Seminar Thomas Shea

Abstract

Groupings of multiple crystals are common in all magma types and likely serve as a constructive framework for the assembly of magma mushes and plutonic bodies. Three mechanisms are generally invoked to rationalize the formation of mono- or polyphase clusters: (1) Aggregation by accumulation (settling and compaction); (2) Aggregation by synneusis (the ‘swimming together’ and joining of crystals in a dynamic magma body; (3) Clustering via crystal growth (branching of crystals, twinning and/or epitaxial growth). These mechanisms predict a number of observations (random vs. non-random crystal and lattice orientations, formation of trapped melt pockets, elemental zoning patterns), but individually they lead to non-unique interpretations. For this study, we integrate various analytical and numerical modeling techniques on a monomineralic olivine cluster. We characterize (a) the number and orientation of grains using Diffraction Contrast Tomography, (b) major element zoning patterns expected from diffusive re-equilibration of Fe-Mg using 3D diffusion models, (c) the zoning patterns recorded by growth-preserving elements via EPMA maps in >30 serial sections, and (d) crystal settling dynamics of non-clustered and clustered crystals using 3D numerical simulations. Our results show that single clusters with 40 different grains have crystallographic orientations and zoning patterns most consistent with growth phenomena. Interestingly, we find that the formation of olivine clusters at Kilauea is directly linked with magma recharge: olivine-melt disequilibrium induced during magma mixing causes partial crystal dissolution, and subsequently provides rough surfaces that are ideal for secondary nucleation and growth of new budding crystals.