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Origin of Basaltic Magma

Origin of Basaltic Magma. Generation of Magma Chapter 11. 2 principal types of basalt in the ocean basins. Tholeiitic Basalt and Alkaline Basalt. Common petrographic differences between tholeiitic and alkaline basalts. Table 10-1. Tholeiitic Basalt. Alkaline Basalt.

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Origin of Basaltic Magma

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  1. Origin of Basaltic Magma Generation of Magma Chapter 11

  2. 2 principal types of basalt in the ocean basins TholeiiticBasalt andAlkalineBasalt Common petrographic differences between tholeiitic and alkaline basalts • Table 10-1 Tholeiitic Basalt Alkaline Basalt Usually fine-grained, intergranular Usually fairly coarse, intergranular to ophitic Groundmass No olivine Olivine common Clinopyroxene = augite (plus possibly pigeonite) Titaniferous augite (reddish) Orthopyroxene (hypersthene) common, may rim ol. Orthopyroxene absent No alkali feldspar Interstitial alkali feldspar or feldspathoid may occur Interstitial glass and/or quartz common Interstitial glass rare, and quartz absent Olivine rare, unzoned, and may be partially resorbed Olivine common and zoned Phenocrysts or show reaction rims of orthopyroxene Orthopyroxene uncommon Orthopyroxene absent Early plagioclase common Plagioclase less common, and later in sequence Clinopyroxene is pale brown augite Clinopyroxene is titaniferous augite, reddish rims after Hughes (1982) and McBirney (1993).

  3. Each is chemically distinct Evolve via FX as separate series along different paths • Tholeiites are generated at mid-ocean ridges • Also generated at oceanic islands, subduction zones • Alkaline basalts generated at ocean islands • Also at subduction zones

  4. Sources of mantle material • Ophiolites • Slabs of oceanic crust and upper mantle • Thrust at subduction zones onto edge of continent • Dredge samples from oceanic fracture zones • Nodules and xenoliths in some basalts • Kimberlite xenoliths • Diamond-bearing pipes blasted up from the mantle carrying numerous xenoliths from depth

  5. Lherzolite is probably pristine mantle Dunite and harzburgite are refractory residuum after basalt has been extracted by partial melting Tholeiitic basalt 15 Partial Melting 10 Wt.% Al2O3 5 Figure 10-1 Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/Kluwer. Lherzolite Harzburgite Residuum Dunite 0 0.8 0.4 0.6 0.2 0.0 Wt.% TiO2

  6. 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

  7. Phase diagram for aluminous 4-phase lherzolite: Al-phase = • Plagioclase • shallow (< 50 km) • Spinel • 50-80 km • Garnet • 80-400 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.

  8. Phase diagram for aluminous 4-phase lherzolite: • Basalts with same chemistries and different mineralogies • Different depth of source 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.

  9. How does the mantle melt?? • Increase the temperature • Local hot spots (not common) • Mechanical heating from shearing • Crustal thickening in mountain belts Figure 10-3. Melting by raising the temperature.

  10. 2) Lower the pressure • Adiabatic rise of mantle with no conductive heat loss • Decompression melting could melt at least 30% • Important at divergent plate boundaries Figure 10-4. Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting.

  11. 3) Add volatiles (especially H2O) Figure 10-5. Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.

  12. 15% 20% 50% 100% Fraction melted is limited by availability of water Figure 7-22. Pressure-temperature projection of the melting relationships in the system albite-H2O. From Burnham and Davis (1974). A J Sci., 274, 902-940.

  13. Melts can be created under realistic circumstances • Plates separate and mantle rises at mid-ocean ridges • Adibatic rise ® decompression melting • Hot spots ® localized plumes of melt • Fluid fluxing important in subduction zones and other settings

  14. Reasonable settings for melting the mantle • mid ocean ridge • Hot spots (mantle plume) • Subduction zones • Extension in continental rift zone • Extension in back-arc basin • Collision zone

  15. How to generate all basalt types we see? • Heterogeneous mantle • Not likely • Homogeneous mantle • Need to make tholeiitic and alkaline basalts

  16. Generation of tholeiitic and alkaline basalts from achemically uniformmantle Variables (other than X) • Temperature • % melt generated • Pressure • Depth of melting 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.

  17. Pressure effects: - At what depth are alkaline basalts generated? - At what depth are tholeiitic basalts generated? Figure 10-8 Change in the eutectic (first melt) composition with increasing pressure from 1 to 3 GPa projected onto the base of the basalt tetrahedron. After Kushiro (1968), J. Geophys. Res., 73, 619-634.

  18. Initial Conclusions: • Tholeiites favored by shallower melting • 25% melting at <30 km ® tholeiite • 25% melting at 60 km ® olivine basalt • Tholeiites favored by greater % partial melting • 20 % melting at 60 km ® alkaline basalt • 30 % melting at 60 km ® tholeiite

  19. Summary • A chemically homogeneous mantle can yield a variety of basalt types • Alkaline basalts are favored over tholeiites by deeper melting and by low % PM • At P there is a thermal divide that separates the two series

  20. Why only two depths of melts? • Look at the shape of solidus

  21. Where is magma generated? • Diverging ocean plates • Converging plates

  22. Melting at convergent margins • Island arcs (ocean-ocean) • Less contamination/differentiation • Continental arcs (continent-ocean) • More contamination/differentiation • Location of calc-alkaline rocks

  23. Melting at convergent margins • Slab subducts and “dewaters” • Water from pore spaces and hydrous minerals • Clay minerals, serpentine, micas, amphiboles

  24. Melting at convergent margins • Addition of water initiates melting in mantle wedge • Subducting slab DOESN’T MELT

  25. Melting at convergent margins • Controls on depth of “dewatering” • Age of downgoing slab • Intensity of shear heating • Angle of subduction

  26. Melting at convergent margins • Older lithosphere created far from subduction zone, subducting slowly with no shearing dehydrates deeper

  27. Mantle model circa 1975 Figure 10-16aAfter Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.

  28. Newer mantle model • Upper depleted mantle = MORB source • Lower undepleted & enriched OIB source Figure 10-16b After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.

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