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著者人数: 13名, Accepted: 2018-07-23, 資料番号: SA1180184000

A comprehensive study of the organic chemistry and mineralogy of an ultracarbonaceous micrometeorite (UCAMM D05IB80) collected from near the Dome Fuji Station, Antarctica, was carried out to understand the genetic relationship among organic materials, silicates, and water. The micrometeorite is composed of a dense aggregate of ∼5 µm-sized hollow ellipsoidal organic material containing submicrometer-sized phases such as glass with embedded metal and sulfides (GEMS) and mineral grains. There is a wide area of organic material (∼15 × 15 μm) in its interior. Low-Ca pyroxene is much more abundant than olivine and shows various Mg/(Mg + Fe) ratios ranging from ∼1.0 to 0.78, which is common to previous works on UCAMMs. By contrast, GEMS grains in this UCAMM have unusual chemical compositions. They are depleted in both Mg and S, which suggests that these elements were leached out from the GEMS grains during very weak aqueous alteration, without the formation of phyllosilicates. The organic materials have two textures—smooth and globular with an irregular outline—and these are composed of imine, nitrile and/or aromatic nitrogen heterocycles, and amide. The ratio of nitrogen to carbon (N/C) in the smooth region of the organics is ∼0.15, which is five times higher than that of insoluble organic macromolecules in types 1 and 2 carbonaceous chondritic meteorites. In addition, the UCAMM organic materials are soluble in epoxy and are thus hydrophilic; this polar nature indicates that they are very primitive. The surface of the material is coated with an inorganic layer, a few nanometers thick, that consists of C, O, Si, S, and Fe. Sulfur is also contained in the interior, implying the presence of organosulfur moieties. There are no isotopic anomalies of D, 13C, or 15N in the organic material. Interstellar photochemistry alone would not be sufficient to explain the N/C ratio of the UCAMM organics; therefore, we suggest that a very small amount of fluid on a comet must have been necessary for the formation of the UCAMM. The GEMS grains depleted in Mg and S in the UCAMM prove a very weak degree of aqueous alteration; weaker than that of carbonaceous chondrites. Short-duration weak alteration probably caused by planetesimal shock locally melted cometary ice grains and released water that dissolved the organics; the fluid would likely have not mobilized because of the very low thermal conductivity of the porous icy body. This event allowed the formation of the large organic puddle of the UCAMM, as well as organic matter sulfurization, formation of thin membrane-like layers of minerals, and deformation of organic nanoglobules., アクセプト後にタイトル・アブストラクト等変更あり、著者最終稿は変更前のタイトル"Formation of an Ultracarbonaceous Antarctic Micrometeorite through Minimum Aqueous Alteration in a Small Porous Icy Body", This work was supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Culture, Sports, Science and Technology (No. 22224010, PI: H. Nagahara). The STXM facility at the beamline 5.3.2.2, ALS, is supported by the Department of Energy, Basic Energy Sciences Program.

The pressure response of lattice thermal conduction in MgO periclase has been a matter of interest for many decades to estimate the thermal conductivity profile of the lower mantle. Using the pulsed light heating thermoreflectance technique, we measured the lattice thermal diffusivity of MgO at pressures up to 137 GPa at 300 K to determine its lattice thermal conductivity under deep lower mantle conditions. Considering the temperature effect estimated by previous high-temperature measurements, we calculated the lattice part of the thermal conductivity of MgO to be 17.9 ± 1.1 W/m/K at 135 GPa and 3600 K. Additionally, we observed that the lattice conductivity of MgO has little dependence on its grain size under core-mantle boundary conditions.

The present study aims to constrain the composition of the initial lunar magma ocean (LMO) with fluid dynamic and thermodynamic consideration. A plausible range of the initial LMO composition is investigated by developing an incremental polybaric fractional crystallization model with variable fractionation efficiency to satisfy three conditions for the anorthosite crust formation: (1) the amount of anorthite crystallized from the LMO is abundant enough to form the crust with the observed thickness, (2) the Mg# (=Mg/(Mg + Fe)) of orthopyroxene crystallized with anorthite in the cooling LMO is consistent with that observed in the lunar highland rocks, ferroan anorthosite, and (3) crystallized anorthite separated to float in the turbulent LMO. A plausible range of FeO and Al 2 O 3 contents of the bulk LMO is successfully constrained as a crescent region tight for FeO and loose for Al 2 O 3 . The FeO content must be higher than 1.3 times the bulk silicate Earth (BSE) and lower than 1.8 ×BSE unless the Al 2 O 3 content of the Moon is extremely higher than the Earth. These upper and lower limits for FeO are positively correlated with the initial Al 2 O 3 content and fractionation efficiency. The FeO–rich LMO composition may suggest that the circum-Earth disk just after the giant impact of the Earth–Moon system formation was more oxidizing or the impactor was richer in FeO than the Earth’s mantle.

