Search NASA⌕ Search

SEARCH · Search NASA

Results for “Metamorphism”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS2014

Sm-Nd and Rb-Sr Isotopic Systematics of a Heavily Shocked Martian Meteorite Tissint and Petrogenesis of Depleted Shergottites

Tissint is a very fresh Martian meteorite that fell near the town of Tissint in Morocco on July 18, 2011. It contains abundant olivine megacrysts (~23%) in a fine-grained matrix of pyroxene (~55%), maskelynitized plagioclase (~15%), opaques (~4%) and melt pockets (~3%) and is petrographically similar to lithologies A and C of picritic shergottite EETA 79001 [1,2]. The presence of 2 types of shock-induced glasses and all 7 high-pressure mineral phases that were ever found in melt pockets of Martian meteorites suggests it underwent an intensive shock metamorphism of ~25 GPa and ~2000 C localized in melt pockets [2]. Mineral textures suggest that olivines, pyroxenes and plagioclases probably did not experience such hightemperature. Earlier determinations of its age yielded 596+/-23 Ma [3] and 616+/-67 Ma [4], respectively, for the Sm-Nd system and 583+/-86 Ma for the Lu-Hf system [4], in agreement with the 575+/-18 Ma age of the oldest olivine-phyric depleted shergottite Dho 019 [5]. However, the exposure ages of Tissint (~1 Ma [1, 6, 7]) and Dho 019 (~20 Ma [8]) are very different requiring two separate ejection events. These previously determined Sm-Nd and Lu-Hf ages are older than the Ar-Ar maskelynite plateau age of 524+/-15 Ma [9], reversing the pattern usually observed for Martian meteorites. In order to clarify these age issues and place models for Tissint's petrogenesis on a firm basis, we present new Rb-Sr and Sm- Nd isotopic results for Tissint, and discuss (a) the shock effects on them and the Ar-Ar chronometer, (b) correlation of the determined ages with those of other depleted shergottites, and (c) the petrogenesis of depleted shergottites. Since the meteorite is a recent fall, terrestrial contamination is expected to be minimal, but, the strong shock metamorphism might be expected to compromise the equilibrium of the isotopic systems.

Shih, C.-Y.↗
NASA NTRS2014

Simulation of Seasonal Snow Microwave TB Using Coupled Multi-Layered Snow Evolution and Microwave Emission Models

The accurate quantification of SWE has important societal benefits, including improving domestic and agricultural water planning, flood forecasting and electric power generation. However, passive-microwave SWE algorithms suffer from variations in TB due to snow metamorphism, difficult to distinguish from those due to SWE variations. Coupled snow evolution-emission models are able to predict snow metamorphism, allowing us to account for emissivity changes. They can also be used to identify weaknesses in the snow evolution model. Moreover, thoroughly evaluating coupled models is a contribution toward the assimilation of TB, which leads to a significant increase in the accuracy of SWE estimates.

microwave↗
NASA NTRS2015

Petrology and In Situ Trace Element Chemistry of a Suite of R Chondrites

Rumuruti (R) chondrites are characterized by low chondrule/matrix modal ratios, high oxidation state, small mean chondrule size, abundant sulfides and low metal contents, and are of petrologic types 3 to 6 [1, 2]. LAP 04840 (R5, [3]) and MIL 11207 (R6), contain the high-T hydrous phases amphibole and mica [3, 4]; not all equilibrated R chondrites contain these [2]. R chondrites thus can provide evidence on whether there are compositional effects caused by high-T, high-fluid metamorphism of nebular materials. We are investigating a suite of R chondrites of diverse petrologic grades to further understand the nature of the metamorphic processes that engendered them [5]. We report on our petrological studies, plus preliminary in situ analyses of trace elements in amphibole-bearing R chondrites.

