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Petrology Lecture 6

Petrology Lecture 6. Generation of Basaltic Magma GLY 4310 - Spring, 2012. Magma Series Definition. A group of rocks that share some chemical, and sometimes mineralogical, characteristics

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Petrology Lecture 6

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  1. Petrology Lecture 6 Generation of Basaltic Magma GLY 4310 - Spring, 2012

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

  4. Relationship of Magma Series to Plate Tectonics

  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. 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. 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 Olivine Dunite 90 Peridotites Wehrlite Lherzolite Harzburgite 40 Pyroxenites Olivine Websterite Orthopyroxenite 10 Websterite 10 Clinopyroxenite Orthopyroxene Clinopyroxene Figure 2.2 C After IUGS

  9. Aluminous Lherzolite Phase Diagram • Al – phase • Plagioclase • shallow (< 50 km) • Spinel • 50-80 km • Garnet • 80-400 km • Si ® VI coord. • > 400 km Figure 10.2 Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.

  10. Mantle Melting • Increase in temperature • Problem: No realistic mechanism • Perhaps with local hot spots, with very limited area

  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 Figure 10.5. Dry peridotite solidus compared to several experiments on H2O-saturated peridotites

  13. Effect of Pressure on Initial Melting • Change in eutectic position with increasing pressure • First melting occurs at the eutectic • After Kushiro, 1968

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

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

  16. Partial Melting and Fractional Crystallization

  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. Basalt Petrogenesis

  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 • Low P • Ol then Plag then Cpx as cool • ~70oC T range Figure 10.13Anhydrous P-T phase relationships for a mid-ocean ridge basalt suspected of being a primary magma. After Fujii and Kushiro (1977).Carnegie Inst. Wash. Yearb., 76, 461-465.

  21. Multiple saturation • Low P • Ol then Plag then Cpx as cool • 70oC T range • High P • Cpx then Plag then Ol Figure 10.13Anhydrous P-T phase relationships for a mid-ocean ridge basalt suspected of being a primary magma. After Fujii and Kushiro (1977).Carnegie Inst. Wash. Yearb., 76, 461-465.

  22. Multiple saturation • Low P • Ol then Plag then Cpx as cool • 70oC T range • High P • Cpx then Plag then Ol • 25 km get all at once • =Multiple saturation • Suggests that 25 km is the depth of last equilibrium with the mantle

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

  24. LREE depleted LREE depleted or unfractionated or unfractionated LREE enriched LREE enriched REE Variation • Chondrite-normalized REE diagrams for spinel and garnet lherzolites • After Basaltic Vocanism Study Project, 1981

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

  27. Partial Melting Experiments • “Opx out” and “Cpx out” = degree of melting at which these phases are completely consumed • Left, depleted lherzolites; Right, fertile lherzolites • Dashed lines = % partial melt produced • Shaded area = condition required for the generation of alkaline basaltic magmas

  28. Changing ΔV • ΔV 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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