Corundum, the thermodynamically stable phase of alumina (Al2O3), is one of the most refractory dust species to condense around evolved stars. Presolar alumina in primitive chondrites has survived various kinds of processing in circumstellar environments, the interstellar medium (ISM), the Sun’s parent molecular cloud, and the protosolar disk. The morphology and crystal structure of presolar alumina grains may reflect their formation and evolution processes, but the relative importance of these two types of processes is poorly understood. In this study, we performed detailed morphological observations of 185 alumina grains extracted from unequilibrated ordinary chondrites (Semarkona, Bishunpur, and RC075). We also performed electron back-scattered diffraction analyses of 122 grains and oxygen isotopic analyses of 107 grains. Dissolution experiments on corundum and transition alumina phases were carried out to examine the possibility of the alteration of surface structures of alumina grains by the chemical separation procedures of chondrites. The average size of the alumina grains was 1 μm, and neither whiskers nor extremely flat grains were observed. About one-third of the grains had smooth surfaces, while ∼60% of the grains had rough surfaces with 10–100 nm-sized fine structures. The rough-surface grains have varieties of morphology and crystallinity, suggesting that the rough surface structures are secondary in origin. Electron back-scattered diffraction patterns from 95% of alumina grains matched with α-Al2O3 (corundum), and more than 75% of the alumina grains are single crystals of corundum. Nine presolar alumina grains with anomalous oxygen isotopic compositions were found among 107 alumina grains, and most of them were characterized by rough surface structures. While most of the presolar alumina grains were corundum, the relative abundance of amorphous or low-crystallinity grains is higher in presolar alumina grains than in solar alumina grains. The dissolution experiments showed that all phases except for corundum dissolved during the acid treatments of chondrites. This suggests that smooth surface structures of corundum grains were originally formed in space, and that original surfaces of alumina that had been damaged by energetic particle irradiation in the ISM or the protosolar disk were lost during chemical separations to form the rough surface structures, and that amorphous or low-crystallinity alumina grains in chondrites have acid-resistant structures different from sol–gel-synthesized amorphous alumina. The present results also imply the possible presence of acid-soluble alumina phases, undiscovered by chemical separations, in chondrites.

The interdiffusion coefficient of Mg–Fe in olivine (D Mg–Fe) was obtained at 1,400–1,600 °C at the atmospheric pressure with the oxygen fugacity of 10−3.5–10−2 Pa using a diffusion couple technique. The D Mg–Fe shows the anisotropy (largest along the [001] direction and smallest along the [100] direction), and its activation energy (280–320 kJ/mol) is ~80–120 kJ/mol higher than that estimated at lower temperatures. The D Mg–Fe at temperatures of >1,400 °C can be explained by the cation-vacancy chemistry determined both by the Fe3+/Fe2+ equilibrium and by the intrinsic point defect formation with the formation enthalpy of 220–270 kJ/mol depending on the thermodynamical model for the Fe3+/Fe2+ equilibrium in olivine. The formation enthalpy of 220–270 kJ/mol for the point defect (cation vacancy) in olivine is consistent with that estimated from the Mg self-diffusion in Fe-free forsterite. The increase in the activation energy of D Mg–Fe at >1,400 °C is thus interpreted as the result of the transition of diffusion mechanism from the transition metal extrinsic domain to the intrinsic domain at the atmospheric pressure.

High-precision oxygen three-isotope measurements of olivine and pyroxene were performed on 33 chondrules in the Yamato 81020 CO3.0 chondrite by secondary ion mass spectrometry. In chondrules where oxygen isotopes were measured in both olivine and pyroxene, the majority of grains have similar values, indicating co-magmatic crystallization. However, many chondrules contain relict grains with unique oxygen isotope ratios. A striking feature of Yamato 81020 chondrules is a bimodal distribution of oxygen isotope ratios, as those with Mg# >97 phenocrysts range in Δ17O from −4.8‰ to −6.5‰ (“−5.5‰” group), and those with Mg# 96–36 phenocrysts have Δ17O values of −2.1‰ to −3.0‰ (“−2.5‰” group). A single Mg# 99.6 barred olivine chondrule has a Δ17O of −3.3‰. We discuss that Δ17O ∼−5.5‰ chondrules are derivative of a reservoir with limited dust enrichment (100× Solar System), which yielded a relatively reduced chondrule-forming environment. In contrast, the Δ17O ∼−2.5‰ chondrules may have been influenced by 16O-poor H2O ice that sublimed and then homogenized with precursor material. The addition of H2O, when combined with high dust enrichment (1000× Solar System) and greater bulk Fe content, could have induced an oxidized environment at high temperatures, forming Mg# 96–36 chondrules. Among the 33 chondrules studied, the Al–Mg relative ages of 20 had been obtained previously. Comparing the oxygen isotope ratios and the 26Al ages of these chondrules, it is likely that the “−5.5‰” and “−2.5‰” oxygen isotope reservoirs existed contemporaneously. This implies that the snow line was spatially fixed during chondrule formation, and separated the CO chondrite accretion region into two distinct volumes of precursors.

— The role of oxygen isotope exchange during evaporation and condensation of silicate melt is quantitatively evaluated. Silicate dusts instantaneously heated above liquidus temperature are assumed to cool in gas and experience partial evaporation and subsequent recondensation. The results show that isotopic exchange effectively suppresses mass-dependent O-isotope fractionation even if the degree of evaporation is large, which is the fundamental difference from the case without isotopic exchange. The final composition of silicate melt strongly depends on the initial abundance of oxygen in the ambient gas relative to that in silicate dust, but not on the cooling rate of the system. The model was applied to O-isotope evolution of silicate melts in isotopically distinct gas of the protoplanetary disk. It was found that deviation from a straight mixing line toward the δ18O-rich side on the three-oxygen isotope diagram is inevitable when mass-dependent fractionation and isotopic exchange take place simultaneously; the degree of deviation depends on the abundance of oxygen in an ambient gas and isotopic exchange efficiency. The model is applied to explain O-isotopic compositions of igneous CAIs and chondrules.

