99 results found
Wilson A, Genge M, Krzesińska A, et al., Atmospheric entry heating of micrometeorites at Earth and Mars: implications for the survival of organics, Meteoritics and Planetary Science, ISSN: 1086-9379
The atmospheric entry heating of micrometeorites (MMs) can significantly alter their pre-existing mineralogy, texture and organic material. The degree of heating depends predominantly on the gravity and atmospheric density of the planet on which they fall. For particles falling on Earth the alteration can be significant, leading to the destruction of much of the pre-entry organics, however, the weaker gravity and thinner atmosphere of Mars enhances the survival of MMs and increases the fraction of particles that preserve organic material. This paper investigates the entry heating of MMs on the Earth and Mars in order to examine the micrometeorite population on each planet and give insights into the survival of extraterrestrial organic material. The results show that particles reaching the surface of Mars experience a lower peak temperature compared to Earth and, therefore, experience less evaporative mass loss. Of the particles which reach the surface, 68.2% remain unmelted on Mars compared to only 22.8% on Earth. Due to evaporative mass loss, unmelted particles that reach the surface of Earth are restricted to sizes <70 µm whereas particles >475 µm survive unmelted on Mars. Approximately 10% of particles experience temperatures below ~800 K, i.e. the sublimation temperature of refractory organics found in MMs. On Earth this fraction is significantly lower with less than 1% expected to remain below this temperature. Lower peak temperatures coupled with the larger sizes of particles surviving without significant heating on Mars suggests a much higher fraction of organic material surviving to the martian surface.
Tomkins AG, Genge MJ, Tait AW, et al., High Survivability of Micrometeorites on Mars: Sites With Enhanced Availability of Limiting Nutrients, Journal of Geophysical Research: Planets, ISSN: 2169-9097
Suttle M, Genge M, Salge T, et al., 2019, A microchondrule-bearing micrometeorite and comparison with microchondrules in CM chondrites, Meteoritics and Planetary Science, Vol: 54, Pages: 1303-1324, ISSN: 1086-9379
We report the discovery of a partially altered microchondrule within a fine-grained micrometeorite.This object is circular, <10μm in diameter and has a cryptocrystalline texture, internal zonation and a thin S-bearing rim. These features imply a period of post-accretion parent body aqueous alteration, in which the former glassy igneous texture was subject to hydration and phyllosilicate formation as well as leaching of fluid-mobile elements. We compare this microchondrule to three microchondrules found in two CM chondrites: Elephant Moraine (EET) 96029 and Murchison. In all instances, their formation appears closely linked to the late-stages of chondrule formation, chondrule recycling and fine-grained rim accretion. Likewise, they share cryptocrystalline textures and evidence of mild aqueous alteration and thus similar histories. We also investigate the host micrometeorite’s petrology, which includes an unusually Cr-rich mineralogy, containing both Mn-chromite spinel and low-Fe-Cr rich (LICE) anhydrous silicates. Because these two refractory phases cannot form together in a single geochemical reservoir under equilibrium condensation, this micrometeorite’s accretionary history requires a complex timeline with formation via non-equilibrium batch crystallization or accumulation of materials from large radial distances. In contrast, the bulk composition of this micrometeorite and its internal textures are consistent with a hydrated carbonaceous chondrite source. This micrometeorite is interpreted as a fragment of fine-grained rim material that once surrounded a larger parent chondrule and was derived from a primitive carbonaceous parent body; either a CM chondrite or Jupiter family comet.
