Toward a more quantitative understanding of open magmatic systems (Invited)

Open-system processes are now established as playing major roles in magma evolution, but quantifying the diverse contributions of multiple processes and components remains challenging. Magmatic systems in which recharge and mixing are important also may carry the imprint of assimilation. The crystal cargoes of hybrid magmas generated by open-system evolution may include: (1) multiple populations of phenocrysts, (2) antecrysts from crystal mush residues of the same magmatic episode, (3) xenocrysts entrained during assimilation (including assimilated mafic to ultramafic cumulates), or (4) some combination of these. Differences in conduit/reservoir volume and geometry and variations in the frequency and amplitude of magma recharge are among the most difficult variables to constrain, but these play important thermo-mechanical roles. The minimum requirement for deconvolving such complexities is stratigraphically controlled, high-density sampling wherein textural relations and phase chemistry are integrated with whole-rock chemistry. Mixed or mingled magmas may appear to be the products of syn-eruptive mixing wherein magma recharge directly provoked an eruption, but a quasi-instantaneous cause-and-effect relationship is often difficult to verify independently. Temporal control on the eruptive sequence of diverse magma compositions is critical to assessing magma recharge as an eruption-triggering process, but it is not always sufficient to uniquely constrain system geometry and/or pre-eruptive magma dynamics. The assumption that recharge magmas are less evolved than resident magmas with which they mix may be generally valid, but contrasting eruptive sequences wherein the more evolved hybrids and end-members were erupted either last or first both occur. Well-documented examples of magma mixing will be used to address these issues.