Olivine Phenocryst morphology with glass and mineral inclusions

The x-ray micro computed-tomography (micro-CT) model of this olivine crystal was acquired on a 2010 GE phoenix v|tome|x s240 system using a 180 KV nanofocus transmission X-ray tube. The olivine was placed in a low density polyethylene pipette. To achieve the smallest voxel size (nominally 1.1 µm), the sample was placed as close to the x-ray source as possible without collision (<1mm). In preliminary scans, we determined that the sample was blurring due to thermal expansion from the X-ray source. To combat this we placed the sample in the X-ray beam ~30 minutes prior to acquiring images so that a steady state could be reached. To further reduce image blur and ensure a crisp x-ray images the primary electron beam was manually focused immediately prior to data acquisition. A low accelerating voltage (40 kV) and molybdenum x-ray target was chosen to enhance compositional contrast in the x-ray images. Detector acquisition timing (333 ms per image), and X-ray source current  (325 µA) was optimized to reduce image noise in the densest part of the image while ensuring a total acquisition time under 1 hour. A thin (0.1 mm) copper filter was placed in front of the x-ray beam to attenuate the beam hardening effects of polychromatic x-ray sources in CT images. For each image used in the tomographic reconstruction four images were averaged to reduce signal noise.  A total of 1200 images were acquired for the tomographic reconstruction, resulting in an angular step size of 0.3˚. X-ray images were reconstructed into computed tomography volumes using GE Phoenix Datos software (version 2.3.2), a median kernel filter (3x3) was applied to the images prior to reconstruction to reduce signal noise, a beam hardening correction was applied to reduce artifacts in the Feldkamp cone beam reconstruction algorithm. Volumetric data was segmented and animated using Volume Graphics VG Studio Max 3.4 as well as the Fiji distribution of ImageJ.

The olivine grain is from the terminal scoria deposits of the 1932 eruption of Volcán Quizapu, Chile. The grain demonstrates the complexities associated with studying olivine (green) and its inclusions (here melt inclusion (dark red) with vapor bubble (teal) and oxide inclusion (yellow)). The olivine is partially enclosed in silicate glass (gray) and adhering plagioclase (white) crystals. This model shows the morphology of this olivine crystal that includes subgrains and melt channels (often referred to as embayments or re-entrants) that connect from the outside of the grain and extend into the grain’s center.