Aerts J, van Spanning R, Flahaut J, et al., Microbial Communities in Sediments From Four Mildly Acidic EphemeralSalt Lakes in the Yilgarn Craton (Australia) – Terrestrial Analogs to Ancient Mars, Frontiers in Microbiology, ISSN: 1664-302X
Suttle MD, Folco L, Genge MJ, et al., 2019, Intense aqueous alteration on C-type asteroids: Perspectives from giant fine-grained micrometeorites, GEOCHIMICA ET COSMOCHIMICA ACTA, Vol: 245, Pages: 352-373, ISSN: 0016-7037
Suttle M, Genge M, Folco L, et al., 2018, The atmospheric entry of fine-grained micrometeorites: the role of volatile gases in heating and fragmentation, Meteoritics and Planetary Science, ISSN: 1086-9379
The early stages of atmospheric entry are investigated in four large (250–950 μm) unmelted micrometeorites (three fine‐grained and one composite), derived from the Transantarctic Mountain micrometeorite collection. These particles have abundant, interconnected, secondary pore spaces which form branching channels and show evidence of enhanced heating along their channel walls. Additionally, a micrometeorite with a double‐walled igneous rim is described, suggesting that some particles undergo volume expansion during entry. This study provides new textural data which links together entry heating processes known to operate inside micrometeoroids, thereby generating a more comprehensive model of their petrographic evolution. Initially, flash heated micrometeorites develop a melt layer on their exterior; this igneous rim migrates inwards. Meanwhile, the particle core is heated by the decomposition of low‐temperature phases and by volatile gas release. Where the igneous rim acts as a seal, gas pressures rise, resulting in the formation of interconnected voids and higher particle porosities. Eventually, the igneous rim is breached and gas exchange with the atmosphere occurs. This mechanism replaces inefficient conductive rim‐to‐core thermal gradients with more efficient particle‐wide heating, driven by convective gas flow. Interconnected voids also increase the likelihood of particle fragmentation during entry and, may therefore explain the rarity of large fine‐grained micrometeorites among collections.
Genge MJ, Van Ginneken M, Suttle M, et al., 2018, Accumulation mechanisms of micrometeorites in an ancient supra-glacial moraine at Larkman Nunatak, Antarctica, Meteoritics and Planetary Science, Vol: 53, Pages: 2051-2066, ISSN: 1086-9379
We report the discovery of a large accumulation of micrometeorites in a supraglacial moraine at Larkman Nunatak in the Grosvenor Mountains of the Transantarctic Range in Antarctica. The micrometeorites are present in abundances of ~600 particles Kg-1 of moraine sediment and include a near complete collection of micrometeorite types similar to those observed in Antarctic blue ice and within bare-rock traps in the Antarctic. The size distribution of the observed particles is consistent with those collected from snow collections suggesting the moraine has captured a representative collection of cosmic spherules with significant loss of only the smallest particles (<100 m) by wind. The presence of microtektites with compositions similar to those of the Australasian strewn field suggests the moraine has been accumulating for 780 ka with dust-sized debris. On the basis of this age estimate it is suggested that accumulation occurs principally through ice sublimation. Direct in-fall of fines is suggested to be limited by snow layers that act as barriers to accumulation and can be removed by wind erosion. Micrometeorite accumulation in many areas in Antarctica, therefore, may not be continuous over long periods and can be subject to climatic controls. On the basis of the interpretation of microtektites as Australasian, Larkman Nunatak deposit is the oldest known supraglacial moraine and its survival through several glacial maxima and interglacial periods is surprising. We suggest that stationary ice produced by the specific ice flow conditions at Larkman Nunatak explains its longevity and provides a new type of record of the East Antarctic ice sheet.
Genge MJ, 2018, Electrostatic levitation of volcanic ash into the ionosphere and its abrupt effect on climate, Geology, Vol: 46, Pages: 835-838, ISSN: 0091-7613
Large volcanic eruptions cause short-term climate change owing to the convective rise of fine ash and aerosols into the stratosphere. Volcanic plumes are, however, also associated with large net electrical charges that can also influence the dynamics of their ash particles. Here I show that electrostatic levitation of ash from plumes with a net charge is capable of injecting volcanic particles <500 nm in diameter into the ionosphere in large eruptions lasting more than a few hours. Measured disturbances in the ionosphere during eruptions, and the first discovery of polar mesospheric clouds after the A.D. 1883 Krakatau (Indonesia) eruption, are both consistent with levitation of ash into the mesosphere. Supervolcano eruptions are likely to inject significant quantities of charged ash into the ionosphere, resulting in disturbance or collapse of the global electrical circuit on time scales of 102 s. Because atmospheric electrical potential moderates cloud formation, large eruptions may have abrupt effects on climate through radiative forcing. Average air temperature and precipitation records from the 1883 eruption of Krakatau are consistent with a sudden effect on climate.
