Silicate Grains Research Articles

Computational Mechanistic Analysis of the Formation of the Magnesium Silicate Monomers MgSiO3 and Mg2SiO4.

Silicate grains comprise a large fraction of cosmic dust, motivating a need to understand how they form. The current body of work on silicates generally reflects the abundance of silicate grains, yet models for their formation often do not consider silicate chemistry on the smallest scale, which can form species available for dust grain nucleation processes. In order to expand upon previous attempts to bridge this gap in silicate chemistry, novel gas-phase reaction pathways for the magnesium silicate monomers enstatite (MgSiO3) and forsterite (Mg2SiO4) from MgH, H2O, and SiO are presently computed using highly accurate quantum chemical calculations. MgSiO3 and Mg2SiO4 form through a series of reactions that initially excludes silicon addition, creating the elusive species MgOH and Mg2O prior to further reaction. The formation of the two silicate monomers is expected to be efficient with the primary bottleneck being the amount of MgH available for reaction. The addition of these reactions to cosmic chemical networks will add further clarity to the processes that govern dust formation, most significantly for those occurring within stellar outflows of asymptotic giant branch stars.

Understanding the multi-wavelength thermal dust polarisation from the Orion molecular cloud in light of the radiative torque paradigm

Dust grains play a key role in various astrophysical processes and serve as indicators of interstellar medium structures, density, and mass. Understanding their physical properties and chemical composition is a crucial goal in astrophysics. Dust polarisation is a valuable tool for studying these properties. The radiative torque (RAT) paradigm, which includes radiative torque alignment (RAT-A) and radiative torque disruption (RAT-D), is essential to interpreting the dust polarisation data and constraining the fundamental properties of dust grains. However, it has been used primarily to interpret observations at a single wavelength. In this study, we analyse the thermal dust polarisation spectrum obtained from observations with SOFIA/HAWC+ and JCMT/POL-2 in the Orion molecular cloud 1 (OMC-1) region and compare the observational data with our numerical results using the RAT paradigm. In general, we show that the dense gas exhibits a positive spectral slope, whereas the warm regions show a negative one. We demonstrate that a one-layer dust (one-phase) model can only reproduce the observed spectra at certain locations and cannot match those with prominent V-shaped spectra (for which the degree of polarisation initially decreases with wavelength from 54 to ~300µm and then increases at longer wavelengths). To address this, we improved our model by incorporating two dust components (warm and cold) along the line of sight, resulting in a two-phase model. This improved model successfully reproduces the V-shaped spectra. The best model corresponds to a mixture composition of silicate and carbonaceous grains in the cold medium. Finally, by assuming the plausible model of grain alignment, we were able to infer the inclination angle of the magnetic fields in OMC-1. This approach is an important step towards a better understanding the physics of grain alignment and constraining 3D magnetic fields using dust polarisation spectra.

Experimental sticking coefficients of CO and N_2 on sub-micrometric cosmic grain analogs

Measuring the sticking coefficient of molecules pertinent to astrochemistry - such as CO - on substrates that mimic interstellar dust grains is crucial for the comprehensive understanding of gas-grain chemical processes. Although astrochemical models assume a sticking coefficient of 1, recent laboratory experiments on H2O and CO2 have revealed significantly lower values when measured on small grain analogs. As the effect of grain size on molecular adsorption has been largely ignored to date, further experiments are needed to determine the accretion rates of species known to freeze out on dust grains. Our aim is to determine the sticking coefficients of CO and N2 on sub-micrometric silicate and carbon grains. By quantifying realistic sticking coefficients on these dust grain analogs, we can improve the accuracy of astrochemists' predictions of molecular abundances as affected by gas-grain interactions. The molecules of interest were added to various substrates at 10 K in an ultra-high vacuum. The amount of adsorbate that stuck to the substrate was quantified using X-ray photoelectron spectroscopy. These quantities were compared to a reference with a sticking coefficient of 1, allowing the deduction of the sticking coefficient for each substrate. The average sticking coefficients of CO and N2 on grain analogs are 0.17 for CO and 0.14 for N2 on olivine powder, and 0.05 for CO and 0.07 on N2 on soot, instead of the presumed 1. This is in line with the low values previously reported for H2O and CO2 These laboratory results indicate that CO and N2 in addition to H2O and CO2 also exhibit a low sticking coefficient on dust grain analogs. It is thus necessary to reconsider the interactions between gaseous species and dust particles as a low-efficiency process. This reduction in accretion and reaction rates has important implications for how we understand astrochemistry.

