Browsing by Subject "Tektites"

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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%).
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    New Insights Into Impact Glass Formation and Evolution Using Machine Learning and Aerodynamic Levitation Laser Heating Experiments
    (2022-09) Marrs, Ian James; Macris, Catherine; Barth, Andrew; Druschel, Gregory
    Impact processes, where a meteor strikes a planetary body’s surface, are ubiquitous in the Solar System. These highly energetic events require study by both computational methods and experimental investigation. An impact process of particular interest to our study is the impact plume, a collection of vaporized rock and superheated gases that is produced during an impact event. Tektites are silica rich (roughly rhyolitic), extremely dry, and often contain both lechatelierite inclusions (amorphous SiO2) and flow textures (schlieren) and are an impact product of particular interest to this study. Tektites likely form either very early in the impact process or within the impact plume itself as condensates, and therefore offer a unique insight into the early stages of the impact cratering process. Here, we present both the results of the statistical analysis of published tektite geochemistry and the geochemical analysis of a variety of glasses produced in an aerodynamic levitation laser furnace. The major findings of the statistical analysis are that the variance of tektite geochemistry is broadly controlled by MgO, CaO, K2O, and Na2O, that the Australasian strewn field (an extensive region of tektite distribution) is best subdivided into five geochemical subgroups, and that random forest classification models can predict the strewn field or geochemical subgroup of an unknown tektite with >94% accuracy. In terms of our heating experiments, in nearly all cases, Na2O and K2O are rapidly lost from the melt due to evaporation, while Al2O3, CaO, and TiO2 become progressively enriched. Volatility is far more dependent on peak heating temperature than on heating time. Additionally, the chemical constituents of basalts are less readily volatilized than those of rhyolites or loess, with few exceptions. We also find that the volatility of the chemical constituents of non-standard samples is far more variable than for standard samples and that oxygen fugacity has a strong influence over elemental volatility in the aerodynamic levitation laser furnace. Changes in oxygen fugacity can either result in variable, exaggerated, or even opposite volatility trends depending on the material and oxide in question.