Shah J, Williams W, Almeida TP, et al., 2018, The oldest magnetic record in our solar system identified using nanometric imaging and numerical modeling, Nature Communications, Vol: 9, ISSN: 2041-1723
Recordings of magnetic fields, thought to be crucial to our Solar System’s rapid accretion, are potentially retained in unaltered nanometric low-Ni kamacite (~metallic Fe) grains encased within dusty olivine crystals, found in the chondrules of unequilibrated chondrites. However, most of these kamacite grains are magnetically non-uniform, so their ability to retain four-billion-year-old magnetic recordings cannot be estimated by previous theories, which assume only uniform magnetization. Here, we demonstrate that non-uniformly magnetized nanometric kamacite grains are stable over Solar System timescales and likely the primary carrier of remanence in dusty olivine. By performing in-situ temperature-dependent nanometric magnetic measurements using off-axis electron holography, we demonstrate the thermal stability of multi-vortex kamacite grains from the chondritic Bishunpur meteorite. Combined with numerical micromagnetic modeling, we determine the stability of the magnetization of these grains. Our study shows that dusty olivine kamacite grains are capable of retaining magnetic recordings from the accreting Solar System.
Van Ginneken M, Genge MJ, Harvey R, 2018, A new type of highly-vaporized microtektite from the Transantarctic Mountains, Geochimica et Cosmochimica Acta, Vol: 228, Pages: 81-94, ISSN: 0016-7037
We report on the discovery of microtektites (microscopic impact glass spherules) in a glacial moraine near Larkman Nunatak in the Transantarctic Mountains, Antarctica. The microtektites were identified based on their physical and chemical properties. Major and trace element compositions of the particles suggest that they may be related to the Australasian strewn field. This would further extend the current strewn field ~800 km southward. Depletion in volatiles and enrichment in refractory elements in Larkman Nunatak microtektites fit the volatilization trend defined by Australasian microtektites, suggesting that they may represent a new highly vapor fractionated member thereof. This observation is supported by their low vesicularity and absence of mineral inclusions. This discovery has strong implications for the formation of microtektites (i.e. their evolution with respect to the distance from the source crater). Finally, the discovery of potentially old (i.e. 0.8 Ma) microtektites in moraine has implications for the stability of the East Antarctic Ice Sheet in the Larkman Nunatak area over the last ~1 Ma and, as a consequence, the high efficiency of such moraines as traps for other extraterrestrial materials (e.g. micrometeorites and meteoritic ablation debris).
Genge MJ, Van Ginneken M, 2017, Comment on “Unmelted Cosmic Metal Particles in the Indian Ocean” by Prasad et al., Meteoritics and Planetary Science, Vol: 53, Pages: 326-332, ISSN: 1086-9379
Genge MJ, Davies B, Suttle M, et al., 2017, The mineralogy and petrology of I-type cosmic spherules: Implications for their sources, origins and identification in sedimentary rocks, Geochimica et Cosmochimica Acta, Vol: 218, Pages: 167-200, ISSN: 0016-7037
I-type cosmic spherules are micrometeorites that formed by melting during atmospheric entry and consist mainly of iron oxides and FeNi metal. I-types are important because they can readily be recovered from sedimentary rocks allowing study of solar system events over geological time. We report the results of a study of the mineralogy and petrology of 88 I-type cosmic spherules recovered from Antarctica in order to evaluate how they formed and evolved during atmospheric entry, to constrain the nature of their precursors and to establish rigorous criteria by which they may be conclusively identified within sediments and sedimentary rocks. Two textural types of I-type cosmic spherule are recognised: (1) metal bead-bearing (MET) spherules dominated by Ni-poor (<1.5 w%) wüstite and FeNi metal (10-95 wt% Ni) with minor magnetite, and (2) metal bead-free (OX) spherules dominated by Ni-rich wüstite (0.5-22.5 wt%) and magnetite. Two varieties of OX spherule are distinguished, magnetite-poor dendritic spherules and magnetite-rich coarse spherules. Six OXMET particles having features of both MET and OX spherules were also observed. The wüstite to magnetite ratios and metal contents of the studied particles testify to their formation by melting of extraterrestrial FeNi grains during progressive oxidation in the atmosphere. Precursors are suggested to be mainly kamacite and rare taenite grains. Vesicle formation within metal beads and extrusion of metallic liquid into surrounding wüstite grain boundaries suggests an evaporated iron sulphide or carbide component within at least 23% of particles. The Ni/Co ratios of metal vary from 14 to >100 and suggest that metal from H-group ordinary, CM, CR and iron meteorites may form the majority of particles. Oxidation during entry heating increases in the series MET<magnetite-poor OX<magnetite-rich OX spherules owing to differences in particle size, entry angle and velocity. Magnetite-poor OX spherules are s
