magmatic differentiation

Magmatic differentiation is a complex process whereby a single melt can produce a wide variety of different igneous rocks. Some degree of differentiation typically develops across space and time in exposed magma bodies (intrusive or extrusive).

Most melts develop in the lower crust or in the asthenosphere (upper mantle), so, such melts have a primitive mafic or basaltic composition, whereas melts developing in the upper crust have a higher initial silica content.

Regardless of depth of formation, melts ultimately produce igneous rocks that depend on the composition of the original melt, on the properties of wall rocks encountered during ascent, and on rate/s of cooling. The main processes involved in differentiation are assimiliation, exchange of volatiles, fractional crystallization, and magmatic mixing.

▪ Bowen's Reaction Series

[links: animations: fractional crystallization/magmatic settling]

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volatiles

Volatiles found in varying amounts in nearly all wall rocks and magmas figure prominently in the magmatic differentiation process of assimiliation. These volatiles include CO2, SO2, O2, Cl2, and particularly H2O.

Water is available in wall rocks of the mid-crust, both as free water and within the hydrated minerals commonly found at depths. Some assimilated water enters hydration reactions with predominantly anhydrous melt components, but most water simply accumulates in the ever-shrinking, residual silicate melt. When a melt has taken on sufficient water, that magma will develop a water-saturated silicate fraction and a separate water-based fluid phase.

Under some conditions, water-saturated silicate fractions can release a whitish fine-grained vein-filling slurry of quartz and feldspars termed aplite. The water-based phase easily assimilates trace elements that do not accommodate well into most silicate crystals. These trace elements include beryllium, lithium, niobium, tantalum, tin, uranium, thorium, tungsten, zirconium and the rare earths. Ore deposits can form when hot, pressurized, mineral-laden hydrous fluid permeates fractured country rock and cools into veins of pegmatite—an very coarse-grained igneous rock containing megacrysts of quartz, feldspars, and sometimes, highly prized minerals. Pegmatite and aplite dikes and veins are common around intrusions.