Understanding crystal populations and their significance through the magma plumbing system

Abstract

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.

The antecryst compositional influence on Cretaceous alkaline lamprophyre dykes, SE Brazil

“…In particular, effects of megacrysts on the bulk chemical composition and their meaning have been widely discussed, especially in porphyritic varieties Larrea et al 2012). Recently, the term 'antecrysts' has been systematically used (e.g., Charlier et al 2005;Davidson et al 2007;Jerram & Martin 2008) referring to a specific crystal type that differs from those identified as phenocrysts (larger early-crystallized phases that are in equilibrium with the liquid) and xenocrysts (crystals that are foreign to the magmatic liquid and have no genetic association with it). Antecrysts may be defined as crystals that did not crystallize directly from the host magma in which they are contained, although they maintain a genetic relationship with the same system.…”

Igneous Rock Associations 17. Advances in the Textural Quantification of Crystalline Rocks

“…Technological advances have enabled increasingly detailed studies of the crystallographic, chemical and isotopic compositions of individual crystals in plutonic, and more commonly in volcanic rocks (Davidson et al 2007;Jerram and Martin 2008). The term 'crystal stratigraphy' is now used to describe such compositional variations as recorded by zonation.…”

Mécanismes de solidification des magmas basaltiques :

“…The deformed olivine crystals from MU lavas can be considered as 'antecrysts', in the sense recently proposed by Jerram & Martin (2008): the term 'antecryst' is "becoming the accepted term used to define crystals that have been 'reincorporated' into the final magma", and are "directly associated with the active magmatic system". These antecrysts are interpreted here to have started as cumulates from earlier Kilauea magmas, which were subsequently disrupted and incorporated into the later magmas.…”