1. Geol 2312 Igneous and Metamorphic Petrology
Lecture 11
Diversification of Magmas
February 18, 2009

2. 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.

3. Crustal Processes creating Magma Diversity
Magmatic Differentiation driven by:
Fractional Crystallization
Diffusion-controlled Crystallization
Volatile Transport
Liquid Immiscibility
Crustal Melting (Anatexis)
Magma Mixing
Crustal Assimilation

4. Magmatic Differentiation driven by Fractional Crystallization
System – slow cooled mafic intrusions
Process – seperation of crystal from parent liquid
gravity settling
filter pressing
flow segregation

5. Magmatic Differentiation driven by Fractional Crystallization Crystal Density vs Magma
(Mg,Ca)
Sparks and Huppert (1984)
Kushiro(1980)

6. 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

7. 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

8. Magmatic Differentiation driven by Fractional Crystallization The Classic Skaergaard Intrusion of East Greenland

9. Polybaric Differentiation
Basalts of the NSVG
Lo-P
Hi-P
Shifts in phase equilibrium (i.e. positions of cotectics and eutectics) due to pressure
Primitive Ol Tholeiite

10. Magmatic Differentiation driven by Diffusion-Driven Crystallization

11. Magmatic Differentiation driven by Liquid Immiscibility A Rare Occurence

12. Magmatic Differentiation driven by Volatile Transport
Vapor/Fluid can easily flux in and out of magma systems;
Controlled by temperature and chemical gradients
Flux into magma from dehydration of wall rocks
Flux out of system by degassing of magma
Build up of volatiles by fractional crystallization B C A

13. Crustal Melting (Anatexis)
Winter (2001) Figure 11-3 Position of the H2O-saturated ternary eutectic in the albite-orthoclase-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).

14. 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)

15. Magma Mixing in the Sonju ?
Zone of Mixing?

16. Mixing or Fractional Crystallization
Mixing or Fractional Crystallization? Mixing should generate straight lines

17. Assimilation Selective Contamination of Magma by country Rock

18. Creation of the Cu-Ni-PGE Sulfide Deposits of the Duluth Complex
Pd + Pt Au S S