how magma forms - wordpress.com · 2017. 11. 28. · how magma forms • sources of heat for...
TRANSCRIPT
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How Magma Forms
• Sources of heat for melting rocks• Factors that control melting temperatures• Other considerations:
– Volatiles– Change in Pressure (Decompression Melting)
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Heat Flow on EarthAn increment of heat, q, transferred into a body produces aproportional incremental rise in temperature, T, given by
q = Cp * T
where Cp is called the molar heat capacity of J/mol-degreeat constant pressure; similar to specific heat, which is basedon mass (J/g-degree).
1 calorie = 4.184 J and is equivalent to the energy necessaryto 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.
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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.
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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.
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Magmatic Examples of Heat TransferThermal Gradient = T betweenadjacent hotter and cooler masses
Heat Flux = rate at which heat isconducted over time from a unitsurface area
Heat Flux = Thermal Conductivity * T
Thermal Conductivity = K; rockshave very low values and thusdeep heat has been retained!
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convection in the mantle
models
observed heat flow warm: near ridges cold: over cratons
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
from: http://www-personal.umich.edu/~vdpluijm/gs205.html
Global Heat Flow
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Earth’s Geothermal GradientA
ppro
xim
ate
Pres
sure
(GPa
=10
kbar
)
Average Heat Flux is0.09 watt/meter2
or 90 mW/m2
Geothermal gradient = T/ z
Viscosity, which measuresresistance to flow, of mantlerocks is 1018 times tar at 24°C !
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Crustal Geothermal Gradients
Crustal Rocks Melt!
20-30 °C/km in orogenic belts; gradient cannot remain constant with depth!
At 200 km would be 4000°C
In contrast, gradient is ~7 °C/km in trenches
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Causes of Mantle Melting
-Increase T
-Decrease P
-Add Water
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Plagioclase Water-saturated vs. Dry Solidi
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Alkaline vs. Sub-alkaline Rocks
Analyses of a global sample of 41,000 igneous rocks of all ages
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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?
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How Magmas of Different � Compositions Evolve
• Sequence of Crystallization and Melting• Differentiation• Partial Melting• Assimilation• Mixing of Magmas
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Bowen’s Reaction Series
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Magmatic �Differentiation: �Crystal Settling
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Sedimentary Structures �in Layered Igneous Intrusions
From: http://www.uoregon.edu/~dogsci/kays/313/plutonic.html
Harzburgite bands in Josephine Ophiolite, Oregon
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Magmatic Cross-Beds �in Skaergaard Layered Intrusion
From: http://www.uoregon.edu/~dogsci/kays/313/plutonic.html
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Binary Eutectic Phase Relations
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Magmatic Differentiation: Assimilation
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Evidence for Assimilation - Adirondacks
From: http://s01.middlebury.edu/GL211A/FieldTrip2.htm
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Magmatic�Differentiation:�Magma �Mixing
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Melt Inclusions in Quartz in Pantellerite
From: http://wrgis.wr.usgs.gov/lowenstern/Mahood and Lowenstern, 1991
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Evidence for Magma Mixing - Adirondacks
From: http://s01.middlebury.edu/GL211A/FieldTrip3.htm
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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
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Earth’s Plates
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MORB �Decompression �Melting
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Decompression Melting and MORB Genesis
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Mantle Plumes - “Hot” or “Wet” Spots?
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Seismic Tomographic Image of Iceland Plume
From: ICEMELT Seismic Experiment - Wolfe et al., 1997
Contour of -2.5%shear wavevelocity anomaly
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Numerical Simulation of Plume Melting
From: http://www.geophysik.uni-frankfurt.de/geodyn/island/tp2_en.html
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Dynamic Plume Models
From: http://www.geophysik.uni-frankfurt.de/geodyn/island/tp2_en.html
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Super Plumes?
From: www.seismo.berkeley.edu/~gung/_Qplume/
Volcanic Hot Spots on Earth’s Surface (dots)
Global shear wave velocity anomalies in deep mantle
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Volatile Fluxing Mantle Wedge
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Volatile Fluxing of Mantle Wedge
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Downgoing Slab Crustal Melting
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Primitive Mantle Melts vs. �Remelting of the Lower Crust
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Igneous Rocks and Plate Tectonic Setting