L2 - Igneous Petrology and Binary Phase Diagrams

Bowen's Reaction Series

Bowen's reaction series is a diagram indicating the temperatures at which minerals crystallize out of a melt, which is essential for understanding the formation of igneous rocks. This series describes the order in which minerals form as magma cools, influencing the composition and texture of the resulting rocks. As temperature decreases (from 1200°C to 750°C), minerals crystallize in a specific order, following two main branches: the discontinuous and continuous series.

Discontinuous:

Olivine
Pyroxene
Amphibole
Biotite
Orthoclase, muscovite, quartz

Continuous:

Ca-rich plagioclase → Na-rich plagioclase

Discontinuous Series

The discontinuous series involves minerals that react with the melt to form new minerals at progressively lower temperatures. Each mineral is stable only within a specific temperature range, and as the magma cools, it transforms into the next mineral in the series. The typical sequence is Olivine → Pyroxene → Amphibole → Biotite Mica.

Continuous Series

The continuous series, exemplified by plagioclase feldspar, involves a gradual change in mineral composition with decreasing temperature. At high temperatures, calcium-rich plagioclase (anorthite) crystallizes, and as the temperature decreases, it continuously reacts with the melt to form sodium-rich plagioclase (albite).

Mafic melts (e.g., Gabbro, Basalt) crystallize at higher temperatures than felsic melts (e.g., Granite, Rhyolite).

Mineral Proportions in Igneous Rocks

Different rock types contain varying percentages of minerals, which determine their classification and properties. The mineral composition reflects the conditions under which the magma cooled and solidified.

Ultramafic Rocks (Peridotite): Mostly olivine. These rocks are commonly found in the Earth's mantle.
Gabbros and Basalts: Predominantly pyroxene and plagioclase. They may also contain olivine.
Diorites and Andesites: Mix of plagioclase, pyroxene, and amphibole. The specific proportions vary depending on the magma's composition and cooling history.
Granites and Rhyolites: Rich in orthoclase (potassium feldspar) and quartz, with some plagioclase and amphibole. These rocks are characteristic of continental crust.

Key Mineralogy

Basalt/Gabbro:

Composed of clinopyroxene and plagioclase. The ratio of these minerals affects the rock's density and color.
May also contain olivine and orthopyroxene. Olivine-rich basalts are common in oceanic settings.
Gabbro is intrusive (cooled slowly), while basalt is extrusive (cooled quickly). The cooling rate influences crystal size and texture.

Andesite/Diorite:

Contain plagioclase and amphibole. The presence of amphibole indicates higher water content in the magma.
May include biotite, pyroxene, and quartz. The specific mineral assemblage reflects the magma's composition and pressure-temperature conditions.
Andesite is extrusive, diorite is intrusive. These rocks are commonly found in subduction zones.

Granite/Rhyolite:

Composed of orthoclase, plagioclase and quartz. The high silica content gives these rocks their light color and high viscosity when molten.
Granite is intrusive, rhyolite is extrusive. Granite forms large crystals due to slow cooling deep within the crust, while rhyolite cools rapidly on the surface, forming small crystals or glass.
Potassium feldspar typically appears salmon pink. This color is due to trace amounts of iron.

Crystal Structure and Element Compatibility

The crystal structures of minerals become more open at lower temperatures, influencing element compatibility. Compatible elements (small, moderate charge) prefer to be in solids, while incompatible elements (large size, high charge) prefer to be in the melt.

Olivine (high-temperature): Accepts magnesium and iron, excludes rare earth elements. The structure of olivine is compact, favoring smaller, highly charged cations.
Micas and Feldspars (low-temperature): Accommodate incompatible elements like potassium. The layered structure of sheet silicates and the more open framework of feldspars allow for the incorporation of larger ions.

Weathering Resistance

Minerals that crystallize at lower temperatures are more resistant to weathering because they are closer to their crystallization point at Earth's surface conditions. Quartz, for example, is highly resistant due to its strong silicon-oxygen bonds.

Thermodynamics and Gibbs Free Energy

Gibbs Free Energy is related to a system's internal energy ( $U$ ), pressure ( $P$ ), volume ( $V$ ), temperature ( $T$ ), and entropy ( $S$ ) via the equation: $\Delta G = \Delta U + P\Delta V - T\Delta S$ .

The Gibbs Free Energy equation shows how changes in internal energy, pressure, volume, temperature, and entropy affect the spontaneity of a process. A negative $\Delta G$ indicates a spontaneous process.

