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Geol 2312 Igneous and Metamorphic Petrology

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!. How does this happen?. But aren’t there different types of “mafic” magmas?. Olivine.

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Geol 2312 Igneous and Metamorphic Petrology

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  1. Geol 2312 Igneous and Metamorphic Petrology Lecture 10 Mantle Melting and the Generation of Basaltic Magma February 16, 2009

  2. What is this made of? Melting the Mantle Makes Mafic Magma ALWAYS! How does this happen? But aren’t there different types of “mafic” magmas?

  3. Olivine Dunite 90 Peridotites Wehrlite Lherzolite Harzburgite 40 Pyroxenites Orthopyroxenite Olivine Websterite 10 Websterite 10 Clinopyroxene Orthopyroxene Clinopyroxenite Composition of the MantleLherzolite • 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 Tholeiitic basalt 15 20% Partial Melting • + Al-bearing Phase • Plagioclase <30km • Spinel 30-80 km • Garnet >80km 10 Wt.% Al2O3 Melt 5 Lherzolite Harzburgite Residuum Dunite Mantle 0 0.8 0.4 0.6 0.2 0.0 Wt.% TiO2

  4. 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.

  5. Melting the MantleIncreasing Temperature – Mantle Plumes Zone of Melting Normal Geotherm Plume- influenced Geotherm

  6. Melting the MantleAdiabatic Decompression(Rise of the Mantle with no Conductive Heat Loss) Adiabatic Geotherm

  7. Melting the MantleRole of Volatilesdramatically lowers the liquidus T of the Mantle “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.

  8. Melting a Hydrated Mantle Ocean Geothermal Gradient

  9. Melting a Hydrated Mantle Problem: 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) Exceeding the melting points of amphibole (b) and phlogopite (c) will release H2O and allow small degrees (<1%) of melting if saturation is achieved.

  10. Thermal Divide 1713 Liquid Tr + L Ne + L Ab + L Ab + L 1070 Ne + Ab Ab + Tr Q Ne Ab Creating Compositional Types of Mafic Magmas in Non-Subduction SettingsAlkaline and Subalkaline (Tholeiitic) Incompatible Elements in the Mantle Qtz-under saturated Qtz-saturated

  11. Creating Compositional Types of Mafic Magmas in Non-Subduction SettingsChanging Pressure (96 km) (65 km) (34 km)

  12. Ne P = 2 GPa CO2 H2O dry Ab Highly undesaturated (nepheline-bearing) alkali olivine basalts Oversaturated (quartz-bearing) Undersaturated tholeiitic basalts tholeiitic basalts Fo En SiO2 Creating Compositional Types of Mafic Magmas in Non-Subduction SettingsChanging Volatile Content Not really applicable to non-subduction settings

  13. Creating Compositional Types of Mafic Magmas in Non-Subduction SettingsChanging Degree of Incongruent Partial Melting Winter (2001) Figure 10.9 , After Green and Ringwood (1967). Earth Planet. Sci. Lett. 2, 151-160.

  14. Incongruent Partial Melting of the Mantle Mantle Gt Lherzolite Fo65Py20Di15 (Fo57En17Py14Di12 ) Batch Melting Experiments of Natural Lherzolites Melt – Fo5Py45Di50 +En From Yoder (1976)

  15. Creating Compositional Types of Mafic Magmas in Non-Subduction SettingsChanging Degree of Partial Melting and Depth of Melting Winter (2010), Figure 10.11 After Kushiro (2001).

  16. Creating Compositional Types of Mafic Magmas in Non-Subduction SettingsFractional 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.

  17. Creating Compositional Types of Mafic Magmas in Non-Subduction SettingsWhat is a Primary Magma? • Magma that last equilibrated with the mantle and was not subsequently modified by fractional crystallization or assimilation • Modified magmas are termed derivative or evolved. • Criteria - Most Primitive Composition • Highest mg# (MgO/(MgO+FeO) = 66-75 • Highest compatible element concentrations (Cr > 1000 ppm, Ni > 400-500 ppm) • Lowest incompatible element abundances • Lowest radiogenic isotope ratios • Multiply saturated

  18. Determining the depth of Primary Magma Formation by the inverse method of finding MULTIPLE SATURATION Involves melting a rock suspected of corresponding to a primary magma and finding the P&T of multiple saturation. This corresponds to “eutectic” point of the multi-component magma. Winter (2010) Figure 10.13), Anhydrous P-T phase relationships for a mid-ocean ridge basalt suspected of being a primary magma. After Fujii and Kushiro (1977). Carnegie Inst. Wash. Yearb., 76, 461-465.

  19. Creating Compositional Types of Mafic Magmas in Non-Subduction SettingsCompositionally Heterogeneous Mantle Two Layer Mantle Convection Model Upper deplete mantle = MORB source Lower undepleted mantle = enriched OIB source From Courtillot et al. (2003). Whole Mantle Convection Model Winter (2010) Figure 10-17b After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.

