Draw a neat labelled sketch for the albite-anorthite phase diagram (1 atm, dry). Trace the course of crystallization of an initial melt Ab20-An80 within this system. How can you interpret zoning in plagioclase with this system?

Albite-Anorthite Phase Diagram (1 atm, dry)

The albite-anorthite phase diagram illustrates the melting and crystallization relationships of the plagioclase solid solution series under atmospheric pressure and dry conditions. It is a binary phase diagram with temperature on the y-axis and composition (from 0% Anorthite/100% Albite to 100% Anorthite/0% Albite) on the x-axis.

The diagram features two distinct curves:

• Liquidus Curve: This curve represents the temperatures above which the system is entirely molten (liquid).
• Solidus Curve: This curve represents the temperatures below which the system is entirely solid (crystalline plagioclase).

Between the liquidus and solidus curves lies the two-phase field where both liquid melt and solid plagioclase crystals coexist in equilibrium. A key characteristic is that anorthite (An) has a higher melting point (~1553°C) than albite (Ab) (~1118°C), reflecting its higher position in Bowen's Reaction Series.

(Note: A diagram cannot be drawn in text format, but a description of its key features is provided. In an exam, a neat, hand-drawn, labelled sketch would be essential.)

Sketch Description:

• X-axis: Composition (mole % Anorthite, from Ab (0% An) on the left to An (100% An) on the right).
• Y-axis: Temperature (°C), decreasing upwards.
• Liquidus Curve: A gently sloping curve connecting the melting point of pure Anorthite (approx. 1553°C) to the melting point of pure Albite (approx. 1118°C), concave upwards.
• Solidus Curve: A similar, but lower, gently sloping curve running from the melting point of pure Anorthite to the melting point of pure Albite, also concave upwards.
• Fields:
  • Above liquidus: Liquid (L) only.
  • Between liquidus and solidus: Liquid + Solid (L+S).
  • Below solidus: Solid (S) only.
• Labels: Liquidus, Solidus, L, L+S, S, Temperature (°C), Composition (Ab <-> An).

Course of Crystallization of an Initial Melt Ab₂₀-An₈₀

Let's trace the crystallization path for a melt with an initial composition of Ab₂₀An₈₀ (i.e., 80 mole % Anorthite), assuming equilibrium crystallization (where crystals continuously react with the melt). Let the initial melt be at a high temperature, entirely liquid.

1. Initial Cooling (Point M): The melt, with bulk composition Ab₂₀An₈₀ (let's call it M), starts at a temperature well above the liquidus and is entirely liquid. As it cools, its temperature decreases without any phase change until it intersects the liquidus curve.
2. First Crystal Formation (Point L₁ and S₁):
   • When the melt cools to a temperature T₁ and intersects the liquidus curve at point L₁, the first solid crystals begin to form.
   • To find the composition of these first crystals (S₁), draw a horizontal line (tie-line) from L₁ to intersect the solidus curve at point S₁.
   • The composition at S₁ will be significantly richer in anorthite than the initial melt (e.g., approximately Ab₅An₉₅), because anorthite has a higher crystallization temperature.
   • At this stage, the system consists of a small amount of An-rich crystals and a large amount of melt with the original composition Ab₂₀An₈₀.
3. Continuous Crystallization (T₂ to T₃):
   • As cooling continues to lower temperatures (e.g., T₂), more crystals form. The overall composition of the system remains Ab₂₀An₈₀.
   • However, the composition of the melt in equilibrium with the crystals shifts along the liquidus curve towards more albite-rich compositions (e.g., L₂).
   • Simultaneously, the crystals growing in equilibrium with this melt become progressively more albite-rich, and their composition shifts along the solidus curve (e.g., S₂).
   • The proportions of liquid and solid at any temperature can be determined using the Lever Rule. The crystals are continuously reacting with the remaining melt to maintain equilibrium.
4. Last Melt Solidification (Point S₄ and L₄):
   • This process continues until the temperature reaches T₄, where the last bit of melt solidifies.
   • At this point, the composition of the last remaining melt (L₄) will be more albite-rich than the initial melt, and the composition of the plagioclase crystals (S₄) will have continuously changed until their average composition matches the original bulk composition, Ab₂₀An₈₀.
   • Below T₄, the entire system is solid plagioclase with the bulk composition Ab₂₀An₈₀.

Interpretation of Zoning in Plagioclase with this System

Plagioclase zoning refers to the compositional variation within a single plagioclase crystal, typically from a calcium-rich core to a more sodium-rich rim. This phenomenon is a clear indication of disequilibrium crystallization, where the crystals do not fully react with the melt as crystallization progresses. The albite-anorthite phase diagram helps interpret different types of zoning:

1. Normal Zoning (Anorthite-rich core to Albite-rich rim)

• Mechanism: This is the most common type and occurs when cooling is relatively rapid, preventing complete reaction between earlier-formed, calcic crystals and the evolving, more sodic melt. As crystallization proceeds, the melt becomes progressively enriched in Na (moves down the liquidus towards Ab-rich compositions).
• Interpretation: The initial crystals formed are highly anorthite-rich (high temperature, according to the solidus curve). As temperature drops and the melt evolves, subsequent layers deposited on the crystal become progressively richer in albite (lower temperature, more sodic), creating concentric zones. This "freezes in" the compositional changes of the melt over time.
• Significance: Indicates progressive cooling and crystallization within a magma chamber without significant mixing events.

2. Reverse Zoning (Albite-rich core to Anorthite-rich rim)

• Mechanism: This type of zoning is less common and suggests an increase in temperature or an influx of hotter, more primitive (more anorthite-rich) magma into the magma chamber.
• Interpretation: If a magma chamber experiences an influx of hotter, more calcic magma, or if pressure decreases rapidly, the melt composition might shift towards a more An-rich composition, or the equilibrium temperature for a given composition might rise. This causes later-formed plagioclase rims to be more anorthite-rich than the core.
• Significance: Records events like magma mixing, recharge, or pressure changes within the magmatic system.

3. Oscillatory Zoning

• Mechanism: Characterized by repetitive, rhythmic alternations between more calcic and more sodic layers.
• Interpretation: This type of zoning suggests rapid and repeated fluctuations in the physicochemical conditions of the magma. These fluctuations could be due to:
  • Pressure changes: Sudden drops in pressure can cause rapid crystallization of more sodic plagioclase, followed by a return to original conditions.
  • Magma mixing: Repeated small influxes of different magmas.
  • Convection currents: Crystals moving between zones of varying temperature and composition within a convecting magma chamber.
  • Kinetic effects: The rate of diffusion and crystal growth may outpace the rate of homogenization.
• Significance: Provides a detailed record of dynamic processes within the magma chamber, often linked to volcanic eruptions.

In essence, the albite-anorthite phase diagram, while depicting equilibrium conditions, serves as a crucial reference. Deviations from the equilibrium crystallization path, often driven by kinetic factors (e.g., rapid cooling, slow diffusion), lead to zoning. By observing the compositional changes across plagioclase crystals, geologists can infer the magmatic processes and thermal histories of igneous rocks.