Explain with suitable examples the implications of albite-anorthite solid solutions in the understanding of crystallisation of magma.

Understanding Albite-Anorthite Solid Solutions

A solid solution is a homogeneous mixture of two or more crystalline substances. In the case of albite and anorthite, the isomorphic substitution of Na⁺ and Ca²⁺ ions within the plagioclase structure allows for a continuous variation in composition. This substitution isn’t random; it’s governed by factors like ionic radius, charge, and the overall stability of the crystal structure. The composition of plagioclase is typically expressed as an anorthite percentage (An%), representing the proportion of anorthite in the solid solution. For example, An₅₀ represents a plagioclase with 50% anorthite and 50% albite.

Implications for Magma Crystallization

The presence of albite-anorthite solid solutions has several key implications for understanding magma crystallization:

• Fractional Crystallization: As magma cools, minerals begin to crystallize. Plagioclase, being a common early-forming mineral, often crystallizes as a solid solution. As the magma evolves, the composition of the remaining liquid changes. If early-formed plagioclase is removed (through settling or flow), the remaining magma becomes enriched in albite. Subsequent plagioclase crystallization will then be more sodium-rich. This process, known as fractional crystallization, is a major mechanism for generating magmatic differentiation.
• Magma Series: Different magma series (e.g., tholeiitic, calc-alkaline) exhibit distinct plagioclase compositions. Calc-alkaline magma series, commonly associated with subduction zones, typically show a wider range of plagioclase compositions (from An₉₀ to An₃₀) compared to tholeiitic series. This difference reflects the varying sources and evolutionary histories of these magmas.
• Cooling Rate and Zoning: The rate at which magma cools significantly influences the development of compositional zoning within plagioclase crystals. Rapid cooling leads to disequilibrium and the formation of normal zoning, where the core of the crystal is more calcium-rich (An%) than the rim. This is because Ca²⁺ has a lower diffusion rate and enters the crystal structure more readily at higher temperatures. Slow cooling allows for greater equilibration and can result in reverse zoning or the absence of zoning altogether.
• Pressure and Temperature Conditions: The stability of different plagioclase compositions is also dependent on pressure and temperature. At higher pressures, anorthite is favored, while at lower pressures, albite is more stable. Therefore, the composition of plagioclase can be used as a geobarometer (to estimate pressure) and geothermometer (to estimate temperature) under certain conditions.

Examples in Igneous Rocks

Several igneous rocks provide excellent examples of the implications of albite-anorthite solid solutions:

• Gabbro and Basalt: These mafic rocks often contain calcium-rich plagioclase (An_60-90), indicating crystallization from relatively high-temperature, deep-seated magmas.
• Granite and Rhyolite: These felsic rocks typically contain sodium-rich plagioclase (An_10-30), reflecting crystallization from more evolved, silica-rich magmas.
• Oceanic Crust: The oceanic crust is largely composed of gabbro and basalt, with plagioclase being a major constituent. The plagioclase compositions in these rocks provide insights into the processes occurring at mid-ocean ridges, such as mantle melting and magma fractionation.
• Anorthosites: These are intrusive igneous rocks consisting almost entirely of plagioclase, typically An_90-95. Their formation is linked to the accumulation of early-formed, calcium-rich plagioclase crystals in magma chambers.

Using Plagioclase Composition as a Tracer

The compositional variations within plagioclase crystals can act as a tracer for understanding magma chamber processes. For instance, the presence of multiple zoning patterns can indicate episodes of magma mixing or recharge. Furthermore, the distribution of trace elements within plagioclase can provide information about the source of the magma and the processes that have affected it.