Suttle M, Genge MJ, 2017, Diagenetically altered fossil micrometeorites suggest cosmic dust is common the geological record, Earth and Planetary Science Letters, Vol: 476, Pages: 132-142, ISSN: 0012-821X
We report the discovery of fossil micrometeorites from Late Cretaceous chalk. Seventy-six cosmic spherules were recovered from Coniacian (87±1 Ma) sediments of the White Chalk Supergroup. Particles vary from pristine silicate and iron-type spherules to pseudomorphic spherules consisting of either single-phase recrystallized magnetite or Fe-silicide. Pristine spherules are readily identified as micrometeorites on the basis of their characteristic mineralogies, textures and compositions. Both magnetite and silicide spherules contain dendritic crystals and spherical morphologies, testifying to rapid crystallisation of high temperature iron-rich metallic and oxide liquids. These particles also contain spherical cavities, representing weathering and removal of metal beads and irregular cavities, representing vesicles formed by trapped gas during crystallization; both features commonly found among modern Antarctic Iron-type (I-type) cosmic spherules. On the basis of textural analysis, the magnetite and Fe-silicide spherules are shown to be I-type cosmic spherules that have experienced complete secondary replacement during diagenesis (fossilization). Our results demonstrate that micrometeorites, preserved in sedimentary rocks, are affected by a suite of complex diagenetic processes, which can result in disparate replacement minerals, even within the same sequence of sedimentary beds. As a result, the identification of fossil micrometeorites requires careful observation of particle textures and comparisons with modern Antarctic collections. Replaced micrometeorites imply that geochemical signatures the extraterrestrial dust are subject to diagenetic remobilisation that limits their stratigraphic resolution. However, this study demonstrates that fossil, pseudomorphic micrometeorites can be recognised and are likely common within the geological record.
Genge MJ, Russell SS, A MICROCHONDRULE-BEARING MICROMETEORITE., 80th Annual Meeting of the Meteoritical Society 2017
Introduction: The Earth receives a continuous flux of cosmic dust derived from young disrupted asteroids andsublimating short period comets [1,2]. Occasionally unique micrometeorites with distinct petrographies are reported. These samples expand the inventory of asteroid parent bodies and provide further clues to the formation andevolution of the solar system. In this study we report the discovery of an unusual dehydroxylated fine-grained micrometeoritecontaining a devitrified microchondrule droplet. This spherule contains a volatile-rich composition indicatingformation in an energetic high density plume.Methods: A single micrometeorite, recovered from the Cap Prudhomme blue-ice micrometeorite collection was analysed by SEM-EMPA, BSE, X-ray element mapping and mid-IR spectroscopy.Results: Particle CP94-050-182 (78x108μm) is a fine-grained micrometeorite, surrounded by a partial magnetiterim. A single spherical object (<10μm diameter) composed of devitrified glass is located near the micrometeorite’sperimeter and contains a non-chondritic volatile-rich composition. Relative to Ivuna, Na, K and Cl concentrations areelevated an order of magnitude above chondritic values at 7.8, 19.8 and 9.4 times, while refractory Al and Ca showapproximately chondritic abundances. The internal mineralogy is a heterogeneous mix of anhydrous silicates suspendedin a fine-grained, porous, nanocrystalline groundmass. Anhydrous silicates are present either as low-Fe(<3wt%), Ca-rich (12-15wt%) pyroxene or high-Fe (~18wt%), Ca-poor (<0.5wt%) olivine. Pyroxene grains containmoderate Mn (0.8-1.7wt%), Cr (~1.4wt%) and Al (~2.0wt%) concentrations, while olivines contain minimal traceelement contamination (Mn,Cr,Al<0.8wt%). Both anhydrous phases appear as anhedral clusters or isolated grains,intergrown with, or mantled by, a coarse non-stoichiometric Fe-rich phase. The surrounding groundmass containsrandomly orientated dehydration cracks and rounded submicron vesicles.
