The parent bodies of fine-grained micrometeorites: A petrologic & spectroscopic perspective

Micrometeorites are millimetre-scale cosmic dust grains, derived from asteroids and comets. They represent the largest flux of extraterrestrial material currently falling to Earth, with an estimated contribution of 20,000-60,000 tons per year. In this thesis, the geological history, parent body properties and atmospheric entry of fine-grained micrometeorites are investigated through micro-analysis and spectroscopic techniques. The degree of aqueous alteration within fine-grained micrometeorites was investigated using criteria initially developed for CM chondrites (Chapt.3). This revealed that most particles are intensely altered, with petrologic subtypes <CM2.3. Textural and geochemical evidence of aqueous alteration is seen in the form of hydrated CAIs, hydrated sulfides, pseudomorphic chondrules and complex intergrown and cross-cut assemblages of phyllosilicate, which attest to extended periods in contact with liquid water. Likewise, the apparent overabundance of CM-like matrix and the relative paucity of C2 chondrule material among fine-grained micrometeorites suggest that the parent bodies of fine-grained micrometeorites are predominantly intensely aqueously altered bodies. This study also identified the first evidence for shock deformation in fine-grained micrometeorites (Chapt.6). Weak, pervasive petrofabrics, formed by aligned phyllosilicates and inferred from dehydration crack orientations were observed in the majority of micrometeorites studied (21). This requires relatively low peak pressures (<5GPa) and is most likely achieved by successive low-intensity impact events. The presence of a single micrometeorite containing brittle deformation cataclasis fabrics also provides evidence for brittle deformation shock processing of micrometeorites. The first near-IR spectra of micrometeorites were collected and directly compared against the NIR spectra of young C-type asteroids (Chapt.8). Although these comparisons proved inconclusive, owing to limitations in the quality of the micrometeorite spectra, this study identified the first evidence of hydroxyl-group absorption bands at NIR wavelengths in Veritas family asteroids, suggesting the presence of intact phyllosilicates on their surfaces and thereby adding support to the genetic link between fine-grained micrometeorites and C-type asteroids. Mid-IR spectroscopy revealed how micrometeorite mineralogy evolves during flash heating in the upper atmosphere, demonstrating that solid state recrystallization preserves pre-atmospheric textures, despite major changes in the mineralogy (Chapt.4). Spatially resolved Raman spectroscopy was used to investigate thermal gradients within micrometeorites during atmospheric entry and revealed that most micrometeorite cores preserve low-temperature (<300°C) carbonaceous phases inherited from their parent asteroid (Chapt.5). The development of secondary interconnected porosity was described for the first time, detailing how the growth and expansion of dehydration cracks driven by the out-gassing of volatiles leads to the formation of branching and sinuous channels (Chapt.7). These channels play an important role in the efficient heating of micrometeorite cores resulting in partial melting as scoriaceous micrometeorites are formed. In addition, the development of secondary porosity significantly lowers the mechanical strength of micrometeoroids, promoting their disruption in the atmosphere. Finally, a small-scale study, attempting to retrieve fine-grained micrometeorites preserved in ancient sedimentary rocks was trailed (Chapt.9). This led to the recovery of a new collection of fossil micrometeorites derived from Cretaceous chalk. Although no unmelted micrometeorites were discovered, the preserved cosmic spherules are found to have experienced complete diagenetic alteration, resulting in preserved micro-textures and replaced terrestrial mineralogies. A repeat study at a different time period and location also found cosmic spherules with identical preservation styles, suggesting that diagenetically altered micrometeorites most likely represent the most common form of cosmic dust on Earth.Open Acces

A novel evolutionary strategy revealed in the Phaeoviruses

Phaeoviruses infect the brown algae, which are major contributors to primary production of coastal waters and estuaries. They exploit a Persistent evolutionary strategy akin to a K- selected life strategy via genome integration and are the only known representatives to do so within the giant algal viruses that are typified by r- selected Acute lytic viruses. In screening the genomes of five species within the filamentous brown algal lineage, here we show an unprecedented diversity of viral gene sequence variants especially amongst the smaller phaeoviral genomes. Moreover, one variant shares features from both the two major sub-groups within the phaeoviruses. These phaeoviruses have exploited the reduction of their giant dsDNA genomes and accompanying loss of DNA proofreading capability, typical of an Acute life strategist, but uniquely retain a Persistent life strategy

A systematic review and meta-analysis of topoisomerase inhibition in pre-clinical glioma models

