Slide 1Creating Diverse Magma CompositionsCrustal Processes creating Magma DiversityMagmatic Differentiation driven by Fractional CrystallizationMagmatic Differentiation driven by Fractional Crystallization Crystal Density vs MagmaMagmatic Differentiation driven by Fractional CrystallizationMagmatic Differentiation driven by Fractional CrystallizationMagmatic Differentiation driven by Fractional Crystallization The Classic Skaergaard Intrusion of East GreenlandSlide 9Magmatic Differentiation driven by Diffusion-Driven CrystallizationMagmatic Differentiation driven by Liquid Immiscibility A Rare OccurenceMagmatic Differentiation driven by Volatile TransportCrustal Melting (Anatexis)Magma Mixing Difficult to Blend Mafic and Felsic Magmas because of Highly Contrasting Viscosity (Effectively Immiscible)Magma Mixing in the Sonju ?Mixing or Fractional Crystallization? Mixing should generate straight linesAssimilation Selective Contamination of Magma by country RockSlide 18GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGYLecture 11Diversification of MagmasFebruary 18, 2009CREATING DIVERSE MAGMA COMPOSITIONSProcesses 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 DIVERSITYMagmatic Differentiation driven by:o Fractional Crystallizationo Diffusion-controlled Crystallizationo Volatile Transporto Liquid ImmiscibilityCrustal Melting (Anatexis)Magma MixingCrustal AssimilationMAGMATIC DIFFERENTIATION DRIVEN BY FRACTIONAL CRYSTALLIZATIONSystem – slow cooled mafic intrusionsProcess – seperation of crystal from parent liquid• gravity settling• filter pressing • flow segregationMAGMATIC DIFFERENTIATION DRIVEN BY FRACTIONAL CRYSTALLIZATIONCRYSTAL DENSITY VS MAGMA(Mg,Ca)Sparks and Huppert (1984)Kushiro(1980)MAGMATIC DIFFERENTIATION DRIVEN BY FRACTIONAL CRYSTALLIZATIONExtracting a mineral assemblage that has a different composition than the parent magma will force the remaining magma to change composition- i.e., differentiateMAGMATIC DIFFERENTIATION DRIVEN BY FRACTIONAL CRYSTALLIZATION+ Fe-oxideTroctoliteDuniteGabbroCryptic Layering in the Layered Series at DuluthTypical Igneous Stratigraphy of Shallow Tholeiitic Mafic Layered IntrusionsMAGMATIC DIFFERENTIATION DRIVEN BY FRACTIONAL CRYSTALLIZATIONTHE CLASSIC SKAERGAARD INTRUSION OF EAST GREENLANDLo-PHi-PPrimitive Ol TholeiitePolybaric DifferentiationPolybaric DifferentiationBasalts of the NSVGBasalts of the NSVGShifts in phase equilibrium (i.e. positions of cotectics and eutectics) due to pressureMAGMATIC DIFFERENTIATION DRIVEN BY DIFFUSION-DRIVEN CRYSTALLIZATIONMAGMATIC DIFFERENTIATION DRIVEN BY LIQUID IMMISCIBILITYA RARE OCCURENCEMAGMATIC DIFFERENTIATION DRIVEN BY VOLATILE TRANSPORT Vapor/Fluid can easily flux in and out of magma systems; Controlled by temperature and chemical gradientsA. Flux into magma from dehydration of wall rocksB. Flux out of system by degassing of magmaC. Build up of volatiles by fractional crystallizationABCWinter (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 graniteMAGMA MIXING IN THE SONJU ?Zone of Mixing?MIXING OR FRACTIONAL CRYSTALLIZATION?MIXING SHOULD GENERATE STRAIGHT LINESASSIMILATIONSELECTIVE CONTAMINATION OF MAGMA BY COUNTRY ROCKCreation of the Cu-Ni-PGE Sulfide Deposits of the Duluth ComplexSSCu Ni Co Pd + Pt