We present evidence for both early- and late-stage magnetic activity on the CV and L/LL parent bodies respectively from chondrules in Vigarano and Bjurböle. Using micro-CT scans to re-orientate chondrules to their in-situ positions, we present a new micron-scale protocol for the paleomagnetic conglomerate test. The paleomagnetic conglomerate test determines at 95% confidence, whether clasts within a conglomerate were magnetized before or after agglomeration, i.e., for a chondritic meteorite whether the chondrules carry a pre- or post-accretionary remanent magnetization. We found both meteorites passed the conglomerate test, i.e., the chondrules had randomly orientated magnetizations. Vigarano's heterogeneous magnetization is likely of shock origin, due to the 10 to 20 GPa impacts that brecciated its precursor material on the parent body and transported it to re-accrete as the Vigarano breccia. The magnetization was likely acquired during the break-up of the original body, indicating a CV parent body dynamo was active ∼9 Ma after Solar System formation. Bjurböle's magnetization is due to tetrataenite, which transformed from taenite as the parent body cooled to below 320 °C, when an ambient magnetic field imparted a remanence. We argue either the high intrinsic anisotropy of tetrataenite or brecciation on the parent body manifests as a randomly orientated distribution, and a L/LL parent body dynamo must have been active at least 80 to 140 Ma after peak metamorphism. Primitive chondrites did not originate from entirely primitive, never molten and/or differentiated parent bodies. Primitive chondrite parent bodies consisted of a differentiated interior sustaining a long-lived magnetic dynamo, encrusted by a layer of incrementally accreted primitive meteoritic material. The different ages of carbonaceous and ordinary chondrite parent bodies might indicate a general difference between carbonaceous and ordinary chondrite parent bodies, and/or format
Genge MJ, Suttle M, Van Ginneken, 2017, Thermal shock fragmentation of Mg silicates within scoriaceous micrometeorites reveal hydrated asteroidal sources, Geology, Vol: 45, Pages: 891-894, ISSN: 1943-2682
Scoriaceous micrometeorites are highly vesicular extraterrestrial dust particles that have experienced partial melting during atmo-spheric entry. We report the occurrence of clusters of anhedral relict forsterite crystals within these particles that testify to in situ frag-mentation. The absence of similar clusters within unmelted micro-meteorites suggests that fragmentation occurs during atmospheric entry rather than by parent body shock reprocessing. Clusters of broken forsterite crystals are shown to form as a result of fracturing owing to thermal stress developed during entry heating and require thermal gradients of >200 K μm–1 in order for differential thermal expansion to exceed the critical shear strength of olivine. Thermal gradients of this magnitude significantly exceed those resulting from thermal conduction and require the endothermic decomposition of phyllosilicates. Fragmented relict forsterite within scoriaceous micro-meteorites, therefore, indicate that the precursor grains were similar to CI and CM2 chondrites and retained phyllosilicate prior to atmo-spheric entry and thus were not dehydrated on the parent asteroid by shock or thermal metamorphism. Explosive fragmentation of hydrous asteroids during collisions, therefore, does not significantly bias the interplanetary dust population.
