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Browsing by Author "Duncan, Megan S."

Now showing 1 - 4 of 4
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    Modeling the Thermal and Chemical Evolution of the Martian Lithosphere Over Time
    McGroarty, Fiona Clare (Virginia Tech, 2021-11-16)
    Mars is an ideal planet to study planetary evolution and development, as its crust has been preserved over its history, rather than continuously recycled through subduction, as has happened on Earth. In order to attain a more coherent understanding of martian evolution, we focused on the thermal and petrologic history of the martian lithosphere. We developed a model that calculates the thermal state and melt composition of Mars over time. This model provides insight into the planet's history and enables us to describe how the density and seismic properties have evolved over time. We calculated the temperature profile through the lithosphere and then fit an equation to pre-existing experimental data in order to produce a model to predict the composition of melt produced as a function of pressure and temperature. From the melt model, we see a trend from ultramafic to mafic composition over time. We calculated the density and seismic properties of the lithosphere and found that they increase over time, but decrease with depth, which is consistent with the recent observations of NASA's InSight mission.
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    Temperature and Density on the Forsterite Liquid-Vapor Phase Boundary
    Davies, E. J.; Duncan, Megan S.; Root, S.; Kraus, R. G.; Spaulding, D. K.; Jacobsen, S. B.; Stewart, S. T. (2021-04)
    The physical processes during planet formation span a large range of pressures and temperatures. Giant impacts, such as the one that formed the Moon, achieve peak pressures of 100s of GPa. The peak shock states generate sufficient entropy such that subsequent decompression to low pressures intersects the liquid-vapor phase boundary. The entire shock-and-release thermodynamic path must be calculated accurately in order to predict the post-impact structures of planetary bodies. Forsterite (Mg2SiO4) is a commonly used mineral to represent the mantles of differentiated bodies in hydrocode models of planetary collisions. Here, we performed shock experiments on the Sandia Z Machine to obtain the density and temperature of the liquid branch of the liquid-vapor phase boundary of forsterite. This work is combined with previous work constraining pressure, density, temperature, and entropy of the forsterite principal Hugoniot. We find that the vapor curves in previous forsterite equation of state models used in giant impacts vary substantially from our experimental results, and we compare our results to a recently updated equation of state. We have also found that due to under-predicted entropy production on the principal Hugoniot and elevated temperatures of the liquid vapor phase boundary of these past models, past impact studies may have underestimated vapor production. Furthermore, our results provide experimental support to the idea that giant impacts can transform much of the mantles of rocky planets into supercritical fluids.
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    The volatile contents of melt inclusions and implications for mantle degassing and ocean island evolution
    Moore, Lowell (Virginia Tech, 2019-09-03)
    The amount of volatile elements dissolved in silicate melts is a controlling factor in a range of geologic processes, which include hazardous volcanic eruptions, economically-significant ore-forming systems, and global-scale volatile fluxes, which contribute to planetary evolution. While melt volatile contents are important, estimating the origin and fate of volatiles distributed within magmas is challenging because volatiles exsolve from the melt during eruption and are transferred into the atmosphere. Therefore, the stratigraphic record of volcanic and intrusive deposits does not contain direct information regarding the pre-eruptive volatile content of the melt. However, melt inclusions trapped within growing phenocrysts present an opportunity to sample the melt before it has completely degassed. Analysis of melt inclusions is challenging owing to a range of processes which occur after the melt inclusion is trapped and which overprint the original texture and composition of the inclusion at the time of entrapment. Thus, efforts to accurately determine the current composition of the melt inclusion sample and then infer the original composition of the trapped melt which that inclusion represents require a combination of microanalytical, numerical, and/or experimental methods. In Chapter 1, we present a pedagogical approach for estimating the processes that affect the CO2 content of a magma from its origin during melting a C-bearing source material to its exsolution into a free fluid phase during crystallization and degassing. In Chapter 2, we explore different experimental, microanalytical, and numerical methods which may be used to estimate the CO2 contents of melt inclusions that contain fluid bubbles and describe the advantages and disadvantages of each approach. In Chapter 3, we apply some of the methods discussed in the previous chapters to estimate the pre-eruptive volatile content of Haleakala Volcano (Maui) and assess different melting mechanisms that may be active in the Hawaiian plume.