Congruent evaporation of a crystalline material in vacuum is an extreme reaction in that backward reactions and transport processes in the reactant can be neglected. The evaporation is strongly governed by surface processes and intrinsic nature of the substance. A thorough knowledge of the atomistic evaporation mechanism is fundamental for better understanding reaction kinetics between gas and condensed materials in general. We have conducted a series of evaporation experiments of forsterite in vacuum for crystallographically oriented surfaces at 1500 to 1810 °C. The (100), (010), and (001) surfaces developed their own morphology characterized by evaporation pits and grooves originated from dislocation outcrops. Nominal overall evaporation rate (average retreat rate of a surface) shows significant anisotropy with the maximum difference by a factor of five below 1740 °C. The overall evaporation rates for individual surfaces are fitted with respective Arrhenius relationships, giving the highest activation energy for (100), intermediate for (001), and the lowest for (010). The anisotropy decreases to within 50% at ~1800 °C, which is caused by enhancement of evaporation from (010) owing to preferential evaporation around dislocation outcrops. “Intrinsic evaporation rates” estimated by subtracting contributions of initial roughness and the preferential evaporation around dislocations from the nominal overall evaporation rates show substantial anisotropy even at ~1800 °C. The “intrinsic evaporation rate” for (010) is adequately fitted by an Arrhenius relationship over the examined temperature range giving a single activation energy of 655 kJ/mol. The prevalence of steps with submicrometer to nanometer-scale height shows that forsterite evaporates mostly by layer-by-layer mechanism. The only exception is the (001) surface above ~1650 °C, on which such steps are absent except for surface-parallel minor facets which are rapidly diminishing with time. The (001) surface is inferred to evaporate by direct detachment mechanism at high temperatures. The change of evaporation mechanisms for (001) at around 1650 °C corresponds to a rough-smooth transition kinetically induced by an atomistic evaporation process.

We report high precision SIMS oxygen three isotope analyses of 36 chondrules from some of the least equilibrated LL3 chondrites, and find systematic variations in oxygen isotope ratios with chondrule types. FeO-poor (type I) chondrules generally plot along a mass dependent fractionation line (D 17 O 0.7&), with d 18 O values lower in olivine-rich (IA) than pyroxenerich (IB) chondrules. Data from FeO-rich (type II) chondrules show a limited range of d 18 O and d 17 O values at d 18 O = 4.5&, d 17 O = 2.9&, and D 17 O = 0.5&, which is slightly 16 O-enriched relative to bulk LL chondrites (D 17 O 1.3&). Data from four chondrules show 16 O-rich oxygen isotope ratios that plot near the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Glass analyses in selected chondrules are systematically higher than co-existing minerals in both d 18 O and D 17 O values, whereas high-Ca pyroxene data in the same chondrule are similar to those in olivine and pyroxene phenocrysts. Our results suggest that the LL chondrite chondrule-forming region contained two kinds of solid precursors, (1) 16 O-poor precursors with D 17 O > 1.6& and (2) 16 O-rich solid precursors derived from the same oxygen isotope reservoir as carbonaceous chondrites. Oxygen isotopes exhibited open system behavior during chondrule formation, and the interaction between the solid and ambient gas might occur as described in the following model. Significant evaporation and recondensation of solid precursors caused a large mass-dependent fractionation due to either kinetic or equilibrium isotope exchange between gas and solid to form type IA chondrules with higher bulk Mg/Si ratios. Type II chondrules formed under elevated dust/gas ratios and with water ice in the precursors, in which the ambient H2O gas homogenized chondrule melts by isotope exchange. Low temperature oxygen isotope exchange may have occurred between chondrule glasses and aqueous fluids with high D 17 O (5&) in LL the parent body. According to our model, oxygen isotope ratios of chondrules were strongly influenced by the local solid precursors in the proto-planetary disk and the ambient gas during chondrule melting events. 2010 Elsevier Ltd. All rights reserved.

Crystalline silicates are observed in many protoplanetary disks and some dust shells around evolved stars. The peak positions of infrared (IR) spectra of forsterite, which is the most abundant circumstellar silicate, vary with dust temperature, composition, size, and crystallinity. However, there is another important factor that affects IR spectra, which is the shape with a specific crystallographic orientation called the crystallographically anisotropic shape. We focused on anisotropic evaporation of crystalline forsterite as one of the possible processes that change the crystallographically anisotropic shape of forsterite grains, and carried out evaporation experiments of single crystals of forsterite in hydrogen gas (0.01-10 Pa) and at temperatures of 1150-1660°C. Forsterite evaporated anisotropically in all experimental conditions, and the anisotropy depended on temperature and hydrogen gas pressure. The results enabled us to calculate crystallographically anisotropic shapes of heated forsterite as a function of temperature and hydrogen pressure, and their corresponding IR spectra. Distinctly, different sets of peak positions were seen in IR spectra of grains with different combination of shapes and their orientation reflecting the heating conditions. The results were applied to the IR spectrum of a protoplanetary disk, HD100546, which suggests that forsterite dust particles that experienced evaporation exist with dominant secondarily fragmented forsterite formed by small-body collisions. We propose that detailed IR spectroscopy of forsterite, and probably other anisotropic crystals, is a new tool to estimate temperature and pressure conditions of circumstellar environments where dust formed.

We have conducted systematic investigations of formation age, chemical compositions, and mineralogical characteristics of ferromagnesian chondrules in Yamato-81020 (CO3.05), one of the most primitive carbonaceous chondrites, to get better understanding of the origin of chemical groups of chondrites. The 26 Al– 26 Mg isotopic system were measured in fourteen FeO-poor (Type I), six FeO-rich (Type II) and two aluminum-rich (Al-rich) chondrules using a secondary ion mass spectrometer. Excesses of 26 Mg in plagioclase (1.0–13.5‰) are resolved with sufficient precision (mostly 0.4–6.6‰ at 2 σ level) in all the chondrules studied except one. Chemical zoning of Mg and Na in plagioclase were investigated in detail in order to evaluate the applicability of 26 Al– 26 Mg chronometer. We conclude that the Al–Mg isotope system of the chondrules in Y-81020 have not been disturbed by parent-body metamorphism and can be used as chronometer assuming homogeneous distribution of 26 Al. Assuming an initial 26 Al/ 27 Al ratio of 5 × 10 −5 in the early solar system, 26 Al– 26 Mg ages were found to be 1.7–2.5 Ma after CAI formation for Type I, 2.0–3.0 Ma for Type II and 1.9 and 2.6 Ma for Al-rich chondrules. The formation ages of ferromagnesian chondrules in Y-81020 are in good agreement with those of L and LL (type 3.0–3.1) chondrites in the literature, which indicates that common chondrules in the CO chondrite were formed contemporaneously with those in L and LL chondrites. The concurrent formation of chondrules of CO and L/LL chondrites suggests that the chemical differences between CO and L/LL chondrites might be caused by spatial separation of chondrule formation environments in the protoplanetary disk.