Theoretical Rotational and Vibrational Spectral Data for the Hypermagnesium Oxide Species Mg2O and Mg2O.

While magnesium is astronomically observed in small molecules, it largely serves as a contributor to silicate grains, though how these grains form is not well-understood. The smallest hypermagnesium oxide compounds (Mg O/Mg O ) may play a role in silicate formation, but little vibrational reference data exist. As such, anharmonic spectroscopic data are computed for Mg O, Mg O, and Mg O using quartic force fields (QFFs). Explicitly-correlated coupled-cluster QFFs for the neutral species perform well, implying that full multireference treatment may not be necessary for such systems if enough electron correlation is included. Equation-of-motion ionization potential (EOMIP) methods for Mg O QFFs circumvent previous symmetry breaking issues even in explicitly-correlated coupled-cluster results, motivating the need for EOMIP treatments at minimum for such systems. All three species are found to have high-intensity vibrational frequencies. Even so, the highly intense frequency ( Mg O: 894.7 cm-1/11.18 μm; Mg O: 915.0 cm-1/10.91 μm) for either neutral state may be astronomically obscured by the polycyclic aromatic hydrocarbon 11.2 μm band. Mg O may be less susceptible to such obfuscation, and its intensity is computed to be a massive 4793 km mol-1.

Galaxies with grains: unraveling dust evolution and extinction curves with hydrodynamical simulations

Dust in galaxies is an important tracer of galaxy properties and their evolution over time. The physical origin of the grain size distribution, the dust chemical composition, and, hence, the associated ultraviolet-to-optical extinctions in diverse galaxies remains elusive. To address this issue, we introduce a model for dust evolution in the RAMSES code for simulations of galaxies with a resolved multiphase interstellar medium. Dust is modelled as a fluid transported with the gas component, and is decomposed into two sizes, 5 nm and 0.1 μm, and two chemical compositions for carbonaceous and silicate grains. This dust model includes the growth of dust by accretion of elements from the gas phase and by the release of dust in stellar ejecta, the destruction by thermal sputtering, supernovae, and astration, and the exchange of dust mass between the two main populations of grain sizes by coagulation and shattering. Using a suite of isolated disc simulations with different masses and metallicities, the simulations can explore the role of these processes in shaping the key properties of dust in galaxies. The simulated Milky Way analogue reproduces the dust-to-metal mass ratio, depletion factors, size distribution and extinction curves of the Milky Way. Galaxies with lower metallicities reproduce the observed decrease in the dust-to-metal mass ratio with metallicity at around a few 0.1 Z⊙. This break in the dust-to-metal ratio corresponds to a galactic gas metallicity threshold that marks the transition from an ejecta-dominated to an accretion-dominated grain growth, and that is different for silicate and carbonaceous grains, with ≃0.1 Z⊙ and ≃0.5 Z⊙ respectively. This leads to more Magellanic Cloud-like extinction curves, i.e. with steeper slopes in the ultraviolet and a weaker bump feature at 2175 Å, in galaxies with lower masses and lower metallicities. Steeper slopes in these galaxies are caused by the combination of the higher efficiency of gas accretion by silicate relative to carbonaceous grains and by the low rates of coagulation that preserves the amount of small silicate grains. Weak bumps are due to the overall inefficient accretion growth of carbonaceous dust at low metallicity, whose growth is mostly supported by the release of large grains in SN ejecta. We also show that the formation of CO molecules is a key component to limit the ability of carbonaceous dust to grow, in particular in low-metallicity gas-rich galaxies.