Mittlefehldt, D. W.↗
NASA NTRS2016

Mineralogy of Experimentally Heated Tagish Lake

Since the 1980s, more than 20 thermally metamorphosed carbonaceous chondrites (TMCCs) have been found in Antarctica and in hot deserts. The petrology of TMCCs suggests that some C-type asteroids were heated and dehydrated after aqueous alteration. Besides, previous studies indicate that the conditions of thermal metamorphism experienced by these meteorites may have been quite variable. It reflects that metamorphism of the TMCCs was complex.

Nakato, A.↗
NASA NTRS2016

Noble Metal Arsenides and Gold Inclusions in Northwest Africa 8186

CK carbonaceous chondrites are a highly thermally altered group of carbonaceous chondrites, experiencing temperatures ranging between approx.576-867 C. Additionally, the mineralogy of the CK chondrites record the highest overall oxygen fugacity of all chondrites, above the fayalite-magnetite-quartz (FMQ) buffer. Metallic Fe-Ni is extremely rare in CK chondrites, but magnetite and Fe,Ni sulfides are commonly observed. Noble metal-rich inclusions have previously been found in some magnetite and sulfide grains. These arsenides, tellurides, and sulfides, which contain varying amounts of Pt, Ru, Os, Te, As, Ir, and S, are thought to form either by condensation from a solar gas, or by exsolution during metamorphism on the chondritic parent body. Northwest Africa (NWA) 8186 is a highly metamorphosed CK chondrite. This meteorite is predominately composed of NiO-rich forsteritic olivine (Fo65), with lesser amounts of plagioclase (An52), augite (Fs11Wo49), magnetite (with exsolved titanomagnetite, hercynite, and titanohematite), monosulfide solid solution (with exsolved pentlandite), and the phosphate minerals Cl-apatite and merrillite. This meteorite contains coarse-grained, homogeneous silicates, and has 120deg triple junctions between mineral phases, which indicates a high degree of thermal metamorphism. The presence of NiO-rich olivine, oxides phases all bearing Fe3+, and the absence of metal, are consistent with an oxygen fugacity above the FMQ buffer. We also observed noble metal-rich phases within sulfide grains in NWA 8186, which are the primary focus of the present study.

Srinivasan, P.↗
NASA NTRS2016

Noble Metal Arsenides and Gold Inclusions in Northwest Africa 8186

CK carbonaceous chondrites are a highly thermally altered group of carbonaceous chondrites, experiencing temperatures ranging between approximately 576-867 degrees Centigrade. Additionally, the mineralogy of the CK chondrites record the highest overall oxygen fugacity of all chondrites, above the fayalite-magnetite-quartz (FMQ) buffer. Me-tallic Fe-Ni is extremely rare in CK chondrites, but magnetite and Fe,Ni sulfides are commonly observed. Noble metal-rich inclusions have previously been found in some magnetite and sulfide grains. These arsenides, tellurides, and sulfides, which contain varying amounts of Pt, Ru, Os, Te, As, Ir, and S, are thought to form either by condensation from a solar gas, or by exsolution during metamorphism on the chondritic parent body. Northwest Africa (NWA) 8186 is a highly metamorphosed CK chondrite. This meteorite is predominately composed of NiO-rich forsteritic olivine (Fo65), with lesser amounts of plagioclase (An52), augite (Fs11Wo49), magnetite (with exsolved titanomagnetite, hercynite, and titanohematite), monosulfide solid solution (with exsolved pentlandite), and the phosphate minerals Cl-apatite and merrillite. This meteorite contains coarse-grained, homogeneous silicates, and has 120-degree triple junctions between mineral phases, which indicates a high degree of thermal metamorphism. The presence of NiO-rich olivine, oxides phases all bearing Fe3 plus, and the absence of metal, are consistent with an oxygen fugacity above the FMQ buffer. We also observed noble metal-rich phases within sulfide grains in NWA 8186, which are the primary focus of the present study.

