Prevalence of growth twins among anhedral plagioclase microlites

Abstract

Crystal textures of volcanic rocks record the processes involved in magma storage and eruptive ascent. Syn-eruptive crystallization, in which groundmass crystals form and grow according to environmental factors such as thermodynamic undercooling, strongly influences the texture of erupted magma. This stage is difficult to isolate for study in natural rocks, but well-suited for laboratory experiments because the chemical compositions and crystallization timescales of eruptive processes can be emulated. This study examines the incipient stages of plagioclase crystallization in hydrous rhyodacite magma undergoing decompression-driven degassing. Experimental samples in which crystal growth at both near-equilibrium and far-from equilibrium conditions were examined using electron backscatter diffraction (EBSD) analysis to ascertain crystallographic lattice orientations of individual crystals. The crystal orientation investigation affirms a common assumption invoked in textural studies of crystal number density: contiguous crystals with parallel faces are crystallographically continuous, whereas contiguous crystals with non-parallel faces have unrelated crystal lattice orientations and as such, represent separate crystals. In the highly undercooled sample, twinning is identified in ~87% of the crystals examined; in the near-equilibrium sample, 38% of the crystals are twinned. We find that the observed twinning is unlikely to be the result of deformation, transformation, or synneusis, but rather a result of growth defects introduced during the incipient stages of crystallization. We suggest internal structural defects (twins) control macroscopic morphological defects (embayments, swallowtails, and melt inclusions) as a result of the high energy of the twin plane boundary. Formation of twins during the incipient stages of plagioclase crystallization is the single most important factor contributing to anhedral morphologies of feldspar microlites growing during magma decompression, and plays a role in the development of some plagioclase-hosted melt inclusions.

Experimental Constraints on Plagioclase Crystallization during H2O- and H2O–CO2-Saturated Magma Decompression

“…The latter may be an artifact of the imaging process, reflecting magnification scales too high to capture statistically meaningful numbers of the largest crystals; it may also record a lack of late nucleation. Overturns at small size classes have been variously attributed to ripening (e.g., Higgins & Roberge 2003;Pupier et al, 2008), under-compensation for intersection probability (e.g., Brugger & Hammer 2010b), or insufficient resolution (e.g., Cashman & Ferry 1988;Hammer et al 1999), although our image resolution is more than adequate to capture crystals >1 µm in length. The most pronounced feature of our size distributions is, however, their curvature, which has been ascribed to a plethora of natural processes in the volcanological literature (e.g., magma mixing, crystal accumulation, textural coarsening, two-stage crystallization, or accelerating nucleation and growth; Marsh 1988;Armienti et al 1994;Higgins 1998;Bindeman 2003;Higgins & Roberge 2003;Shea et al 2010a), or to artifacts produced by the 2D-3D conversion because of variations in crystal shape (Castro et al 2004).…”

“…In particular, choices of characteristic crystal length L vary considerably (e.g., major axis length, caliper length, crystal width, square root of crystal area), as do approaches to stereological conversion (Peterson, 1996;Sahagian & Proussevitch, 1998;Higgins, 2000). Following Brugger & Hammer (2010b), we adopt methods and assumptions that have been widely used in recent textural studies of igneous systems. Our aim is to facilitate comparison of our experimental results with past (and future) datasets.…”

To sink, swim, twin, or nucleate: A critical appraisal of crystal aggregation processes

“…The large lattice mismatches, and associated energy penalties, generated by heterogeneous nucleation may drive the detachment of a poorly orientated unit, followed by reattachment in a more-favorable orientation (Buerger, 1945). In certain cases, it may even be more energetically favorable for the components of the misaligned daughter crystal to remain in the liquid state (Brugger and Hammer, 2015).…”

Experimentally determined subsolidus metal-olivine element partitioning with applications to pallasites

“…FeNiCo charges were prepared for EBSD analysis by light polishing to remove the carbon coat, followed by ultra-fine polishing using colloidal silica on a VibroMet ™ vibrating polisher. For EBSD measurements, the SEM was operated at low vacuum (11 Pa) with 20 kV accelerating voltage and 5 nA beam current and 70° tilted stage (Brugger and Hammer, 2015). Five to ten point analyses were conducted on each sample to ensure that single crystals were analyzed.…”