Developing Levitation Laser-Fused Glasses as Proxies for Lower Mantle Experiments: a Methodological Approach

Abstract

Observations of heterogeneities in Earth’s mantle motivate studies of mantle phase assemblages with variable composition. As samples cannot be directly collected from these regions, synthetic glasses can act as analogues for mantle melt and starting materials for high-pressure synthesis of stable mantle minerals in experiments. Here, I develop a specific methodology to produce homogeneous glasses that accurately span the composition space from enstatite (MgSiO3) to forsterite (Mg2SiO4), as well as Fe-bearing enstatite ((Mg0.1Fe0.9)SiO3 and ((Mg0.95Fe0.05)(Si0.95Fe0.05)O3) with variable oxidation states. This study systematically tests and iterates upon glass synthesis methods using an aerodynamic levitation laser furnace, in which a spherical glass sample levitates on a gas stream flowing vertically through a conical nozzle, while being heated from above with a 400 W CO2 laser. With sample diameters of 0.6-2.0 mm, shutting off the laser results in supercooling of levitated spheres at rates between 350 and 1350 °C/s. Sample preparation begins with grinding and mixing pure oxide powders in an agate mortar and pestle, followed by heating in a high temperature oven to devolatilize the mixture. Powders (0.5-7 mg aliquots) are fused into spheres in a copper hearth plate. To tune Fe valency and vitrify each sphere, samples are then levitated on flows of Ar, O2, 5% CO in Ar, 5% CO2 in Ar, or combinations of two of these gases, while being heated with the laser to temperatures above the liquidus for each composition for ~10 s before quenching. After x-ray diffraction (XRD) analyses confirm vitrification, a dual polish is applied, exposing parallel flat polished surfaces for scanning electron microscope (SEM) and electron probe microanalyzer analyses (EPMA). Back-scattered electron images and energy-dispersive x-ray spectroscopy (EDS) analyses of the spheres are obtained first on the SEM to gauge compositional accuracy and homogeneity, then EPMA analyses determine quantitatively the samples’ compositions. Once fully characterized, these glasses can be used in diamond anvil cell experiments, where they can act as proxies for an otherwise inaccessible area of the Earth. In addition to the development of this methodology, two web applications produced during this research aid in visualization of both data logs and analytical results.