Download F08 4 Igneous

760 kA - last eruption
Geohazzards and the public
Igneous Rocks
Melting Earth Materials
Magma
Igneous rocks are those formed from partially molten
(melted) materials
Partially molten rock is called magma as it moves
through the Earth, and lava when it is extruded at the
surface.
From where does magma originate?
Partial melting of the mantle and crust due to
•Influxes of heat
•Decreasing pressure
•The movement of water or other fluids
Pressure & Temperature
Most materials
have positive
slopes between
their states
For rocks, we
need to consider
the solid-liquid
changes. Adding
water changes
melting
relationship
SiO2 and melting
Partial melting in S.W. Maine
Tectonic thinning of the crust
When plates move away from each other, it
stretches and thins the crust, reducing
pressure on the mantle.
Tectonic increase of
crustal thickness
Thermochemical plumes
Plume theory
Earth is hottest at its
core, the release of heat
may be convective mass transferred. Some
masses may move as
buoyant ductile bodies.
Diagram by J. Tarney
End of the ride… if such plumes exist, they would
become more or less stalled at the transition to
brittle mantle.
Diagram by J. Tarney
Crystal-liquid interfaces
Hornblende
Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH, F)2
Four crystals growing
in melt. Note angular
faces typical of the 2/m
crystals as seen from
the 001 plane.
Planes (facets) result from
energy minimization along
a crystallographic plane depends on T, P, and X.
Crystallization
On this phase
diagram, there
are two phases,
a solid and a
liquid.
The line
represents the
conditions
where both will
be present at
equilibrium
From Blackburn & Dennen,
1998
Growth interface
Diffusional growth
Crystal growth results from diffusion of components
to the crystal surface
Crystals can grow in
any medium - solids,
liquids, gasses,
supercritical fluids.
Liquids and fluids may
be melted rocks, C-OH or aqueous fluids, or
a mixture. All are
contingent on
component transport.
Reactive growth
New minerals may form from
recrystallization of reaction of
preexisitng grains.
Overgrowth, mantling, coronas
This is a diffusion driven process.
This may result from a change in
conditions or composition during
growth
Compositional zoning
Changes in physical
conditions or in
surrounding composition
may be reflected in
crystals
Ti variations as an
impurity in quartz (CL
image by D. Wark)
REE variations as an
impurity in zircon (BSE &
CL images by E.B.
Watson)
Bowen’s reaction series
Mineral names carry
a certain mystique rock names seem
more utilitarian
Igneous nomenclature
“archaic” texture discrimination
boundary at 1mm.
An alkali feldspar and quartz
dominant rock with an average
grainsize >1 mm is a granite, <1
mm is a rhyolite.
> 1mm Phaneritic
< 1 mm Aphanitic
Glassy - vitrophyric
Common rock names
Dacite
Diorite
Tonalite
Granite
Andesite
Rhyolite
Gabbro
Basalt
Anorthosite
Phonolite
Foid Syenite
Mean grain size > 1mm
Mean grain size < 1mm
After Strekeisen, 1978
mineral.galleries.com
SiO2
Quartz | Coesite | Stishovite
De minuscules cristaux de quartz ont une
disposition radiale autour de la coesite : ceci
montre que le quartz se forme au d師riment de la
coesite (LPA)
High birefringence Stishovite in coesite, synthetically grown
by J. Mosenfelder, CalTech.
Feldspars
Plagioclase Feldspar
Anorthite
(CaAl2Si2O8)
Alkali Feldspar
Albite (NaAlSi3O8)
Orthoclase (KAlSi3O8)
Images from mineral.galleries.com
Feldspathoids
dkimages
Leucite (KAlSi2O6)
Nepheline (NaAlSiO4)
Sodalite (Na4Al3Si3O12Cl)
Image from mineral.galleries.com
Grain size origins
Surface extrusion (volcanism)
forces molten material to cool
rapidly in the air. Grain sizes
are small.
Intrusion (plutonism)
forces molten material
to cool slowly deep
underground. Grain
sizes are large.
Cooling rapidly can limit the organization of atoms into
lattices. Fast = glass, slower = crystalline, slower still =
bigger crystals
From Blackburn & Dennen,
1998
Ultramafic (very Mg-Fe rich) rocks
Coarse: Dunite, Lherzolite, Harzburgite, Periodotite
Fine: Kimberlite
Minerals: these are rocks largely comprised of olivine
(isolated silicate) and pyroxene (single-chain silicate)
Origin: solidification of early Earth
Location: the mantle
Univ. North Carolina, Atlas of rocks,
minerals, and textures website
Classification
Nesosilicates
Olivine
Forsterite Mg2SiO4
Fayalite Fe2SiO4
Image from mineral.galleries.com
Inosilicates (singles)
Pyroxene
Orthopyroxene - hypersthene
Enstatite Mg2Si2O6
Orthoferrosilite Fe2Si2O6
Clinopyroxene - Augite
Diopside CaMgSi2O6
Hedenbergite CaFeSi2O6
Image from mineral.galleries.com
Kimberlite, lamprophyre
Volcanic ultramafic
rock. Very rare, and
sourced straight from
the mantle - eruption
rates must be
impressive. Some are
diamondiferous.