  20. Creating Compositional Types of Mafic Magmas in Non-Subduction SettingsCompositionally Heterogeneous Mantle Ocean Island Basalt (plume-influenced) Mid-ocean Ridge Basalt (normal upper mantle) increasing incompatibility

  21. Creating Compositional Types of Mafic Magmas in Non-Subduction SettingsCompositionally Heterogeneous Mantle increasing incompatibility Winter (2010) Figure 10.14b. Spider diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

  22. Creating Compositional Types of Mafic Magmas in Non-Subduction SettingsCompositionally Heterogeneous Mantle Melting “Fertile” Mantle Melting “Infertile” (previously melted) Mantle – MORB?

  23. LREE depleted LREE depleted or unfractionated or unfractionated LREE enriched LREE enriched From Rollinson (1996) Winter (2010) Figure 10.15 Chondrite-normalized REE diagrams for spinel (a) and garnet (b) lherzolites. After Basaltic Volcanism Study Project (1981).

  24. Creating Compositional Types of Mafic Magmas in Non-Subduction SettingsCompositionally Heterogeneous Mantle = EMORB/OIB = MORB % partial melting % normative olivine Winter (2010) Figure 10-18a. Results of partial melting experiments on depleted and enriched lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which these phases are completely consumed in the melt. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73, 287-310

  25. Melting the Mantle makes Mafic Magma Partially Melting the Heterogeneous Mantle makes Various Types of Mafic Magma • A chemically homogenous mantle can yield a variety of basalt types • Alkaline basalts are favored over tholeiites by deeper melting and by lower % partial melting • Crystal fractionation at moderate to great depths in the mantle can also create alkaline basalts from tholeiites • At mod to high P, there is a thermal divide that separates the two series • Mantle varies in bulk composition and fertility due to prior melting events (upper – depleted; lower undepleted)

  26. Present-Day Mid-Ocean RidgesBirthplaces of MORB

  27. Oceanic Crust and Upper Mantle Structure EVIDENCE • Seismic Velocities • Deep Sea Drilling Program • Ophiolites • Dredging of Fracture Zone Scarps

  28. Oceanic Crust and Upper Mantle formation Winter (2010) Figure 13.15. After Langmuir et al. (1992). AGU.

  29. Composition of MORB Fractional Crystallization Primitive Magma

  30. Compositional Variability in MORB Fractional crystallization appears to play a minor role. Suggests instead, incompatible element-rich and incompatible element-poor mantle source regions for MORB magmas • N-MORB (normal MORB) taps the depleted upper mantle source • Mg# > 65: K2O < 0.10 TiO2 < 1.0 • E-MORB (enriched MORB) taps undepleted (deeper?) mantle • Mg# > 65: K2O > 0.10 TiO2 > 1.0 FC

  31. Origin of NMORB and EMORB

  32. X-Sectional View Distribution of NMORB & EMORB at Fast Spreading RidgesPromotes development of larger magma chambers more fractionated basalt compositions Winter (2001) Figure 13-15, After Perfit et al. (1994) Geology, 22, 375-379. Longitudinal View Winter (2001) Figure 13-16 ; After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.

  33. 2 Rift Valley 4 Depth (km) Gabbro 6 Transition zone Moho Mush 8 10 10 5 0 5 Distance (km) Slow-spreading Ridges Smaller, crystal-rich, dike-like magma bodies results is less fractionation  less evolved MORB Winter (2001) Figure 13-16 After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.

  34. Ocean Island Basalts (OIB) Plume-influenced Volcanism Figure 14.1. Map of relatively well-established hotspots and selected hotspot trails (island chains or aseismic ridges). Hotspots and trails from Crough (1983) with selected more recent hotspots from Anderson and Schramm (2005). Also shown are the geoid anomaly contours of Crough and Jurdy (1980, in meters). Note the preponderance of hotspots in the two major geoid highs (superswells).

  35. Hawaiian VolcanismStaging of Alkaline and Tholeiitic Magma Series

  36. Major Element Chemistry of OIB Variable alkalinity likely reflects variable depths and degrees of partial melting in the plume and variable degrees of mixing and re-equilbration as magma rises through the mantle plume to the ocean crust. Winter (2001) Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.

  37. REE Compositions of OIB Alkali basalts strongly LREE-enriched  low degrees of partial melting Similar to E-MORB, but stronger depletion of HREE for both tholeiitic and alkali magmas series  deep garnet-bearing source for both with re-equilibration at shallower depths Lack of Eu anomaly indicates no significant fractionation of plagioclase

  38. Trace Elements OIB / MORB Winter (2001) Figure 14-3. An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

  39. A Model for Ocean Magmatism Chondritic (undepleted) mantle From recycled ocean crust Low Sr87/Sr86 mantle Mantle Reservoirs (based on radiogenic isotopes) Low Nd143/Nd144 mantle Previously melted mantle Winter (2001) Figure 14-10. Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993). U-enriched mantle Chondritic (undepleted) mantle

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