Malignant glioma is a devastating disease affecting both adults and children with limited treatment strategies. Pre-clinical animal studies are critical to the development and planning of novel treatment designs for human clinical trials. Topoisomerases has been a target of interest in the treatment of high grade gliomas, such as glioblastoma, in the past years. Here we assess pre-clinical glioma literature with the aim to identify predictive variables that favour treatment outcomes from topoisomerase inhibition. Data was extracted from 90 experimental comparisons, this was divided based on available survival (n= 61) and tumor volume (n= 29) data. The meta-analysis revealed that the overall effect of topoisomerase inhibition prolonged survival by a factor of 1.33 (95% CI: 1.23-1.43) and reduced tumor growth by a factor of 3.21 (95% CI: 1.99-5.88), with considerable between-study heterogeneity. Multivariable meta-regression identified glioma model, type of control, route of drug administration and drug of choice to be predictive of improved survival outcome. Publication bias assessment by contour-enhanced funnel plots, Egger's regression test and trim and fill analysis showed evidence of publication bias in all studies. This study identified multiple study design factors that should be taken into consideration to improve the translation of pre-clinical investigation of topoisomerase inhibition into clinical use.</p

Thermal shock fragmentation of Mg silicates within scoriaceous micrometeorites reveal hydrated asteroidal sources

Scoriaceous micrometeorites are highly vesicular extraterrestrial dust particles that have experienced partial melting during atmospheric entry. We report the occurrence of clusters of anhedral relict forsterite crystals within these particles that testify to in situ fragmentation. The absence of similar clusters within unmelted micrometeorites 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 micrometeorites, therefore, indicate that the precursor grains were similar to CI and CM2 chondrites and retained phyllosilicate prior to atmospheric 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

A seasonal diary of phytoplankton in the North Atlantic

In recent years new biological and physical controls have been suggested to drive phytoplankton bloom dynamics in the North Atlantic. A better understanding of the mechanisms driving primary production has potentially important implications for the understanding of the biological carbon pump, as it has for prediction of the system in climate change scenarios. However, the scientific discussion regarding this topic has generally failed to integrate the different drivers into a coherent picture, often rendering the proposed mechanisms exclusive to each other. We feel that the suggested mechanisms are not mutually exclusive, but rather complementary. Thus, moving beyond the “single mechanism” point of view, here we present an integrated conceptual model of the physical and biological controls on phytoplankton dynamics in the North Atlantic. Further we believe that the acclimation of physiological rates can play an important role in mediating phytoplankton dynamics. Thus, this view emphasizes the occurrence of multiple controls and relates their variations in impact to climate change

Micrometeorites: Insights into the flux, sources and atmospheric entry of extraterrestrial dust at Earth

Micrometeorites (MMs) provide constraints on the flux and sources of extraterrestrial dust falling on Earth as well as recording the processes occurring during atmospheric entry. Collections of micrometeorites have been recovered from a wide variety of environments including Antarctic moraine, rock traps, ice and snow and on roof tops in urban areas. Studies of the mineralogy and composition of MMs suggest that most particles (>98%) >50 ​μm in diameter have asteroidal sources, whilst ~50% of particles smaller than 50 ​μm are likely to be derived from comets. The relative abundance of S(IV)-type asteroid materials, similar to ordinary chondrites increases with size, although C-type asteroidal materials, similar to carbonaceous chondrites dominate over all. Although MMs provide excellent evidence on the nature and abundance of extraterrestrial dust at the Earth’s orbit they are not without bias and uncertainty. Mineralogical and compositional change during atmospheric entry makes the exact nature of their precursors uncertain complicating evaluation of source beyond basic classes of material. This is particularly true at larger sizes when complete melting to form cosmic spherules occurs, however, unmelted MMs >50 ​μm in size are also often thermally altered. Mixing with atmospheric oxygen and mass fractionation by evaporation furthermore complicates the use of oxygen isotope compositions in identifying parent bodies. All MM collections are suggested to exhibit biases owing to: (1) collection method, (2) terrestrial weathering, (3) terrestrial contamination, and (4) erosion and deposition by terrestrial surface processes. Even in the least biased collections, those collected by dedicated melting of Antarctic snow, erosive loss of material is suggested here to make fluxes uncertain by factors of up to ~2. The abundance of asteroid-derived MMs observed in collections contradicts models of the orbital evolution of interplanetary dust to Earth, which suggests >70% should be provided by comets

