The Physical Properties of Volcanic and Impact Melt

The emplacement mechanisms of lunar impact melt flows, that form from hypervelocity impact events, have been a subject of debate in the lunar science community, because of their unique physical properties that separate them from other geologic features. Understanding how lunar impact melt flows were emplaced on the surface of the Moon will not only grant us new information about the flow dynamics of impact melt but provide insight into the production and distribution of impact melt and how it built and modified the surfaces of planetary surfaces. Lunar impact melt flows exhibit surface roughness textures and morphologies that are analogous to terrestrial lava flows. For this reason, we seek to quantify the surface roughness of terrestrial lava flows using synthetic aperture radar (SAR) at two localities, Craters of the Moon National Monument and Preserve, Idaho and the 2014-2015 Holuhraun lava flow-field. We focus on using SAR data in this study for two reasons, (1) improve our understanding on how radar surface roughness can be connected to the emplacement mechanisms of volcanic and impact melt, and (2) to highlight the techniques capabilities and limitations for differentiating different lava flow types and lava facies. Impact melt has contrasting intrinsic properties and geologic origins to lava flows, so we include the analysis of a physical property of impact melts that influences melt behaviour. To complement our radar surface roughness analysis, we seek to constrain the temperature of the Mistastin Lake impact structure impact melt deposits by analyzing the crystallographic orientations and microstructures of zircon grains and zirconia crystals encased in melt-bearing impactites. We demonstrate in this work that without entirely understanding the capabilities and limitations of using SAR for lava flow differentiation, we will struggle to interpret the eruption dynamics and history of volcanic landforms on terrestrial bodies, which in turn limits what we can learn about impact melt emplacement. Furthermore, we discover that high temperature and pressure conditions can be constrained from an impact environment that was once superheated, which has strong implications for discovering high P-T shock indicators in other terrestrial impact structures and also in lunar impactites. In addition, our work has strong applications towards addressing high priority science goals established by research groups such as the Lunar Exploration Analysis Group

Hot Rocks: Constraining the Thermal Conditions of the Mistastin Lake Impact Melt Deposits Using Zircon Grain Microstructures

The production of superheated melt during hypervelocity impact events has been proposed to be a common occurrence on terrestrial planetary bodies. Recent direct evidence of superheated impact melt temperatures exceeding \u3e2370°C from the Kamestastin (Mistastin Lake) impact structure, Canada, was based on a single impact glass sample. Such high superheated melt temperatures have strong implications for the evolution of crustal material, the thermal history of impact cratering events, and the rheology of impact melt. However, although widely predicted in previous studies, with the exception of the Mistastin Lake impact glass, there is little direct evidence for superheated temperatures in multiple settings across an impact structure. Therefore, an outstanding question is how heterogeneous are superheated conditions across a single impact structure. In this work, we analyze the crystallographic orientations and microstructures of zircon grains and the precursor parent phases of baddeleyite crystals, from four different samples representing the entire melt-bearing stratigraphy at Mistastin: an impact glass, a vesicular clast-poor impact melt rock, a clast-rich impact melt rock, and a glass-bearing impact breccia. Using electron microprobe analysis followed by electron backscatter diffraction, we discovered that four zircon grains with vermicular coronae of baddeleyite crystals from the impact glass contain evidence for a cubic zirconia precursor, indicative of temperature conditions \u3e2370°C. We also report evidence of superheating up to 1673°C in the glass-bearing impact breccia. In addition, we also report the first occurrence at Mistastin of the high-pressure zircon polymorph reidite and former reidite in granular neoblastic (FRIGN) zircon in grains from the glass-bearing impact breccia, implying minimum peak shocks from 30–40 GPa. The identification of superheating from two localities at Mistastin demonstrates (1) that superheating is not restricted solely to rapidly cooled impact melt rock samples and is therefore more distributed across impact structures, and (2) we can investigate the P-T evolution pathways of impact melt from different impact settings, providing a clearer picture of the thermal conditions and history of the impact structure

Interpretations of Lava Flow Properties from Radar Remote Sensing Data

The surface morphology and roughness of a lava flow provides insight on its lava properties and emplacement processes. This is essential information for understanding the eruption history of lava fields, and magmatic processes beneath the surface of Earth and other planetary bodies such as the Moon. The surface morphology is influenced by lava properties such as viscosity, temperature, composition, and rate of shear. In this work, we seek to understand how we can interpret the emplacement processes and lava properties of lava flows using remote sensing data. Craters of the Moon (COTM) National Monument and Preserve in Idaho hosts a suite of compositionally diverse lava flows with a wide range of surface roughness making it the ideal case study. Lava flows there have surface morphologies consistent with smooth pāhoehoe, slabby pāhoehoe, hummocky pāhoehoe, rubbly pāhoehoe, ‘a’ā, block-`a’ā, and blocky textures. The variation in surface roughness across the lava field reflects changes in lava properties and/or emplacement processes over space and time. We investigate geochemical and petrographic variations of the different lava flow morphologies and analyse how they relate to airborne radar data. Results show L-Band (24 cm) radar circular polarization ratios (CPR) distinguish the contrasting surface roughness at COTM, separating the smoother (primitive; low SiO2 and alkali) and rougher (evolved; high SiO2 and alkali) lava flows. However, ambiguities are present when comparing the CPR values for rubbly pāhoehoe and block-`a’ā flow. Even though their CPR values appear similar at the decimetre scale, they have distinct morphologies that formed under different emplacement processes. Without ground-truth information, the rubbly pāhoehoe and block-`a’ā lava flows could therefore be misinterpreted to be the same type of flow morphology, which would lead to false interpretations about their lava properties and emplacement processes. This is important when comparing these flows to lava flows on other planetary bodies that share similar CPR values, such as the Moon. Thus, using terrestrial analogues such as those at COTM can provide an improved understanding of the surface morphology and emplacement processes of lunar lava flows. This will lead to more refined interpretations about past volcanic processes on the Moon

Differentiating Fissure-Fed Lava Flow Types and Facies Using RADAR and LiDAR: An Example from the 2014–2015 Holuhraun Lava Flow-field

Distinguishing between lava types and facies using remote sensing data is important for interpreting the emplacement history of lava flow-fields on Earth and other planetary bodies. Lava facies typically include a mixture of lava types and record the collective emplacement history of material preserved at a particular location. We seek to determine if lava facies in the 2014–2015 Holuhraun lava flow-field are discernible using radar roughness analysis. Furthermore, we also seek to distinguish between lava types using high resolution Light Detection and Ranging (LiDAR) data. We extracted circular polarization ratios (CPR) from the Uninhabited Aerial Vehicle Synthetic Aperture Radar and cross-polarization (VH/VV) data from the Sentinel-1 satellite to analyze the surface roughness of three previously mapped lava facies: rubbly, spiny, and undifferentiated rubbly–spiny. Using the Kruskal-Wallis test, we reveal that all but one pair of the facies are statistically separable. However, the populations overlap by 88%–89% for CPR and 64%–67% for VH/VV. Therefore, owing to large sample populations (n \u3e 2 × 105), slight differences in radar data may be used to probabilistically infer the presence of a particular facies, but not directly map them. We also calculated the root-mean-square slope and Hurst exponents of five different lava types using LiDAR topography (5 cm/pixel). Our results show minute differences between most of the lava types, with the exception of the rubbly pāhoehoe, which is discernible at 1σ. In brief, the presence of “transitional” lava types (e.g., rubbly pāhoehoe) within fissure-fed lava flow-fields complicates remote sensing-based mapping