Diagenetic variability in Tertiary, syn-rift mixed siliciclastic‑carbonate depositional system (Lower Musayr Formation), Red Sea, Saudi Arabia

Mixed siliciclastic‑carbonate depositional systems are prone to differential diagenesis due to lithological heterogeneity. However, unlike pure carbonate and/or siliciclastic counterparts, the diagenesis within mixed clastic‑carbonate successions remains poorly known. This study integrates sedimentological and petrographic analysis with porosity-permeability measurements to understand diagenetic variability and its impact on fluid flow in the Oligocene-Miocene mixed clastic‑carbonate system (lower part of Musayr Fm) in Midyan Basin, Red Sea, Saudi Arabia. Sedimentological observations highlight that siliciclastic intervals comprise conglomerates, coarse- to fine-grained sandstones and subordinate mudstones whereas mixed siliciclastic‑carbonate intervals are composed of shell, ooids and microbialites-dominated facies. Petrographic analysis indicates presence of several diagenetic processes in siliciclastic intervals including dissolution of unstable silicate grains (feldspar), formation of pore filling and/or grain-replacive kaolinite, precipitation of calcite and/or dolomite, and replacement of dolomite by silica. The siliciclastic strata exhibit minimal compaction fabrics with no stylolites and are characterized by higher permeability (average = 1884 md) and porosity (average = 18.7 %). On the other hand, mixed siliciclastic‑carbonate intervals underwent micritization, sparry calcite formation, dolomitization of micrite and bioclasts, and replacement of sparry calcite by pyrite and Fe-oxides. Wavy stylolite seams were also observed in ooid facies reflecting moderate chemical compaction. In addition, the porosity (average = 6.6 %) and permeability (average = 93 md) are magnitudes lower than siliciclastic counterparts. Significant differences between porosity and permeability of siliciclastic and mixed intervals are partly linked to relatively intense diagenetic alteration (higher cementation and chemical compaction) in mixed units. Understanding such diagenetic heterogeneity in mixed systems has important implications for identification of reservoir and non-reservoir zones and may provide useful insights for hydrocarbon exploration and carbon sequestration in the analogous strata in the subsurface.

Gas-phase Condensation of Carbonated Silicate Grains

Reports on the detection of carbonates in planetary nebulae (PNe) and protostars have suggested the existence of a mechanism that produces these compounds in stellar winds and outflows. A subsequent laboratory study has reported a possible mechanism by presenting the non-thermodynamic-equilibrium (TE), gas-phase condensation of amorphous silicate grains with amorphous calcium carbonate inclusions. The authors concluded that water vapor was necessary for the formation of the carbonates. We present a laboratory study with pulsed laser ablation of a MgSi target in O2 and CO2 gases and report, in the absence of water vapor, the non-TE, gas-phase condensation of amorphous carbonated magnesium silicate dust. It consists of amorphous silicate grains with the formula MgSiO3, which comprise carbonate groups homogeneously dispersed in their structure. The IR spectra of the grains show the characteristic bands of amorphous silicates and two bands at ∼6.3 and ∼7.0 μm, which we assign to the carbonate groups. The silicate bands are not significantly affected at an estimated Si:C ratio of 9:1–9:2. Such grains could form in winds and outflows of evolved stars and PNe if C atoms are present during silicate condensation. Additionally, we find that Lyα radiation dissociates the carbonate groups at the surface of the carbonated silicate grains and we estimate the corresponding photodissociation cross section of (0.04 ± 0.02) ×10−16 cm2. Therefore, photodissociation would limit the formation of carbonate groups on grains in winds and outflows of stars emitting vacuum ultraviolet photons, and the carbonates observed in protostars have not formed by gas-phase condensation.