Two glassy refractory Al-rich chondrules in Semarkona (LL3.0), the most primitive unequilibrated ordinary chondrite, provide direct evidence for condensation of Si and Mg on melt droplets during cooling. The chondrules are completely rounded, rich in Ca and Al, and poor in Fe and alkalis. They have extraordinarily abundant glass (70–80 vol%) with a subordinate amount of forsterite as the only crystalline phase that occurs mostly rimming the chondrule edge. The groundmass glass is concentrically zoned in terms of Si with an outward increase, which is overlapped with local heterogeneity of Mg and Al induced by crystallization of forsterite. The outward increase of Si, mostly compensated by Al, cannot be formed solely by crystallization of forsterite from a homogeneous melt in a closed system. Combined with skeletal or dendritic morphology and sector zoning of forsterite, it is suggested that Si condensed onto totally molten droplets (“initial melts”) accompanied by nucleation and rapid growth of forsterite with lowering temperature. The “initial melts”, the compositions of which were estimated from the Ca contents of the first crystallized forsterite, are very similar to Type C CAI but are notably poorer in Mg and Si than the bulk chondrules, indicating condensation of Mg in addition to Si with an atomic ratio of Mg:Si ∼ 3:2. The condensation after the nucleation of forsterite took place below ∼1300 °C under cooling at ∼70 °C/h and amounted to 30 wt% of the current chondrule. This study suggests a model that a short-time and local shock heating event induced melting of Type C CAI and concomitant evaporation of dusts, ferromagnesian chondrules of earlier generation, and their fragments to generate Mg and Si-rich gas, which condensed onto the melt droplets upon cooling accompanying condensation of Type I chondrules.

Accepted: 2016-08-02, 資料番号: SA1160151000

Symplectites, fine vermicular intergrowth (< 10 μm) of orthopyroxene, clinopyroxene, and spinel, and the sur- rounding lenticular coarser-grained (100-200 μm) aggregate (seam) with the same mineral assemblage of the symplectites define remarkable foliation and lineation in spinel lherzolites of the Horoman complex, northern Japan. They are inferred to be products of reaction between garnet and olivine during decompression of the host peridotite accompanying deformation. Microstructure of a symplectite was investigated with automated electron backscattered diffraction (EBSD) analysis using a field -emission gun SEM (FE-SEM) in order to clarify reaction and deformation mechanisms and thereby better constraining mechanical interaction between the Earth's upper mantle and lower crust. The symplectite is composed of two segments with a large misorien- tation angle of ~ 60° only for spinel, and the two spinel crystals are in mirror symmetry with the segment boundary approximately parallel to the mirror plane. The segment boundary is interpreted as spinel law twin formed during phase transition from garnet. Each segment is further subdivided into several sectors with grad- ual lattice distortion smaller than a few degrees/mm and intra-sector misorientation mostly smaller than 25° for all constituent minerals and with misorientation axes nearly perpendicular to the lineation and parallel to the foliation. The sector boundaries are inferred to be subgrain boundaries formed by dislocation creep of py- roxenes and spinel in the spinel stability field. The spinel twin suggests that a garnet was decomposed directly into entangled aggregate of pyroxenes and spinel, which grew from an embryo nucleated on the surface of the reactant garnet. The symplectite minerals in each sector show systematic crystallographic orientations (topo- taxy) with each other. The topotaxial relationship in the fine intergrowth with subgrain structure demonstrates that the systematic crystallographic orientations were acquired when the crystals grew by decomposing the garnet and were later modified by deformation during the consecutive ascent of the complex.

第6回極域科学シンポジウム[OA] 南極隕石11月17日（火） 国立国語研究所 2階 講堂

Evaporation of solid materials under low-pressure conditions could play important roles in chemical and isotopic fractionations in the early solar system. We have studied anisotropy of isotopic fractionation of 26Mg and 25Mg during kinetic evaporation of forsterite (Mg2SiO4), which is potentially a powerful tool to understand thermal histories of crystals in the early solar system. Ion-microprobe depth profiling revealed that the Mg isotopic zoning profiles of forsterite evaporated at 1500–1700 °C are notably differing along the a-, b-, and c-axes, which can be attributed to anisotropy in self-diffusion coefficient of Mg (D) and an isotopic fractionation factor for evaporation of Mg (α). The D and α were obtained from zoning profiles by applying the diffusion-controlled isotopic fractionation model of Wang et al. [1999. Evaporation of single crystal forsterite: Evaporation kinetics, magnesium isotope fractionation, and implications of mass-dependent isotopic fractionation of a diffusion-controlled reservoir. Geochim. Cosmochim. Acta 63(6), 953–966.]. The D is largest and smallest along the a- and c-axes, respectively. The activation energy of 560–670 kJ/mol indicates that Mg diffusion at 1500–1700 °C occurred in the intrinsic diffusion regime. The α seems to be larger along the a- or c-axes than along the b-axis. The α along the a- or c-axes show weak temperature dependence. The α along all the crystallographic orientations is closer to unity than that expected from the kinetic theory of gases. These lines of evidence suggest that surface processes such as breaking of bonds and surface diffusion are responsible for the isotopic fractionation.

