Download Igneous Rocks II: Heat, magma generation, and differentiation

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
Thermal Conductivity = K; rocks
have very low values and thus
deep heat has been retained!
Heat Flux = Thermal Conductivity * T
Crustal Geothermal Gradients
Crustal Rocks Melt!
Approximate Pressure (GPa=10 kbar)
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 !
models
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
Global Heat Flow
convection in the mantle
observed heat flow
warm: near ridges
cold: over cratons
from: http://www-personal.umich.edu/~vdpluijm/gs205.html
Causes of Mantle Melting
-Increase T
-Decrease P
-Add Water
Plagioclase Water-saturated vs. Dry Solidi
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
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
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•
•
•
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Sequence of Crystallization and Melting
Differentiation
Partial Melting
Assimilation
Mixing of Magmas
Bowen’s Reaction Series
Binary Eutectic Phase Relations
Magmatic
Differentiation:
Crystal Settling
Sedimentary Structures
in Layered Igneous Intrusions
Harzburgite bands in
Josephine Ophiolite, Oregon
From: http://www.uoregon.edu/~dogsci/kays/313/plutonic.html
Magmatic Cross-Beds
in Skaergaard Layered Intrusion
From: http://www.uoregon.edu/~dogsci/kays/313/plutonic.html
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
Contour of -2.5%
shear wave
velocity anomaly
From: ICEMELT Seismic Experiment - Wolfe et al., 1997
Numerical Simulation of Plume Melting
From: http://www.geophysik.uni-frankfurt.de/geodyn/island/tp2_en.html
Dynamic Plume Models
QuickTime™ and a GIF decompressor are needed to see this picture.
From: http://www.geophysik.uni-frankfurt.de/geodyn/island/tp2_en.html
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/
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