PDRs4All

Context. Interstellar dust particles, in particular carbonaceous nano-grains (like polycyclic aromatic hydrocarbons, fullerenes, and amorphous hydrogenated carbon), are critical players for the composition, energy budget, and dynamics of the interstellar medium (ISM). The dust properties, specifically the composition and size of dust grains are not static; instead, they exhibit considerable evolution triggered by variations in local physical conditions such as the density and gas temperature within the ISM, as is the case in photon-dominated regions (PDRs). The evolution of dust and its impact on the local physical and chemical conditions is thus a key question for understanding the first stages of star formation. Aims. From the extensive spectral and imaging data of the JWST PDRs4All program, we study the emission of dust grains within the Orion Bar – a well-known, highly far-UV (FUV)-irradiated PDR situated at the intersection between cold, dense molecular clouds, and warm ionized regions. The Orion Bar because of its edge-on geometry provides an exceptional benchmark for characterizing dust evolution and the associated driving processes under varying physical conditions. Our goal is to constrain the local properties of dust by comparing its emission to models. Taking advantage of the recent JWST data, in particular the spectroscopy of dust emission, we identify new constraints on dust and further previous works of dust modelling. Methods. To characterize interstellar dust across the Orion Bar, we follow its emission as traced by JWST NIRCam (at 3.35 and 4.8 μm) and MIRI (at 7.7, 11.3, 15.0, and 25.5 μm) broad band images, along with NIRSpec and MRS spectroscopic observations. First, we constrain the minimum size and hydrogen content of carbon nano-grains from a comparison between the observed dust emission spectra and the predictions of the Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS) coupled to the numerical code DustEM. Using this dust model, we then perform 3D radiative transfer simulations of dust emission with the SOC code (Scattering with OpenCL) and compare to data obtained along well chosen profiles across the Orion Bar. Results. The JWST data allows us, for the first time, to spatially resolve the steep variation of dust emission at the illuminated edge of the Orion Bar PDR. By considering a dust model with carbonaceous nano-grains and submicronic coated silicate grains, we derive unprecedented constraints on the properties of across the Orion Bar. To explain the observed emission profiles with our simulations, we find that the nano-grains must be strongly depleted with an abundance (relative to the gas) 15 times less than in the diffuse ISM. The NIRSpec and MRS spectroscopic observations reveal variations in the hydrogenation of the carbon nano-grains. The lowest hydrogenation levels are found in the vicinity of the illuminating stars suggesting photo-processing while more hydrogenated nano-grains are found in the cold and dense molecular region, potentially indicative of larger grains.

Search for Chirality in Hydrogenated Magnesium Nanosilicates: A DFT and TD-DFT Investigation.

The formation of silicate grains in the interstellar medium (ISM), especially those containing chiral surfaces such as clinopyroxenes, is poorly understood. Moreover, silicate interactions with various forms of hydrogen-proton (H+), neutral H (HI), and molecular hydrogen (H2) are of high importance as hydrogen comprises >90% of the ISM gas budget, and these species play important roles in the formation of new molecules in space. Furthermore, silicate surfaces catalyze the formation of H2 in the interstellar medium formed on dust grain surfaces by H-H association. The technical difficulty of in situ laboratory investigations of nanosilicate nucleation using astrophysically relevant environmental conditions makes computational chemistry a useful tool for studying potential nanosilicate structures. Furthermore, chiral surfaces interacting with chiral organic molecules could serve as templates that lead to the enantiomeric excess of l-amino acids and d-polyols detected in carbonaceous meteorites. However, in order for this effect to take place, an excess of one chiral form of a mineral is required to break the symmetry. This symmetry-breaking event could have been due to the asymmetric absorption of circularly polarized light by the nanosilicate as it traverses star-forming regions. We investigate this possibility using a metastable chiral form of an enstatite dimer as an input for density functional theory (DFT) and time-dependent (TD)-DFT calculations to obtain various properties and circular dichroism spectra. All in all, twenty-six magnesium nanosilicate structures were studied using varying degrees of hydrogenation: none, with HI, with H+, and with H2. The HSE06/aug-cc-pVQZ level of theory was used for the DFT calculations. TD-DFT calculations utilized the CAM-B3LYP/cc-pVQZ and ωB97X-D3/cc-pVQZ functional and basic set pairings. Results show that (1) all twenty-six structures have absorption bands that fall within the 0.6-28.3 μm range available with the newly launched James Webb Space Telescope and (2) there is a small enantioselective effect by UV-CPL on the eight chiral enstatite dimers (predicted g-values of up to 0.007). While the observed effect is small, it opens up the possibility that it is the inorganic material that becomes enantiomerically biased by UV-CPL, driving chiral enhancements in meteoric organic molecules.

