Petrologic perspectives on the architecture of active magmatic reservoirs

Abstract:

Conceptual models change to integrate new observations, particularly those enabled by technological advances. We are currently witnessing such a change in our views of magmatic systems, spurred in large part by developments in micro-analytical capabilities that allow us to dissect, in detail, the history of individual crystals. As has long been suspected from petrographic examination of thin sections, there is now abundant evidence that many (most?) erupted magmas bring with them samples - in the form of single crystals, glomerocrysts, and cumulate nodules - of mid-upper crustal magma storage regions. These observations have spawned a proliferation of detailed petrologic studies, on the one hand, and conceptual models of trans-crustal magmatic systems, on the other. Using the observations to constrain the models is, however, a fundamental challenge.

One concept that underlies most models is that of a magma storage system that can be divided into two rheologically distinct components: (eruptible) melt-rich magma and (non-eruptible) crystal-rich mush. Here we question this discrete division. In fact, interactions between magma and mush zones are not well understood, and are likely to involve different types of interactions over different time scale. These interactions include not only compaction and melt extraction but also melt percolation and reactive flow, as well as fracture, entrainment and mingling over time scales of volcanic unrest and eruption. For example, extensive syn-eruptive mingling between notionally eruptible and non-eruptible components of a magmatic system is illustrated by glomerocrysts entrained in the eruptive products of Fuego volcano, Guatemala, which show a correlation between melt content (~6 to >30%), melt composition and extent of deformation. More broadly, bimodal volcanism in calderas indicates that recharge magma may either mingle with resident magma or traverse reservoirs to reach the surface. This suggests that magma-mush interactions are controlled by both the physical state of the magmatic system, and the dynamics of perturbations to the system. Using eruptive products to recognize and correctly interpret the full range of subvolcanic processes is important for understanding patterns of volcanic activity, and explaining the diversity of igneous rock compositions.