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How Magma Forms

How Magma Forms. Sources of heat for melting rocks Factors that control melting temperatures Other considerations: Volatiles Change in Pressure (Decompression Melting). Heat Flow on Earth. An increment of heat ,  q, transferred into a body produces a

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How Magma Forms

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  1. How Magma Forms • Sources of heat for melting rocks • Factors that control melting temperatures • Other considerations: • Volatiles • Change in Pressure (Decompression Melting)

  2. Heat Flow on Earth An increment of heat, q, transferred into a body produces a proportional incremental rise in temperature, T, given by q = Cp * T where Cp is called the molar heat capacity of J/mol-degree at constant pressure; similar to specific heat, which is based on mass (J/g-degree). 1 calorie = 4.184 J and is equivalent to the energy necessary to raise 1 gram of of water 1 degree centigrade. Specific heat of water is 1 cal /g°C, where rocks are ~0.3 cal /g°C.

  3. Heat Transfer Mechanisms • Radiation: involves emission of EM energy from the surface of hot body into the transparent cooler surroundings. Not important in cool rocks, but increasingly important at T’s >1200°C • Advection: involves flow of a liquid through openings in a rock whose T is different from the fluid (mass flux). Important near Earth’s surface due to fractured nature of crust. • Conduction: transfer of kinetic energy by atomic vibration. Cannot occur in a vacuum. For a given volume, heat is conducted away faster if the enclosing surface area is larger. • Convection:movement of material having contrasting T’s from one place to another. T differences give rise to density differences. In a gravitational field, lower density (generally colder) materials sink.

  4. Earth’s Energy Budget • Solar radiation:50,000 times greater than all other energy sources; primarily affects the atmosphere and oceans, but can cause changes in the solid earth through momentum transfer from the outer fluid envelope to the interior. • Radioactive decay:238U, 235U, 232Th, 40K, and 87Rb all have t1/2 that >109 years and thus continue to produce significant heat in the interior; this may equal 50 to 100% of the total heat production for the Earth. Extinct short-lived radioactive elements such as 26Al were important during the very early Earth. • Tidal Heating: Earth-Sun-Moon interaction; much smaller than radioactive decay. • Primordial Heat: Also known as accretionary heat; conversion of kinetic energy of accumulating planetismals to heat. • Core Formation: Initial heating from short-lived radioisotopes and accretionary heat caused widespread interior melting (Magma Ocean) and additional heat was released when Fe sank toward the center and formed the core.

  5. Magmatic Examples of Heat Transfer Thermal Gradient=T between adjacent hotter and cooler masses Heat Flux = rate at which heat is conducted over time from a unit surface area Thermal Conductivity = K; rocks have very low values and thus deep heat has been retained! Heat Flux = Thermal Conductivity * T

  6. Crustal Geothermal Gradients Crustal Rocks Melt!

  7. Earth’s Geothermal Gradient Average Heat Flux is 0.09 watt/meter2 or 90 mW/m2 Geothermal gradient = T/ z 20-30°C/km in orogenic belts; Cannot remain constant w/depth At 200 km would be 4000°C ~7°C/km in trenches Viscosity, which measures resistance to flow, of mantle rocks is 1018 times tar at 24°C ! Approximate Pressure (GPa=10 kbar)

  8. Global Heat Flow models from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270 convection in the mantle observed heat flow warm: near ridges cold: over cratons from: http://www-personal.umich.edu/~vdpluijm/gs205.html

  9. Causes of Mantle Melting • -Increase T • -Decrease P • -Add Water

  10. Plagioclase Water-saturated vs. Dry Solidi

  11. Alkaline vs. Sub-alkaline Rocks 46.7% widely scattered <- Basalts 53.3% tightly clustered in a central band Analyses of a global sample of 41,000 igneous rocks of all ages

  12. Attributes of Total Alkalies Diagram • Magmatic rocks constitute a continuous chemical spectrum, i.e. no breaks or discontinuities. Other elemental combinations show similar trends. • Questions? • How is such a chemical spectrum created? • Is there a similar range in liquid (magma) compositions? • What processes of magma generation from solid rocks can give rise to the observed range? • Could this spectrum be generated from a much narrower source range and the derived liquids modified to yield the observed diversity?

  13. How Magmas of Different Compositions Evolve • Sequence of Crystallization and Melting • Differentiation • Partial Melting • Assimilation • Mixing of Magmas

  14. Bowen’s Reaction Series

  15. Binary Eutectic Phase Relations

  16. Magmatic Differentiation: Crystal Settling

  17. Sedimentary Structures in Layered Igneous Intrusions Harzburgite bands in Josephine Ophiolite, Oregon From: http://www.uoregon.edu/~dogsci/kays/313/plutonic.html

  18. Magmatic Cross-Beds in Skaergaard Layered Intrusion From: http://www.uoregon.edu/~dogsci/kays/313/plutonic.html

  19. Magmatic Differentiation: Assimilation

  20. Evidence for Assimilation - Adirondacks From: http://s01.middlebury.edu/GL211A/FieldTrip2.htm

  21. MagmaticDifferentiation:Magma Mixing

  22. Melt Inclusions in Quartz in Pantellerite From: http://wrgis.wr.usgs.gov/lowenstern/ Mahood and Lowenstern, 1991

  23. Evidence for Magma Mixing - Adirondacks From: http://s01.middlebury.edu/GL211A/FieldTrip3.htm

  24. The Relationship of Igneous Activity to Tectonics • Igneous Processes at Divergent Boundaries • MORB genesis and decompression melting • Intraplate Igneous Activity • “Hot” or “Wet” spots and mantle plumes • Igneous Processes at Convergent Boundaries • Downing plate crustal melting or volatile flux melting in the mantle wedge

  25. Earth’s Plates

  26. MORB Decompression Melting

  27. Decompression Melting and MORB Genesis

  28. Mantle Plumes - “Hot” or “Wet” Spots?

  29. Seismic Tomographic Image of Iceland Plume Contour of -2.5% shear wave velocity anomaly From: ICEMELT Seismic Experiment - Wolfe et al., 1997

  30. Numerical Simulation of Plume Melting From: http://www.geophysik.uni-frankfurt.de/geodyn/island/tp2_en.html

  31. Dynamic Plume Models From: http://www.geophysik.uni-frankfurt.de/geodyn/island/tp2_en.html

  32. Super Plumes? Volcanic Hot Spots on Earth’s Surface (dots) Global shear wave velocity anomalies in deep mantle From: www.seismo.berkeley.edu/~gung/_Qplume/

  33. Volatile Fluxing Mantle Wedge

  34. Volatile Fluxing of Mantle Wedge

  35. Downgoing Slab Crustal Melting

  36. Primitive Mantle Melts vs. Remelting of the Lower Crust

  37. Igneous Rocks and Plate Tectonic Setting

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