— Calcium‐aluminum‐rich inclusions (CAIs) were among the first solids in the solar system and were, similar to chondrules, created at very high temperatures. While in chondrules, trapped noble gases ...have recently been detected, the presence of trapped gases in CAIs is unclear but could have important implications for CAI formation and for early solar system evolution in general. To reassess this question, He, Ne, and Ar isotopes were measured in small, carefully separated and, thus, uncontaminated samples of CAIs from the CV3 chondrites Allende, Axtell, and Efremovka.
The 20Ne/22Ne ratios of all CAIs studied here are <0.9, indicating the absence of trapped Ne as, e.g., Ne‐HL, Ne‐Q, or solar wind Ne. The 21Ne/22Ne ratios range from 0.86 to 0.72, with fine‐grained, more altered CAIs usually showing lower values than coarse‐grained, less altered CAIs. This is attributed to variable amounts of cosmogenic Ne produced from Na‐rich alteration phases rather than to the presence of Ne‐G or Ne‐R (essentially pure 22Ne) in the samples. Our interpretation is supported by model calculations of the isotopic composition of cosmogenic Ne in minerals common in CAIs. The 36Ar/38Ar ratios are between 0.7 and 4.8, with fine‐grained CAIs within one meteorite showing higher ratios than the coarse‐grained ones. This agrees with higher concentrations of cosmogenic 36Ar produced by neutron capture on 35Cl with subsequent β−‐decay in finer‐grained, more altered, and thus, more Cl‐rich CAIs than in coarser‐grained, less altered ones.
Although our data do not strictly contradict the presence of small amounts of Ne‐G, Ne‐R, or trapped Ar in the CAIs, our noble gas signatures are most simply explained by cosmogenic production, mainly from Na‐, Ca‐, and Cl‐rich minerals.
— The Portales Valley meteorite provides an opportunity to investigate and compare the microstructure in Fe‐Ni metal of the metallic particles in the chondritic portion and in the metal veins. The ...low‐temperature phase decomposition of Fe‐Ni metal was investigated using scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The microstructure is formed as the Portales Valley meteorite cooled from high temperatures and includes the outer taenite rim, the cloudy zone, clear taenite, and martensite. Martensite in turn decomposes into a fine admixture of fcc rods in a bcc matrix. The width of the island phase of the cloudy zone in the metal particles of the chondritic portion and the metal veins can be used to estimate a low‐temperature cooling rate. The microstructural evidence indicates that the chondritic portions and the metal veins in the Portales Valley meteorite cooled together as a mixture with a cooling rate of roughly 6.5 K/Ma.
— A transmission electron microscope (TEM) study of three coarse‐grained Type A Ca, Al‐rich inclusions (CAIs) from Allende, Acfer 082 and Acfer 086 (all CV3 chondrites) was performed in order to ...decipher their origin and effects of possible metamorphism. The constituent minerals of the CAIs are found to exhibit very similar microstructural characteristics in each of the inclusions studied. In general, the minerals show a well‐developed equilibrium texture with typical 120° triple junctions. Melilites are clearly considerably strained and characterized by high dislocation densities up to 3 × 1011 cm−2. The dislocations have Burgers vectors of 001, 110 or 011 and often form subgrain boundaries subparallel {100}. Melilite in the Allende CAI additionally contains thin amorphous lamellae mostly oriented parallel to {001}. Fassaite (Al‐Ti‐diopside) is almost featureless even on the TEM scale. Only a few subplanar dislocation walls composed of dislocations with Burgers vectors 001 and 1/2 110 were detected. Although enclosed within the highly strained melilites, the euhedral spinels contain only low dislocation densities (<2 × 104 cm−2). In the Allende CAI, spinels were found twinned on {111}. Perovskite is also characterized by a low number of linear lattice defects. All grains possess orthorhombic symmetry and are commonly twinned according to a 90° rotation around 101. Many crystals exhibit typical domain structures as well as curved twin walls where two orthogonal sets intersect. In addition to the mineral phases described above, tiny inclusions of the simple oxides CaO and TiO2 were found within melilite (CaO), spinel (CaO, TiO2) and perovskite (CaO, TiO2). Based on these observations, it is assumed that at the beginning of the formation of the CAIs a condensed solid precursor was present. Euhedral spinels poikilitically enclosed within melilites suggest that this solid aggregate was then molten. If the pure oxides represent relict condensates, their presence proves that this melting was incomplete. While still plastic, the CAIs were shocked by microimpacts causing the high dislocation densities in melilite as well as diaplectic melilite glass and twinned spinels in the Allende CAI. In Acfer 082 and 086, the deformation took place at elevated temperatures, preventing the solid phase transition and mechanical twinning. The absence of linear lattice defects in spinel, fassaite and perovskite most probably reflects inhomogeneous pressure distribution in the polycrystalline CAI as well as the different strengths of the minerals. According to cooling‐rate experiments on perovskite by Keller and Buseck (1994), the dominating (101) twins in the CAI perovskites point to cooling rates ≤50 °C/min. Finally, after crystallization of the CAI was complete, mild thermal metamorphism caused the formation of subgrain boundaries, 120° triple junctions and chemical homogenization of the melilites.
