THE ROLE OF FRACTIONAL VAPORIZATION AS AN IGNEOUS DIFFERENTIATION PROCESS DURING THE FORMATION OF ROCKY PLANETARY BODIES

Igneous differentiation describes how the bulk composition of a magma changes through processes like partial melting or fractional crystallization during cooling, emplacement, or eruption. Fractional vaporization is another potential process of igneous differentiation that is relevant under the extremely high temperature (>2000 °C) and low-pressure conditions associated with the early formation stages of rocky planetary bodies. We hypothesize that vaporization of moderately volatile elements (MVEs) changes the bulk magma composition, modifying the crystallization sequence of a magma. Here we investigate fractional vaporization as an igneous differentiation process using high temperature, low pressure evaporation experiments with a high-temperature conical nozzle levitator (HT-CNL™) and thermodynamic modeling. We used binary and multi-component silicate melt compositions relevant to planetary processes, including An20, Fo30, and a bulk silicate Earth (BSE). The high temperature experiments are conducted in three phases. (1) Glass spheroids are heated to 2000 °C for up to 60 seconds to evaporate MVEs. (2) The liquid spheres are then held isothermally at a temperature close to the liquidus temperatures of the initial glass compositions for up to 480 s. (3) The liquids are quenched to glasses at rates >400 °C s⁻¹, preserving liquid compositions and any crystalline textures that develop. Preliminary results from experiments using the An20 composition show a decrease in Na concentrations with increasing laser dosage, crystalline textures on the surface of levitated spheres and increase in density suggesting partial crystallization. We also modeled the effects of equilibrium fractional vaporization on the crystallization sequence of a lunar magma ocean (LMO) composition using MAGMA and alphaMELTS software. We find (1) increased liquidus temperatures of the magma and higher plagioclase onset crystallization temperatures with increasing vaporization, and (2) a 48% increase in the modal abundance of plagioclase after crystallizing a melt that experienced 25% fractional vaporization. Our results suggest that fractional vaporization can be an effective igneous differentiation process during the early formation of planetary bodies with implications for the formation of lunar highland crust.