Amorphous silicates are abundant in extraterrestrial objects such as interplanetary dust particles and primitive chondrites. They are thought to be formed through condensation and possibly later ...exposed to thermal processes in the nebula before being accreted within an asteroid and/or comet.
We aim to constrain the conditions that prevailed during thermal events in the nebula, through experimental work on the chemical and structural evolution of condensed amorphous silicate.
We conducted coupled condensation and heating experiments of Fe-Mg-silicate thin films using the pulsed laser deposition technique. We compared samples condensed at room temperature and annealed in a second step with samples directly condensed on heated substrate, at 450 °C and 700 °C.
For both processes, at temperature as low as 450 °C, iron-rich nanoparticles and Mg-rich domains form, evidencing the high reactivity of the condensed amorphous silicate. This reactivity was found to be even higher for the process of condensation on heated substrate. We also evidence the persistence of amorphous silicate up to 700 °C, in spite of the chemical evolution and the demixion into MgO and SiO2 domains.
These results imply that amorphous silicates condensed from a plasma (and possibly from any process producing atoms in an excited state) are more reactive than quenched glasses of similar composition. In complement to high temperature events that occurred at the time of solar system formation and that formed chondrules for instance, this work emphasizes the importance of mild heating on dust evolution before accretion within parent(s) body(ies). It helps to place chemical and structural constraints on the thermal evolution of amorphous silicate found in primitive chondrites: i) iron segregation as metallic nanoparticles can be generated within a silicate groundmass at temperature as low as 450 °C (and possibly even below) ii) iron-rich chondritic amorphous silicate can persist up to 700 °C.
•We performed experiments using condensed amorphous silicate at 450 °C and 700 °C•This material shows high reactivity at low temperature (450 °C).•These products share similarities with chondritic amorphous silicate and glass embedded with metal and sulfides (GEMS).
Metamorphic fluids, faults, and shear zones are carriers of carbon from the deep Earth to shallower reservoirs. Some of these fluids are reduced and transport energy sources, like H
and light ...hydrocarbons. Mechanisms and pathways capable of transporting these deep energy sources towards shallower reservoirs remain unidentified. Here we present geological evidence of failure of mechanically strong rocks due to the accumulation of CH
-H
-rich fluids at deep forearc depths, which ultimately reached supralithostatic pore fluid pressure. These fluids originated from adjacent reduction of carbonates by H
-rich fluids during serpentinization at eclogite-to-blueschist-facies conditions. Thermodynamic modeling predicts that the production and accumulation of CH
-H
-rich aqueous fluids can produce fluid overpressure more easily than carbon-poor and CO
-rich aqueous fluids. This study provides evidence for the migration of deep Earth energy sources along tectonic discontinuities, and suggests causal relationships with brittle failure of hard rock types that may trigger seismic activity at forearc depths.
Planar deformation features (PDFs) in quartz are a commonly used and well‐documented indicator of shock metamorphism in terrestrial rocks. The measurement of PDF orientations provides constraints on ...the shock pressure experienced by a rock sample. A total of 963 PDF sets were measured in 352 quartz grains in 11 granite samples from the basement of the Chicxulub impact structure’s peak ring (IODP‐ICDP Expedition 364 drill core), with the aim to quantify the shock pressure distribution and a possible decay of the recorded shock pressure with depth, in the attempt to better constrain shock wave propagation and attenuation within a peak ring. The investigated quartz grains are highly shocked (99.8% are shocked), with an average of 2.8 PDF sets per grain; this is significantly higher than in all previously investigated drill cores recovered from Chicxulub and also for most K‐Pg boundary samples (for which shocked quartz data are available). PDF orientations are roughly homogenous from a sample to another sample and mainly parallel to {101-3} and {101-4} orientations (these two orientations representing on average 68.6% of the total), then to {101-2} orientation, known to form at higher shock pressure. Our shock pressure estimates are within a narrow range, between ~16 and 18 GPa, with a slight shock attenuation with increasing depth in the drill core. The relatively high shock pressure estimates, coupled with the rare occurrence of basal PDFs, i.e., parallel to the (0001) orientation, suggest that the granite basement in the peak ring could be one of the sources of the shocked quartz grains found in the most distal K‐Pg boundary sites.
