MAGMA COMPOSITION - THE DETERMINING FACTORS
Composition of the source - The composition of the source material is the dominant factor that determines the composition of a primary magma. Pressure, temperature, and factors such as water content play an important but lesser role.
Evolution - Composition of a magma can change as it passes through the crust and is emplaced within a magma chamber by magmatic differentiation, mixing of magmas, or assimilation of overlying rocks.
EVOLUTIONARY MECHANISMS
I. Magmatic Differentiation - includes all processes by which the composition of an initially homogeneous magma changes (without the introduction of other material)
A) Fractional crystallization - any mechanical process by which early-formed crystals are prevented from equilibration with the liquid from which they grew. As minerals precipitate, the remaining (residual) melt changes composition.
1) Gravity separation - sinking of heavy minerals in the magma. For example, olivine and pyroxene have density of ~3.1 g cm-3 can sink in a less dense basaltic melt (density ~ 2.7 g cm-3). Magma progressively changes composition and layered zones of different rock types are produced at the bottom of the intrusion. Mostly occurs in basaltic magma which have low viscosity, so that crystals can settle. Examples include the Skaergaard, Bushweld, Dufek intrusions.
2) Flow differentiation - as magma flows, crystals will tend to migrate toward the middle of a dike. Thus, the magma becomes progressively differentiated outward.
3) Filter pressing - at advanced stages of crystallization the crystals form a mush with residual liquid in the pore space. Collapsing of crystals or fracturing of the mush during vapor exsolution can lead to expulsion of the residual liquid to form a separate differentiated magmatic body.
In general, fractional crystallization leads to
i) enrichment of the residual melt in Si, K, and the ratios of Fe/Mg and Na/Ca ratio because the early-formed minerals tend to have high Ca/Na and Mg/Fe ratios and are relatively poor in Si (e.g. olivine, pyroxene, anorthitic plagioclase).
ii) increased concentration of water leading to the crystallization of hydrous minerals, including hornblende, biotite, and muscovite.
Bowen’s Reaction Series (Evolution of Igneous Rocks, 1928)
Bowen attributed all rock types to fractional crystallization of a basaltic magma. However, fractional crystallization of a basalt leads to at most 5-10% of a granite magma. This volume cannot account for the volume of granite found in the continental crust. Nevertheless, his ideas illustrate important principles by which minerals precipitate from a melt.
Types of reactions:
a) discontinuous - mineral reacts with the melt to form a new mineral
b) continuous - mineral continuously changes composition by reaction with the melt but retains its original basic structure.
B) Liquid miscibility - separation of a melt into two melts of different composition. Mostly used to explain the formation of iron-phosphorus or carbonate rich magmas.
II. Magma Mixing- on occasion, magmas of different composition can mix in a conduit or inside a magma chamber. Mixing may be retarded by immiscibility. Thought to have occurred in the Tuolumne Intrusive Series, Yosemite National Park and elsewhere
III. Assimilation of wall rocks - magmas can assimilate wall-rocks by two processes:
1) melting - melting of wall-rocks can occur only if there is a large difference in the heat content between the magma and the wall-rocks. To melt any material, head of fusion must be supplied, which is in excess of just raising temperature. Moreover, magmas usual contain some crystals, which means that their temperatures are fixed to the liquidi and therefore magmas cannot superheated. Thus they have only a limited supply of heat.
2) diffusion - solid-state diffusion is very slow, therefore the effect may be limited. However, the break-up of wall-rocks (stoping) can significantly increase the surface-area and diffusion can become effective.