Browsing by Subject "Evaporation"

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    Evaporating Planetesimals: A Modelling Approach
    (2021-10) Hogan, Arielle Ann; Macris, Catherine A.; Barth, Andrew P.; Druschel, Gregory K.
    This thesis is a comprehensive investigation into the mechanics of evaporation experienced by planetesimals during accretion, a planet-building process. The evaporation events that these rocky bodies experience govern their chemical evolution, impacting the chemistry of the final body – a planet. Studying these planet-building processes is notoriously difficult (e.g., Sossi et al., 2019). There are still many unknowns surrounding what controls the degree of evaporation these bodies experience, and the resulting chemical signatures. The current study was designed to attempt to define some important parameters that govern silicate melt evaporation. Here, we isolate and evaluate the effects of (1) pressure, (2) oxygen fugacity and (3) the activity coefficient of MgO on evaporating planetesimals through a series of computational models. The model introduced in this study, the ƒO2 Modified KNFCMAS Model, uses a robust stepwise routine for calculating evaporative fluxes from a shrinking sphere. The modelling results are then compared to data from partial evaporation experiments of synthetic chondrite spheres to demonstrate the validity of this model, and to expose unknowns about the physicochemical conditions of high temperature silicate melts experiencing evaporation (in this case, the effective pressure, and the activity coefficient of MgO). Major element-oxide and isotope data from the models yielded two main conclusions concerning planetesimals: (1) the rate of evaporation is controlled by pressure and oxygen fugacity and (2) the chemical composition of the residual melt is controlled by oxygen fugacity and the activity coefficient of MgO. Results from computational modelling and evaporation experiments were used to determine an approximation for the activity coefficient of MgO in a simplified chondritic composition, as well as the effective pressure experienced by the evaporating spheres during the partial evaporation experiments. This study outlines the controls on planetesimal chemistry during evaporation and provides a more accessible means of studying these complex processes.
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    Evaporation-induced copper isotope fractionation: Insights from laser levitation experiments
    (Elsevier, 2021-04) Ni, Peng; Macris, Catherine A.; Darling, Emilee A.; Shahar, Anat; Health Sciences, School of Health and Human Sciences
    As a transition metal that is moderately volatile at high temperatures, copper shows limited isotopic fractionation in terrestrial mantle-derived rocks but significant enrichment in its heavier isotope (up to 12.5‰ for 65Cu/63Cu) in objects that experienced volatile loss during formation, such as tektites, trinitite glasses, and lunar rocks. Previous efforts to model the Cu isotope fractionation trend from measurements of δ65Cu in tektites found that the trend cannot be explained by the theoretical isotope fractionation factor (α) for free evaporation of Cu, making it necessary to experimentally study Cu isotope fractionation under conditions similar to tektite formation. Here we present new experimental data of elemental (Na, K, Cu) and isotopic (Cu) fractionation during evaporation. Our experiments, conducted by laser-heating an aerodynamically levitated glass sphere to 1750, 2000, and 2150 °C, show rapid loss of Na, K, and Cu from the molten glass. In particular, > 99.99% of Cu was lost within 60 seconds. The evaporation induced loss of Cu is accompanied by progressive enrichment in its heavier isotope in the residue glass, with a maximum fractionation in δ65Cu of ∼18‰ relative to the synthesized initial sample. The empirical fractionation factor (α) calculated from our laser levitation data is 0.9960 ± 0.0002. Compared to similar experiments conducted for Zn, Cu appears to be significantly more volatile and show higher degrees of Cu isotope fractionation, consistent with observations in natural tektites. Comparing isotopic fractionation in a range of moderately volatile elements among laser levitation experiments, tektites, trinitites, and the bulk silicate Moon suggest that they experienced evaporation under various degrees of effective vapor saturation (∼74%, 93%, ∼99%, ∼99%), which depart significantly from free-evaporation (0%).