We have studied the relationship between bulk chemical compositions and relative formation ages inferred from the initial 26Al/27Al ratios for sixteen ferromagnesian chondrules in least equilibrated ordinary chondrites, Semarkona (LL3.0) and Bishunpur (LL3.1). The initial 26Al/27Al ratios of these chondrules were obtained by Kita et al. (2000) and Mostefaoui et al. (2002), corresponding to relative ages from 0.7 ± 0.2 to 2.4 -0.4/+0.7 Myr after calcium-aluminum-rich inclusions (CAIs), by assuming a homogeneous distribution of 26Al in the early solar system. The measured bulk compositions of the chondrules cover the compositional range of ferromagnesian chondrules reported in the literature and, thus, the chondrules in this study are regarded as representatives of ferromagnesian chondrules. The relative ages of the chondrules appear to correlate with bulk abundances of Si and the volatile elements (Na, K, Mn, and Cr), but there seems to exist no correlation of relative ages neither with Fe nor with refractory elements. Younger chondrules tend to be richer in Si and volatile elements. Our result supports the result of Mostefaoui et al. (2002) who suggested that pyroxene-rich chondrules are younger than olivine-rich ones. The correlation provides an important constraint on chondrule formation in the early solar system. It is explained by chondrule formation in an open system, where silicon and volatile elements evaporated from chondrule melts during chondrule formation and recondensed as chondrule precursors of the next generation.

Motivated by recent detection of transiting high-density super-Earths, we explore the detectability of hot rocky super-Earths orbiting very close to their host stars. In the environment hot enough for their rocky surfaces to be molten, they would have the atmosphere composed of gas species from the magma oceans. In this study, we investigate the radiative properties of the atmosphere that is in the gas/melt equilibrium with the underlying magma ocean. Our equilibrium calculations yield Na, K, Fe, Si, SiO, O, and O$_2$ as the major atmospheric species. We compile the radiative-absorption line data of those species available in literature, and calculate their absorption opacities in the wavelength region of 0.1--100~$\mathrm{\mu m}$. Using them, we integrate the thermal structure of the atmosphere. Then, we find that thermal inversion occurs in the atmosphere because of the UV absorption by SiO. In addition, we calculate the ratio of the planetary to stellar emission fluxes during secondary eclipse, and find prominent emission features induced by SiO at 4~$\mathrm{\mu m}$ detectable by Spitzer, and those at 10 and 100~$\mathrm{\mu m}$ detectable by near-future space telescopes., Comment: 15 pages, 12 figures, accepted for publication in ApJ

We performed a systematic high precision SIMS 26Al-26Mg isotopic study for 11 ferromagnesian chondrules from the highly unequilibrated ordinary chondrite Bishunpur (LL3.1). The chondrules are porphyritic and contain various amounts of olivine and pyroxene and interstitial plagioclase and/or glass. The chemical compositions of the chondrules vary from FeO-poor to FeO-rich. Eight chondrules show resolvable 26Mg-excesses with a maximum δ26Mg of ~1% in two chondrules. The initial 26Al/27Al ratios inferred for these chondrules range between (2.28 ± 0.73) x 10^(-5) to (0.45 ± 0.21) x 10^(-5). Assuming a homogeneous distribution of Al isotopes in the early solar system, this range corresponds to ages relative to CAIs between 0.7 ± 0.2 My and 2.4^(-0.4/+0.7) My. The inferred total span of the chondrule formation ages is at least 1 My, which is too long to form chondrules by the X-wind. The initial 26Al/27Al ratios of the chondrules are found to correlate with the proportion of olivine to pyroxene suggesting that olivine-rich chondrules formed earlier than pyroxene-rich chondrules. Though we do not have a completely satisfactory explanation of this correlation we tentatively interpret it as a result of evaporative loss of Si from earlier generations of chondrules followed by addition of Si to the precursors of later generation chondrules.

The mass-dependent fractionation laws that describe the partitioning of isotopes are different for kinetic and equilibrium reactions. These laws are characterized by the exponent relating the fractionation factors for two isotope ratios such that α2/1 = α3/1β. The exponent β for equilibrium exchange is (1/m1 − 1/m2)/(1/m1 − 1/m3), where mi are the atomic masses and m1

Variations in bulk Mg/Si ratios in the various groups of chondritic meteorites indicate that Mg/Si fractionation occurred in the primitive solar nebula. Enstatite (MgSiO3) evaporates incongruently forming forsterite (Mg2SiO4) as an evaporation residue; therefore, evaporation of enstatite produces Mg/Si variations in solid (Mg-rich) and gas (Si-rich) and must be considered as a probable process responsible for Mg/Si fractionation recorded in chondrites. To understand the evaporation kinetics of enstatite, incongruent evaporation experiments on enstatite single crystals have been carried out in vacuum and in hydrogen gas at temperatures of 1300 to 1500°C. A polycrystalline forsterite layer is formed on the surface of enstatite by preferential evaporation of the SiO2 component, both in vacuum and in hydrogen gas. The thickness of the forsterite layer in vacuum increases with time in the early stage of evaporation and later the thickness of the forsterite layer remains constant (several microns). This is due to the change in the rate limiting process from surface reaction plus nucleation and growth to diffusion in the surface forsterite layer. The activation energy of the diffusion-controlled evaporation rate constant of enstatite is 457 (±58) kJ/mol. A thinner forsterite layer is formed on the surface of enstatite in hydrogen gas than in vacuum. Evaporation of enstatite in hydrogen gas is also considered to be controlled by diffusion of ions through the forsterite layer. The thin forsterite layer formed in hydrogen gas is ascribed to the enhanced evaporation rate of forsterite in the presence of hydrogen gas. The results are applied to incongruent evaporation under the solar nebular conditions. The steady thickness of the forsterite of nebular pressure-temperature conditions is estimated to be submicron because of the enhanced evaporation rate of forsterite under hydrogen-rich nebular conditions if evaporated gases are taken away immediately and no back reaction occurs (an open system). Because enstatite grains in the solar nebula would be comparable to the estimated steady thickness of forsterite, evaporation of such enstatite grains under kinetic conditions could play an important role in producing variations in Mg/Si ratios between solid and gas in the solar nebula.

