Basalt Crystallization: Phase Diagram Analysis

Understanding the Diopside-Anorthite Phase Diagram

The diopside-anorthite phase diagram at 1 atm pressure is fundamental to understanding the crystallization behavior of Ca-rich basaltic magmas. It illustrates the stability fields of diopside (CaMgSi₂O₆), anorthite (CaAl₂Si₂O₈), and various intermediate solid solutions. Key features include:

• A eutectic point representing the lowest temperature at which a liquid can exist in the system.
• Solid solution series between diopside and anorthite.
• The diagram shows that at higher temperatures, a more diopside-rich composition can coexist with a more anorthite-rich composition in the liquid phase before crystallization begins.

Rock 1: Phenocrysts of Ca-rich Plagioclase in Diopside Matrix

This rock indicates that plagioclase crystallized before diopside. Based on the phase diagram, this suggests:

• Initial Melt Composition: The initial melt was likely anorthite-rich, falling within the anorthite-rich side of the phase diagram.
• Temperature: Crystallization began at a relatively high temperature, where anorthite is stable. As the magma cooled, it crossed the anorthite solvus, leading to the precipitation of anorthite phenocrysts.
• Crystallization Behavior: As cooling continued, the remaining liquid became increasingly enriched in diopside component. Eventually, the liquid reached the diopside field, resulting in the crystallization of diopside as the matrix. This implies fractional crystallization, where anorthite was removed from the melt before diopside.

Rock 2: Phenocrysts of Diopside in Ca-rich Plagioclase Matrix

This rock indicates that diopside crystallized before plagioclase. Applying the phase diagram:

• Initial Melt Composition: The initial melt was likely diopside-rich, falling within the diopside-rich side of the phase diagram.
• Temperature: Crystallization started at a high temperature where diopside is stable. As the magma cooled, it crossed the diopside solvus, leading to the precipitation of diopside phenocrysts.
• Crystallization Behavior: Continued cooling shifted the liquid composition towards the anorthite field, resulting in the crystallization of anorthite as the matrix. This also suggests fractional crystallization, but with diopside being removed first.

Rock 3: Coexisting Phenocrysts of Ca-rich Plagioclase and Diopside

The simultaneous presence of both minerals as phenocrysts suggests a more complex crystallization history:

• Initial Melt Composition: The initial melt composition was likely intermediate, falling near the eutectic point on the diopside-anorthite phase diagram.
• Temperature: Crystallization began at a temperature where both anorthite and diopside were stable. The simultaneous nucleation and growth of both minerals indicate a relatively slow cooling rate, allowing both phases to crystallize concurrently.
• Crystallization Behavior: The magma cooled through the two-phase field, resulting in the formation of both anorthite and diopside phenocrysts. The matrix composition would depend on the extent of crystallization and any subsequent fractional crystallization events. This could also indicate equilibrium crystallization, where both minerals grew in proportion to their stability in the melt.

Implications for Magma Source and Evolution

The variations in crystallization behavior observed in these three rocks suggest different magma sources or different evolutionary pathways for a single magma source. For example, variations in the degree of partial melting in the mantle, or differences in the assimilation of crustal material, could lead to variations in the initial melt composition. Fractional crystallization processes, as evidenced by the sequential crystallization of plagioclase and diopside, play a significant role in modifying the magma composition during its ascent and cooling.