Doctor of Philosophy
Planetary Science and Exploration
Osinski, Gordon, R.
Impact melts physically and texturally resemble endogenic igneous melts but are known to be initially superheated. However, the exact amount of superheating and, therefore, the temperatures experienced by superheated impact melts are not very well constrained. Zircon is a refractory mineral that can survive high pressure-temperature events and has been known to persist even in the extreme conditions of impact events. Additionally, zircon (ZrSiO4) dissociates to zirconia (ZrO2) and silica (SiO2) at temperatures > ~1670 ℃, and the pressure-temperature phase relationships of zircon, zirconia, and silica have been well studied.
Zircons in different types of impactites from the West Clearwater Lake impact structure were investigated using various microanalytical techniques to: (1) identify the presence of crystallographic deformation; (2) to assess the orientation relationships arising from polymorphic transformations of high pressure or high temperature phases using a phase heritage approach, and (3) to constrain initial pressure-temperature conditions with the help of phase relationships. We documented evidence for superheating > 2370 ℃ in an impact glass sample, shock pressures of >30 GPa in some impactites, and observed a variety of microstructures ranging from fully dissociated and granular zircons to undeformed zircons in all impactite samples. Our results also demonstrate that impact melt evolution is complex and the evidence for superheating in zircons appears to be differentially preserved in different impactite settings. Furthermore, we also investigated the variation in textures and geochemistry of a thick impact melt deposit (Discovery Hill) at the Mistastin Lake impact structure, which was recently shown to contain evidence for superheating in excess of 2370 ℃. It is proposed that the textures seen in this melt rock are a result of a complex cooling history governed by the initially superheated melt that interacted with cooler country rock. Because such textures are analogous to those seen in some lunar impact melts, this thesis has implications for improving our knowledge of the cratering process on the Moon.
Summary for Lay Audience
Craters formed by hypervelocity meteorite impacts are ubiquitous on rocky planetary bodies of our Solar System, and the process of impact cratering had a significant role in modifying the surface, atmosphere, and biosphere of primordial Earth. During crater formation, decompression of target material after the passage of a shock wave causes a volume of target rock (and the projectile) to melt and/or vaporize, leading to the formation of impact-generated melt rocks. Melts produced during impact cratering experience superheated temperatures that are higher than the liquidus of the constituent minerals, and the degree of superheating is a function of residual heat in the system after passage of the shock wave. However, few constraints exist on the exact amount of heat that the melt experiences. This is partly due to the fact that preservation of impact melts in terrestrial craters is rare, and most traditional minerals that are used for obtaining thermobarometric constraints on rocks do not survive the process of impact cratering. This thesis presents analyses of impactites from the West Clearwater Lake impact structure using zircon and zirconia microstructures to obtain pressure-temperature constraints on the impact melts at this crater. Zircon is an accessory mineral that commonly occurs in most rock types and survives for long geologic time periods. Zircon is especially useful for obtaining pressure-temperature estimates because it can survive very high pressure -temperature events; it chemically dissociates at temperatures >1670 ℃ and transforms to reidite at high pressures. The presence of such high pressure-temperature indicators in zircon was studied using electron backscatter diffraction and was inferred using crystallographic relationships. We also explored the petrogenesis and geochemistry of an impact melt deposit at the Mistastin Lake impact structure, where previous workers had obtained temperature estimate of >2370 ℃, implying that this melt rock is the hottest known rock on the Earth’s surface. Our work illustrates the crystallization history of an initially superheated impact melt rock. Because impact melts are far better preserved on the Moon, and lunar impact melts are critical in improving our knowledge of the Earth-Moon system, we also comment on implications of our work for lunar impact melts.
Chinchalkar, Neeraja, "Superheated melts in hypervelocity meteorite impact craters: Evidence from terrestrial craters" (2023). Electronic Thesis and Dissertation Repository. 9744.
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