Assimilation of Ultramafic Rock in Subduction-Related Magmatic Arcs

Abstract

Assimilation of ultramafic rock by fractionating magma may be an important process in the genesis of subduction-related magmatic arcs. Physical conditions, characterized by low viscosity, fractionating mafic magma, and high wall rock temperatures, are favorable for interaction between magma and rock in the upper mantle. Kinetic and equilibrium constraints determine the availability of wall rock for reaction and the ratio of mass assimilated vs. mass crystallized (Ma/Mc). The exact value of Ma/Mc is difficult to predict but, in general, should vary from near 1.0 to about 0.4 for mafic magma reacting with peridotite at high temperature. For any positive value of Ma/Mc, the effect of assimilation of magnesian rock in fractionating magma is to produce a less iron-enriched, more alkaline derivative liquid than would be produced by crystal fractionation alone. A method is presented for evaluating the effect of combined assimilation and crystal fractionation (AFC) which permits continuously changing values for the crystal/liquid distribution coefficient, Ma/Mc, and the composition of the assimilate, unlike modified versions of the integrated Rayleigh equations in which these parameters must be constant over the entire crystallization interval. A comparison of the concentration of compatible vs. incompatible elements shows that, in principle, even in the absence of isotopic or trace element discriminants, magma series produced by AFC may be distin- guished from those derived by crystal fractionation alone. In AFC, the concentration of a compatible element in many cases approaches a steady state value, while the concentration of an incompatible element increases exponentially.