Characterization of achondritic cosmic spherules from the Widerøefjellet micrometeorite collection (Sør Rondane Mountains, East Antarctica)

Achondritic micrometeorites represent one of the rarest (ca. 0.5–2.1%) particle types among Antarctic micrometeorite collections. Here, we present major, trace element and oxygen isotope compositions on five vitreous, achondritic cosmic spherules (341–526 µm in size) recovered from the Widerøefjellet sedimentary trap in the Sør Rondane Mountains (SRMs) of East Antarctica. We also present the first iron isotope data for four of these achondritic cosmic spherules. The particles were initially identified based on the atomic concentrations of Fe-Mg-Mn and their distribution in Fe/Mg versus Fe/Mn space, spanning a relatively wide range in Fe/Mg ratios (ca. 0.48–1.72). The Fe/Mn ratios cover a more restricted range (22.4–31.7), comparable to or slightly below the values measured for howardite-eucrite-diogenite (HED) and martian meteorites. One particle (WF1801-AC3) displays an elevated Fe/Mn ratio of ∼78, comparable to the values determined for lunar rocks. The negative correlation observed between the CaO + Al2O3 contents and the Fe/Si ratios of achondritic spherules reflects both the mineralogy of the precursor materials, as well as the extent of volatilization experienced during atmospheric entry heating. This trend suggests that the primary mineralogy of precursor materials may have been compositionally similar to basaltic achondrites. Based on their distribution in Ca/Si versus Al/Si space, we argue that the majority of achondritic cosmic spherules predominantly sample pyroxene- and/or plagioclase-rich (i.e., basaltic) precursor bodies. Such precursor mineralogy is also inferred from their rare earth element (REE) patterns, which show resemblances to fine-grained basaltic eucrites or Type 1 achondritic spherules (n = 3 – av. REEN = 11.2–15.5, (La/Yb)N = 0.93–1.21), pigeonite-rich equilibrated eucrite precursors or Type 2 achondritic spherules (n = 1 – av. REEN = 27.9, (La/Yb)N = 0.10), and possibly Ca-phosphates from (primitive) achondritic bodies (n = 1 – av. REEN = 58.8, (La/Yb)N = 1.59). This is clearly demonstrated for particle WF1801AC-1, which was likely inherited from a fine-grained eucritic precursor body. The pre-atmospheric oxygen isotope composition was reconstructed through compensation of mass-dependent fractionation processes as well as mixing with atmospheric oxygen, using iron isotope data. Two particles (WF1801AC-2, WF1801-AC4) display corrected oxygen isotope compositions (δ18O = 3.7–4.4‰) largely consistent with HED meteorites and may thus originate from HED-like parent bodies. The corrected oxygen isotope compositions (δ18O = 12.6–12.8‰) of the remaining particles (WF1801-AC3, WF1801-AC5) do not correspond to known meteorite fields and may represent two distinct types of unknown achondritic parent bodies or residual atmospheric entry effects. Finally, the abundance (ca. 0.5%) of achondritic cosmic spherules within the Widerøefjellet sedimentary trap is comparable to that observed in the South Pole Water Well (SPWW – ca. 0.5%), Novaya Zemlya glacier (ca. 0.45%) and Transantarctic Mountain (TAM) (ca. 2.1%) collections, confirming their overall rarity in micrometeorite collections. Unambiguous evidence for micrometeorites from the Moon or Mars remains absent from collections to date

Australasian microtektites across the Antarctic continent: Evidence from the Sør Rondane Mountain range (East Antarctica)

