assimilation

Assimilation is that process of magmatic differentiation whereby ascending magmas evolve chemically by recruiting easily melted or dissolved components (fusibles) from the walls of their conduits.

Under the agency of heat and magmatic fluids, ascending magmas pick up volatiles, silica, trace elements, and occassionally fragments of wall rock. The heat that the melt gains by leaving behind quick-freezing refractories (an exothermic process) is typically sufficient to compensate for heat lost in the endothermic reactions required for the assimilation (melting) of country rock components. This trade-off ensures that assimilation can proceed without causing the melt to freeze (solidify).

Any wall rock fragments that survive more or less intact, without completely melting or dissolving into the magma, are called xenoliths. Surviving wall rock crystals are called xenocrysts. Together, xenoliths and xenocrysts provide invaluable information about rarely exposed lower crust and mantle levels by carrying these materials up within the ascending magma.

▪ Bowen's Reaction Series

| 0 Guide-Glossary

fractional crystallization

Fractional crystallization (fractionation) is that process of magmatic differentiation that accompanies the failure of early-forming crystals to react to the melt that remains. The process of fractional crystallization is responsible for the bulk of differentiation that is occurs in igneous rocks.

As ascending melts cool and react with country rock, those minerals in the melt that have the highest melting points or the lowest solubilities (quick-freezing refractories, like olivine and pyroxene) crystallize out first, leaving minerals with the lowest melting points or solubilities (quick-melting fusibles, like silica) behind in the melt to freeze out last.

Latent heat associated with phase change is released by the crystallization of refractories, replacing heat lost by conduction to the surrounding country rocks, lost to melting of country rock, and lost to the assimilation of fusibles in the country rock. Fusibles enter and refractories leave the melt at characteristic temperatures and pressures, and these exchanges tend to occur at specific depths along the ascent. The remaining melt loses volume as it rises, rendering its fusibles increasingly concentrated. Thus, exchanges within ascending magma leave behind a trail of solid refractories and country rock alterations.

Gravitative differentiation is the commonest form of fractionation, and results from the phenomenon that most solid minerals are denser than their parent melts. As denser crystals settle to the bottom of the magma body, they become segregated from the residual melt. Rocks that are formed by settling crystals are termed cumulates, and the rocks are often zoned, with the densest, first-formed crystals accumulated at the base of the magma chamber. Cumulates formed by the lighter crystals occasionally float to the top, with the lightest at the very top. This process produces layering in igneous rocks. The crystals of cumulate rocks are typically cemented by residual magmatic fluids.

▪ Bowen's Reaction Series

[links: animations: fractional crystallization/magmatic settling; webpages: Kurt Hollocher's webpage on Greenland's Eocene Skaergaard Intrusion has a gallery of excellent photographs of microrhythmic layering (and other interesting phenomena); Mining: Rock Formation]

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]

| 0 Guide-Glossary