Srinivasan, P.↗
NASA NTRS2018

Development of High-Performance Solar Cells for the Jupiter and Saturn Environments

Many of the mission targets that NASA and the planetary-science community are interested in are located in deep space, in the 5-10AU range. This provides compelling motivation to develop solar cells and arrays that are highly efficient in low irradiance low temperature (LILT) environments. We give several examples of the iterative process our team has employed to develop cell designs that optimize the performance at LILT. We also provide results on advanced-architecture devices that have already demonstrated very high efficiencies in the Jupiter and Saturn LILT and radiation environments, specifically four-junction inverted metamorphic and triple-junction upright metamorphic solar cells, respectively.

Boca, Andreea↗
NASA NTRS2018

Physical, Chemical, and Petrological Characteristics of Chondritic Materials and Their Relationships to Small Solar System Bodies: Chapter - 2

Chondrite materials with varying abundances of volatile-bearing phases are expected at the destinations for the asteroid sample-return missions Hayabusa2 and OSIRIS-REx. The targets of the missions are 162173 (1999 JU3) Ryugu and 101955 (1999 RQ36) Bennu. Spectroscopic analyses of these asteroids suggest that their surface materials are related to types 1 and 2 carbonaceous chondrites. Some studies suggest that the parent bodies of these chondrites may have also experienced some thermal and/or shock metamorphism. The physical properties of boulders at asteroid surfaces and fine particles in asteroid regoliths are consequences of the diverse processes that fragmented them, mobilized them, and redeposited them in unique accumulations. Sample-return missions are likely to encounter a broad range of carbonaceous chondrite (CC)-like materials, to which aqueous alteration, thermal, and shock metamorphism imparted changes affecting their sub-micron- to meter-scale physical properties. Consequently, implementation of scale-dependent analytical techniques to the study of the chemical, physical, and geotechnical characteristics of these CC-like materials is fundamental to safe mission operations, sample selection, and return. However, most of the available knowledge for informing and formulating expectations about regolith processes, products, and properties on carbonaceous small bodies comes from missions that studied anhydrous (e.g., Itokawa studied by Hayabusa) and/or much larger asteroids (e.g., Vesta studied by Dawn). No previous mission is likely directly relevant to small ice-free carbonaceous NEOs 162173 Ryugu or 101955 Bennu, although the Rosetta Spaceraft performed a flyby of the large asteroid Lutetia which has variously been classified as M and C type (Pätzold et al., 2011). Carbonaceous chondrites carry the best record of the history, distribution, and activity of water in the early solar system. Ordinary and Enstatite chondrites carry only partial records, but these are still critical to understanding the full story. We will describe the records of water-rock interactions on asteroids, as recorded in these meteorites, with particular emphasis on the timing, nature, settings, and fluid compositions. An integral part of this story is the rare, but fortunate, preservation of actual early solar system water as aqueous fluid inclusions.

Zolensky, Michael E.↗
NASA NTRS2019

Creating Calibration Curves to Determine Shock Pressure in Clinopyroxene

Impact cratering is an important geological process that occurs on every rocky body in the solar system. It alters the texture and mineralogy of rocks via shock metamorphism. The peak shock pressures experienced by a rock are traditionally evaluated using qualitative optical methods however, quantitative methods do exist. One such method was developed by Uchizono et al., who used X-ray Diffraction (XRD) to measure lattice strain (ε) in several artificially shocked olivine grains using XRD peak broadening as a function of tan θ, where θ is the diffraction angle. They plotted the ε values against the known peak shock pressures experienced by the olivine grains. Using this calibration curve, the precise shock pressure experienced by a grain of olivine can be determined using its measured ε value. Another method was developed by McCausland et al. and Izawa et al., who used in situ XRD to measure strain-related mosaicity (SRM) of olivine in several ordinary chondrites and enstatite in enstatite chondrites, respectively. They plotted these results against the shock stage estimates for these meteorites. Using these plots, meteorites can be assigned to shock stage bins by measuring the SRM of olivine and/or enstatite. Both methods are useful for evaluating shock metamorphism, however, they have limitations. Uchizono et al.’s calibration curve has been successfully applied to martian meteorites, however it can only be applied to olivine-bearing rocks. McCausland et al.’s and Izawa et al.’s SRM method is uncalibrated and is limited to binning meteorites by shock stage. This work aims to expand on both methods by creating calibration curves for clinopyroxene (CPX): one for ε, similar to Uchizono et al.’s calibration curve for olivine, and one for SRM. This will extend the application of shock calibration methods to a greater variety of rock types. Preliminary results are presented herein.

