DEVELOPING LEVITATION LASER-FUSED GLASSES AS PROXIES FOR LOWER MANTLE EXPERIMENTS: A METHODOLOGICAL APPROACH

Observations of heterogeneities in Earth’s mantle, including provinces near the core-mantle boundary, 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. However, a specific methodology producing accurate and homogeneous glasses with arbitrary enstatite-forsterite, and both ferrous and ferric Mg-Fe pyroxene chemical compositions has not been developed. This study systematically tests glass synthesis using an aerodynamic levitation laser furnace, in which a spherical sample is positioned within a gas stream that flows vertically through a conical nozzle, allowing samples to levitate while being heated from above with a 400 W CO₂ laser. With sample diameters of 0.6-2.0 mm, shutting off power to 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. In order 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 XRD analyses confirm vitrification, a dual polish is applied, exposing parallel flat polished surfaces for SEM and EPMA analyses. Back-scattered electron images and 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 characterized, these glasses will be used in diamond anvil cell experiments, where they will act as proxies for an otherwise inaccessible area of the Earth.