The ~790 ka Australasian (micro)tektite strewn field is one of the most recent and best-known examples of impact ejecta emplacement as the result of a large-scale cratering event across a considerable part of Earth's surface (>10% in area). The Australasian strewn field is characterized by a tri-lobe pattern consisting of a large central distribution lobe, and two smaller side lobes extending to the west and east. Here, we report on the discovery of microtektite-like particles in sedimentary traps, containing abundant micrometeorite material, in the Sør Rondane Mountain (SRM) range of East Antarctica. The thirty-three glassy particles display a characteristic pale yellow color and are predominantly spherical in shape, except for a single dumbbell-shaped particle. The vitreous spherules range in size from 220 to 570 μm, with an average diameter of ~370 μm. This compares relatively well with the size distribution (75–778 μm) of Australasian microtektites previously recovered from the Transantarctic Mountains (TAM) and located ca. 2500–3000 km from the SRM. In addition, the chemical composition of the SRM particles exhibits limited variation and is nearly identical to the ‘normal-type’ (i.e., <6% MgO) TAM microtektites. The Sr and Nd isotope systematics for a single batch of SRM particles (n = 26) strongly support their affiliation with TAM microtektites and the Australasian tektite strewn field in general. Furthermore, Sr isotope ratios and Nd model ages suggest that the target material of the SRM particles was composed of a plagioclase- or carbonate-rich lithology derived from a Paleo- or Mesoproterozoic crustal unit. The affiliation to the Australasian strewn field requires long-range transportation, with estimated great circle distances of ca. 11,600 km from the hypothetical source crater, provided transportation occurred along the central distribution lobe. This is in agreement with the observations made for the Australasian microtektites recovered from Victoria Land (ca. 11,000 km) and Larkman Nunatak (ca. 12,000 km), which, on average, decrease in size and alkali concentrations (e.g., Na and K) as their distance from the source crater increases. The values for the SRM particles are intermediate to those of the Victoria Land and Larkman Nunatak microtektites for both parameters, thus supporting this observation. We therefore interpret the SRM particles as ‘normal-type’ Australasian microtektites, which significantly extend the central distribution lobe of the Australasian strewn field westward. Australasian microtektite distribution thus occurred on a continent-wide scale across Antarctica and allows for the identification of new, potential recovery sites on the Antarctic continent as well as the southeastern part of the Indian Ocean. Similar to volcanic ash layers, the ~790 ka distal Australasian impact ejecta are thus a record of an instantaneous event that can be used for time-stratigraphic correlation across Antarctica

Evidence for the presence of chondrule‐ and CAI‐derived material in an isotopically anomalous Antarctic micrometeorite

We report the discovery of a unique, refractory phase‐bearing micrometeorite (WF1202A‐001) from the Sør Rondane Mountains, East Antarctica. A silicate‐rich cosmic spherule (~400 µm) displays a microporphyritic texture containing Ca‐Al‐rich inclusion (CAI)‐derived material (~5–10 area%), including high‐Mg forsterite (Fo98‐99) and enstatite (En98‐99, Wo0‐1). The micrometeorite also hosts a spherical inclusion (~209 µm), reminiscent of chondrules, displaying a barred olivine texture. Oxygen isotopic compositions of the micrometeorite groundmass (δ17O = –3.46‰, δ18O = 10.43‰, ∆17O = –1.96‰) are consistent with a carbonaceous chondrite precursor body. Yet, a relict forsterite grain is characterized by δ17O = –45.8‰, δ18O = –43.7‰, ∆17O = –23.1‰, compatible with CAIs. In contrast, a relict low‐Ca pyroxene grain (δ17O = –4.96‰, δ18O = –4.32‰, ∆17O = –2.71‰) presumably represents a first‐generation silicate grain that accreted 18O‐rich gas or dust in a transient melting scenario. The spherical inclusion displays anomalous oxygen isotope ratios (δ17O = –0.98‰, δ18O = –2.16‰, ∆17O = 0.15‰), comparable to anhydrous interplanetary dust particles (IDPs) and fragments from Comet 81P/Wild2. Based on its major element geochemistry, the chondrule size, and oxygen isotope systematics, micrometeorite WF1202A‐001 likely sampled a carbonaceous chondrite parent body similar to, but distinct from CM, CO, or CV chondrites. This observation may suggest that some carbonaceous chondrite bodies can be linked to comets. The reconstructed atmospheric entry parameters of micrometeorite WF1202A‐001 suggest that the precursor particle originated from a low‐inclination, low‐eccentricity source region, most likely either the main belt asteroids or Jupiter family comets (JFCs)

Cosmic spherules from Widerøefjellet, Sør Rondane Mountains (East Antarctica)

