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The Extraterrestrial Dust Flux: Size Distribution and Mass Contribution Estimates Inferred From the Transantarctic Mountains (TAM) Micrometeorite Collection
Full text linkThis study explores the long‐duration (0.8–2.3 Ma), time‐averaged micrometeorite flux (mass and size distribution) reaching Earth, as recorded by the Transantarctic Mountains (TAM) micrometeorite collection. We investigate a single sediment trap (TAM65), performing an exhaustive recovery and characterization effort and identifying 1,643 micrometeorites (between 100 and 2,000 μm). Approximately 7% of particles are unmelted or scoriaceous, of which 75% are fine‐grained. Among cosmic spherules, 95.6% are silicate‐dominated S‐types, and further subdivided into porphyritic (16.9%), barred olivine (19.9%), cryptocrystalline (51.6%), and vitreous (7.5%). Our (rank)‐size distribution is fit against a power law with a slope of −3.9 (R2 = 0.98) over the size range 200–700 μm. However, the distribution is also bimodal, with peaks centered at ~145 and ~250 μm. Remarkably similar peak positions are observed in the Larkman Nunatak data. These observations suggest that the micrometeorite flux is composed of multiple dust sources with distinct size distributions. In terms of mass, the TAM65 trap contains 1.77 g of extraterrestrial dust in 15 kg of sediment (<5 mm). Upscaling to a global annual estimate gives 1,555 (±753) t/year—consistent with previous micrometeorite abundance estimates and almost identical to the South Pole Water Well estimate (~1,600 t/year), potentially indicating minimal variation in the background cosmic dust flux over the Quaternary. The greatest uncertainty in our mass flux calculation is the accumulation window. A minimum age (0.8 Ma) is robustly inferred from the presence of Australasian microtektites, while the upper age (~2.3 Ma) is loosely constrained based on 10Be exposure dating of glacial surfaces at Roberts Butte (6 km from our sample site)
The Thermal Decomposition of Fine-grained Micrometeorites, Observations from Mid-IR Spectroscopy
Full text linkWe analysed 44 fine-grained and scoriaceous micrometeorites. A bulk mid-IR spectrum (8–13 lm) 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 lm. 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 CIlike
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 lm, and since the surfaces of C-type asteroids are expected to be heated via impact gardening, we suggest that
future spectroscopic investigations should attempt to identify dehydroxylate signatures in the reflectance spectra of young carbonaceous
asteroid families
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Melting experiments of an L6 ordinary chondrite: Implications for the formation of alkali-rich achondrites
No full textWe conducted high-pressure (1 GPa) melting experiments (1100–1400 °C) on the equilibrated ordinary chondrite DAV 01001 (L6) to investigate partial melting scenarios of planetary embryo in the early solar system. At 1100 °C, no melting of the silicate phase is observed, and the initial chondritic texture is preserved, but the metallic-sulphidic phases formed two immiscible Fe–Ni and S-rich liquids. Melting of silicate minerals began at 1200 °C, progressing from plagioclase to high-Ca and low-Ca pyroxene and olivine. As melting advanced, the formation of new olivine and low-Ca pyroxene resulted in the production of trachy-andesitic melt at 1200 °C, basaltic trachy-andesitic melt at 1300 °C, and andesitic melt at 1400 °C. These silicate melts have chemical similarities with some anomalous achondrites (e.g., GRA 60128/9). At the same time, minerals of new formation resemble those of primitive achondrites (e.g., brachinites, ureilites, IAB silicate inclusions, acapulcoites and lodranites). The rapid mineral-liquid re-equilibration suggests that basaltic liquids can form only above 1400 °C and that relatively high degrees of melting (>20 %) and crystallisation are necessary to explain the observed diversity of achondritic lithologies. These findings suggest that partial melting and recrystallization processes within planetary embryos could have played a critical role in the early solar system, contributing to the early differentiation of planetary bodies and the diversity of achondritic lithologies, including (but not limited to) alkali-rich achondrites
Flying too close to the Sun – The viability of perihelion-induced aqueous alteration on periodic comets
Full text linkComets are typically considered to be pristine remnants of the early solar system. However, by definition they evolve significantly over their lifetimes through evaporation, sublimation, degassing and dust release. This occurs once they enter the inner solar system and are heated by the Sun. Some comets (e.g. 1P/Halley, 9P/Tempel and Hale-Bopp) as well as chondritic porous cosmic dust – released from comets – show evidence of minor aqueous alteration resulting in the formation of phyllosilicates, carbonates or other secondary phases (e.g. Cu-sulphides, amphibole and magnetite). These observations suggest that (at least some) comets experienced limited interaction with liquid water under conditions distinct from the alteration histories of hydrated chondritic asteroids (e.g. the CM and CR chondrites). This synthesis paper explores the viability of perihelion-induced heating as a mechanism for the generation of highly localised subsurface liquid water and thus mild aqueous alteration in periodic comets. We draw constraints from experimental laboratory studies, numerical modelling, spacecraft observations and microanalysis studies of cometary micrometeorites. Both temperature and pressure conditions necessary for the generation and short-term (hour-long) survival of liquid water are plausible within the immediate subsurface (<0.5 m depth) of periodic comets with small perihelia (<1.5 A.U.), low surface permeabilities and favourable rotational states (e.g. high obliquities and/or slow rotational periods). We estimate that solar radiant heating may generate liquid water and perform aqueous alteration reactions in 3–9% of periodic comets. An example of an ideal candidate is 2P/Encke which has a small perihelion (0.33 A.U.), a high obliquity and a short orbital period. This comet should therefore be considered a high priority candidate in future spectroscopic studies of comet surfaces. Small quantities of phyllosilicate generated by aqueous alteration may be important in cementing together grains in the subsurface of older dormant comets, thereby explaining observations of unexpectedly high tensile strength in some bodies. Most periodic comets which currently pass close to the Sun are dormant, having experienced surface heating, significant cometary activity and dust release in the past. These bodies may be responsible for the partially hydrated cometary micrometeorites we find at the Earth's surface and their aqueous alteration histories may have been produced by perihelion-induced subsurface heating. This is in contrast to radiogenic and impact heating that operated during the early solar system on asteroids. This study has implications for the alteration history of the active asteroid Phaethon, the target of JAXA's DESTINY+ mission
