg.

    13: 13 MORB Variation Diagrams

    14: 14 Glass Composition: Slow vs. Fast Spreading Ridges

    15: 15 K2O vs. Mg for MAR MORB

    16: 16 REE Patterns for MAR MORBS

    17: 17 LREE vs. Mg#

    18: 18 143Nd/ 144Nd vs. 87Sr/ 86Sr

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

    20: 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 Relatively large (~ 5 km wide and 9 km deep)Relatively large (~ 5 km wide and 9 km deep)

    21: 21 Semi-Permanent Axial Magma Chamber Problem: Recent seismic work has failed to detect any chambers of this size at ridges, thus causing a fundamental shift away from this traditional view of axial magma chambers as large, steady-state, predominantly molten bodies of extended duration Problem: Recent seismic work has failed to detect any chambers of this size at ridges, thus causing a fundamental shift away from this traditional view of axial magma chambers as large, steady-state, predominantly molten bodies of extended duration

    22: 22 Axial Magma Chamber, Fast-Spreading Ridge Combines the magma chamber geometry proposed by Sinton and Detrick (1992) with the broad zone of volcanic activity noted by Perfit et al. (1994) Completely liquid body is a thin (tens to hundreds of meters thick) and narrow (< 2 km wide) sill-like lens 1-2 km beneath the seafloor Provides reflector noticed in detailed seismic profiles shot along and across sections of the EPR Melt surrounded by a wider mush and transition zone of low seismic velocity (transmits shear waves, but may still have a minor amount of melt) “Magma chamber” = melt + mush zone (the liquid portion is continuous through them) As liquid ? mush the boundary moves progressively toward the liquid lens as crystallization proceeds Lens maintained by reinjection, much like the “infinite onion” Combines the magma chamber geometry proposed by Sinton and Detrick (1992) with the broad zone of volcanic activity noted by Perfit et al. (1994) Completely liquid body is a thin (tens to hundreds of meters thick) and narrow (< 2 km wide) sill-like lens 1-2 km beneath the seafloor Provides reflector noticed in detailed seismic profiles shot along and across sections of the EPR Melt surrounded by a wider mush and transition zone of low seismic velocity (transmits shear waves, but may still have a minor amount of melt) “Magma chamber” = melt + mush zone (the liquid portion is continuous through them) As liquid ? mush the boundary moves progressively toward the liquid lens as crystallization proceeds Lens maintained by reinjection, much like the “infinite onion”

    23: Crystal Mush Zone Seismic velocities are still low beyond the mush (transition zone where the partially molten material grades to cooler solid gabbro) The small sill-like liquid chamber seems difficult to reconcile with the layered gabbros and cumulates, which appear to be more compatible with a large liquid chamber In situ crystallization in the mush zone, however may be a viable alternative for gabbro formation Much of the layering of ophiolite gabbros may be secondary, imposed during deformation of the spreading seafloor, and not by crystal settlingSeismic velocities are still low beyond the mush (transition zone where the partially molten material grades to cooler solid gabbro) The small sill-like liquid chamber seems difficult to reconcile with the layered gabbros and cumulates, which appear to be more compatible with a large liquid chamber In situ crystallization in the mush zone, however may be a viable alternative for gabbro formation Much of the layering of ophiolite gabbros may be secondary, imposed during deformation of the spreading seafloor, and not by crystal settling

    24: 24 Discontinuous Axial Magma Chamber

    25: 25 Axial Magma Chamber, Slow-Spreading Ridge

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

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

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