A newly discovered sedimentary accumulation of micrometeorites in the Sør Rondane Mountains of East Antarctica, close to the Widerøefjellet summit at ~2750 meter above sea level, is characterized in this work. The focus here lies on 2099 melted cosmic spherules larger than 200 μm, extracted from 3.2 kg of sampled sediment. Although the Widerøefjellet deposit shares similarities to the micrometeorite traps encountered in the Transantarctic Mountains, both subtle and more distinct differences in the physicochemical properties of the retrieved extraterrestrial particles and sedimentary host deposits are discernable (e.g., types of bedrock, degree of wind exposure, abundance of metal-rich particles). Unlike the Frontier Mountain and Miller Butte sedimentary traps, the size fraction below 240 μm indicates some degree of sorting at Widerøefjellet, potentially through the redistribution by wind, preferential alteration of smaller particles, or processing biases. However, the cosmic spherules larger than 300 μm appear largely unbiased following their size distribution, frequency by textural type, and bulk chemical compositions. Based on the available bedrock exposure ages for the Sør Rondane Mountains, extraterrestrial dust is estimated to have accumulated over a time span of ~1 to 3 Ma at Widerøefjellet. Consequently, the Widerøefjellet collection reflects a substantial reservoir to sample the micrometeorite influx over this time interval. Petrographic observations and 3D microscopic CT imaging are combined with chemical and triple-oxygen isotopic analyses of silicate-rich cosmic spherules larger than 325 μm. The major element composition of 49 cosmic spherules confirms their principally chondritic parentage. For 18 glassy, 15 barred olivine, and 11 cryptocrystalline cosmic spherules, trace element concentrations are also reported on. Based on comparison with evaporation experiments reported in literature and accounting for siderophile and chalcophile element losses during high-density phase segregation and ejection, the observed compositional sequence largely reflects progressive heating and evaporation during atmospheric passage accompanied by significant redox shifts, although the influence of (refractory) chondrite mineral constituents and terrestrial alteration cannot be excluded in all cases. Twenty-eight cosmic spherules larger than 325 μm analyzed for triple-oxygen isotope ratios confirm inheritance from mostly carbonaceous chondritic precursor materials (~55% of the particles). Yet, ~30% of the measured cosmic spherules and ~50% of all glassy cosmic spherules are characterized by oxygen isotope ratios above the terrestrial fractionation line, implying genetic links to ordinary chondrites and parent bodies currently unsampled by meteorites. The structural, textural, chemical, and isotopic characteristics of the cosmic spherules from the Sør Rondane Mountains, and particularly the high proportion of Mg-rich glass particles contained therein, imply a well-preserved and representative new sedimentary micrometeorite collection from a previously unstudied region in East Antarctica characterized by distinct geological and exposure histories

Petrographic and geochemical characterization of the micrometeorite collection from the Sør Rondane Mountains: Nature and origin of the extraterrestrial flux to Earth