Suttle MD, Genge MJ, 2017, THE DIAGENESIS AND REPLACEMENT OF COSMIC DUST IN THE GEOLOGICAL RECORD., 80th Annual Meeting of the Meteoritical-Society, Publisher: WILEY, Pages: A338-A338, ISSN: 1086-9379
Suttle MD, Genge MJ, Russell SS, 2017, Shock fabrics in fine-grained micrometeorites, Meteoritics & Planetary Science, Vol: 52, Pages: 2258-2274, ISSN: 1086-9379
The orientations of dehydration cracks and fracture networks in fine-grained, unmelted micrometeorites were analyzed using rose diagrams and entropy calculations. As cracks exploit pre-existing anisotropies, analysis of their orientation provides a mechanism with which to study the subtle petrofabrics preserved within fine-grained and amorphous materials. Both uniaxial and biaxial fabrics are discovered, often with a relatively wide spread in orientations (40°–60°). Brittle deformation cataclasis and rotated olivine grains are reported from a single micrometeorite. This paper provides the first evidence for impact-induced shock deformation in fine-grained micrometeorites. The presence of pervasive, low-grade shock features in CM chondrites and CM-like dust, anomalously low-density measurements for C-type asteroids, and impact experiments which suggest CM chondrites are highly prone to disruption all imply that CM parent bodies are unlikely to have remained intact and instead exist as a collection of loosely aggregated rubble-pile asteroids, composed of primitive shocked clasts.
van Ginneken M, Gattacceca J, Rochette P, et al., 2017, The parent body controls on cosmic spherule texture: Evidence from the oxygen isotopic compositions of large micrometeorites, Geochimica et Cosmochimica Acta, Vol: 212, Pages: 196-210, ISSN: 0016-7037
High-precision oxygen isotopic compositions of eighteen large cosmic spherules (>500 µm diameter) from the Atacama Desert, Chile, were determined using IR-laser fluorination – Isotope Ratio Mass spectrometry. The four discrete isotopic groups defined in a previous study on cosmic spherules from the Transantarctic Mountains (Suavet et al., 2010) were identified, confirming their global distribution. Approximately 50% of the studied cosmic spherules are related to carbonaceous chondrites, 38% to ordinary chondrites and 12% to unknown parent bodies. Approximately 90% of barred olivine (BO) cosmic spherules show oxygen isotopic compositions suggesting they are related to carbonaceous chondrites. Similarly, ∼90% porphyritic olivine (Po) cosmic spherules are related to ordinary chondrites and none can be unambiguously related to carbonaceous chondrites. Other textures are related to all potential parent bodies. The data suggests that the textures of cosmic spherules are mainly controlled by the nature of the precursor rather than by the atmospheric entry parameters. We propose that the Po texture may essentially be formed from a coarse-grained precursor having an ordinary chondritic mineralogy and chemistry. Coarse-grained precursors related to carbonaceous chondrites (i.e. chondrules) are likely to either survive atmospheric entry heating or form V-type cosmic spherules. Due to the limited number of submicron nucleation sites after total melting, ordinary chondrite-related coarse-grained precursors that suffer higher peak temperatures will preferentially form cryptocrystalline (Cc) textures instead of BO textures. Conversely, the BO textures would be mostly related to the fine-grained matrices of carbonaceous chondrites due to the wide range of melting temperatures of their constituent mineral phases, allowing the preservation of submicron nucleation sites. Independently of the nature of the precursors, increasing peak temperatures form glassy textures
Genge MJ, 2017, The entry heating and abundances of basaltic micrometeorites, Meteoritics & Planetary Science, Vol: 52, Pages: 1000-1013, ISSN: 1086-9379
Basaltic micrometeorites (MMs) derived from HED-like parent bodies have been found amongst particles collected from the Antarctic and from Arctic glaciers and are to date the only achondritic particles reported amongst cosmic dust. The majority of Antarctic basaltic particles are completely melted cosmic spherules with only one unmelted particle recognised from the region. This paper investigates the entry heating of basaltic MMs in order to predict the relative abundances of unmelted to melted basaltic particles and to evaluate how mineralogical differences in precursor materials influence the final products of atmospheric entry collected on the Earth's surface. Thermodynamic modelling is used to simulate the melting behaviour of particles with compositions corresponding to eucrites, diogenites and ordinary chondrites in order to evaluate degree of partial melting and to make a comparison between the behaviour of chondritic particles that dominate the terrestrial dust flux and basaltic micrometeroids. The results of 120,000 simulations were compiled to predict relative abundances and indicate that the phase relations of precursor materials are crucial in determining the relative abundances of particle types. Diogenite and ordinary chondrite materials exhibit similar behaviour, although diogenite precursors are more likely to form cosmic spherules under similar entry parameters. Eucrite particles, however, are much more likely to melt due to their lower liquidus temperatures and small temperature interval of partial melting. Eucrite MMs, therefore, usually form completely molten cosmic spherules except at particle diameters <100 m. The low abundance of unmelted basaltic MMs compared with spherules, if statistically valid, is also shown to be inconsistent with a low velocity population (12 km s-1) and is more compatible with higher velocities which may suggest a Near Earth Asteroid sources dominates the current dust production of basaltic MMs.