The Paris chondrite provides an excellent opportunity to study CM chondrules and refractory inclusions in a more pristine state than currently possible from other CMs, and to investigate the earliest ...stages of aqueous alteration captured within a single CM bulk composition. It was found in the effects of a former colonial mining engineer and may have been an observed fall. The texture, mineralogy, petrography, magnetic properties and chemical and isotopic compositions are consistent with classification as a CM2 chondrite. There are ∼45vol.% high-temperature components mainly Type I chondrules (with olivine mostly Fa0–2, mean Fa0.9) with granular textures because of low mesostasis abundances. Type II chondrules contain olivine Fa7 to Fa76. These are dominantly of Type IIA, but there are IIAB and IIB chondrules, II(A)B chondrules with minor highly ferroan olivine, and IIA(C) with augite as the only pyroxene. The refractory inclusions in Paris are amoeboid olivine aggregates (AOAs) and fine-grained spinel-rich Ca–Al-rich inclusions (CAIs). The CAI phases formed in the sequence hibonite, perovskite, grossite, spinel, gehlenite, anorthite, diopside/fassaite and forsterite. The most refractory phases are embedded in spinel, which also occurs as massive nodules. Refractory metal nuggets are found in many CAI and refractory platinum group element abundances (PGE) decrease following the observed condensation sequences of their host phases. Mn–Cr isotope measurements of mineral separates from Paris define a regression line with a slope of 53Mn/55Mn=(5.76±0.76)×106. If we interpret Cr isotopic systematics as dating Paris components, particularly the chondrules, the age is 4566.44±0.66Myr, which is close to the age of CAI and puts new constraints on the early evolution of the solar system. Eleven individual Paris samples define an O isotope mixing line that passes through CM2 and CO3 falls and indicates that Paris is a very fresh sample, with variation explained by local differences in the extent of alteration. The anhydrous precursor to the CM2s was CO3-like, but the two groups differed in that the CMs accreted a higherproportion of water. Paris has little matrix (∼47%, plus 8% fine grained rims) and is less altered than other CM chondrites. Chondrule silicates (except mesostasis), CAI phases, submicron forsterite and amorphous silicate in the matrix are all well preserved in the freshest domains, and there is abundant metal preserved (metal alteration stage 1 of Palmer and Lauretta (2011)). Metal and sulfide compositions and textures correspond to the least heated or equilibrated CM chondrites, Category A of Kimura et al. (2011). The composition of tochilinite–cronstedtite intergrowths gives a PCP index of ∼2.9. Cronstedtite is more abundant in the more altered zones whereas in normal highly altered CM chondrites, with petrologic subtype 2.6–2.0 based on the S/SiO2 and ∑FeO/SiO2 ratios in PCP or tochilinite–cronstedtite intergrowths (Rubin et al., 2007), cronstedtite is destroyed by alteration. The matrix in fresh zones has CI chondritic volatile element abundances, but interactions between matrix and chondrules occurred during alteration, modifying the volatile element abundances in the altered zones. Paris has higher trapped Ne contents, more primitive organic compounds, and more primitive organic material than other CMs. There are gradational contacts between domains of different degree of alteration, on the scale of ∼1cm, but also highly altered clasts, suggesting mainly a water-limited style of alteration, with no significant metamorphic reheating.
ABSTRACT The origin of hydrogen in chondritic components is poorly understood. Their isotopic composition is heavier than the solar nebula gas. In addition, in most meteorites, hydrous silicates are ...found to be lighter than the coexisting organic matter. Ionizing irradiation recently emerged as an efficient hydrogen fractionating process in organics, but its effect on H-bearing silicates remains essentially unknown. We report the evolution of the D/H of hydrous silicates experimentally irradiated by electrons. Thin films of amorphous silica, amorphous "serpentine," and pellets of crystalline muscovite were irradiated at 4 and 30 keV. For all samples, irradiation leads to a large hydrogen loss correlated with a moderate deuterium enrichment of the solid residue. The entire data set can be described by a Rayleigh distillation. The calculated fractionation factor is consistent with a kinetically controlled fractionation during the loss of hydrogen. Furthermore, for a given ionizing condition, the deuteration of the silicate residues is much lower than the deuteration measured on irradiated organic macromolecules. These results provide firm evidence of the limitations of ionizing irradiation as a driving mechanism for D-enrichment of silicate materials. The isotopic composition of the silicate dust cannot rise from a protosolar to a chondritic signature during solar irradiations. More importantly, these results imply that irradiation of the disk naturally induces a strong decoupling of the isotopic signatures of coexisting organics and silicates. This decoupling is consistent with the systematic difference observed between the heavy organic matter and the lighter water typically associated with minerals in the matrix of most carbonaceous chondrites.
