Trace element compatibility during igneous fractionation is governed by the bulk distribution coefficient (bulk D) of a rock, calculated as the weighted average of each constituent mineral's individual distribution coefficient (D) for that element, weighted by the mineral's weight fraction in the rock; an element is considered compatible in a given rock (i.e., preferentially retained in the solid rather than partitioning into a coexisting melt) when the calculated bulk D exceeds 1.