G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 9a Diversification of Magmas February 17, 2016

C REATING D IVERSE M AGMA C OMPOSITIONS 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.

C RUSTAL P ROCESSES CREATING M AGMA D IVERSITY Magmatic Differentiation driven by: o Fractional Crystallization o Diffusion-controlled Crystallization o Volatile Transport o Liquid Immiscibility Crustal Melting (Anatexis) Magma Mixing Crustal Assimilation

M AGMATIC D IFFERENTIATION DRIVEN BY F RACTIONAL C RYSTALLIZATION System – slow cooled mafic intrusions Process – seperation of crystal from parent liquid gravity settling filter pressing flow segregation

M AGMATIC D IFFERENTIATION DRIVEN BY F RACTIONAL C RYSTALLIZATION C RYSTAL D ENSITY VS M AGMA (Mg,Ca) Sparks and Huppert (1984) Kushiro(1980)

M AGMATIC D IFFERENTIATION DRIVEN BY F RACTIONAL C RYSTALLIZATION Extracting a mineral assemblage that has a different composition than the parent magma will force the remaining magma to change composition- i.e., differentiate

M AGMATIC D IFFERENTIATION DRIVEN BY F RACTIONAL C RYSTALLIZATION + Fe-oxide Troctolite Dunite Gabbro Cryptic Layering in the Layered Series at Duluth Typical Igneous Stratigraphy of Shallow Tholeiitic Mafic Layered Intrusions

M AGMATIC D IFFERENTIATION DRIVEN BY F RACTIONAL C RYSTALLIZATION T HE C LASSIC S KAERGAARD I NTRUSION OF E AST G REENLAND

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

M AGMATIC D IFFERENTIATION RESULTING FROM D IFFUSION -D RIVEN C RYSTALLIZATION

M AGMATIC D IFFERENTIATION DRIVEN BY L IQUID I MMISCIBILITY A R ARE O CCURENCE

M AGMATIC D IFFERENTIATION DRIVEN BY V OLATILE T RANSPORT 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). C RUSTAL M ELTING (A NATEXIS )

M AGMA M IXING D IFFICULT TO B LEND M AFIC AND F ELSIC M AGMAS BECAUSE OF H IGHLY C ONTRASTING V ISCOSITY (E FFECTIVELY I MMISCIBLE ) 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

M AGMA M IXING IN THE S ONJU ? Zone of Mixing?

M IXING OR F RACTIONAL C RYSTALLIZATION ? M IXING SHOULD GENERATE STRAIGHT LINES

A SSIMILATION S ELECTIVE C ONTAMINATION OF M AGMA BY COUNTRY R OCK

Creation of the Cu-Ni-PGE Sulfide Deposits of the Duluth ComplexS Cu Ni Co Pd + Pt Au S