GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 11

Diversification of Magmas

February 18, 2009

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 Crystallizationo Diffusion-controlled Crystallizationo Volatile Transporto 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 CRYSTALLIZATIONCRYSTAL 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

+ Fe-oxide

Troctolite

Dunite

Gabbro

Cryptic Layering in the Layered Series at Duluth

Typical Igneous Stratigraphy of Shallow Tholeiitic Mafic Layered Intrusions

MAGMATIC DIFFERENTIATION DRIVEN BY FRACTIONAL CRYSTALLIZATION

THE CLASSIC SKAERGAARD INTRUSION OF EAST GREENLAND

Lo-PHi-P

Primitive Ol Tholeiite

Polybaric DifferentiationPolybaric DifferentiationBasalts of the NSVGBasalts of the NSVG

Shifts in phase equilibrium (i.e. positions of cotectics and eutectics) due to pressure

MAGMATIC DIFFERENTIATION DRIVEN BY DIFFUSION-DRIVEN

CRYSTALLIZATION

MAGMATIC DIFFERENTIATION DRIVEN BY LIQUID IMMISCIBILITYA 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

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

A

B

C

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).

CRUSTAL MELTING (ANATEXIS)

MAGMA MIXINGDIFFICULT TO BLEND MAFIC AND FELSIC

MAGMAS BECAUSE OF HIGHLY CONTRASTING VISCOSITY (EFFECTIVELY IMMISCIBLE)

Basalt pillows accumulating at the bottom of a in granitic magma chamber, Vinalhaven Island,

Maine (From Winter, 2001)

Upper “Chilled” margin of the Layered Series at Duluth with bulbous fine gabbro in medium granite

MAGMA MIXING IN THE SONJU ?

Zone of Mixing?

MIXING OR FRACTIONAL CRYSTALLIZATION?MIXING SHOULD GENERATE STRAIGHT LINES

ASSIMILATIONSELECTIVE CONTAMINATION OF MAGMA BY COUNTRY

ROCK

Creation of the Cu-Ni-PGE Sulfide Deposits of the Duluth Complex

SSCu Ni Co Pd + Pt Au

SS