From Binary Mixing to Magma Chamber Simulator - Geochemical Modeling of
Assimilation in Magmatic Systems
Abstract
Magmas readily react with their surroundings, which may be other magmas
or solid rocks. Such reactions are important in the chemical and
physical evolution of magmatic systems and the crust, for example, in
inducing volcanic eruptions and in the formation of ore deposits. In
this contribution, we conceptually distinguish assimilation from other
modes of magmatic interaction and discuss and review a range of
geochemical (+/- thermodynamical) models used to model assimilation. We
define assimilation in its simplest form as an end-member mode of
magmatic interaction in which an initial state (t0) that includes a
system of melt and solid wallrock evolves to a later state (tn) where
the two entities have been homogenized. In complex natural systems,
assimilation can refer more broadly to a process where a mass of magma
wholly or partially homogenizes with materials derived from wallrock
that initially behaves as a solid. The first geochemical models of
assimilation used binary mixing equations and then evolved to
incorporate mass balance between a constant-composition assimilant and
magma undergoing simultaneous fractional crystallization. More recent
tools incorporate energy and mass conservation in order to simulate
changing magma composition as wallrock undergoes partial melting. For
example, the Magma Chamber Simulator utilizes thermodynamic constraints
to document the phase equilibria and major element, trace element, and
isotopic evolution of an assimilating and crystallizing magma body. Such
thermodynamic considerations are prerequisite for understanding the
importance and thermochemical consequences of assimilation in nature,
and confirm that bulk assimilation of large amounts of solid wallrock is
limited by the enthalpy available from the crystallizing resident magma.
Nevertheless, the geochemical signatures of magmatic systems-although
dominated for some elements (particularly major elements) by
crystallization processes-may be influenced by simultaneous assimilation
of partial melts of compositionally distinct wallrock.