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Geol 2312 Igneous and Metamorphic Petrology

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)

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Geol 2312 Igneous and Metamorphic Petrology

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  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 CrystallizationCrystal 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 CrystallizationThe 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 ImmiscibilityA 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 MixingDifficult 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 should generate straight lines

  17. AssimilationSelective Contamination of Magma by country Rock

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

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