Jenkins, Laura E.↗
NASA NTRS2019

Outstanding Natural Occurrence of TiO2-II at the Chicxulub Crater - Anatomy of A Shock-Produced High-Pressure Polymorph

Scientific drilling of the end-Cretaceous, approximately 180 kilometers-diameter Chicxulub crater (Yucatan Peninsula, Mexico) during IODP–ICDP (Integrated Ocean Drilling Program - International Continental Scientific Drilling Program) Expedition 364 has provided new insights into the formation, shock metamorphism, structural evolution, and thermal history of peak rings in large complex impact craters. An outstanding feature in uplifted granitoid rocks of the Chicxulub peak ring is the preservation of TiO2-II, an orthorhombic high-pressure polymorph of TiO2 with an alpha-PbO2 structure, produced during the impact from rutile and/or anatase at shock pressures of approximately 12.5-17.5 gigapascals. Unlike other mostly micro- and cryptocrystalline occurrences of TiO2-II at terrestrial impact sites, ejecta deposits, and in rare ultra-high pressure metamorphic rocks, TiO2-II at Chicxulub occurs as abundant euhedral crystals greater than or equal to 70 microns in size within aggregates of altered magmatic titanite. This mode of occurrence provides an excellent opportunity to investigate the crystallography and transformation kinetics of the shock-produced high-pressure polymorph using scanning electron microscopic, micro-Raman, electron backscatter diffraction (EBSD), focused ion beam (FIB), as well as transmission-EBSD and transmission electron microscopic (TEM) techniques. Here we present refined microstructural and new crystallographic results for TiO2-II at the Chicxulub crater.

Schmieder, Martin↗
NASA NTRSDate not supplied

Shock-Induced Deformation Recorded in the Regolith Particles from C-Type Asteroid Ryugu

The Hayabusa 2 mission successfully returned with samples of C-type near-Earth asteroid (162173) Ryugu on December of 2020. Ryugu samples contain large quantities of volatiles such as water and noble gases. Studying Ryugu samples is a great opportunity to understand the shock metamorphism on water-bearing rubble-pile asteroids. In this study, we have examined shock metamorphism on sixteen coarse mm-sized particles recovered by the Hayabusa 2 mission.

M. Kikuiri↗
NASA NTRSDate not supplied

Phase Transitions and Melting in the Venusian Basaltic Crust: Implications for Crustal Recycling

Understanding phase transitions and melting in the Venusian crust and the associated changes in density are critical to constrain crustal thickness, recycling, and remelting processes. For example, average surface conditions on Venus of 92 bars and 460 °C correspond to low-grade metamorphic conditions on Earth equivalent to hornfels- and greenschist-facies. Further, the primary igneous mineralogy in the Venusian crust should be replaced by metamorphic minerals over time and potentially melt, as temperatures and pressures increase with depth along geothermal gradients. Here, we use Perple_X (Connolly, 2005), a Gibbs free energy minimization program to calculates table phase equilibria over a range of pressure and temperature conditions for whole-rock compositions of a dry basalt, alkali basalt, and peridotite. Further, we extract mineral abundancies along five geotherms of 5, 10, 15, 20 and 25 °C/km and calculate the rock density after the extraction of melt. On the coldest geotherm of 5 °C/km, melting would start at ~ 197 km and ~121 km in the basalt and alkali basalt, respectively, and therefore at depths that significantly exceed the estimated thickness of the Venusian crust (8-25 km). A density cross-over, where the crustal density would exceed that of the mantle could potentially induce delamination and is estimated to occur at ~ 40 km for both compositions. On the 10 °C/km geotherm, melting of the basalt starts at 65 km and melt extraction only causes a gradual densification due the formation of initially lower melt proportions. In contrast, the alkali basalt starts to melt at a depth of ~ 57 km and the subsequent densification of the residual composition could trigger delamination at ~ 60km depth. For hotter geotherms (i.e., 25 °C/km gradient), melting would occur at shallower depths of 21 – 23 km. However, the basalt will not get significantly denser than the mantle while the residuum of the alkali basalt would reach a density cross-over at 30 km. Hence, differences in Venusian crustal compositions can significantly influence the thickness of the crust on individual geotherms. Our results demonstrate that phase transitions and melting are strongly dependent on the assumed geotherm and could, combined with geodynamic models, further constrain crustal parameters.

