Diversification of magmas
(Chapter
(Ch
11)
Fractal image: Elaine Normandy: www.stardel.com/fractals17/320x240_magma.html
Magmatic Differentiation
Any process by which magma diversifies producing a
magma or rock of different composition
T essential
Two
i l processes
1. Create compositional difference
in one or more phases (generally
a melt and solid) – due to
element partitioning
2. Preserves chemical difference byy
segregating (or fractionating)
physically- and chemicallydistinct portions
Image source:
http://www.es.mq.edu.au/GEMOC/AnnualReport/an
nrep2004/Reshighlights04.htm
1. Partial Melting
Separation of partially melted
d liquid
d from solid
d residue
d
Effects of removing liquid at various stages of melting
Eutectic systems
• First melt always = eutectic
composition
• Major element composition of
eutectic melt is constant until one
source mineral p
phase is
consumed
Image source: Winter, 2001
• Once a phase is consumed,
consumed next
increment of melt will be
different X and T
1. Partial Melting
Separation of partially melted
d liquid
d from solid
d residue
d
• requires a critical melt %
Sufficient melt must be
produced
d d ffor iit to:
• Form continuous,
interconnected film
• Have enough interior volume
that it is not all adsorbed to
crystal surfaces
Image source: Winter, 2001
1. Partial Melting
Separation of a partially melted
d liquid
d from solid
d residue
d
• requires a critical melt %
Sufficient melt must be
produced for it to:
p
• Ability to form interconnected
film - dependent upon dihedral
angle (θ ) - property of melt
• For dihedral angles <60
<60˚
interconnectivity occurs and
extraction is possible - e.g. 50˚ in
mafic rocks but 50-60˚ in
rhyolites.
Image source: Winter, 2001
1. Partial Melting
Separation of a partially melted
d liquid
d from solid
d residue
d
• requires a critical melt %
Rheological
Rh
l i l critical
iti l melt
lt
percentage ( RCMP) for
most theoretical static
situations is 30-50%, but it
has additional influences:
• Gravitational effects (buoyant
liquid)
• Fil
Filter pressing,
i
off crystall mush
h
(this reduces RCMP to 26%)
• Shear
• Function of T, viscosity,
composition add volatile content
Image source: Winter, 2001
2. Crystal Fractionation
Dominant mechanism - once formed, differentiate?
Variation
diagram for
lavas of 1959
Kilauea
Kil
eruption.
Illustrate
fractional
crystallization
based on
settling of
olivine
li i
Image source: Winter, 2001
2. Crystal Fractionation
Gravity settling
Differential motion of crystals and liquid under influence of gravity due to
differences in density
• evidence found in the sedimentary-type structures and cumulate textures
Example:
Cool point a to form olivine layer at
base of pluton - if first olivine sinks
results in dunite
a
Next get ol+cpx layer (troctolite)
finally get ol+cpx+plag layer (olivine
gabbro)
Image source: Winter, 2001
Fractional crystallization cannot explain
p
all igneous
g
rocks e.g.
g the large
g granitic
g
batholiths could not have been formed by fractionation of basalt because there
is not enough basalt in the area i.e. it should be 10x more than the granitic rocks
3. Volatile Transport
T
Three
ways to introduce
d
a vapor
1.
Vapor released by heating of hydrated
or carbonated
b
d wall
ll rocks
k
2.
As volatile-bearing (but
undersaturated) magma rises and P is
reduced, the magma may become
saturated in the vapor, and a free
vapor phase will be released and rises
to top of magma chamber
• generally H2O producing
hydrothermal alteration and/or
CO2 alteration
• Important for economic minerals
Image source: Winter, 2001
3. Volatile Transport
T
Three
ways to introduce
d
a vapor
3.
Late-stage fractional crystallization
• enriches late melt in H2O as well as
incompatible, LIL, and nonlithophile elements
• Many
M
concentrate further
f h iin vapor
• Particularly enriched with
resurgent boiling (melt already
evolved when vapor phase released)
3. Volatile Transport
Three ways to introduce a vapor
3.
Pink aplite cross-cutting Silvermines granite (St. Francois Mtns, MO) Image source:
http://web.missouri.edu/~whittingtona/photos/SEMO2005/index2.html
Late-stage fractional crystallization
Vapor and melt escape along fractures
as dikes
dik
• Silicate melt produces quartz and
feldspar as small dikes of aplite
(sugary texture)
• Vapor phase produces dikes or pods
of pegmatite (coarse crystals due to
poor nucleation
n cleation and high diffusivity
diff si it
in H2O-rich phase)
Concentrate incompatible elements (e.g. Li,
B, P, K, Na, Rb, REEs, Be ...)
Complex: varied mineralogy (some gems)
• May display concentric zonation
• Miarolitic cavities are smaller fluid
segregations
4. Magma Mixing
End member
b r mixing
i in fforr a suite
it off rrocks
k
Variation on
Harker-type
diagrams should
li on straight
lie
i h li
line
between the two
most extreme
compositions
(most are curved and
explained by fractional
crystallization of
minerals)
i
l )
4. Magma Mixing
End member mixing for a suite of rocks
Contrasting magmas will have distinct
T X,
T,
X density and viscosity - influence
magma mixing
Comingled basalt-Rhyolite Mt. McLoughlin, Oregon
Magma mixing at North Peak Village in the Sunapee Lake area,
western New Hampshire (photo by Tim Allen, NEIGC 2003).
Magma mixing in MOR may be
common and produce
disequilibrium textures (e.g.
resorption,
i
or reverse zoning)
i )
Resorbed plagioclase: (photo: wwwodp.tamu.edu/.../209_IR/chap_10/c10_f29.htm)
5. Assimilation
In rp r ti n off wall
Incorporation
ll rrocks
k (diff
(diffusion,
i n xenoliths)
n lith )
Assimilation by melting is limited by heat available in
m m
magma
Zone melting
g
• Crystallizing igneous material at
base equivalent to amount melted
at the top
• Transfer heat byy convection
Image source: www.radiointel.com/phil/borg2.jpg
5. Assimilation
In rp r ti n off wall
Incorporation
ll rrocks
k (diff
(diffusion,
i n xenoliths)
n lith )
Assimilation by melting is limited by heat available in
m m
magma
Detecting and assessing assimilation
• Isotopes
I
are generally
ll b
best
• Continental crust becomes
progressively enriched in 87Sr/86Sr
and depleted in 143Nd/144Nd
Image source: www.radiointel.com/phil/borg2.jpg
• U-Th-Pb system as indicator of
continental contamination is
particularly useful
6. Boundary Layer Crystallization
b nd r llayers
boundary
r n
nearr ttop
p and
nd sides
id off magma chambers
h b r
Langmuir model
z
z
Image source: Winter (2001)
Thermal gradient at wall and
cap results in variation in %
crystallized
Compositional convection
leads to evolved magmas
from boundary layer to cap
(or mix into interior)