Origin of Basaltic Magma Seismic evidence -> basalts are generated in the mantle Partial melting of mantle material Probably can derive most other magmas from this primary magma by fractional crystallization, assimilation, etc. Basalt is the most common magma If we are going to understand the origin of igneous rocks, it’s best to start with the generation of basalt from the mantle

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.

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

Kimberlite xenoliths Photo of Kimberley diamond min (South Africa) and two examples of mantle xenoliths (peridotite [top] and garnet Peridotite [bottom] from a kimberlite.

Lherzolite is probably fertile (undepleted) unaltered mantle Dunite and harzburgite are refractory residuum after basalt has been extracted by partial melting 15 Tholeiitic basalt Ultramafic rocks 10 Partial Melting 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 0.2 0.4 0.6 0.8 Wt.% TiO2

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

Phase diagram for aluminous 4-phase lherzolite: Al-phase = Plagioclase shallow (< 50 km) Spinel 50-80 km Garnet 80-400 km Si ® VI coord. > 400 km Note: the mantle will not melt under normal ocean geotherm! 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.

How does the mantle melt?? 1) Increase the temperature No realistic mechanism for the general case Local hot spots OK very limited area Figure 10-3. Melting by raising the temperature.

2) Lower the pressure Adiabatic rise of mantle with no conductive heat loss Decompression melting could melt at least 30% Adiabatic rise of mantle with no conductive heat loss Steeper than solidus Intersects solidus D slope = heat of fusion as mantle melts Decompression melting could melt at least 30% 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.

3) Add volatiles (especially H2O) Remember solid + water = liq(aq) and LeChatelier Dramatic lowering of melting point of peridotite Figure 10-4. Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.

Melts can be created under realistic circumstances Plates separate and mantle rises at mid-ocean ridges, or at continental rifts Adibatic rise ® decompression melting Hot spots ® localized plumes of melt Fluid fluxing Important in subduction zones

Figure 9-8. (a) after Pearce and Cann (1973), Earth Planet, Sci. Lett Figure 9-8. (a) after Pearce and Cann (1973), Earth Planet, Sci. Lett., 19, 290-300. (b) after Pearce (1982) in Thorpe (ed.), Andesites: Orogenic andesites and related rocks. Wiley. Chichester. pp. 525-548, Coish et al. (1986), Amer. J. Sci., 286, 1-28. (c) after Mullen (1983), Earth Planet. Sci. Lett., 62, 53-62.