Slide 1Melting the Mantle Makes Mafic Magma ALWAYS!Composition of the Mantle LherzolitePhase Diagram of Normal MantleMelting the Mantle Increasing Temperature – Mantle PlumesMelting the Mantle Adiabatic Decompression (Rise of the Mantle with no Conductive Heat Loss)Melting the Mantle Role of VolatilesMelting a Hydrated MantleSlide 9Creating Compositional Types of Mafic Magmas in Non-Subduction Settings Alkaline and Subalkaline (Tholeiitic)Creating Compositional Types of Mafic Magmas in Non-Subduction Settings Changing PressureCreating Compositional Types of Mafic Magmas in Non-Subduction Settings Changing Volatile ContentCreating Compositional Types of Mafic Magmas in Non-Subduction Settings Changing Degree of Partial MeltingCreating Compositional Types of Mafic Magmas in Non-Subduction Settings Fractional Crystallization during AscentSlide 15Creating Compositional Types of Mafic Magmas in Non-Subduction Settings Compositionally Heterogeneous MantleSlide 17Melting the Mantle makes Mafic MagmaGEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGYLecture 10Mantle Melting and the Generation of Basaltic MagmaFebruary 16, 2009MELTING THE MANTLE MAKES MAFIC MAGMA ALWAYS!How does this happen?But aren’t there different types of “mafic” magmas?What is this made of?COMPOSITION OF THE MANTLELHERZOLITEEvidence: •OphiolitesSlabs of oceanic crust and upper mantleObducted onto edge of continent at convergent zones•Dredge samples from oceanic fracture zones•Nodules and xenoliths in some basalts•Kimberlite xenolithsPipe-like intrusions quickly intruded from the deep mantle carrying numerous xenolithsOlivineClinopyroxeneOrthopyroxeneLherzoliteHarzburgiteWehrliteWebsteriteOrthopyroxeniteClinopyroxeniteOlivine WebsteritePeridotitesPyroxenites90401010Dunite1510500.00.20.4 0.60.8Wt.% Al2O3Wt.% TiO2DuniteHarzburgiteLherzoliteTholeiitic basalt20% Partial MeltingResiduum+ Al-bearing Phase• Plagioclase <30km• Spinel 30-80 km• Garnet >80kmMantleMeltPHASE DIAGRAM OF NORMAL MANTLEWinter (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. Mantle should not melt under”normal” geothermal conditionsHow to get it to melt?MELTING THE MANTLEINCREASING TEMPERATURE – MANTLE PLUMESZone of MeltingNormal GeothermPlume-influenced GeothermMELTING THE MANTLEADIABATIC DECOMPRESSION(RISE OF THE MANTLE WITH NO CONDUCTIVE HEAT LOSS)AdiabaticGeothermMELTING THE MANTLEROLE 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 A HYDRATED MANTLEOceanGeothermal GradientMELTING A HYDRATED MANTLEProblem: Water content of the mantle typically <0.2% (far from saturated) and it is structurally locked into hydrous mineral phases like amphibole and phlogopite (biotite)CREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGSALKALINE AND SUBALKALINE (THOLEIITIC)CREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGSCHANGING PRESSURECREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGSCHANGING VOLATILE CONTENTNeFo EnAbSiO2Oversaturated(quartz-bearing)tholeiitic basaltsHighly undesaturated(nepheline-bearing)alkali olivinebasaltsUndersaturatedtholeiitic basaltsCO2H2OdryP = 2 GPaNot really applicable to non-subduction settingsCREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGSCHANGING DEGREE OF PARTIAL MELTINGCREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGSFRACTIONAL CRYSTALLIZATION DURING ASCENTWinter (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 SETTINGSFRACTIONAL CRYSTALLIZATION DURING ASCENTincreasing incompatibilityOcean Island Basalt(plume-influenced)Mid-ocean Ridge Basalt(normal upper mantle)CREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGSCOMPOSITIONALLY HETEROGENEOUS MANTLEMelting “Fertile” MantleMelting “Infertile” (previously melted)MantleCREATING COMPOSITIONAL TYPES OF MAFIC MAGMAS IN NON-SUBDUCTION SETTINGSCOMPOSITIONALLY HETEROGENEOUS MANTLEUpper deplete mantle=MORB sourceLower undepleted mantle=enriched OIB sourceMELTING THE MANTLE MAKES MAFIC MAGMAPartiallyPartially Melting the Melting the Heterogeneous Heterogeneous Mantle Mantle makes makes Various Types of Various Types of Mafic MagmaMafic Magmao A chemically homogenous mantle can yield a variety of basalt typeso Alkaline basalts are favored over tholeiites by deeper melting and by low % partial meltingo Crystal fractionation at moderate to great depths in the mantle can also create alkaline basalts from tholeiiteso At low P, there is a thermal divide that seperates the two serieso Mantle varies in bulk composition and fertility due to prior melting events (upper – depleted; lower