Univ. North Carolina, Atlas of rocks,
minerals, and textures website
IUGS Crystaline
Coarse
crystaline rock
classification note that the
scheme
precludes the
coexistence of
foids and SiO2.
IUGS Fine grained
Fine grained
rock
classification compare with
the previous
slide.
This is a nice
diagram, but
the fine-grained
nature of these
rocks can
impede its use.
IUGS TAS
If you really want to classify a volcanic rock - powder
it and analyze its composition. How does this relate
to the previous slide? What of plutonic rocks?
Mafic (Mg-Fe rich) Rocks
Coarse: Gabbro, Anorthosite
Fine: Basalt
Minerals: Largely olivine (isolated), pyroxene (singlechain), and feldspar (framework).
Origin: Partial melting of the mantle
Oceanic crust, oceanic islands, the moon.
Anorthosite
Univ. North Dakota, Plutonic Images
Univ. North Carolina, Atlas of rocks, minerals,
and textures website
Basalt
Gabbro
Univ. North Carolina, Atlas of rocks, minerals,
and textures website
If most of the heat needed for
melting is deep within the earth,
why are there volcanoes?
Magma is driven upwards.
Parameters - density, viscosity
Recall density = mass / volume
Viscosity is a measure of resistance to flow maple syrup is more viscous than water
Ascent of Magmas
Hogan et al., 1998
Kilauea, Hawaii
March, 1996
Hawaiian magmatism
Two plumes?
A cross section of a shield volcano
Generally low Si
Low volatile
High rate
Figure 20.19
20-406
Mauna Kea, Hawaii
Viscosity and Lava
Pu’u Hulu
Mauna Loa, Hawaii
Pahoehoe
Viscosity - Resistance to flow
Compare water to maple syrup - the latter is many times
more viscous.
What makes for runny (less viscous) magma
Temperature
Viscocity is almost always reduced in hotter fluids
Lower silica content
Lower silica content means fewer tetrahedra sharing
oxygens - think about the minerals in mafic rocks
Volatile (H2O, CO2, HF, H2SO4) content
Many of these components work to depolymerize the
magma.
Fluorine and evolution
“To many petrologists a
volatile component…
does just what one
wants it to do.”
N.L. Bowen, 1928
High -T crystals
largely cannot
incorporate large
amounts of
components like H2O,
OH, F, Cl, and SO2.
Oceanic rifting
Continental Rift
http://jsc.nasa.gov
STS-32
Example: the East African Rift
Rifting - Birth of an ocean
Diagram by J. Tarney
Question 4: how does fluorine influence rift
magmatism?
Cartoon to left shows
some general attributes
of rift formation
Presumably, the tension
that drives rifting is not
well understood.
Diagram by J. Tarney
The minimum liquid fractions at each temperature
and pressure give the melting curve that we use
for mantle rocks. DJ DePaolo
By adding a small amount of water, the
degree of melting increases significantly.
With water - melting starts much deeper
Intermediate rocks (less Fe-Mg, more Na-K-Si-Al)
Coarse: Granodiorite, Tonalite Fine: Andesite, Dacite
Minerals: Feldspar (framework), pyroxene (single
chain), amphibole (double chain), and quartz
(framework)
Origin: liquid fraction of crystallizing mafic liquids.
Location: Convergent tectonic settings
Tonalite
Subduction: Island Arcs
Example: the Aleutians
Subduction: Continental Arcs
Example: the Cascades
Mount Saint Helens:
May 18, 1980 0832
View from South Rim, June 1991
Q: What is a
volcanic dome?
Crater Lake
Oregon
Felsic (K, Na, and Si rich) rocks
Coarse: Granite
Fine: Rhyolite
Minerals: Feldspar and quartz (framework), with small
amounts of amphibole (double chain) and mica (sheet).
Origins: Partial melting of crustal rocks and liquid
fraction of crystallizing intermediate liquids.
Location: continental magmatism
Granite
Mount Scott Granite
An intergrowth of
quartz and feldspar
Likely result of too few
nucleation sites
Undercooling
Viscosity contrasts
Rapid diffusion
Making granites?
Metamorphic
materials that
exceed the
solidus for
the system
start to melt
Migmatite
Photo by Mike Brown (UMD)
Felsic rocks
Univ. North Carolina, Atlas of rocks,
minerals, and textures website
Rhyolite
©Pam Gore, GPC
Vitrophyre
Pegmatite
Pegmatites are
typically coarsely
crystalline - may
contain rare
minerals. Most
result from
extreme
fractionation.
Slow cooling
Low nucleation
Lots of H2O