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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)
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.
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.
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.
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
Heat Flux = Thermal Conductivity * T
Thermal Conductivity = K; rocks have very low values and thus deep heat has been retained!
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
Earth’s Geothermal Gradient A
ppro
xim
ate
Pres
sure
(GPa
=10
kbar
)
Average Heat Flux is 0.09 watt/meter2
or 90 mW/m2
Geothermal gradient = T/ z
Viscosity, which measures resistance to flow, of mantle rocks is 1018 times tar at 24°C !
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
Causes of Mantle Melting
-Increase T
-Decrease P
-Add Water
Plagioclase Water-saturated vs. Dry Solidi
Alkaline vs. Sub-alkaline Rocks
Analyses of a global sample of 41,000 igneous rocks of all ages
<- Basalts
46.7% widely scattered
53.3% tightly clustered in a central band
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?
How Magmas of Different � Compositions Evolve
• Sequence of Crystallization and Melting • Differentiation • Partial Melting • Assimilation • Mixing of Magmas
Bowen’s Reaction Series
Magmatic �Differentiation: �Crystal Settling
Sedimentary Structures �in Layered Igneous Intrusions
From: http://www.uoregon.edu/~dogsci/kays/313/plutonic.html
Harzburgite bands in Josephine Ophiolite, Oregon
Magmatic Cross-Beds �in Skaergaard Layered Intrusion
From: http://www.uoregon.edu/~dogsci/kays/313/plutonic.html
Binary Eutectic Phase Relations
Magmatic Differentiation: Assimilation
Evidence for Assimilation - Adirondacks
From: http://s01.middlebury.edu/GL211A/FieldTrip2.htm
Magmatic�Differentiation:�Magma �Mixing
Melt Inclusions in Quartz in Pantellerite
From: http://wrgis.wr.usgs.gov/lowenstern/ Mahood and Lowenstern, 1991
Evidence for Magma Mixing - Adirondacks
From: http://s01.middlebury.edu/GL211A/FieldTrip3.htm
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
Earth’s Plates
MORB �Decompression �Melting
Decompression Melting and MORB Genesis
Mantle Plumes - “Hot” or “Wet” Spots?
Seismic Tomographic Image of Iceland Plume
From: ICEMELT Seismic Experiment - Wolfe et al., 1997
Contour of -2.5% shear wave velocity anomaly
Numerical Simulation of Plume Melting
From: http://www.geophysik.uni-frankfurt.de/geodyn/island/tp2_en.html
Dynamic Plume Models
From: http://www.geophysik.uni-frankfurt.de/geodyn/island/tp2_en.html
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
Volatile Fluxing Mantle Wedge
Volatile Fluxing of Mantle Wedge
Downgoing Slab Crustal Melting
Primitive Mantle Melts vs. �Remelting of the Lower Crust
Igneous Rocks and Plate Tectonic Setting