Mantle Melting Some slides from Mary Leech

Table 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, Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Wt.% Al 2 O 3 Wt.% TiO 2 Dunite Harzburgite Lherzolite Tholeiitic basalt Partial Melting Residuum Lherzolite is probably fertile (undepleted) unaltered mantle Harzburgite ± Dunite are refractory residuum after basalt has been extracted by partial melting Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/Kluwer. Slide from Mary Leech. Ultramafic rocks

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

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

Result? Basalt

l MgO and FeO l Al 2 O 3 and CaO l SiO2 l Na 2 O, K 2 O, TiO 2, P 2 O 5 Basaltic glasses from the Afar region of the MAR. Note different ordinate scales. From Stakes et al. (1984) J. Geophys. Res., 89, What is M id O cean R idge B sasalt ?

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

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