Transcript Igneous Textures - McGill University

Crystal Growth & Volcanic Textures
and Primary Magmas
Francis 2013
Volcanic
A few coarse-grained phenocrysts
in a fine to extremely fine-grained matrix
Plutonic
Coarse-grained equigranular rocks
olivine
basalt
Volcanic Rocks – relatively fine-grained
but
commonly phenocyrstic
andesite
Mafic
Intermediate
Felsic
rhyolite
Little happens to a superheated melt that is cooled
to its liquidus temperature because, although
ΔGo
xyl = 0, there is a positive free energy
associated with maintaining an interface between
melt and a newly formed crystal nucleii, and thus
homogeneous nucleation does not occur.
Crystallization begins when the degree of
undercooling ΔT is such that the negative ΔGxyl
offsets the positive Gsurface. For a spherical crystal:
ΔG
xyl
× 4/3 π r3 / Vm > Gsurface× 4 π r2
r* = 3 × Vm × Gsurface / ΔGxyl
Crystal nulceii with radii greater than r*
will grow. As the degree of undercooling
ΔT increases, the size distribution of
spontaneously forming and dissolving
nucleii shifts to larger r, while the
magnitude of r* decreases. Thus as ΔT
increases, the proportion of spontaneous
nucleii with r > r* will increase. These will
continue to grow into larger crystals rather
than being resorbed.
Boundary Layer Effects
A boundary layer develops at
the interface of a growing
crystal, across which both
chemical constituents and heat
must diffuse. Boundary layer
stagnation may occur when the
residual liquid produced by
crystallization in the boundary
layer becomes too evolved to
crystallize at the ambient
temperature.
Crystallization
ceases, but begins again once
the relic boundary layer has
been dissipated by convection
in the body of the intrusion.
Chemical Boundary
Layers
Diffusion versus Growth Rate
In most cases crystal nucleation and growth is delayed to
some degree of undercooling ΔT. The greater the ΔT, however,
the higher the crystal growth rate in comparison to the rate of
diffusion of chemical constituents and heat in the melt.
The shape and size of crystals is controlled by the density of
nucleii and the ratio of crystal growth rate to the diffusion
rate of chemical constituents in the adjacent melt.
At low ratios of growth rate to diffusion rate, surface
nucleation is the controlling factor, and crystals tend to
grow layer by layer and have equant dimensions with
well developed crystal faces.
High ratios of growth rate to diffusion rate leads to
diffusion-controlled growth in which layer by layer
growth breaks down and any protuberance on a crystal
face will grow and be enhanced because it has access to
higher nutrient concentrations and lower temperatures.
Cooling Rate
Versus
Grain Size and Shape
~ 1500 oC/hr
Rapid cooling rates leads to large undercooling,
high nucleation and growth rates and therefore
diffusion controlled growth
0.5 oC/hr
~ 80 oC/hr
olivine
~ 20 oC/hr
aa
pahoehoe
glassy
pillow
margin
palagonite
OLIVINE :
#l:
MgO(NM) + 1/2SiO2(NF) = MgSi0.5O2
#2:
FeO(NM) + 1/2SiO2(NF) = FeSi0.5O2
NM: Network modifying cations
NF: Network forming cations
K1 = aFo / (aMgOLiq × (aSiO2Liq)1/2) = XMgoliv / (XMg(NM))×(XSi(NF))1/2 )
K2 = aFa / (aFeOLiq × (aSiO2Liq)1/2) = XFeoliv / (XFe(NM))×(XSi(NF))1/2)
ln K =
a+b
T
a
6,700
b
-3.73
K2 6,874
-4.97
K1
aFo = XMgoliv = Mg / (Mg + Fe)oliv
aMgOliq = Mg / ∑NM
aFa = XFeoliv = Fe / (Mg + Fe)oliv
aSiO2liq = Si / ∑NF
NF = Network Formers Σ Si + Na + K
NM = Network Modifiers Σ Mg + Fe + Ni + Ca + Mn + Ti + Cr + Al - (Na + K)