An experimental simulation of oxygen isotope exchange reaction between amorphous silicate dust and carbon monoxide gas in the early Solar System

The reaction mechanism and kinetics of oxygen isotope exchange between tens of nanometer-sized amorphous silicate grains with forsterite composition (amorphous forsterite) and low-pressure carbon monoxide (CO) gas (PCO) of 0.05–1 Pa at 643–883 K were examined to investigate oxygen isotopic evolution in the protosolar disk that led to the mass-independent oxygen isotopic variation of planetary materials. Both CO gas supply- and diffusion-controlled isotope exchange reactions were observed. At 753–883 K and PCO of 0.05–1 Pa, the supply of CO gas controls the isotope exchange reaction, and its rate is 2–3 orders of magnitude smaller than that of the H2O supply-controlled isotope exchange reaction. The diffusion-controlled isotope exchange occurred at 643–703 K and PCO of 0.3 Pa, and the reaction rate of D (m2/s) = (3.1 ± 2.3) × 10−23 exp[−41.7 ± 9.6 (kJ mol−1) R−1 (1/T − 1/1200)] was obtained.We found that the oxygen isotope exchange rates of amorphous forsterite with CO and H2O gases are larger than those of gaseous isotope exchange between CO and H2O gases at a wide range of temperatures, wherein amorphous forsterite crystallization does not precede the isotope exchange reaction of amorphous forsterite with these gases. The most sluggish isotope exchange rate between H2O and CO in the gas phase suggests that amorphous forsterite would play a role in accelerating gaseous isotopic equilibrium through the isotope exchange of amorphous forsterite with both CO and H2O. We found that the oxygen isotopic equilibrium between 0.1 μm-sized amorphous forsterite, CO, and H2O would be accomplished through the isotope exchange of amorphous forsterite at temperatures as low as ∼600–700 K in the dynamically accreting protosolar disk, which is significantly lower than expected for the case of gaseous isotope exchange (>∼800 K).

Trapped Water on Silicates in the Laboratory and in Astrophysical Environments

The existence of strongly bound water molecules on silicate surfaces, above the desorption temperature of water ice, was first predicted by computational studies and has recently been demonstrated by laboratory experiments. Such trapped water may be present in various astrophysical environments and there is now evidence for its presence in the diffuse interstellar medium (ISM) and in extraterrestrial particles. We present here new results of a laboratory study of the phenomenon of trapping (strong bonding) of water molecules by silicates. We show that the efficiency of the trapping is strongly dependent on the properties and composition of the surface. Our results point out that the presence of trapped water should be due to the hydrophilic properties of the silicate surface and that the nature of the trapping is physical (physisorption rather than chemisorption). We demonstrate that water can be trapped on silicates up to temperatures of about 470 K, which speaks for the presence of wet silicate grains in the terrestrial planet formation zone in planet-forming disks. Studying the thermal and UV stability of trapped water, we conclude that the detection of trapped water in the diffuse ISM speaks for its efficient continuous formation. We discuss our results as relevant to fundamental scientific questions, such as the oxygen depletion problem, the origin of water on Earth, and the formation of rocky planets.

Submillimeter-wavelength Polarimetry of IRC+10216

We present SCUBA-2/POL-2 850 μm polarimetric observations of the circumstellar envelope (CSE) of the carbon-rich asymptotic giant branch (AGB) star IRC+10216. Both far-IR (FIR) and optical polarization data indicate grains aligned with their long axis in the radial direction relative to the central star. The 850 μm polarization does not show this simple structure. The 850 μm data are indicative, albeit not conclusive, of a magnetic dipole geometry. Assuming such a simple dipole geometry, the resulting 850 μm polarization geometry is consistent with both Zeeman observations and small-scale structure in the CSE. While there is significant spectral-line polarization contained within the SCUBA-2 850 μm passband for the source, it is unlikely that our broadband polarization results are dominated by line polarization. To explain the required grain alignment, grain mineralogy effects, due to either fossil silicate grains from the earlier oxygen-rich AGB phase of the star or due to the incorporation of ferromagnetic inclusions in the largest grains, may play a role. We argue that the most likely explanation is due to a new alignment mechanism wherein a charged grain, moving relative to the magnetic field, precesses around the induced electric field and therefore aligns with the magnetic field. This mechanism is particularly attractive as the optical, FIR, and submillimeter-wave polarization of the carbon dust can then be explained in a consistent way, differing simply due to the charge state of the grains.

