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Mantle Melting

Mantle Melting. Some slides from Mary Leech.

colette-barton
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Mantle Melting

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  1. Mantle Melting Some slides from Mary Leech

  2. Table 18-4. A Classification of Granitoid Rocks Based on Tectonic Setting. After Pitcher (1983) in K. J. Hsü (ed.), Mountain Building Processes, Academic Press, London; Pitcher (1993), The Nature and Origin of Granite, Blackie, London; and Barbarin (1990) Geol. Journal, 25, 227-238. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  3. Lherzolite is probably fertile (undepleted) unaltered mantle Harzburgite ± Dunite are refractory residuum after basalt has been extracted by partial melting Tholeiitic basalt 15 Ultramafic rocks Partial Melting 10 Wt.% Al2O3 5 Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/Kluwer. Slide from Mary Leech. Lherzolite Harzburgite Residuum Dunite 0 0.8 0.4 0.6 0.2 0.0 Wt.% TiO2

  4. Lherzolite: A type of peridotite with Olivine > Opx + Cpx Olivine Dunite 90 Peridotites Wehrlite Harzburgite Lherzolite 40 Pyroxenites Olivine Websterite Orthopyroxenite 10 Websterite 10 Clinopyroxenite Orthopyroxene Clinopyroxene Figure 2-2 C After IUGS

  5. How does the mantle melt?? 2) Lower the pressure • Adiabatic rise of mantle with no conductive heat loss • Decompression melting could melt at least 30% Phase diagram of aluminous lherzolite with melting interval (pink), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153. 1) Increase the temperature 3) Add volatiles (especially H2O) Phase diagram for aluminous 4-phase lherzolite: Alminous phase = • Plagioclase shallow (< 50 km) • Spinel 50-80 km • Garnet 80-400 km • Si VI coord. > 400 km Where does mantle melting occur?

  6. Result? Basalt

  7. What is Mid Ocean Ridge Bsasalt? • MgO and FeO • Al2O3 and CaO • SiO2 • Na2O, K2O, TiO2, P2O5 Basaltic glasses from the Afar region of the MAR. Note different ordinate scales. From Stakes et al. (1984) J. Geophys. Res., 89, 6995-7028.

  8. Ternary Variation Diagrams Example: AFM diagram (alkalis-FeO*-MgO) AFM diagram for Crater Lake volcanics, Oregon Cascades. From Mary Leech

  9. Conclusions about MORBs, and the processes beneath mid-ocean ridges • MORBs are not the completely uniform magmas that they were once considered to be • They show chemical trends consistent with fractional crystallization of olivine, plagioclase, and perhaps clinopyroxene • MORBs cannot be primary magmas, but are derivative magmas resulting from fractional crystallization (~ 60%)

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