GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 9a
Diversification of Magmas
February 17, 2016
CREATING DIVERSE MAGMA COMPOSITIONS
Processes Occurring in the Mantle
• different degrees of partial melting of the
mantle
• mantle melting at different pressures (depth)
• role of volatiles in lowering the melting
temperature
• fractional crystallization in the mantle
• melting different compositions on mantle
(fertile vs. depleted)
THESE ACCOUNT FOR VARIABLE MAFIC
COMPOSITIONS (e.g. ALKALIC – THOLEIITIC), BUT NOT
THE WIDE RANGE OF IGNEOUS COMPOSITIONS
OBSERVED IN THE CRUST (e.g., MAFIC – FELSIC).
To explain this diversity, we need to consider other processes
that occur in the crust.
CRUSTAL PROCESSES CREATING MAGMA
DIVERSITY
Magmatic Differentiation driven by:
o Fractional Crystallization
o Diffusion-controlled Crystallization
o Volatile Transport
o Liquid Immiscibility
Crustal Melting (Anatexis)
Magma Mixing
Crustal Assimilation
MAGMATIC DIFFERENTIATION DRIVEN BY
FRACTIONAL CRYSTALLIZATION
System – slow cooled
mafic intrusions
Process – seperation of
crystal from parent liquid
• gravity settling
• filter pressing
• flow segregation
MAGMATIC DIFFERENTIATION DRIVEN BY
FRACTIONAL CRYSTALLIZATION
CRYSTAL DENSITY VS MAGMA
(Mg,Ca)
Sparks and Huppert (1984)
Kushiro(1980)
MAGMATIC DIFFERENTIATION DRIVEN BY
FRACTIONAL CRYSTALLIZATION
Extracting a mineral assemblage
that has a different composition
than the parent magma will force
the remaining magma to change
composition- i.e., differentiate
MAGMATIC DIFFERENTIATION DRIVEN BY
FRACTIONAL CRYSTALLIZATION
Typical Igneous Stratigraphy of Shallow
Tholeiitic Mafic Layered Intrusions
Cryptic Layering in the
Layered Series at Duluth
Troctolite
Gabbro
Dunite
+ Fe-oxide
MAGMATIC DIFFERENTIATION DRIVEN BY
FRACTIONAL CRYSTALLIZATION
THE CLASSIC SKAERGAARD INTRUSION OF EAST GREENLAND
Polybaric Differentiation
Lo-P
Hi-P
Basalts of the NSVG
Shifts in phase equilibrium (i.e. positions of
cotectics and eutectics) due to pressure
Primitive Ol Tholeiite
MAGMATIC DIFFERENTIATION RESULTING
FROM DIFFUSION-DRIVEN CRYSTALLIZATION
MAGMATIC DIFFERENTIATION DRIVEN BY
LIQUID IMMISCIBILITY
A RARE OCCURENCE
MAGMATIC DIFFERENTIATION DRIVEN BY
VOLATILE TRANSPORT
Vapor/Fluid can
easily flux in and out
of magma systems;
Controlled by
temperature and
chemical gradients
B
C
A
A. Flux into
magma from
dehydration of
wall rocks
B. Flux out of
system by
degassing of
magma
C. Build up of
volatiles by
fractional
crystallization
CRUSTAL MELTING (ANATEXIS)
Winter (2001) Figure 11-3 Position of the
H2O-saturated ternary eutectic in the albiteorthoclase-silica system at various pressures.
The shaded portion represents the
composition of most granites. Included are
the compositions of the Tuolumne Intrusive
Series (Figure 4-32), with the arrow showing
the direction of the trend from early to late
magma batches. Experimental data from
Wyllie et al. (1976).
MAGMA MIXING
DIFFICULT TO BLEND MAFIC AND FELSIC MAGMAS
BECAUSE OF HIGHLY CONTRASTING VISCOSITY
(EFFECTIVELY IMMISCIBLE)
Upper “Chilled” margin of the Layered Series at
Duluth with bulbous fine gabbro in medium granite
Basalt pillows accumulating at the bottom of a in
granitic magma chamber, Vinalhaven Island,
Maine (From Winter, 2001)
MAGMA MIXING IN THE SONJU ?
Zone of Mixing?
MIXING OR FRACTIONAL CRYSTALLIZATION?
MIXING SHOULD GENERATE STRAIGHT LINES
ASSIMILATION
SELECTIVE CONTAMINATION OF MAGMA BY COUNTRY ROCK
Creation of the Cu-Ni-PGE Sulfide Deposits of the Duluth Complex
S
Cu
S
Ni
Co
Pd + Pt
Au