The disk of the eruptive protostar V900 Mon

Aims. In this work, we study the silicate dust content in the disk of one of the youngest eruptive stars, V900 Mon, at the highest angular resolution, probing down to the inner 10 au of said disk, and study the historical evolution of the system, traced in part by a newly discovered emission clump. Methods. We performed high angular resolution MIR interferometric observations of V900 Mon with MATISSE/VLTI with a spatial coverage ranging from 38 to 130-m baselines, and compared them to archival MIDI/VLTI data. We also mined and re-analyzed archival optical and infrared photometry of the star to study its long-term evolution since its eruption in the 1990s. We complemented our findings with integral field spectroscopy data from MUSE/VLT. Results. The MATISSE/VLTI data suggest a radial variation in the silicate feature in the dusty disk, whereby at large spatial scales (≥10 au) the protostellar disk’s emission is dominated by large-sized (≥1μm) silicate grains, while at smaller spatial scales and closer to the star (≤5 au) silicate emission is absent, suggesting self-shielding. We propose that the self-shielding may be the result of small dust grains at the base of the collimated CO outflow previously detected by ALMA. A newly discovered knot in the MUSE/VLT data, located at a projected distance approximately 27 000 au from the star, is co-aligned with the molecular gas outflow at a P.A. of 250°(±5°) consistent with the position angle and inclination of the disk. The knot is seen in emission in Hα [N II], and the [S II] doublet and its kinematic age is about 5150 yr. This ejected material could originate from a previous eruption.

Diatoms microfossils and their microstructural effects on the sensitivity of metastable silty-clayey soils

In 2013, an accident occurred in Port of Santana, located at the mouth of the Amazon River, in the State of Amapa, Brazil, comprising a large volume of soft silty-clay soil flowing very fast, for a great distance, towards the bottom of the river. Field tests performed afterwards identified the occurrence of sensitive soil layers at the site. Subsequent laboratory tests verified the presence of diatom microfossils in layers with highest liquidity indices. Owing to peculiarities of the accident, a large research programme started at PUC-Rio, aiming to investigate the role of these biomineral grains on soil behaviour. Thus, based on physical-chemical-mineralogical tests performed on samples from the site, higher sensitivity natural soils from Port of Santana were reconstituted. Chemical, mineralogical and grain size reconstitution was carried out using manufactured soils containing equal proportions of minerals as in the natural soils, but with a difference in the portion of non-plastic siliceous grains: a reconstituted soil used quartz, in silt size, and the other used diatomaceous earth. This article presents and discuss results of undrained shear strength data obtained through laboratory fall cone and vane tests, as well as microstructure analyses - including scanning electron microscopy, mercury intrusion porosimetry and X-ray microtomography - in high sensitivity natural diatomaceous soils and reconstituted soils, in the liquidity index range of 1.0 ≤ LI ≤ 1.6. It is shown the fundamental role of the diatom microfossils in microstructural metastability, impacting the high sensitivity measured in soil reconstituted with diatoms and in natural diatomaceous soils.

Magnetism of the Acapulco Primitive Achondrite and Implications for the Evolution of Partially Differentiated Bodies

AbstractPrimitive achondrites like the acapulcoites‐lodranites (AL) clan are meteorites that formed on bodies in the process of forming a metallic core, providing a unique window into how early solar system processes transformed unmelted material into differentiated bodies. However, the size and structure of the parent body of ALs and other primitive achondrites are largely unknown. Paleomagnetism can establish the presence or absence of a metallic core by looking for evidence of a dynamo field. We conducted a magnetic study of the Acapulco acapulcoite to determine its ferromagnetic minerals and their recording properties. This is the first detailed rock magnetic and first paleomagnetic study of a primitive achondrite group. We determined that metal inclusions inside silicate grains consist of two magnetic minerals, kamacite and tetrataenite, which have robust recording properties. However, the mechanisms and timing by which these minerals acquired any natural remanent magnetization are unknown. Despite this, Acapulco has not been substantially remagnetized since arriving on Earth and therefore should retain a record dating to 4.55 billion years ago. Future studies could characterize this record by using high‐resolution magnetometry measurements of individual populations of grains and developing an understanding of how and when they became magnetized. Our discovery of tetrataenite in ALs provides the first mineralogical evidence for slow cooling [<∼5–10 × 103°C per million years (Ma−1)] of the AL parent body at low temperatures (∼320°C). Its presence suggests the AL parent body is unlikely to have been catastrophically disrupted at AL peak temperatures (∼1,200°C) without subsequent reaccretion.