3D electron diffraction for the mineralogical characterization of micro-meteorites and impact micro ejecta
No full textThe investigation of cutting-edge topics in Earth and planetary sciences
often requires the study of cryptocrystalline polyphasic materials, typically
characterized by nano-scale mineral intergrowths associated with
high-pressure/high-temperature transformations, fast and non-equilibrium
processes and small amounts of available specimens. Conventional optical
imaging and X-ray crystallographic tools may be not sufficient for the proper
characterization of such samples. The development of efficient probes able to
investigate the nanoworld is therefore crucial to further our understanding of the
mineralogical and geochemical processes that regulate Earth and extraterrestrial
environments. Over the last ten years, electron diffraction (ED) evolved from a
qualitative method restricted to few TEM users, to a robust protocol for phase
identification and ab-initio structure determination. Such changes have been
possible due to the development of automatic and semi-automatic routines for 3D
data collection (Gemmi et al., 2019). This methodology is in principle equivalent to
single-crystal X-ray diffraction, but can be performed on crystals 10 to 1000 times
smaller. In this contribution, we show recent applications of ED in planetary
sciences. In particular, how ED allowed the mineralogical screening of the
carbonaceous chondrite CM Paris (Pignatelli et al., 2018) and of a hydrated
chondritic micrometeorite (CP94-050-052) through the polytypic description of
sub-micrometric phyllosilicate grains. Moreover, we will present an extensive
petrographic and crystallographic study of quartz-coesite mineralogical association
in impact ejecta from Kamil Crater, Egypt (Folco et al., 2018), and from the
Australasian tektite strewn field (Campanale et al., 2019). We believe that the
extensive application of modern ED techniques on micro-to-nanometer
extraterrestrial samples has the potential for significant breakthroughs in our
understanding of the Solar System’s formation and evolution. Also, it will allow the
thorough exploitation of the evidence already enclosed in the micrometeorite
collection recovered within the Progetto Nazionale Ricerche in Antartide (PNRA)
and in the forthcoming European space missions. ED will also significantly support
other sources of information based on remote sensing and spectroscopy and will
therefore ensure better constraints in numerical modeling studies
X-ray computed tomography: Morphological and porosity characterization of giant Antarctic micrometeorites
No full textGiant micrometeorites (MMs; 400–2000 μm) are exceedingly rare and scientifically valuable. Three-dimensional nondestructive characterization by X-ray computed tomography (X-CT) provides information on the petrography and thus petrogenesis of MMs and serves as a guide to maximize subsequent multi-analytical studies on such precious planetary materials. Here, we discuss the results obtained by X-CT on 22 giant MMs and the classification based on their 3-D density contrast images. Scoriaceous and unmelted MMs have distinct porosity ranges (10–40 vol% versus 0–25 vol%, respectively). We observe a porosity variation inside scoriaceous MMs, which allows their atmospheric entry flight history to be resolved. For the first time, spinning entry is explicitly demonstrated for four partially melted MMs. Furthermore, we are able to resolve the thermal gradient in a single particle, based on porosity variation (seen as a progressive increase in pore abundance and size with higher peak temperatures). Moreover, we explore parent body alteration through the 3-D analysis of pores distribution, showing that shock fabrics are either absent or weakly developed in our data set. Finally, owing to the detection of pseudomorphic chondrules, we estimate that the intensively aqueously altered C1 or CI-like material could represent 18% of the MM flux at this size fraction (400–1000 μm)
The aqueous alteration of GEMS-like amorphous silicate in a chondritic micrometeorite by Antarctic water
No full textWe analysed the heterogenous fine-grained (sub-μm) matrix of a small (58x93μm), unmelted and minimally heated (<350°C) micrometeorite (CP94-050-052) recovered from Antarctic blue ice. This particle contains some unaltered highly primitive phases, including refractory anhydrous high-Mg silicates and submicron crystalline needle-shaped acicular grains interpreted as enstatite whiskers. The particle also contains an abundance of micron-sized Fe-rich grains, which span a compositional and textural continuum between amorphous oxygen-rich silicate and poorly crystalline Fe-rich phyllosilicate (cronstedtite). These Fe-rich grains are here interpreted as secondary phases formed by aqueous alteration. Their inferred anhydrous precursors were likely primitive “GEMS-like” amorphous Fe-Mg-silicates. This micrometeorite’s bulk chemical composition and mineralogy suggest either a carbonaceous chondrite or cometary origin. However, the particle’s average O-isotope composition (δ17O: -12.4‰ [±5.0‰], δ18O: -24.0‰ [±2.3‰] and Δ17O at +0.1‰ [±4.8‰] is distinct from all previously measured chondritic materials. Instead this value is intermediate between primitive chondritic materials and the composition of Antarctic water – strongly implying that the particle was heavily affected by Antarctic alteration.