Julia Semprich↗
NASA NTRS2023

An evolutionary system of mineralogy, Part VI: Earth’s earliest Hadean crust (> 4370 Ma)

Part VI of the evolutionary system of mineralogy catalogs 262 kinds of minerals, formed by 18 different processes, that we suggest represent the earliest solid phases in Earth’s crust. All of these minerals likely formed during the first tens of millions of years following the global-scale disruption of the Moon-forming impact prior to ~4.4 Ga, though no samples of terrestrial minerals older than ~4.37 Ga are known to have survived on Earth today. Our catalog of the earliest Hadean species includes 80 primary phases associated with ultramafic and mafic igneous rocks, as well as more than 80 minerals deposited from immiscible S-rich fluids and late-stage Si-rich residual melts. Earth’s earliest crustal minerals also included more than 200 secondary phases of these primary minerals that were generated by thermal metamorphism, aqueous alteration, impacts, and other processes. In particular, secondary mineralization related to pervasive near-surface aqueous fluids may have included serpentinization of mafic and ultramafic rocks, hot springs and submarine volcanic vent mineralization, hydrothermal sulfide deposits, zeolite and associated mineral formation in basaltic cavities, marine authigenesis, and hydration of subaerial lithologies. Additional Hadean minerals may have formed by thermal metamorphism of lava xenoliths, sublimation at volcanic fumaroles, impact processes, and volcanic lightning. These minerals would have occurred along with more than 180 additional phases found in the variety of meteorites that continuously fell to Earth’s surface during the early Hadean Eon.

Philosophy of mineralogy↗
NASA NTRS2023

STXM-C-XANES, IR Microspectroscopy, and Raman Microspectroscopy Analysis of Organic Matter in the Chwichiya 002 C3.00-Ungrouped Chondrite

Chwichiya 002 is a C3.00-ungrouped carbonaceous chondrite discovered in Western Sahara in 2018. The presence of hydrated phases in Chwichiya 002 was not confirmed by XRD analysis, and only a faint water and Si-O band attributed to phyllosilicate were observed by infrared transmission spectroscopy, indicating a very low degree of aqueous alteration (Meteoritical Bulletin, 2020). The structural order of polycyclic aromatic hydrocarbons evaluated by Raman spectroscopy indicates that the effect of thermal metamorphism is less severe than in the Semarkona meteorite, an LL3.00 ordinary chondrite (Meteoritical Bulletin, 2020). Chiwichiya 002 is a very primitive carbonaceous chondrite and an important information source for the early stage of the evolution of organic matter in the solar system. In this study, we investigated the molecular structure and distribution of organic matter in Chiwichiya 002 by Carbon X-ray Absorption Near Edge Structure (C-XANES) analysis using a Scanning Transmission X-ray Microscope (STXM). The STXM-C-XANES analysis has a spatial resolution of 20~100 nm and can observe the spatial distribution of organic materials in microstructures. In addition, to investigate the degree of thermal metamorphism of the organic matter, micro-IR, and micro-Raman spectroscopic analyses were performed. In addition, the Murchison CM2 chondrite was also analyzed for comparison. To evaluate contamination during analytical procedures, antigorite, which was heated in air at 500°C for 5 hours to eliminate organic matter was also analyzed in the same manner. The samples were milled into 100 nm-thick, thin sections using a Focused Ion Beam (FIB), and C-XANES spectra were obtained using the STXM at the High Energy Accelerator Research Organization, Photon Factory (PF), BL-19B. The obtained C-XANES spectra were smoothed and then fitted with Gaussian functions using the method of Le Guillou et al. (2018). The C-XANES showed that the ratio of oxygenated functional group (ketone, phenolic) carbons in Chwichiya 002 was lower than in Murchison, whereas the ratio of aliphatic carbons was almost the same. This may reflect the low degree of aqueous alteration of Chwichiya 002. In addition, contamination peaks were detected at 287.3-287.7 eV and 288.3-288.7 eV in the C-XANES spectra of some of the antigorite samples. The source of these peaks is under investigation. Raman spectroscopic analysis showed that the intensity ratio of the D band to the G band of Chwichiya 002 was slightly higher than that of Murchison. IR absorption spectra showed aromatic absorption at 1630 cm -1 , but there was little absorption due to aliphatic C-H around 2900 cm -1 . These results suggest that the organic matter in Chwichiya 002 may be more heated than in Murchison.