Photophoretic motion can transport illuminated particles in protoplanetary disks. In a previous paper we focused on the modeling of steady state photophoretic forces based on the compositions derived ...from tomography and heat transfer. Here, we present microgravity experiments which deviate significantly from the steady state calculations of the first paper. The experiments on average show a significantly smaller force than predicted with a large variation in absolute photophoretic force and in the direction of motion with respect to the illumination. Time-dependent modeling of photophoretic forces for heat-up and rotation show that the variations in strength and direction observed can be well explained by the particle reorientation in the limited experiment time of a drop tower experiment. In protoplanetary disks, random rotation subsides due to gas friction on short timescales and the results of our earlier paper hold. Rotation has a significant influence in short duration laboratory studies. Observing particle motion and rotation under the influence of photophoresis can be considered as a basic laboratory analog experiment to Yarkovsky and YORP effects.
Photophoresis is a physical process that transports particles in optical thin parts of protoplanetary disks, especially at the inner edge and at the optically surface. To model the transport and ...resulting effects in detail, it is necessary to quantify the strength of photophoresis for different particle classes as a fundamental input. Here, we explore photophoresis for a set of chondrules. The composition and surface morphology of these chondrules was measured by X-ray tomography. Based on the three-dimensional models, heat transfer through illuminated chondrules was calculated. The resulting surface temperature map was then used to calculate the photophoretic strength. We found that irregularities in particle shape and variations in composition induce variations in the photophoretic force. These depend on the orientation of a particle with respect to the light source. The variations of the absolute value of the photophoretic force on average over all chondrules is \(4.17\%\). The deviation between the direction of the photophoretic force and illumination is \(3.0^\circ \pm 1.5^\circ\). The average photophoretic force can be well approximated and calculated analytically assuming a homogeneous sphere with a volume equivalent mean radius and an effective thermal conductivity. We found an analytic expression for the effective thermal conductivity. The expression depends on the two main phases of a chondrule, and decreases with the amount of fine-grained devitrified, plagioclase-normative mesostasis up to factor of three. For the chondrule sample studied (Bjurb\"ole chondrite), we found a dependence of the photophoretic force on chondrule size.
In a set of 16 drop tower experiments the motion of sub-mm to mm-sized particles under microgravity was observed. Illumination by a halogen lamp induced acceleration of the particles due to ...photophoresis. Photophoresis on dust-free chondrules, on chondrules, glass spheres and metal spheres covered with SiC dust and on pure SiC dust aggregates was studied. This is the first time that photophoretic motion of mm-sized particles has been studied experimentally. The absolute values for the photophoretic force are consistent with theoretical expectations for spherical particles. The strength of the photophoretic force varies for chondrules, dust covered particles and pure dust from low to strong, respectively. The measurements support the idea that photophoresis in the early Solar System can be efficient to transport solid particles outward.
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