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Petrology Lecture 6

2. Magma Series Definition. A group of rocks that share some chemical, and sometimes mineralogical, characteristicsThey share patterns on chemical variation diagrams which suggests a genetic relationship

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Petrology Lecture 6

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    1. 1 Petrology Lecture 6 Generation of Basaltic Magma GLY 4310 - Spring, 2011

    2. 2 Magma Series Definition A group of rocks that share some chemical, and sometimes mineralogical, characteristics They share patterns on chemical variation diagrams which suggests a genetic relationship beyond the scope of this course

    3. 3 Major Magma Series J.P. Iddings, 1892 Alkaline Sub-alkaline C.E. Tilley (1950) split sub-alkaline into: Tholeittic Calc-alkaline

    4. 4 Relationship of Magma Series to Plate Tectonics

    5. 5 Geotherms Estimated ranges of oceanic (hatched) and continental (solid) steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface

    6. 6 Sources of Upper Mantle Rocks Ophiolites Dredge Samples from oceanic fracture zones Nodules in basalts Autoliths Restites Xenoliths in kimberlites Stony meteorites analogous to upper mantle of a broken planet

    7. 7 Basaltic Partial Melt TiO2 vs, Al2O3 Extraction of a basaltic partial melt from lherzolite can result in solid refractory harzburgite or dunite Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/Kluwer.

    8. Lherzolite: A type of peridotite with Olivine > Opx + Cpx 8

    9. 9 Aluminous Lherzolite Phase Diagram Al phase Plagioclase shallow (< 50 km) Spinel 50-80 km Garnet 80-400 km Si VI coord. > 400 km

    10. Mantle Melting 10

    11. 11 Decompression Melting Decompression melting under adiabatic conditions When adiabat crosses solidus, melting begins Dashed lines represent approximate % melting At least 30% melting is realistic

    12. Addition of Volatiles 12 Remember solid + water = liq(aq) LeChatelier principle leads to dramatic lowering of melting point of peridotite Remember solid + water = liq(aq) LeChatelier principle leads to dramatic lowering of melting point of peridotite

    13. 13 Effect of Pressure on Initial Melting Change in eutectic position with increasing pressure First melting occurs at the eutectic After Kushiro, 1968 Increased pressure moves the ternary eutectic minimum from the oversaturated tholeiite field to the under-saturated alkaline basalt field Alkaline basalts are thus favored by greater depth of melting Increased pressure moves the ternary eutectic minimum from the oversaturated tholeiite field to the under-saturated alkaline basalt field Alkaline basalts are thus favored by greater depth of melting

    14. 14 Pyrolite Melting Nature of the liquids and refractory residua associated with partial melting of pyrolite After Green and Ringwood (1967)

    15. 15 Fractional Crystallization of Basaltic Magmas Minerals fractionating are listed near arrows After Wyllie, 1971

    16. 16 Partial Melting and Fractional Crystallization

    17. 17 Trends in Partial Melts Spinel lherzolites, from 6 to 35% partial melt Low melt leads to alkaline basalt, higher % melts to more tholeittic compositions After Hirose and Kushiro, 1991

    18. 18 Basalt Petrogenesis

    19. 19 Magma Types Primary Magma - one that forms by melting at depth, without any later modification Derivative - A primary magma that has been modified by some magma differentiation process on the way to the surface Parental - Most primitive magma type within a given magma series, it may or may not be primary

    20. Multiple saturation

    22. Multiple saturation Multiple saturation implies that the liquid corresponding to the melted basalt in the experiment was in equilibrium with Ol + Cpx + Plag at 25 km depth This is the appropriate mineralogy for a lherzolite at this depthMultiple saturation implies that the liquid corresponding to the melted basalt in the experiment was in equilibrium with Ol + Cpx + Plag at 25 km depth This is the appropriate mineralogy for a lherzolite at this depth

    23. 23 OIB and MORB Upper, REE diagram Lower, Spider diagram Data from Sun and McDonough, 1989

    24. 24 REE Variation Chondrite-normalized REE diagrams for spinel and garnet lherzolites After Basaltic Vocanism Study Project, 1981

    25. 25 143Nd/144Nd vs. 87Sr/86Sr Upper, oceanic basalts Lower, ultramafic xenoliths from subcontinental mantle MAR = Mid-Atlantic Ridge EPR = East Pacific Rise IR = Indian Ocean Ridge

    26. 26 Mantle Convection Models After Basaltic Vocanism Study Project, 1981

    27. 27 Partial Melting Experiments Left, depleted lherzolites; Right, fertile lherzolites Dashed lines = % partial melt produced Shaded area = condition required for the generation of alkaline basaltic magmas

    28. 28 Changing ?V ?V approaches zero on going from solid to liquid as pressure increases Causes both solidus and liquidus to change slope as depth increases

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