Chwichiya 002↗
NASA NTRS2024

Magma-Sediment Hydrothermal Systems on Mars Explored Through Thermochemical Modeling and Analogue Studies

High temperature impact-generated and magmatic hydrothermal systems on Noachian Mars may have produced habitable environments by providing heat, energy, and volatiles necessary to support microbial life. Magmatic hydrothermal systems formed when mafic magma intruded into sedimentary rocks are of primary interest when investigating the habitability of high-temperature aqueous environments on Mars because contact metamorphism favors the alteration of the country rocks and the release of bound fluids in minerals and fluids in ice and pore spaces, and mobilization of bio-essential elements. Magma-sediment hydrothermal systems on Mars are difficult to identify from orbit and only limited in-situ investigations have been performed. Thus, terrestrial analogues need to be investigated. Recent studies focused on a magmatic hydrothermal system characterized by near-neutral Cl-S-CO 2 -rich fluids produced by a mafic dike intruding the Jurassic Entrada sandstone (Colorado Plateau, UT). Chemical and mineralogical changes identified in the dike and in the sediments were attributed to high-temperature metamorphism (> 700 °C), and low-temperature regimes (< 200 °C) when the system cooled down, and became potentially habitable [6]. However, compositional differences exist between the terrestrial dike and basaltic rocks on Mars, making difficult a comparison between secondary minerals and fluid chemistries of terrestrial and Martian systems. Here, we use thermochemical modelling to explore how differences in bulk dike composition will affect reaction pathways, secondary mineralogy, fluid chemistry, and bio-essential element availability.

S Cogliati↗
NASA NTRSDate not supplied

Insights Into the CM Parent Body's Interior: A Comparative Study With Bennu's Bright Boulders

CM (Mighei-like) chondrites, primitive and water-rich asteroid samples from the early solar system, are significant for their potential role in delivering water and organic compounds to Earth [1-2]. Their resemblance to near-Earth asteroids Bennu and Ryugu, the targets of recent sample return missions, highlights their importance. What we learn from CM chondrites can be applied to these intriguing celestial bodies. Initial studies indicated that phyllosilicate-rich C chondrites underwent low-temperature aqueous alteration (<150°C) [3-5]. Subsequent research revealed that some experienced post-hydration thermal metamorphism at temperatures between ∼200°C and >750°C, resulting in dehydrated minerals and reduced volatile elements [6-9].This finding challenged the previous assumption that these chondrites had not undergone significant thermal metamorphism.

