Understanding crystal populations and their significance through the magma plumbing system
Dougal A. Jerram, Victoria M. Martin, 2008. "Understanding crystal populations and their significance through the magma plumbing system", Dynamics of Crustal Magma Transfer, Storage and Differentiation, Catherine Annen, Georg F. Zellmer
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Crystals are rarely composed of a single crystal population that have grown solely from the batch of magma in which they are resident on emplacement, either by eruption or shallow intrusion. Close investigation of the majority of crystal populations reveal that they comprise up to four main components: phenocrysts, crystals co-genetic with their magmatic host; xenocrysts, crystals wholly, or in part, foreign to the magmatic host and magma system; antecrysts, crystals which are recycled one or several times before inclusion in the host magma but have an origin within the magmatic system; and microlites, which represent small co-genetic crystals which nucleate and grow rapidly on decompression and eruption. Textural analysis techniques are employed to quantify key aspects of the crystal population, including crystal shape, crystal size distributions, spatial distribution patterns and textural modification using dihedral angles. Santorini provides a case study of an active volcanic system where a combined textural analysis study has been developed, highlighting how the crystal population is being continuously modified by a series of replenishment and mixing events. Developing textural and microgeochemical techniques provides the next stage in the interrogation of crystal populations, linking textures to isotopic heterogeneities and providing fingerprints of where crystals are sourced and re-cycled.
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Magmas are subject to a series of processes that lead to their differentiation during transfer through, and storage within, the Earth’s crust. The depths and mechanisms of differentiation, the crustal contribution to magma generation through wall-rock assimilation, the rates and timescales of magma generation, transfer and storage, and how these link to the thermal state of the crust are subject to vivid debate and controversy. This volume presents a collection of research articles that provide a balanced overview of the diverse approaches available to elucidate these topics, and includes both theoretical models and case studies. By integrating petrological, geochemical and geophysical approaches, it offers new insights to the subject of magmatic processes operating within the Earth’s crust, and reveals important links between subsurface processes and volcanism.