Depositional and sequence stratigraphic controls on diagenesis in the Upper Cambrian-Lower Ordovician Barik Formation, central Oman: Implications for prediction of reservoir porosity in a hybrid-energy delta system

This study aims to employ an integrated depositional and sequence stratigraphic approach to assess the control of diagenesis on reservoir porosity of a hybrid-energy delta system. The study focuses on examining the outcropped Upper Cambrian-Lower Ordovician Barik Formation representing a highstand system tract of a hybrid-energy delta in the Haushi-Huqf region of Central Oman, which is equivalent to the subsurface gas reservoirs within the Interior Oman Salt Basins. The petrographic, mineralogical, and geochemical assessments revealed that the Barik Formation primarily consists of moderately to very well-sorted coarse-grained siltstones to fine-grained sandstones, with a composition mainly comprisingfeldspathic- and subfeldspathic arenite and the reservoir porosity is controlled by various degrees of diagenetic processes. Eodiagenetic alterations comprise mechanically infiltrated clays and kaolinitization of silicate grains, including detrital feldspars, micas, and mud matrix. The mechanically infiltrated clays (e.g., smectite) were introduced into the sandstones by the percolation of muddy waters under the control of tidal pumps. Although these clays may affect reservoir porosity during eodiagenesis, they positively preserve reservoir porosity during deep burial (mesodiagenesis) by inhibiting cementation by feldspar and quartz overgrowths. Kaolitization of silicate detrital grains was facilitated by the influx of meteoric waters into the sandstones as the delta progrades during the highstand system tract. During progressive burial and lack of extensive eodiagenetic cement, compaction continued and reduced reservoir porosity. Illitization and chloritization of the mechanically infiltrated smectitic clays took place; however, they limited the cementation by quartz and feldspar overgrowths. The quartz and feldspar overgrowths on detrital grains with discontinued smectitic clay coats supported the detrital grains from further compaction and, thus, helped in reservoir porosity preservation. Moreover, extensive feldspar dissolution and kaolinitization has occured during mesodiagenesis and enhanced the reservoir porosity. Subordinate amounts of calcite, dolomite, and gypsum cement have a minor reduction in the reservoir porosity of the Barik Formation. Hence, it can be concluded that, on the one hand, lack of eodiagenetic cementation favored intense reservoir porosity reduction due to compaction. Nevertheless, on the other hand, the mechanically infiltrated clays during the eodiagenetic stage helped preserve the reservoir porosity by hindering the formation of quartz and feldspar overgrowths during the mesodiagenetic stage. This study may serve as an analog model for a similar hybrid-energy delta sandstone reservoirs to better understand the type, and spatial and temporal of diagenetic alterations and their role in controlling the reservoir quality in deeply buried sandstone reservoirs.

Rapid Dust Growth during Hydrodynamic Clumping due to Streaming Instability

Streaming instability is considered to be one of the dominant processes in promoting planetesimal formation by the gravitational collapse of dust clumps. The development of streaming instability is expected to form dust clumps in which the local dust density is strongly enhanced and even greater than the Roche density. The resulting clumps can collapse to form planetesimals. Recent simulations showed that such strong clumping occurs in a wider parameter space than previously expected. However, the indicated timescale for strong clumping can be on the order of tens to hundreds of Keplerian periods. In this paper, we estimate the growth time of dust grains during the pre-clumping phase. We find that the dust growth proceeds considerably before the strong clumping because even the moderate clumping due to streaming instability increases the local dust-to-gas ratio to ≳10. Depending on the sound speed, the dust collision velocity can be kept below ∼1 m s−1 once sufficiently strong dust clumping occurs. Thus, even silicate grains might have the potential to grow safely toward the size whose Stokes number is unity during the clumping. Our results demonstrate the importance of local dust coagulation during dust clumping due to streaming instability.

