ABSTRACT 
Craig Lundstrom, Department of Geology, University of Illinois, Champaign, USA

How do magmas differentiate?

Igneous compositional diversity mostly reflects crustal depth magma differentiation. Clearly, magma evolution follows control of mineral-melt equilibrium as indicated by compositional trends from basalts through granites. Yet the paradigm that magmas evolve by fractional crystallization or partial melting (e.g. mechanical separation of melt from crystals) does not readily explain the most basic observation of silicic differentiates: these processes imply granites are minimum melts (on the NaAlSi3O8-SiO2-KAlSi3O8 ternary) extracted from a quartz bearing residue (Tuttle and Bowen, 1958). This explanation does not explain how the quartz residue originally formed and thus leaves open fundamental questions about how silicic continental crust came to be.

A non-mechanical process that can produce minimum melt-like bulk compositions is wet thermal migration (mineral-melt equilibrium driven diffusion in a temperature gradient). We have shown that andesite + 4wt% H2O in a 950-350°C temperature gradient evolves to a granite at the low temperature end (400°C) following a typical calc-alkaline differentiation trend. Indeed, compositional trends in this experiment closely follow those of the catastrophic eruption of Mt. Mazama. Why differentiation proceeds to <400°C is explained by phase equilibria in the system Na2O-K2O-Al2O3-SiO2-H2O. Experiments show that a Qtz-Albite-Orthoclase assemblage coexists with very hydrous (40wt% H2O) per alkaline melt at 330-400°C and 0.5-1 kbar. Significantly, differentiation within a thermal gradient produces an isotopic signature of Fe and Si which can be used to fingerprint the process. Results from a variety of experiments and natural observations will be presented to make the case that thermal migration coupled to incremental magma emplacement by adding sills top-down provides a consistent explanation for differentiation that gets beyond the problems posed by mechanical separation processes.