A kinetic model for evaporation of a multi-component condensed phase with a fixed rate constant of the reaction is developed. A binary system with two isotopes for one of the components undergoing simple thermal histories (e.g., isothermal heating) is investigated in order to evaluate the extent of isotopic and chemical fractionations during evaporation. Diffusion in the condensed phase and the effect of back reaction from ambient gas are taken into consideration. Chemical and isotopic fractionation factors and the Peclet number for evaporation are the three main parameters that control the fractionation. Dust enrichment factor (η), the ratio of the initial dust quantity to that required for attainment of gas-dust equilibrium, is critical when back reactions become significant. Dust does not reach equilibrium with gas at η 0). In the former case, a quasi-steady state in the diffusion boundary layer is maintained for isotopic fractionation but not for chemical fractionation. In the latter case, the back reaction brings the strong isotopic fractionation generated in the earlier stage of evaporation back to a negligibly small value in the later stage before complete evaporation. The model results are applied to cosmochemical fractionation of volatile elements during evaporation from a condensed phase that can be regarded as a binary solution phase. The wide range of potassium depletion without isotopic fractionation in various types of chondrules (Alexander et al., 2000) is explained by instantaneous heating followed by cooling in a closed system with various degrees of dust enrichment (η = 0.001–10) and cooling rates of less than ∼5°C/min. The extent of decoupling between isotopic and chemical fractionations of various elements in chondrules and matrix minerals may constrain the time scale and the conditions of heating and cooling processes in the early solar nebula.

The Hf-W chromometer that was developed recently is a very powerful tool because of high time resolution due to short half life and significant partitioning of Hf into the mantle and W into the core. We have developed a mass conservation equation, which includes three important parameters, mass accretion rate, metal separation rate, and redox reaction rate, and searched for the conditions to satisfy the Hf/W and W isotopic compositions of the present Earth's mantle. Although previous work has suggested slow accretion (accretion time > 50 Myr), it could be shorter by an order of magnitude if the metal separation time or redox reaction rate is longer than ∼100 Myr. The slow metal separation rate with rapid accretion is, however, inconsistent with constraints from thermal evolution and fluid dynamics, and slow redox reaction of oxidized materials is the plausible mechanism.

The 26 Al– 26 Mg systems of five ferromagnesian chondrules from the least metamorphosed ordinary chondrite Semarkona (LL3.0) were studied using a secondary ion mass spectrometer. Their glass or plagioclase portions contain excesses of 26 Mg, and in two chondrules the 26 Mg excesses are well correlated with 27 Al/ 24 Mg, which demonstrates the in-situ decay of 26 Al. The initial 26 Al/ 27 Al ratios in these chondrules obtained from the slope of isochrons show a narrow range of between 6 × 10 −6 and 9 × 10 −6 , indicating their short formation duration of less than 1 My. If the solar nebula was initially homogeneous in Al isotopes, the chondrule formation ages are ∼2 My younger than those of CAIs. Our results based on the study of the least metamorphosed UOC are consistent with the previous studies on Al-rich chondrules that the chondrule formation started at least 2 My after CAIs. Alternatively, the older records before 2 My were erased by chondrule recycling process. It further suggests that the young apparent ages (3 to >5 My after CAIs) for chondrules in type 3.4 UOCs are due to the disturbance of the 26 Al– 26 Mg system, possibly during parent body metamorphism. The result is not consistent with the extended nebular time scale of >5 My and the chondrule formation by planetary processes. The Ni isotopic analysis of the FeO-rich olivines in a type II chondrule in Semarkona did not show any detectable excess 60 Ni in spite of their high Fe/Ni ratios. The upper limit of the initial 60 Fe/ 56 Fe ratio of the solar system was estimated to be 3.4 × 10 −7 , which is consistent with the previous estimate (0.2–1.9 × 10 −7 ) from eucrites. This result confirms that the 60 Ni excess previously observed from CAIs was not due to the decay of the short-lived nuclide 60 Fe, but a Ni isotopic anomaly of nucleosynthetic origin.

Development of isotopic fractionation in a condensed phase during congruent evaporation by abrupt heating is modeled to estimate heating conditions in the solar nebula. Effects of elemental diffusion in the condensed phase, back reaction, and grain shape are taken into consideration in the model. Isotopic mass fractionation of an element during evaporation is governed by five critical parameters: evaporation Peclet number Pe, fractionation factor α, dust enrichment factor η, volume expansion factor e, and initial gas–dust ratio ω0. Three modes of isotopic fractionation are recognized in terms of Pe: at Pe 1000, a steady diffusion boundary layer quickly develops near the surface, which significantly suppresses isotopic fractionation. Free evaporation conditions can accordingly be divided into “Rayleigh”, “intermediate”, and “no” fractionation regimes, respectively. Parameters η, e, and ω0 control the degree of back reaction; higher η and ω0 and lower e represent an extensive back reaction. Very small dust enrichment factor (η ∼10) leads to free evaporation. The conditions for attainment of gas–dust equilibrium are given by η(ω0+1)>1.0 and e

