Igneous and Metamorphic Petrology - Week 7b Notes

Week 7b – Melting and Differentiation

Magma Differentiation

• Magma differentiation is influenced by several factors:
  • Melting of distinct sources.
  • Varying degrees of partial melting from one source.
  • Fractional crystallization.
  • Magma mixing.
  • Crustal assimilation or contamination.
  • Liquid immiscibility.

Harker Diagrams

• Harker diagrams plot the evolution of igneous rocks against $SiO_2$, which serves as a trace of differentiation.
• In Harker diagrams:
  • $SiO_2$ behaves as an incompatible element, increasing with differentiation.
  • Other element concentrations are controlled by fractionating phases.
  • Compatible elements decrease in the residual melt.
  • Incompatible elements increase in the residual melt.
• Kinks/inflections in Harker diagrams indicate changes in phase assemblage during basalt crystallization.
  • Early olivine (olv) and clinopyroxene (cpx) crystallization removes $MgO$ from the melt.
  • Plagioclase (plag) crystallization decreases $Al<em>2O</em>3$ and $CaO$.
  • $Na<em>2O$/$K</em>2O$ increases until alkali feldspars appear.

Element Compatibility

• Element compatibility determines how an element evolves during magma differentiation.
• It's defined by the distribution constant ($K_D$):
  • $K_D = \frac{\text{element in solid}}{\text{element in melt}}$
  • If K_D > 1, the element prefers the solid (compatible).
  • If K_D < 1, the element prefers the melt (incompatible).

Pseudo-Harker Diagrams

• Pseudo-Harker diagrams use $MgO$ and have a reverse scale.
• The slope of differentiation indicates what is crystallizing and the proportions.

Cumulates

• Cumulates are products of fractional crystallization in magma chambers.
  • $\Delta$ (density difference) controls whether crystals float or sink.
  • Stoke’s law controls the rate of settling.
• Mafic phases tend to sink, while feldspars can float in mafic magma.

Cumulate Textures

• Crystal settling leads to cumulate formation.
• Perfect spherical close packing has ~25% empty volume.
• A real cumulate pile resulting from crystal settling alone would have > 25% porosity.
• Orthocumulates:
  • Contain > 25% interstitial crystals/liquid.
  • Typically contain euhedral cumulus phases surrounded by anhedral postcumulus phases forming a poikilitic texture.
• Mesocumulates:
  • Contain < 25% and > 7% interstitial material.
• Adcumulates:
  • Contain < 7% interstitial material due to compaction, densification, Oswald ripening, and melt migration.

Cooling in Magma Chambers

• Cooling occurs from the outside inwards, promoting crystallization.

Layered Mafic Intrusions

• Layered mafic intrusions:
  • Are a special group of mafic intrusions.
  • Largest ones are typically Archean, possibly due to higher mantle temperatures.
  • Formed by cooling of one huge batch of magma.

Skaergaard Intrusion

• Located in Greenland and is ~56 Ma (tertiary).
• It experienced relatively few pulses of magma.
• Sections:
  • Marginal Border Group: Crystallized inward from walls.
  • Layered Series: > 2700m of gabbroic rocks crystallized bottom to top.
    • Lower Zone: Olivine-bearing.
    • Middle Zone: Two pyroxenes only (with inverted pigeonite).
    • Upper Zone: Fe-rich olivine.
  • Upper Border Group: Crystallized from top down, similar to layered series mineralogy.
  • Hidden Zone: No surface outcrops, early mafic cumulates similar to Marginal Border Zone.
  • Sandwich Horizon: Final layer of melt between UBZ & LZ.
    • Very evolved liquids heavily enriched in incompatible elements during crystallization.
    • Two immiscible liquids are produced (silicate and sulfide).
    • PGEs (Pt, Pd, Re etc) + Au scavenged into sulfide melt.
    • They crystallized to produce the Platinova Au-Pd reef.
      • 60% of the world’s Pt supply comes from here.
      • Anything > 1ppm Pt is economic.
      • Up to 10ppm Pt in this horizon → 10g of Pt per ton of rock.

Bushveld Complex

• Mafic intrusion > 100,000 $km^2$.
• 8 km thick cumulate sequence.
• A hugely important economic resource.
• Marginal zone: norite (opx + plag rock) with assimilation of surrounding country rock (quartz + biotite).
  • Mineral composition fluctuations demonstrate multiple magma pulses
• Critical zone: rhythmic layered chromite bearing layers
  • Merensky reef is cumulus opx + poikilitic plag with chromite at top/bottom of each layer
  • Chromite believed to result from new mafic magma reacting with remaining interstitial liquids expelled from layer below.
    • Chromite layers associated with massive PGE deposits
• Main zone: 2 pyroxene gabbronorites (also inverted pigeonite).
• Upper zone: magnetite ($Fe<em>3O</em>4$) appears (mined for V).
  • Multiple magma pulses followed by mixing and fractional crystallisation + late stage mush compaction.
• Late stage liquid-liquid immiscibility:
  • Observed in inclusions in apatite (which is a cumulus phase in the Upper Zone).

Reading Resources

• Essential:
  • Chapter 10 Essentials of Igneous & Metamorphic Petrology (10.1-10.3 inclusive)
  • Chapter 16 (section 16.5) Principles of Igneous and Metamorphic Petrology
• Optional:
  • Chapter 7 Open Petrology (section 7.6.3 only)
  • Chapter 15 (section 15.6) Principles of Igneous and Metamorphic Petrology