Analysis of the micrometeorite’s H-isotopes reveals low deuterium abundances (δD: -217‰ to -173‰ [±43-47‰]) paired with high H abundances (and thus high water contents [<25wt.%]). Although both water contents and H-isotope compositions overlap with those reported in CM chondrites, the datapoints measured from CP94-050-052 extend to more extreme values. Further supporting the idea that the aqueous alteration that affected this micrometeorite operated under different environmental conditions to asteroidal settings.
These data collectively demonstrate partial isotopic exchange with light (δ18O-poor, δD-poor) terrestrial fluids whilst the micrometeorite resided in Antarctica. Although this micrometeorite may have been aqueously altered whilst on its parent body this cannot be conclusively demonstrated due to the extent of the weathering overprint. Antarctic alteration operated at significantly higher water-to-rock ratios than chondritic settings. Despite these differences the extent of secondary replacement and the duration of alteration were limited with mafic silicates remaining unaffected. The combined alteration conditions for this particle likely operated over short timescales (<24hrs), under mildly alkaline conditions (∼pH8) and at low temperatures (<50°C), this could have occurred during the micrometeorite’s extraction from blue ice
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The aqueous alteration of GEMS-like amorphous silicate in a chondritic micrometeorite by Antarctic water
No full textWe analysed the heterogenous fine-grained (sub-μm) matrix of a small (58 × 93 μm), unmelted and minimally heated (<350 °C) micrometeorite (CP94-050-052) recovered from Antarctic blue ice. This particle contains some unaltered highly primitive phases, including refractory anhydrous high-Mg silicates and submicron crystalline needle-shaped acicular grains interpreted as enstatite whiskers. The particle also contains an abundance of micron-sized Fe-rich grains, which span a compositional and textural continuum between amorphous oxygen-rich silicate and poorly crystalline Fe-rich phyllosilicate (cronstedtite). These Fe-rich grains are here interpreted as secondary phases formed by aqueous alteration. Their inferred anhydrous precursors were likely primitive “GEMS-like” amorphous Fe-Mg-silicates. This micrometeorite's bulk chemical composition and mineralogy suggest either a carbonaceous chondrite or cometary origin. However, the particle's average O-isotope composition (δ17O: −12.4‰ [±5.0‰], δ18O: −24.0‰ [±2.3‰] and Δ17O at +0.1‰ [±4.8‰] is distinct from all previously measured chondritic materials. Instead this value is intermediate between primitive chondritic materials and the composition of Antarctic water – strongly implying that the particle was heavily affected by Antarctic alteration. Analysis of the micrometeorite's H-isotopes reveals low deuterium abundances (δD: −217‰ to −173‰ [±43–47‰]) paired with high H abundances (and thus high water contents [<25 wt.%]). Although both water contents and H-isotope compositions overlap with those reported in CM chondrites, the datapoints measured from CP94-050-052 extend to more extreme values. Further supporting the idea that the aqueous alteration that affected this micrometeorite operated under different environmental conditions to asteroidal settings. These data collectively demonstrate partial isotopic exchange with light (δ18O-poor, δD-poor) terrestrial fluids whilst the micrometeorite resided in Antarctica. Although this micrometeorite may have been aqueously altered whilst on its parent body this cannot be conclusively demonstrated due to the extent of the weathering overprint. Antarctic alteration operated at significantly higher water-to-rock ratios than chondritic settings. Despite these differences the extent of secondary replacement and the duration of alteration were limited with mafic silicates remaining unaffected. The combined alteration conditions for this particle likely operated over short timescales (<24 h), under mildly alkaline conditions (∼pH 8) and at low temperatures (<50 °C), this could have occurred during the micrometeorite's extraction from blue ice
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