Evaporation is a process that caused chemical fractionation in the solar nebula. The evaporation rates of meteorite-forming minerals and silicate melts are key parameters for constraining the timescale of high temperature processes in the early solar nebula and for understanding the mechanisms of cosmochemical fractionation. The kinetics of evaporation of olivine (initial Fo = 92), the most common silicate in meteorites, was experimentally investigated at 1414°C for various duration under continuous evacuation. The surface of olivine becomes more magnesian and the interior becomes zoned as evaporation proceeds, indicating preferential evaporation of the fayalite (Fa) component. The weight loss, surface composition, and interior zoning profile were analyzed using a semi-infinite one-dimensional diffusion model with boundary migration, and the controlling model parameters were optimized by fitting the experimental results. The evaporation rate of olivine shows notable composition and orientation dependence. As olivine becomes more magnesian, the evaporation rates of the (100), (010), and (001) surfaces decrease, among which the rate at the (001) surface is greatest (1.0 μm/h at Fo = 91.8), which is about 2 orders of magnitude faster than that of pure synthetic forsterite. The estimated Fe-Mg fractionation factor shows anisotropy ranging from 0.017 to 0.035, which is significantly greater than the equilibrium value of the Fe-Mg distribution coefficient between olivine and gas, suggesting kinetic suppression of evaporation of the Fa component. The Fe-Mg interdiffusion coefficient at 1414°C, which also shows notable composition and orientation dependence, is estimated to be 8.7 × 10−15 m2/sec along the c-axis. The results were applied to constrain the timescale of heating for a spherical Fo50 olivine grain to retain the Fa component. Olivine dust ∼1 μm in diameter can retain 80% of the initial Fa component for one minute in vacuum, and for ∼30 min at ∼10 Pa PH2 when heated at 1414°C. Therefore, Fo50 olivine in chondrite matrices could have survived an incipient heating event depending on the ambient hydrogen pressure, if they were originally present in the solar nebula.

Evaporation mechanism of synthetic pure forsterite and natural iron-bearing forsterite was experimentally studied in vacuum in the temperature range of 1400-1800°C. Forsterite, which is one of the most important minerals in the planetary system, shows anisotropy of evaporation regarding evaporation rates and surface microstructures. The surfaces develop evaporation pits emanating from dislocation outcrops and smooth or stepped evaporation surface of which evaporation is not enhanced by dislocations. The shape of pits is controlled by anisotropy in evaporation rate and/or that in the surface free energy. Although forsterite and iron-bearing olivine have different type, orientation, and density of dislocation, the characteristics of evaporation pits are common for three crystallographically oriented surfaces. It suggests the larger contribution of anisotropy of surface free energy than the stress energy of dislocations on the shape of the evaporation pits. The estimated evaporation rates, on the contrary, indicate the importance of dislocation, which changes the overall evaporation rate by several factors at the experimental temperature. The change of evaporation rate affects the estimation of timescale of chemical fractionation in the solar nebula.

At low pressure conditions of the solar nebula, condensation and evaporation are the major phase transitions that caused chemical and isotopic fractionation, which may be responsible for planetary and meteoritical chemical fractionation. In order to describe the fractionation quantitatively as a time-dependent process, kinetic factors such as evaporation rates, fractionation factors, nucleation rates, reaction rates between gas and solids, and diffusion rates of elements in major minerals and rarely in melts should be obtained. Experimental work on evaporation of minerals and melts has enabled us to understand the roll of various kinetics during evaporation, and their parameters have been obtained for most important reactions. Theoretical treatment of kinetic processes has been also developed for chemical and isotopic fractionation due to evaporation with or without contribution of diffusion in condensed phases. Combination of experimental and theoretical work along with solar nebula modeling becomes successful in describing the development of chemical and isotopic fractionation in the solar system. The recent progress in laboratory evaporation experiments and theoretical studies is reviewed in this paper.

Kinetics of evaporation of forsterite in hydrogen gas was investigated by high temperature vacuum experiments in the pressure range plausible for the solar nebula. The evaporation rate at total pressure (Ptot) below 10−6 bar is nearly constant and is similar to that in vacuum, whereas the rate at 10−6 to 10−3 bar is dependent on Ptot. The evaporation rate, JexpFo, is fitted by JexpFo = 1.72Ptot1.199.87 × 10−7 (g · cm−2 · s−1) for Ptot below 10−4 bar. The condensation coefficient, α, which is a factor related to kinetics of surface reactions, is evaluated by using the Hertz-Knudsen equation for the kinetic theory of gas molecules. The ratio of the experimentally obtained evaporation rate to that calculated from chemical equilibrium in the system Mg2SiO4-H2 gives the α value of 0.06 in vacuum, which increases up to 0.2 with increasing Ptot from 10−3 to 10−4 bar. The apparent increase of forsterite evaporation rate with increasing H2 abundance is due mainly to increase of the equilibrium vapor pressure, which corresponds to increase in the driving force, and partly to increase in α (reduction of the kinetic barrier) for evaporation. The experimental results were applied to understand behavior of forsterite dusts with time in an abruptly heated model nebula mostly comprising forsterite and H2. The nebular system can be divided into complete and partial evaporation regimes, which is defined by a dust enrichment factor. For the complete evaporation regime (low dust enrichment), the minimum time for forsterite grains to totally evaporate is estimated as a function of total pressure, temperature, and initial grain size. The lifetime of forsterite grains (

