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 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.

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 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!

Crustal Geothermal Gradients

Crustal Rocks Melt!

Earth’s Geothermal GradientA

ppro

xim

ate

Pre

ssur

e (G

Pa=

10 k

bar)

Average Heat Flux is0.09 watt/meter2

or 90 mW/m2

Geothermal gradient = / z

C/km in orogenic belts;Cannot remain constant w/depthAt 200 km would be 4000°C

~7°C/km in trenches

Viscosity, which measuresresistance to flow, of mantlerocks is 1018 times tar at 24°C !

convection in the mantle

models

observed heat flowwarm: near ridgescold: over cratons

from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270

from: http://www-personal.umich.edu/~vdpluijm/gs205.html

Global Heat Flow

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 clusteredin 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

Binary Eutectic Phase Relations

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

Magmatic Differentiation: Assimilation

Evidence for Assimilation - Adirondacks

From: http://s01.middlebury.edu/GL211A/FieldTrip2.htm

MagmaticDifferentiation: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 wavevelocity anomaly

Numerical Simulation of Plume Melting

From: http://www.geophysik.uni-frankfurt.de/geodyn/island/tp2_en.html

Dynamic Plume Models

QuickTime™ and aGIF decompressorare needed to see this picture.

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