Transcript G 2312 I M
GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY Lecture 10 Mantle Melting and the Generation of Basaltic Magma February 16, 2009 What is this made of? MELTING THE MANTLE MAKES MAFIC MAGMA ALWAYS! COMPOSITION OF THE MANTLE LHERZOLITE Evidence: •Ophiolites Slabs of oceanic crust and upper mantle Obducted onto edge of continent at convergent zones •Dredge samples from oceanic fracture zones •Nodules and xenoliths in some basalts •Kimberlite xenoliths Pipe-like intrusions quickly intruded from the deep mantle carrying numerous xenoliths + Al-bearing Phase • Plagioclase <30km • Spinel 30-80 km • Garnet >80km Olivine Tholeiitic basalt 15 Dunite Melt 90 Peridotites 10 Lherzolite 40 5 Lherzolite Harzburgite Dunite 0 0.0 0.2 Mantle Pyroxenites Orthopyroxenite Olivine Websterite Residuum Websterite 10 0.4 Wt.% TiO2 10 0.6 0.8 Orthopyroxene Clinopyroxenite Clinopyroxene PHASE DIAGRAM OF NORMAL MANTLE Mantle should not melt under”normal” geothermal conditions How to get it to melt? Winter (2001) 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. MELTING THE MANTLE INCREASING TEMPERATURE – MANTLE PLUMES Zone of Melting Normal Geotherm Plumeinfluenced Geotherm MELTING THE MANTLE ADIABATIC DECOMPRESSION (RISE OF THE MANTLE WITH NO CONDUCTIVE HEAT LOSS) Adiabatic Geotherm MELTING THE MANTLE ROLE OF VOLATILES “Dry” curve has a positive slope because increased P favors lower V phase (solid), increased T favors S phase (liquid) H2O-saturated curve has negative slope because V of liq+vapor (Liqaq) is less than V of solid+vapor (or fluid); change is most extreme at low overall pressures. Melting point is lowered with increasing P by more volatiles being dissolved in the melt MELTING A HYDRATED MANTLE Ocean Geothermal Gradient MELTING A HYDRATED MANTLE Problem: Water content of the mantle Dehydration melting curves typically <0.2% (far from saturated) and it is structurally locked into hydrous mineral phases like amphibole and phlogopite (biotite). However, dehydration melting of these phases will only yield <1% H2O, hardly enough to saturate the mantle CREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGS FIVE WAYS CREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGS 1) CHANGING PRESSURE CREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGS 2) CHANGING VOLATILE CONTENT Ne P = 2 Gpa (80 Km) CO2 dry Highly undesaturated (nepheline-bearing) alkali olivine basalts H2O Ab Oversaturated (quartz-bearing) tholeiitic basalts Not really applicable to non-subduction settings Fo En SiO2 CREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGS 3) CHANGING DEGREE OF PARTIAL MELTING CREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGS 4) FRACTIONAL CRYSTALLIZATION DURING ASCENT Winter (2001) Figure 10-10 Schematic representation of the fractional crystallization scheme of Green and Ringwood (1967) and Green (1969). After Wyllie (1971). The Dynamic Earth: Textbook in Geosciences. John Wiley & Sons. CREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGS 5) COMPOSITIONALLY HETEROGENEOUS MANTLE Melting “Fertile” Mantle Melting “Infertile” (previously melted) Mantle CREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGS 5) COMPOSITIONALLY HETEROGENEOUS MANTLE Upper deplete mantle= MORB source Lower undepleted mantle= enriched OIB source Partially Melting the Heterogeneous Mantle M ELTING THE MANTLE MAKES MAFIC M AGMA makes Various Types of Mafic Magma o A chemically homogenous mantle can yield a variety of basalt types o Alkaline basalts are favoured over tholeiites by deeper melting and by low % partial melting o Crystal fractionation at moderate to great depths in the mantle can also create alkaline basalts from tholeiites o At low P, there is a thermal divide that seperates the two series o Mantle varies in bulk composition and fertility due to prior melting events (upper – depleted; lower undepleted)