The Antarctic continent has traditionally been a successful searching ground for meteoritic material due to its cold and dry climate. Meteorites, and their microscopic analogues micrometeorites, were originally sampled from Antarctic ice and snow. Recently, however, a large collection of micrometeorites was discovered in sedimentary traps and moraine deposits from the Transantarctic Mountains, where extraterrestrial dust particles have accumulated for a prolonged time span (ca. 3–4 Ma). Micrometeorites (or ‘cosmic dust’) show unique chemical and isotopic signatures, which originate from a large and diverse amount of asteroidal and cometary bodies within the Solar System. In addition, they document major events such as the origin and evolution of the Solar System, and provide insight into the source region of their precursor bodies. These sedimentary deposits consequently represent a valuable archive that documents the flux of extraterrestrial material to Earth and ancient meteoritic events over Antarctica.Yet, much of this information is lost during the atmospheric entry stage, where cosmic dust is subjected to frictional heating and is partially or completely molten down. This may significantly alter the original physicochemical and isotopic properties of extraterrestrial dust particles. A thorough understanding of these physicochemical processes is thus required to reconstruct the atmospheric entry of cosmic dust (but also larger objects) and interpret their chemical and isotopic data. During the course of this PhD research, multiple sedimentary deposits from the Sør Rondane Mountains (Dronning Maud Land, East Antarctica) were petrographically examined and chemically-isotopically characterized using state-of-the-art instruments. Furthermore, various experiments and numerical models were constructed to replicate the atmospheric entry stage of both small- and large-sized meteoritic material. This study has demonstrated that the Sør Rondane Mountains sedimentary deposits contain a rich and pristine variety of extraterrestrial- and impact-related materials, including micrometeorites, microtektites and meteoritic condensation spherules. Statistical analysis suggests that the Sør Rondane Mountains micrometeorite collection is representative of the contemporary flux of cosmic dust to Earth. Extraterrestrial material is subjected to a complex interplay of redox and volatilization processes during atmospheric entry heating, which allow to explain the chemical trends observed in cosmic dust. Isotopic studies also suggest that at least a minor fraction of the micrometeorite population has sampled new, unknown types of asteroidal and/or cometary bodies. Microtektites and meteoritic condensation spherules have been linked to major meteoritic events on Earth ca. 790 ka and ca. 430 ka ago, respectively, and underline the importance of the Earth’s atmosphere during their formation. The results of this PhD research emphasize the scientific value of Antarctic sedimentary deposits and provide more insight into the processes taking place during the atmospheric entry of extraterrestrial material.Le continent antarctique a traditionnellement été un terrain de recherche fructueux pour le matériel extraterrestre en raison de son climat froid et sec. Les météorites et leurs analogues microscopiques, les micrométéorites, ont été à l'origine échantillonnés dans la glace et la neige de l'Antarctique. Plus récemment, une grande collection de micrométéorites a été découverte dans des pièges sédimentaires et des dépôts de moraine des montagnes transantarctiques, où des particules de poussière extraterrestres se sont accumulées pendant une période prolongée (environ 3-4 Ma). Les micrométéorites (ou « poussière cosmique ») présentent des signatures chimiques et isotopiques uniques, qui proviennent d'une quantité importante et diversifiée de corps astéroïdes et cométaires au sein du système solaire. En outre, elles documentent des événements majeurs tels que l'origine et l'évolution du système solaire et donnent un aperçu de la région source de leurs corps parents. Ces dépôts sédimentaires représentent par conséquent une archive précieuse qui documente le flux de matière extraterrestre vers la Terre et les événements météoritiques anciens au-dessus de l'Antarctique.Pourtant, une grande partie de cette information est perdue au cours de l'étape d'entrée dans l'atmosphère, où la poussière cosmique est soumise à un chauffage par friction et est partiellement ou complètement fondue. Cela peut altérer considérablement les propriétés physico-chimiques et isotopiques d'origine des particules de poussière extraterrestres. Une compréhension approfondie de ces processus physico-chimiques est donc nécessaire pour reconstituer l'entrée atmosphérique des poussières cosmiques (mais aussi des objets plus gros) et interpréter leurs données chimiques et isotopiques. Au cours de cette recherche de doctorat, plusieurs dépôts sédimentaires des montagnes Sør Rondane (Dronning Maud Land, Antarctique de l'Est) ont été examinés pétrographiquement et caractérisés chimiquement et isotopiquement. En outre, diverses expériences et modèles numériques ont été construits pour reproduire l'étape d'entrée dans l'atmosphère de matériaux météoritiques de petite et de grande taille.Cette étude a démontré que les dépôts sédimentaires des montagnes Sør Rondane contiennent une variété riche et peu altérée de matériaux extraterrestres et de cratères d’impacts, notamment des micrométéorites, des microtektites et des sphérules de condensation météoritique. L'analyse statistique suggère que la collection de micrométéorites des montagnes Sør Rondane est représentative du flux contemporain de poussière cosmique vers la Terre. La matière extraterrestre est soumise à une interaction complexe de processus d'oxydo-réduction et de volatilisation lors de l'entrée dans l'atmosphère, ce qui permet d'expliquer les tendances chimiques observées dans la poussière cosmique. Des études isotopiques suggèrent également qu'au moins une fraction mineure de la population de micrométéorites a échantillonné de nouveaux types inconnus d’astéroïdes et/ou de comètes. Les microtektites et les sphérules de condensation météoritiques ont été liées à des événements météoritiques majeurs sur Terre il y a ~790 ka et ~430 ka, respectivement, et soulignent l'importance de l'atmosphère terrestre lors de leur formation. Les résultats de cette recherche doctorale soulignent la valeur scientifique des dépôts sédimentaires de l'Antarctique et donnent un meilleur aperçu des processus qui se déroulent lors de l'entrée dans l'atmosphère de matière extraterrestreDoctorat en Sciencesinfo:eu-repo/semantics/nonPublishe

Elemental and oxygen isotopic fractionation recorded in highly vaporized cosmic spherules from Widerøefjellet, Sør Rondane Mountains (East Antarctica)

Upon passage through Earth's atmosphere, micrometeorites undergo variable degrees of melting and evaporation. Among the various textural and chemical groups recognized among cosmic spherules, that is, melted micrometeorites, a subset of particles may indicate anomalously high degrees of vaporization based on their chemical and isotopic properties. Here, a selection of such refractory element‐enriched cosmic spherules from Widerøefjellet (Sør Rondane Mountains, East Antarctica) is characterized for their petrographic features, major and trace element concentrations (N = 35), and oxygen isotopic compositions (N = 23). Following chemical classification, the highly vaporized particles can be assigned to either the “CAT‐like” or the “High Ca‐Al” cosmic spherule groups. However, through the combination of major and trace element concentrations and oxygen isotopic data, a larger diversity of processes and precursor materials are identified that lead to the final compositions of refractory element‐enriched particles. These include fragmentation, disproportional sampling of specific mineral constituents, differential melting, metal bead extraction, redox shifts, and evaporation. Based on specific element concentrations (e.g., Sc, Zr, Eu, Tm) and ratios (e.g., Fe/Mg, CaO + Al2O3/Sc + Y + Zr + Hf), and variations of O isotope compositions, “CAT‐like” and “High Ca‐Al” cosmic spherules likely represent a continuum between mineral endmembers from both primitive and differentiated parent bodies that experienced variable degrees of evaporation

