Many caldera-forming (large-volume) ignimbrite sheets display chemical and thermal heterogeneities, reflecting evacuation from a compositionally zoned shallow reservoir. These chemically and thermally complex reservoirs are an expected consequence of open-system processes that are common in magmatic systems. Mixing of magma batches with different physical properties (density, viscosity), assimilation of wall rocks, and internal phase changes (crystallization, gas exsolution) related to cooling and decompression all lead to chemical and thermal gradients within the reservoirs. In contrast to common belief, convection is not an efficient homogenizing agent at all scales in viscous magmatic systems and can produce long-lasting gradients. Recent experiments and numerical studies have shown that heterogeneities (in composition, crystal content, temperature) will arise (or be preserved) as sluggish convection stirs the system. If left active long enough in a chaotic mode, convection can produce homogeneity, except in the case of thermally-induced heterogeneities, which are continuously re-established in cooling magma bodies. Therefore, in most magmatic situations, gradients are inescapable. This inference is on par with volcanic deposits from explosive eruptions (ignimbrites), which, in many cases, show heterogeneities in their whole-rock composition, crystallinity, temperature and volatile contents. Looking at the ignimbrite record, we conclude that most crystal-poor magma chambers will preserve heterogeneities as the system undergoes sluggish convection.
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