Suttle MD, Genge MJ, Folco L, et al., 2017, The thermal decomposition of fine-grained micrometeorites, observations from mid-IR spectroscopy, Geochimica et Cosmochimica Acta, Vol: 206, Pages: 112-136, ISSN: 1872-9533
We analysed 44 fine-grained and scoriaceous micrometeorites. A bulk mid-IR spectrum (8–13 μm) for each grain was collected and the entire micrometeorite population classified into 5 spectral groups, based on the positions of their absorption bands. Corresponding carbonaceous Raman spectra, textural observations from SEM-BSE and bulk geochemical data via EMPA were collected to aid in the interpretation of mid-IR spectra. The 5 spectral groups identified correspond to progressive thermal decomposition. Unheated hydrated chondritic matrix, composed predominantly of phyllosilicates, exhibit smooth, asymmetric spectra with a peak at ∼10 μm. Thermal decomposition of sheet silicates evolves through dehydration, dehydroxylation, annealing and finally by the onset of partial melting. Both CI-like and CM-like micrometeorites are shown to pass through the same decomposition stages and produce similar mid-IR spectra. Using known temperature thresholds for each decomposition stage it is possible to assign a peak temperature range to a given micrometeorite. Since the temperature thresholds for decomposition reactions are defined by the phyllosilicate species and the cation composition and that these variables are markedly different between CM and CI classes, atmospheric entry should bias the dust flux to favour the survival of CI-like grains, whilst preferentially melting most CM-like dust. However, this hypothesis is inconsistent with empirical observations and instead requires that the source ratio of CI:CM dust is heavily skewed in favour of CM material. In addition, a small population of anomalous grains are identified whose carbonaceous and petrographic characteristics suggest in-space heating and dehydroxylation have occurred. These grains may therefore represent regolith micrometeorites derived from the surface of C-type asteroids. Since the spectroscopic signatures of dehydroxylates are distinctive, i.e. characterised by a reflectance peak at 9.0–9.5
Genge MJ, 2017, Vesicular parachutes increase the abundance of micrometeorites from water-rich asteroids on Earth, Geophysical Research Letters, Vol: 44, ISSN: 1944-8007
Micrometeorites (MMs) are extraterrestrial dust particles that survive atmospheric entry and can be recovered from sedimentary rocks. Fossil MMs allow events beyond the Earth, such as the collisional breakup of asteroids, to be identified. Here the effects of vesicle formation during melting of dust are investigated through numerical modeling and observations of Antarctic MMs. Vesicle formation is shown to cause a parachute effect that causes rapid deceleration, decreasing peak temperature. Vesicular parachuting enhances the abundance of melted MMs formed from phyllosilicate-bearing C-type asteroid dust on the Earth surface by a factor of 2. Micrometeorites recovered from the geological record, therefore, are biased toward breakup events involving hydrated C-type asteroids, whilst those involving phyllosilicate-poor particles are diluted by the enhanced background flux of hydrous dust. The parachute effect is also likely to increase the delivery of 3He to ocean sediments by C-type asteroid dust.