Chrome-rich spinels in micrometeorites from modern Antarctic sedimentary deposits

Each year, approximately 5000 tons of extraterrestrial material reaches the Earth's surface as micrometeorites, cosmic dust particles ranging from 10 to 2000 μm in size. These micrometeorites, collected from diverse environments, mainly deep-sea sediments, Antarctic ice, snow and loose sediments, and hot deserts, are crucial in understanding our Solar System's evolution. Chrome-rich spinel (Cr-spinel) minerals have gained attention as proxies for studying the extraterrestrial flux in sedimentary deposits, because these robust minerals occur, in various extraterrestrial materials, with compositions characteristic of their parent bodies. A total of 27 Cr-spinel bearing micrometeorites within the size range of 185–800 μm, were identified from approximately 6000 micrometeorites from the Transantarctic Mountains (n = 23) and the Sør Rondane Mountains (n = 4), in Antarctica, containing Cr-spinel (8–120 μm), were examined in this study for geochemical composition and high-precision oxygen isotope ratios to assess alteration and identify potential parent bodies. Oxygen isotopes in the micrometeorite groundmass and in Cr-spinel grains reveal a predominance of ordinary chondritic precursors, with only 1 in 10 micrometeorites containing Cr-spinel minerals showing a carbonaceous chondritic signature. This may be further confirmed by an elevated Al content (> 12 wt% Al2O3) in Cr-spinel from specific carbonaceous chondrite types, but a more extensive dataset is required to establish definitive criteria. The first Cr-spinel bearing particle, in an Antarctic micrometeorite, that can be linked to R-chondrites based on oxygen isotopes, has been documented, demonstrating the potential for R-chondrites as a source of chrome-rich spinels. The study also highlights the potential for chemical modifications and alteration processes that Cr-spinel minerals may undergo during their time on the parent body, atmospheric entry, and terrestrial residence. In the context of the broader micrometeorite flux, the results align with previous findings, showing a consistent contribution of micrometeorites containing Cr-spinel minerals related to ordinary chondrites over the past 2 to 4 million years. This is however a small fraction (∼ 1 %) of the total micrometeorite flux. The study further confirms that Cr-spinel minerals recovered from sedimentary deposits serve as valuable proxies for tracking events related to ordinary chondritic or achondritic materials. However, it is emphasized that Cr-spinel minerals alone cannot serve as exclusive indicators of the overall extraterrestrial flux, especially during periods dominated by carbonaceous chondritic dust in the inner Solar System. To comprehensively understand the complete extraterrestrial flux, additional proxies are needed to trace dust-producing events associated with various Solar System objects. The intricate nature of Cr-spinel compositions, and the potential for alteration processes emphasize the need for further research to refine our understanding of these extraterrestrial markers

Meteoroid atmospheric entry investigated with plasma flow experiments : petrography and geochemistry of the recovered material

Melting experiments attempting to reproduce some of the processes affecting asteroidal and cometary material during atmospheric entry have been performed in a high enthalpy facility. For the first time with the specific experimental setup, the resulting material has been recovered, studied, and compared with natural analogues, focusing on the thermal and redox reactions triggered by interaction between the melt and the atmospheric gases under high temperature and low pressure conditions. Experimental conditions were tested across a range of parameters, such as heat flux, experiment duration, and pressure, using two types of sample holders materials, namely cork and graphite. A basalt served as asteroidal analog and to calibrate the experiments, before melting a H5 ordinary chondrite meteorite. The quenched melt recovered after the experiments has been analyzed by μ-XRF, EDS-SEM, EMPA, LA-ICP-MS, and XANES spectroscopy. The glass formed from the basalt is fairly homogeneous, depleted in highly volatile elements (e.g., Na, K), relatively enriched in moderately siderophile elements (e.g., Co, Ni), and has reached an equilibrium redox state with a lower Fe3+/Fetot ratio than that in the starting material. Spherical objects, enriched in SiO2, Na2O and K2O, were observed, inferring condensation from the vaporized material. Despite instantaneous quenching, the melt formed from the ordinary chondrite shows extensive crystallization of mostly olivine and magnetite, the latter indicative of oxygen fugacity compatible with presence of both Fe2+ and Fe3+. Similar features have been observed in natural meteorite fusion crusts and in micrometeorites, implying that, at least in terms of maximum temperature reached and chemical reactions, the experiments have successfully reproduced the conditions likely encountered by extraterrestrial material following atmospheric entry