We have characterized by transmission electron microscopy the mineralogy of samples extracted from the walls of the Stardust track 80. More than 500 fragments were studied using conventional imaging, ...electron diffraction and EDX microanalysis. Two categories of particles are distinguishable in equal proportions (wt%). The first one is comprised of relatively large crystalline grains (≈1μm on average), dominated by silicates (olivine and pyroxene). They display a wide range of compositions and microstructures comparable to those found in terminal particles. Minor phases including magnetite and apatite are also present. Their occurrence suggests that the Wild 2 material underwent aqueous alteration to some extent. The second type of particle, called GEMS-like material, is made of silica-rich glassy clumps embedding iron sulfide beads and vesicles. Their microstructure is characteristic of thermally modified particles that have suffered strong interaction with the silica aerogel during the hypervelocity impact. This melted material may form by shedding of melted and vaporized material, but given the shape of the impact track and high diversity of surviving mineral compositions, much of it originated from fine-grained aggregates that disaggregated during the collection. Chemical mapping at the nano-scale allowed the localization of individual components within the silica-rich glass. They are dominated by Mg-rich components with a size less than 300nm. The average composition of this thermally modified material is close to the solar abundance for the major elements Fe, Mg and S. The fine-grained material has probably not been chemically fractionated in the protoplanetary disk before its incorporation in comet Wild 2 unlike the sulfur depleted matrix of chondrites. From these two categories of particles, we deduce that Wild 2 is likely made of an assemblage of relatively large evolved grains (first category) cemented by a fine-grained material with primitive chemistry (second category). The pre-impact configuration of the incident material deduced from this study seems comparable to the matrix of the most primitive chondrites (3.0) or to chondritic porous interplanetary dust particles.
Sample return missions Hayabusa2 (JAXA) and OSIRIS-REx (NASA) found evidence of hydrated silicates on the surface of C and B-type asteroids Ryugu and Bennu. This detection relied on the study of the ...2.7 μm OH-stretching spectral feature revealed from remote sensing observations of the asteroids' surfaces. Laboratory studies simulating the effects of space weathering (SpWe) on primitive bodies have shown that the feature's position, considered as the wavelength of the band minimum, can vary under ion implantation, shifting towards longer wavelengths for implanted surfaces. Since SpWe is a surface process, we investigated how the geometry of observation can affect the hydration feature on space weathered surfaces. Here, we report new laboratory Reflectance Factors (REFF) measurements on pristine and ion-bombarded phyllosilicate pellets, to monitor the evolution of the 2.7 μm feature with varying observation geometry. We found that, as we approach specular reflection, the feature's position for He+ bombarded surfaces shifts towards longer wavelengths. We interpret that the spectral shift is due to chemical and physical changes induced by ion implantation in the first hundreds of nanometers of our phyllosilicate pellets. The diversity in the observed amplitude of the shift means that different competing effects are dominating at different optical configurations, mainly volume and surface scattering. The effects of the ion-implanted matter are especially visible when measuring in near-specular conditions, where the specular component (more sensible to the very top surface (implanted layer) of the sample)dominates - hence the larger shift measured. Our results indicate that the geometry of observation can induce a certain bias in the interpretation of remote sensing data from space-weathered bodies.
•Sample return missions found evidence of hydrated silicates on primitive asteroids.•This detection relies on spectroscopic surveys of the hydration band at 2.7 μm.•Space-weathering can affect the position of the hydration band.•We found that the geometry of observation can also affect the position of the band.•Taking in account these biases can help better constrain surface composition.
Meteorites are a primary source of information about past magnetic field in the solar system. Yet, the small-scale magnetic properties of FeNi metals, which are the magnetic carriers of most ...meteorites, are poorly known. We study here the magnetic microstructures of FeNi metals in two equilibrated chondrites. Two types of tetrataenite-bearing microstructures are revealed: (1) Zoned taenite particles that consist of a “cloudy zone” (20–250
nm large tetrataenite precipitates embedded in Ni-poor matrix) and a 1–10
μm thick tetrataenite rim. (2) Zoneless plessite particles that consist of large tetrataenite grains (>
1
μm) embedded in a kamacite matrix. Magneto-optical imaging of saturation remanence shows that, the submicron-sized tetrataenite islands in cloudy zone carry a much stronger remanence than the μm-sized tetrataenite crystals in the tetrataenite rims and plessite. Micron-scale mapping of coercivity of remanence (
B
cr
) shows that the center part of the cloudy zone has finer tetrataenite grains (20
nm) and higher
B
cr
values (~
1
T) than the outer part (250
nm and 400
mT, respectively). These results suggest that the micron-sized tetrataenite is in a multi domain state, whereas the submicron-sized tetrataenite in the cloudy zone are in a single domain-like state and may be regarded as a potentially good paleomagnetic recorder in meteorites. The stability of the remanent magnetization in ordinary chondrites is a function of the amount of the cloudy zones of the zoned taenite grains rather than the bulk amount of tetrataenite.
