Assimilation of felsic crust by basaltic magma: thermal limits and extents of crustal contamination of mantle-derived magmas

Using the MELTS software package, isenthalpic (heat-balanced) AFC between two common types of basaltic magma and several types of felsic crust at 1 kbar was modelled. The simulations show that during the early stages of isenthalpic AFC, the ratio of rates of assimilation to crystallization (r) may be substantially greater than one (2.0-2.7), allowing assimilation of a mass of country rock up to 5%-18% of the initial magma mass with only 3%-7% crystallization. The second stage of AFC, beginning with plagioclase and/or pyroxene saturation, is characterized by lower r (0.5-1.0). The initial high-r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses. Under certain conditions, even small degrees of crystallization of olivine alone, coupled with cooling of the magma, can accommodate relatively large amounts of crustal assimilation, and cause large shifts in isotopic and trace element geochemical indicates with little differentiation. -from Authors