Shah J, Muxworthy AR, Almeida TP, et al., 2017, Dusty olivine: our oldest record of rock magnetism? (invited), Magnetic Interactions, Edinburgh
Genge MJ, 2016, Vesicle dynamics during the atmospheric entry heating of cosmic spherules, Meteoritics & Planetary Science, Vol: 52, Pages: 443-457, ISSN: 1086-9379
Cosmic spherules are unique igneous objects that form by melting due to gas drag heating during atmospheric entry heating. Vesicles are an important component of many cosmic spherules since they suggest their precursors had finite volatile contents. Vesicle abundances in spherules decrease through the series porphyritic, glassy, barred, to cryptocrystalline spherules. Anomalous hollow spherules, with large off-centre vesicles occur in both porphyritic and glassy spheres. Numerical simulation of the dynamic behaviour of vesicles during atmospheric flight is presented that indicates vesicles rapidly migrate due to deceleration and separate from non-porphyritic particles. Modest rotation rates of tens of radians s-1 are, however, sufficient to impede loss of vesicles and may explain the presence of small solitary vesicles in barred, cryptocrystalline and glassy spherules. Rapid rotation at spin rates of several thousand radians s-1 are required to concentrate vesicles at the rotational axis and leads to rapid growth by coalescence and either separation or retention depending on the orientation of the rotational axis. Complex rapid rotations that concentrate vesicles in the core of particles are proposed as a mechanism for the formation of hollow spherules. High vesicle contents in porphyritic spherules suggest volatile-rich precursors, however, calculation of volatile retention indicates these have lost >99.9% of volatiles to degassing prior to melting. The formation of hollow spherules, by rapid spin, necessarily implies pre-atmospheric rotations of several thousand radians s-1. These particles are suggested to represent immature dust, recently released from parent bodies, in which rotations have not been slowed by magnetic damping.
Genge MJ, Larsen J, Van Ginneken M, et al., 2016, An urban collection of modern-day large micrometeorites: Evidence for variations in the extraterrestrial dust flux through the Quaternary, Geology, Vol: 45, Pages: 119-122, ISSN: 1943-2682
Contrary to expectation we report the discovery of significant numbers (500) of large micrometeorites (MMs; >100 µm) from roof tops in urban areas. The identification of particles as MMs is achieved on the basis of their compositions, mineralogies and textures. All particles are silicate-dominated (S-type) cosmic spherules with sub-spherical shapes that form by melting during atmospheric entry and consist of quench crystals of magnesian olivine, relict crystals of forsterite and iron-bearing olivine within glass. Four particles also contain Ni-rich, metal-sulphide beads. Bulk compositions are chondritic apart from depletions in the volatile, moderately volatile and siderophile elements as observed in MMs from other sources. The reported particles are likely to have fallen on Earth in the past 6 years and thus represent the youngest large MMs collected to date. The relative abundance ratio of barred olivine to cryptocrystalline spherule types amongst the urban particles of 1.45 is shown to be higher than a Quaternary average of ~0.9 suggesting variations in the extraterrestrial dust flux over the last 800 k.y. Changes in the entry velocities of dust caused by quasi-periodic gravitational perturbation during transport to Earth are suggested to be responsible. Variations in cosmic spherule abundance within the geological column are thus unavoidable and can be a consequence of dust transport as well as major dust production events.
Genge MJ, Suttle M, Van Ginneken M, 2016, Olivine settling in cosmic spherules during atmospheric deceleration: An indicator of the orbital eccentricity of interplanetary dust, Geophysical Research Letters, Vol: 43, Pages: 10646-10653, ISSN: 1944-8007
A new type of cosmic spherule is reported with textures suggesting settling of olivine during atmospheric deceleration. Numerical simulations of entry heating reveal that relict forsterite, which survives melting, can settle over the 1-2s of flight at high entry angles and entry velocities up to 16 km s-1. Enhanced crystallisation of phenocrysts by heterogeneous nucleation on accumulated relict forsterites is the most likely origin of the observed cumulate textures in cosmic spherules. Such textures in cosmic spherules reveal interplanetary dust with higher encounter velocity with the Earth that correspond to orbital eccentricities >0.3. The relative abundance of cumulate spherules suggests 14% of ordinary chondrite-related, S(IV)-type asteroid dust over the last 800 kyr had relatively high orbital eccentricity owing to secular and planetary perturbations. The textures of cosmic spherules collected from sediments can therefore be used to trace dust orbital variations with time which may influence terrestrial climate.
Shah J, Muxworthy AR, Almeida TP, et al., 2016, Determining the magnetic recording fidelity of chondrule dusty olivine, 15th Castle Meeting
Shah J, Bates H, Muxworthy AR, et al., 2016, A micro-CT conglomerate test (poster), 15th Castle Meeting
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