Liquid fractionation. Part III: Geochemistry of zoned magmas and the compositional effects of liquid fractionation

Compositional variations of zoned magmas ranging from basalt to rhyolite and basanite to phonolite are compatible with crystal-liquid fractionation. There is no difference in the trends within zoned magmas and those of differentiated suites erupted over long times in the form of separate homogeneous extrusions. Apparent anomalies in the trace-element variations of zoned high-silica magmas can be explained by fractionation of accessory phases. The wide range of incompatible trace-element abundances in silicic magmas requires extended crystal-liquid fractionation. This cannot take place in the zoned, static, and viscous upper parts of silicic magma chambers. Most of the compositional variations must develop during flow of buoyant differentiated liquid up the walls of a magma reservoir. Distribution coefficients measured from zoned magmas cannot be used directly to model fractionation in a flowing boundary layer, because the mineral assemblage crystallizing on the wall differs from the phenocryst assemblage that grows after segregation to form a zoned magma layer under the roof of the chamber, and kinetic effects in the boundary layer tend to increase trace-element enrichments and depletions. Similar effects result from crystallization on the chamber roof, and from periodic extractions of roof-zone magma by eruption or intrusion. Laboratory experiments illustrate the roles of crystallization and diffusion during and after ascent of boundary-layer magmas. Theory suggests that the effects of fractionation and mixing will be dominant in the boundary layer at the wall and under the roof of a magma chamber, but ordinary diffusion may also be detectable in the roof zone. The compositional ranges and volumes of different igneous suites are well explained by multi-stage liquid-fractionation processes. The Soret effect cannot explain the differentiation of zoned magmas unless they are above their liquidus temperatures. Linear correlation of most incompatible elements in zoned magmas suggests that vapor phase transport is not important to their differentiation.