Trace element zoning in garnet as a monitor of crustal melting

X-ray composition maps of Sc, Y, P, and Cr in garnet have been measured using the electron microprobe. Zoning is discontinuous and is correlated with fluid-absent crustal melting reactions. The magnitude of the discontinuity is a function of the amount of muscovite that melts, and the partition coefficients of solids versus melt. Trace element zoning is apparently not modified by diffusive processes, so the zoning can be used to monitor the extent of melting and to examine kinetics of melting and the participation of accessory phases in crustal anatexis.