Nature and timing of a significant reduction event on the L‐chondrite parent asteroid

AbstractAbout 17% of L6 chondrites (15/87) show significant reduction features in BSE images in thin section. Because some thin sections of these meteorites do not show reduction features, this percentage is a lower limit. Reduction features include: (1) 4–5‐μm‐thick BSE‐dark reduction rims on olivine and orthopyroxene grains and along fracture boundaries in these grains, (2) 4–12‐μm‐thick dark bands (probably poorly crystalline pyrrhotite) at the margins and along fractures in troilite grains, and (3) 2–5‐μm‐thick dark rinds of kamacite around some taenite grains. Only one of 70 L‐group chondrites (1.4%) of lower petrologic type exhibits minor reduction. The L6 chondrites showing major reduction have 40Ar/39Ar plateau ages ranging from 156 ± 1 Ma for Guangnan to 4543 ± 3 Ma for Thamaniyat Ajras. Reduction occurred after silicate, sulfide, and metal grains had attained their present sizes during parent‐body thermal metamorphism (and had been fractured by parent‐body collisions). The precise plateau age of Thamaniyat Ajras probably marks the timing of the L6 reduction event. It seems likely the reductant was a low‐viscosity fluid, plausibly CO, derived from oxidation of poorly graphitized and amorphous carbon within fine‐grained matrix. Water‐ice that had accreted to the L‐chondrite asteroid was heated and mobilized during metamorphism, causing oxidation. After peak metamorphism, ~75% of the water had been used up or lost; the remaining water facilitated continuing graphite oxidation so that, after this point, overall reduction effects exceeded those of oxidation. L chondrites of lower petrologic type were less affected by reduction due to their lower metamorphic temperatures.

Descriptors for binding energies at clusters: The case of nanosilicates as models of interstellar dust grains.

Binding energies of radicals and molecules at dust grain surfaces are important parameters for understanding and modeling the chemical inventory of interstellar gas clouds. While first-principles methods can reliably be used to compute such binding energies, the complex structure and varying sizes and stoichiometries of realistic dust grains make a complete characterization of all adsorption sites exposed by their surfaces challenging. Here, we focus on nanoclusters composed of Mg-rich silicates as models of interstellar dust grains and two adsorbates of particular astrochemical relevance; H and CO. We employ a compressed sensing method to identify descriptors for the binding energies, which are expressed as analytical functions of intrinsic properties of the clusters, obtainable through a single first-principles calculation of the cluster. The descriptors are identified based on a diverse training dataset of binding energies at low-energy structures of nanosilicate clusters, where the latter structures were obtained using a first-principles-based global optimization method. The composition of the descriptors reveals how electronic, electrostatic, and geometric properties of the nanosilicates control the binding energies and demonstrates distinct physical origins of the bond formation for H and CO. The predictive performance of the descriptors is found to be limited by cluster reconstruction, e.g., breaking of internal metal-oxygen bonds, upon the adsorption event, and strategies to account for this phenomenon are discussed. The identified descriptors and the computed datasets of stable nanosilicate clusters along with their binding energies are expected to find use in astrochemical models of reaction networks occurring at silicate grain surfaces.

Abundant presolar grains and primordial organics preserved in carbon-rich exogenous clasts in asteroid Ryugu.

Preliminary analyses of asteroid Ryugu samples show kinship to aqueously altered CI (Ivuna-type) chondrites, suggesting similar origins. We report identification of C-rich, particularly primitive clasts in Ryugu samples that contain preserved presolar silicate grains and exceptional abundances of presolar SiC and isotopically anomalous organic matter. The high presolar silicate abundance (104 ppm) indicates that the clast escaped extensive alteration. The 5 to 10 times higher abundances of presolar SiC (~235 ppm), N-rich organic matter, organics with N isotopic anomalies (1.2%), and organics with C isotopic anomalies (0.2%) in the primitive clasts compared to bulk Ryugu suggest that the clasts formed in a unique part of the protoplanetary disk enriched in presolar materials. These clasts likely represent previously unsampled outer solar system material that accreted onto Ryugu after aqueous alteration ceased, consistent with Ryugu's rubble pile origin.