I Kerraouch↗
DOE OSTI2024

InP- and GaAs-Based 0.6 eV GaInAs Devices for Thermophotovoltaics and Laser Power Conversion

Emerging applications such as thermal energy grid storage, waste heat recovery and portable power generation require efficient thermophotovoltaic (TPV) converters tuned to temperatures near 1000 Degrees Celsius or below. Metamorphic GaInAs with larger lattice constants than InP present a promising option for these needs. The ability to grow these devices on GaAs substrates instead of more expensive InP would enhance the scalability of these devices. In this talk, we present inverted metamorphic Ga0.3In0.7As photovoltaic converters with sub-0.60 eV bandgaps grown on InP and GaAs substrates. These devices are realized using InAsP or GaInP/InAsP compositionally graded buffers which exhibit threading dislocation densities of 1.3 +/- 0.6 x 106 cm-2 and 8.9 +/- 1.7 x 106 cm-2 on InP and GaAs, respectively. Despite this difference in defect density, the devices generate similar open-circuit voltages of 0.386 V and 0.383 V, respectively, under irradiance producing a short-circuit current density of -10 A/cm2, with bandgap-voltage offsets of 0.20 and 0.21 V. We estimate their thermophotovoltaic efficiency using these measurements coupled with broadband reflectance measurments. The InP-based cell is estimated to yield 1.09 W/cm2 at 1100 Degrees Celsius vs. 0.92 W/cm2 for the GaAs-based cell, with TPV efficiencies of 16.8 vs. 9.2%. Both devices are limited by sub-bandgap absorption, which we assess largely occurs in the graded buffers. We estimate that the 1100 Degrees Celsius thermophotovoltaic efficiencies would increase to 24.0% and 20.7% in structures with the graded buffer removed, if previously demonstrated reflectance is achieved. These devices also have application as laser power converters in the 2.0-2.3 um atmospheric window. We estimate efficiencies of 36.8% and 32.5% under 2.0 um monochromatic irradiance of 1.86 W/cm2 and 2.81 W/cm2, respectively.

ENGINEERING,MATERIALS SCIENCE,SOLAR ENERGY↗
NASA NTRS2023

Microstructural Changes and Pb Mobility During the Zircon to Reidite Transformation: Implications for Planetary Impact Chronology

Impact events modify and leave behind a complex history of rock metamorphism on terrestrial planets. Evidence for an impact event may be recorded in physical changes to minerals, such as mineral deformation and formation of high P-T polymorphs, but also in the form of chemical fingerprints, such as enhanced elemental diffusion and isotopic mixing. Here we explore laboratory shock-induced physical and chemical changes to zircon and feldspar, the former of which is of interest because its trace elements abundances and isotope ratios are used extensively in geochemistry and geochronology. To this end, a granular mixture of Bishop Tuff sanidine and Kuehl Lake zircon, both with well characterized Pb isotope compositions, was prepared and then shocked via a flat plate accelerator. The peak pressure of the experiment, as calculated by the impedance matching method, was ~24 GPa although a broader range of P-T conditions is anticipated due to starting sample porosity. Unshocked and shocked materials were characterized via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and Raman spectroscopy. These methods show that the starting zircon material had abundant metamict regions, and the conversion of the feldspar to glass in the post-shock material. Analyses of the shocked product also yielded multiple occurrences of the high-pressure ZrSiO 4 polymorph reidite, with some domains up to 300 μm across. The possibility of U-Pb system disturbance was evaluated via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS). The isotopic data reveal that disturbance of the U-Pb geochronometer in the reidite was minimal (<2% for the main U-Pb geochronometers). To better constrain the P-T conditions during the shock experiment, we complement impedance matching pressure calculations with iSALE2D impact simulations. The simulated results yield a range of P-T conditions experienced during the experiment and show that much of the sample may have reached >30 GPa, which is consistent with formation of reidite. In the recovered shocked material, we identified lamellae of reidite, some of which interlock with zircon lamellae. Reidite {112} twins were identified, which we interpret to have formed to reduce stress between the crystal structure of the host zircon and reidite. These two findings support the interpretation that shear transformation enabled the transition of zircon to reidite. The size and presence of reidite found here indicate that this phase is probably common in impact-shocked crustal rocks that experienced ~25 to ~35 GPa, especially when the target material has porosity. Additionally, shock loading of the zircon and transformation to reidite at these pressures in porous materials is unlikely to significantly disturb the U-Pb system in zircon and that the reidite inherits the primary U and Pb elemental and isotopic ratios from the zircon.

Bishop Tuff↗