The aqueous alteration of CM chondrites, a review

The CM chondrites are samples of primitive water-rich asteroids formed during the early solar system. They record significant interaction between liquid water and silicate rock, resulting in a mineralogy dominated by hydrated secondary phases. Their similarity to the near-Earth asteroids Bennu and Ryugu – targets of current sample return space missions – makes the analysis of CM chondrites essential to the interpretation of these enigmatic bodies. Here, we review the aqueous alteration history of the CM chondrite group. Initially, amorphous silicate, metal and sulphides within the matrix were converted into Fe-cronstedtite and tochilinite. Later, the serpentinization of refractory coarse-grained inclusions led to the addition of Mg to the fluid phase. This is reflected in the cation composition of secondary phases which evolved from Fe-rich to Mg-rich. Although most CM meteorites are classified as CM2 chondrites and retain some unaltered anhydrous silicates, a few completely altered CM1s exist (∼4.2% [Meteoritical Bulletin, 2021]). The extent of aqueous alteration can be quantified through various techniques, all of which trace the progression of secondary mineralization. Early attempts employed petrographic criteria to assign subtypes – most notably the Browning and Rubin scales have been widely adopted. Alternatively, bulk techniques evaluate alteration either by measuring the ratio of phyllosilicate to anhydrous silicate (this can be with X-ray diffraction [XRD] or infrared spectroscopy [IR]) or by measuring the combined H abundance/δD compositions. The degree of aqueous alteration appears to correlate with petrofabric strength (most likely arising due to shock deformation). This indicates that aqueous alteration may have been driven primarily by impact rather than by radiogenic heating. Alteration extent and bulk O-isotope compositions show a complex relationship. Among CM2 chondrites higher initial water contents correspond to more advanced alteration. However, the CM1s have lighter-than-expected bulk compositions. Although further analyses are needed these findings could suggest either differences in alteration conditions or initial isotopic compositions – the latter scenario implies that the CM1 chondrites formed on a separate asteroid from the CM2 chondrites. Secondary phases (primarily calcite) act as proxies for the conditions of aqueous alteration and demonstrate that alteration was prograde, with an early period at low temperatures (<70 °C), while later alteration operated at higher temperatures of 100–250 °C. Estimates for the initial water-to-rock ratios (W/R) vary between 0.2–0.7. They are based either on isotopic mass balance or mineral stoichiometry calculations – variability reflects uncertainties in the primordial water and protolith compositions and whether alteration was open or closed system. Some CM chondrites (<36%) experienced a later episode of post-hydration thermal metamorphism, enduring peak temperatures <900 °C and resulting in a dehydrated mineralogy and depleted volatile element abundances. Heating was likely short-duration and caused by impact events. The presence of CM chondrite material embedded in other meteorites, their prominence among the micrometeorite flux and the link between CMs and rubble-pile C-type near-Earth asteroids (e.g. Bennu and Ryugu) implies that the CM parent body was disrupted, leaving second-generation CM asteroids to supply material to Earth.Copyright © 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). The attached file is the published version of the article.NHM Repositor

Cosmic dust fertilization of glacial prebiotic chemistry on early Earth

Earth’s surface is deficient in available forms of many elements considered limiting for prebiotic chemistry. In contrast, many extraterrestrial rocky objects are rich in these same elements. Limiting prebiotic ingredients may, therefore, have been delivered by exogenous material; however, the mechanisms by which exogeneous material may be reliably and non-destructively supplied to a planetary surface remains unclear. Today, the flux of extraterrestrial matter to Earth is dominated by fine-grained cosmic dust. Although this material is rarely discussed in a prebiotic context due to its delivery over a large surface area, concentrated cosmic dust deposits are known to form on Earth today due to the action of sedimentary processes. Here we combine empirical constraints on dust sedimentation with dynamical simulations of dust formation and planetary accretion to show that localized sedimentary deposits of cosmic dust could have accumulated in arid environments on early Earth, in particular glacial settings that today produce cryoconite sediments. Our results challenge the widely held assumption that cosmic dust is incapable of fertilizing prebiotic chemistry. Cosmic dust deposits may have plausibly formed on early Earth and acted to fertilize prebiotic chemistry

Antarctic micrometeorites as a resource to complement the current Solar System inventory: the case of 16O-poor cumulate porphyritic cosmic spherules

The 14th Symposium on Polar Science/Ordinary sessions [OA] Antarctic Meteorite, Tue. 14 Nov. / 3F Multipurpose Conference room, Institute of Statistical Mathematicsconference objec

Rapid heating rates define the volatile emission and regolith composition of (3200) Phaethon

Asteroid (3200) Phaethon experiences extreme solar radiant heating ( ~ 750 °C) during perihelion (0.14 au), leading to comet-like activity. The regolith composition and mechanism of volatile emission are unknown but key to understanding JAXA’s DESTINY+ mission data (fly-by in 2029) and the fate of near-Sun asteroids more generally. By subjecting CM chondrite fragments to fast, open system, cyclic heating (2-20 °C/min), simulating conditions on Phaethon we demonstrate that rapid heating rates combine with the low permeability, resulting in reactions between volatile gases and decomposing minerals. The retention of S-bearing gas limits the thermal decomposition of Fe-sulphides, allowing these minerals to survive repeated heating cycles. Slow escape of S-bearing gases provides a mechanism for repeated gas release from a thermally processed surface and, therefore the comet-like activity without requiring surface renewal to expose fresh material each perihelion cycle. We predict Phaethon regolith is composed of olivine, Fe-sulphides, Ca-sulphates and hematite