The Fe–Ni phase diagram indicates that precursor of tetrataenite is paramagnetic when the metamorphic temperature was above 350–400
°C. Therefore, the remanent magnetization of tetrataenite cannot be an evidence of early magnetic activity on the parent body (e.g., dynamo activity) during the first 10 to 50
Myr after the peak of metamorphism, assuming 900
°C peak temperature and 50–100
°C/Myr cooling rate. Our TEM observations show that tetrataenite has a homogeneous crystallographic orientation in an individual zoned taenite grain. In low or null field, tetrataenite may acquire a spontaneous magnetization whose direction is controlled by this crystallographic orientation, which varies from grain to grain. The small-scale heterogeneity of remanence observed in equilibrated chondrites may imply that no significant magnetic field (e.g., dynamo field) was present during cooling below 350–400
°C.
► Microscopic identification of magnetic minerals in metal-bearing meteorites. ► Microscopic mapping of magnetic properties of FeNi minerals. ► Best magnetic carrier in equilibrated chondrite is fine-grained (<
1
μm) tetrataenite. ► Tetrataenite cannot record early magnetic activity of the parent body.
Fe-rich serpentines are an abundant product of the early aqueous alteration events that affected the parent bodies of CM carbonaceous chondrites. Alteration assemblages in these meteorites show a ...large chemical variability and although water–rock interactions occurred under anoxic conditions, serpentines contain high amounts of ferric iron. To date very few studies have documented Fe valence variations in alteration assemblages of carbonaceous chondrites, limiting the understanding of the oxidation mechanisms. Here, we report results from a nanoscale study of a calcium–aluminum-rich inclusion (CAI) from the Murray chondrite, in which alteration resulted in Fe import and Ca export by the fluid phase and in massive Fe-rich serpentines formation. We combined scanning and transmission electron microscopies and scanning transmission X-ray microscopy for characterizing the crystal chemistry of Fe-serpentines. We used reference minerals with known crystallographic orientations to quantify the Fe valence state in Fe-rich serpentines using X-ray absorption spectroscopy at the Fe L2,3-edges, yielding a robust methodology that would prove valuable for studying oxidation processes in other terrestrial or extra-terrestrial cases of serpentinization. We suggest that aqueous Fe2+ was transported to the initially Fe-depleted CAI, where local changes in pH conditions, and possibly mineral catalysis by spinel promoted the partial oxidation of Fe2+ into Fe3+ by water and the formation of Fe-rich serpentines close to the cronstedtite endmember. Such mechanisms produce H2, which opens interesting perspectives as hydrogen may have reacted with carbon species, or escaped and yield increasingly oxidizing conditions in the parent asteroid. From the results of this nanoscale study, we also propose transformations of the initial cronstedtite, destabilized by later input of Al- and Mg-rich solutions, leading to Fe2+ leaching from serpentines, as well as to random serpentine-chlorite interstratifications. Such transformations towards polysomatic assemblages that are un-equilibrated from the structural, chemical and redox point of views are probably controlled by the various rates of alteration of primary minerals, but also by porosity gradients, as in terrestrial hydrothermal systems. We suggest that the proposed mechanisms may have played a role in the early formation of (Fe2+,Fe3+)-rich serpentines documented in CM chondrites, as well as in their transformation with on-going alteration towards Fe-poorer compositions inferred from previous petrologic, mineralogical and magnetic studies of CM chondrites.
The Allende matrix is dominated by micron‐sized lath‐shaped fayalitic olivine grains with a narrow compositional range (Fa40–50). Fayalitic olivines also occur as rims around forsterite grains in ...chondrules and isolated forsterite fragments in the matrix or as veins cross‐cutting the grains. Allende is a type 3 CV carbonaceous chondrite having experienced a moderate thermal metamorphism. There is therefore a strong chemical disequilibrium between the large forsterite grains and the fayalite‐rich fine‐grained matrix. Chemical gradients at interfaces are poorly developed and thus not accessible using conventional techniques. Here, we used analytical transmission electron microscopy to study the microstructure of the fayalite‐rich matrix grains and interfaces with forsterite fragments. We confirm that fayalitic grains in the matrix and fayalitic rims around forsterite fragments have the same properties, suggesting a common origin after the accretion of the parent body of Allende. Composition profiles at the rim/forsterite interfaces exhibit a plateau in the rim (typically Fa45), a compositional jump of 10 Fa% at the interface, and a concentration gradient in the forsterite grain. Whatever the studied forsterite grain or whatever the nature of the interface, the Fe‐Mg profiles in forsterite grains have the same length of about 1.5 μm. This strongly suggests that the composition profiles were formed by solid‐state diffusion during the thermal metamorphism episode. Time–temperature couples associated with the diffusion process during thermal metamorphism are deduced from profile modeling. Considering the uncertainties on the diffusion coefficient value, we found that the peak temperature in Allende is ranging from 425 to 505 °C.
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