Show diagrammatically the characteristics of binary eutectic system under 1 atmosphere (1 atm pressure). How does one explain the formation of porphyritic basic rock with phenocryst of plagioclase in a groundmass with plagioclase and clinopyroxene with the help of a suitable binary eutectic system? IAS 2024, 10 Marks

Introduction:

• A binary eutectic system consists of two components that form a eutectic mixture at a specific composition and temperature.
• The characteristics of a binary eutectic system under 1 atm pressure can be represented diagrammatically.

Characteristics of Binary Eutectic System Under 1 Atmosphere

Key Features:

• A binary eutectic system involves two components that can coexist as liquid and solid at a specific eutectic composition and temperature under 1 atm pressure.
• Phases: Includes liquid, solid A, and solid B.
• Eutectic Point: The lowest temperature at which the liquid phase can exist; both components crystallize simultaneously in a fixed ratio.

Diagram Explanation:

• The phase diagram consists of:
  
  • Liquidus Curve: Separates the liquid phase from the two-phase liquid + solid region.
  • Solidus Curve: Indicates temperatures below which only solid phases exist.
  • Eutectic Point: Intersection of solidus and liquidus lines where the two solids crystallize simultaneously.

Simplified Diagram:

Formation of Porphyritic Basic Rock with Plagioclase Phenocrysts

Explanation with Binary Eutectic System:

• System: Consider the diopside-anorthite system as a representative binary eutectic system.
• Initial Stage:
  
  • The magma initially cools slowly at depth, leading to the formation of large plagioclase phenocrysts (early crystallization due to supercooling).
  • Plagioclase forms due to the liquidus curve's inclination toward the plagioclase composition.
• Subsequent Cooling:
  
  • As the magma cools further, it reaches the eutectic point.
  • Both plagioclase and clinopyroxene crystallize together from the remaining liquid.
• Final Rock Texture:
  
  • The phenocrysts of plagioclase (formed earlier) are embedded in a fine-grained groundmass composed of plagioclase and clinopyroxene, reflecting the eutectic crystallization.

Relevance of Binary Eutectic System:

• The eutectic system explains the simultaneous crystallization of plagioclase and clinopyroxene in the groundmass.
• The earlier nucleation of plagioclase corresponds to the solidus-liquidus path of a plagioclase-rich composition.

Diagram:

Conclusion:

• The characteristics of a binary eutectic system under 1 atm pressure can be represented diagrammatically.
• The formation of porphyritic basic rock with phenocrysts of plagioclase can be explained using a binary eutectic system with plagioclase and clinopyroxene.