Systems tend to achieve the lowest possible Gibbs free energy for thermodynamic stability. This principle governs the equilibrium state of mineral assemblages under different conditions.

Melting Points and Dilution

The melting point of a substance decreases when it is diluted with another composition (e.g., adding salt to ice). This phenomenon is crucial in understanding magma generation in the Earth's mantle.

Binary Phase Diagrams

Binary phase diagrams illustrate the interactions between two minerals based on their thermodynamic stability. An example includes diopside and anorthite. These diagrams show the temperature and composition ranges under which different phases (solid, liquid, or a mixture) are stable.

Eutectic Point: The intersection of liquidus curves, representing the lowest temperature at which melt can exist in the system. Below this point, everything is solid. The eutectic composition is the specific mixture of minerals that melts at the lowest temperature.

In the context of a mid-ocean ridge, basalt formation involves constant proportions of clinopyroxene and plagioclase. This specific ratio is a result of the eutectic composition for these minerals.

The first melt formed corresponds to the eutectic point, where both pyroxene and plagioclase melt simultaneously. The composition of the starting material is not relevant, since the first melt will always occur at the lowest possible temperature, the eutectic point. This is because the eutectic point represents the thermodynamically most favorable composition for melting.

Gibbs Phase Rule

Gibbs Phase Rule defines the degrees of freedom ( $F$ ) in a system:

$F = C + 2 - \Phi$

where:

$C$ is the number of chemical components.
$\Phi$ is the number of phases.
There are 2 extra degrees of freedom due to varying pressure and temperature

However, in reality, we use the following equation:

$F=C+1-\phi$

because we assume pressure to be constant.

The Gibbs Phase Rule is an essential tool for understanding the conditions under which different mineral assemblages can coexist in equilibrium. It relates the number of components, phases, and degrees of freedom in a thermodynamic system.

At an invariant point (e.g., eutectic), $F = 0$ , indicating no degrees of freedom. This means that the temperature and composition are fixed, and the system must remain at that point until a phase disappears.

Degrees of Freedom

Invariant Point: No freedom to change temperature or composition until the reaction is complete. At this point, the system is fully constrained.
Univariant Line: One degree of freedom, allowing movement along the line but restricted otherwise. Along this line, either temperature or composition can be changed independently, but not both.
Divariant Field: Two degrees of freedom, allowing independent variation in temperature and composition. Within the liquid field, both temperature and composition can be varied independently without changing the number of phases.

Equilibrium Melting

During heating, a solid composition remains unchanged until the eutectic temperature is reached. Once there, melting occurs until one phase is exhausted, then the system moves along the liquidus. The final melt composition matches the initial solid composition. This process is governed by the lever rule, which determines the proportions of solid and liquid phases at any given temperature.

Equilibrium Crystallization

As temperature drops, the liquidus is reached and crystals form. The amount of solid and liquid is determined by the lever rule. Equilibrium crystallization assumes that the crystals remain in contact with the melt, allowing for continuous reaction and equilibration.

Lever Rule

The lever rule is used to determine the proportions of solid and liquid phases at a given temperature. The lever rule is a graphical method for determining the relative amounts of each phase in a two-phase region of a phase diagram.

Eutectic Textures

Anorthite exhibits euhedral shapes, whereas pyroxenes fill in the gaps. A thin section examination reveals the sequence of crystallization based on crystal shapes. The textures provide clues about the cooling history and the relative timing of mineral formation.

Fractional Crystallization

During fractional crystallization, crystals are removed from the system (e.g., sinking due to density). This changes the melt composition and prevents the system from returning to its original state upon melting. Fractional crystallization is a key process in the formation of diverse igneous rock suites.

As crystallization occurs, gravity separates phases, changing the melt composition because the olivine has been removed. The removal of early-formed crystals enriches the remaining melt in incompatible elements.

As different minerals are crystallized, they are also removed, and each extraction alters the composition of the remaining melt. This leads to a progressive change in the composition of the residual liquid.

In this process the pivot point is constantly shifting, unlike during equilibrium crystallization. The changing melt composition continually shifts towards the eutectic point.

Eutectic Intergrowth

Plagioclase is juxtaposed with clinopyroxene with a similar rock composition. This intergrowth texture is characteristic of eutectic crystallization, where two minerals crystallize simultaneously from a melt.

Also potassium feldspar and quartz can also exhibit eutectic intergrowth. This texture is common in granitic rocks and indicates rapid crystallization from a melt at or near the eutectic point.