Low-pressure phase relations and vapor pressures of forsterite and fayalite were studied with the Knudsen method in the temperature range of 1400–1860°C for forsterite and 1100–1160°C for fayalite. The triple points are 5.2 × 10−5 bar (1890°C) and 6.3 × 10−8 bar (1217°C) for forsterite and fayalite, respectively. The enthalpy and entropy of evaporation of forsterite are ΔHv = 543 ± 33 kJ/mol and ΔSv = 169 ± 21 J/(mol·K), and those for fayalite are 502 ± 9 kJ/mol and 199 ± 6 J/(mol·K), respectively. By assuming that gas and olivine are ideal solutions, vaporous and solidus curves of the olivine solid solution system in the pressure range from 10−10 to 10−3 bar were calculated from the thermochemical data. At pressures above the triple point of forsterite, the phase diagram is the same as that at 1 bar, because the vapor field is at higher temperatures than those of liquid-bearing fields. At pressures between the triple points of forsterite and fayalite, liquid-bearing stability fields appear below the gas-solid stability field in the Fe-rich portion of the system. The compositional range of liquid-bearing fields expand with increasing pressure; at 10−4 bar, the liquid-bearing fields cover most of the olivine system (XMg < 0.9). Liquid is absent at pressures below the triple point of fayalite, regardless of the composition. Large MgFe fractionation between gas and solid is more extensive, compared to the solid and liquid relationship. When heated in equilibrium, olivine melts before evaporation in the pressure range of 10−7-10−3 bar. In a solar nebula with total pressure of 10−3 bar, olivine melts when the nebula is enriched in dust by more than an order of magnitude over the solar nebular value and when the (Mg + Si + Fe)H ratio increased. The origin of type IA and II chondrules and matrix olivine in ordinary and carbonaceous chondrites can be explained in terms of the pressure of olivine gas. Type IA chondrules were formed in the gas-solid phase field where volatiles (FeO, Na2O, and others) were lost during heating events responsible for chondrule formation. The residual materials became enriched in refractory elements. Type II chondrules were formed in the temperature range of the liquid-solid phase field. Under these conditions, the original compositions were retained after heating of chondrule formation. Provided that type IA and II chondrules were formed at a similar temperature, type IA chondrules were formed at an olivine pressure one order of magnitude lower than that of type II chondrules. In this model, matrix olivine is a condensate from gas which formed by partial evaporation of chondritic material during type IA chondrule formation.

Yamato-8002 is classified as a "unique" meteorite having a granoblastic texture with olivine and orthopyroxene and interstitial diopside, plagioclase, spinel, kamacite schreibersite, troilite, and rare graphite. Olivine, orthopyroxene, and clinopyroxene are homogeneous with respect to Mg and Fe, but Ca in pyroxenes are zoned. Plagioclase and spinel are heterogeneous within and among grains. Mineral compositions, particularly plagioclase, spinel and trace elements in all minerals along with oxygen isotopic compositions suggest a genetic link to Y-74063,Y-75274,ALH77081,Y-791493,Acapulco, and Lodran. They were derived from a common precursor through different degrees of partial melting; Y-8002 and Y-75274 have the highest Mg/(Mg+Fe) ratios of olivine, pyroxenes, and spinel, the highest An mol% of plagioclase, and the higest Cr/(Cr+Al) ratio of spinel, indicating formation through the highest degree of partial melting. They were reduced during melting, resulting in crystallization of extremely magnesian minerals. Y-74063,ALH77081,Y-791493,Lodran, and Acapulco possess nearly primary composition of the ferromagnesian minerals excepting that some have lost and some have added partial melts. The partial melting model is supported by the REE patterns; Y-74063 and ALH77081 are flat, Y-8002 and Lodran are heavy REE enriched, and Acapulco is light REE enriched. Positive or negative Eu anomalies are general in these meteorites, suggesting addition or extraction of interstitial melt rich in plagioclase component. The precursor is chondritic with higher Mg/(Mg+Fe) ratio and higher Cr and Mn contents than ordinary chondrites, and is tentatively called a "U (nique)" chondrite. Y-74357,Y-78230,and MAC88177 are probably related to the "U" chondrite. Oxygen isotopic heterogeneity of those meteorites may be due to weathering on the Antarctic ice, and the effect of weathering was excluded by assuming that all oxygen in Fe_2O_3 was from Antarctic ice. The calculated compositions are similar to each other, suggesting their derivation from a common precursor.

Yamato-75261 is a breccia with a non-porphyritic clast embedded in the partly porphyritic matrix. The clast consists mainly of closely packed finegrained enstatite with interstitial material rich in Al. Metallic iron and troilite are rare. An anomalous Fe-Mn-Mg-Ca-Cr monosulfied, of which composition lies intermediate between alabandite in EL chondrites and niningerite in EH chondrites, occurs in the interstices of the clast. The heterogeneous matrix consists of enstatite, forsterite, glass, fine-grained materials, and rare troilite. Chemical compositions of bulk meteorite, clast, and matrix obtained by the broad beam analysis of a microprobe are highly fractionated. Both the clast and matrix are especially depleted in siderophile elements, which is in accordance with apparent depletion of metallic iron over the thin section. Aluminum, Fe, Na, Si, Ca, and S are enriched in the matrix portion; on the contrary, Mg is enriched in the clast. Texture, mineral assemblage, and mineral compositions along with oxygen isotopic compositions (T. K. MAYEDA and R. N. CLAYTON; Papers presented to the 14th Symposium on Antarctic Meteorites, Tokyo, Natl Inst. Polar Res., 172,1989) suggest that Y-75261 is a breccia related to enstatite chondrites. The CaO content of enstatite, Ni, Si, and P contents of kamacite, and Ti and Cr contents of troilite indicate a closer affinity to EH chondrites than to EL's in spite of the intermediate composition of the sulfide.

We have investigated seven micrometeorites (MMs) from Antarctic snow collected in 2003 and 2010 by means of electron microscopy, X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy (TEM) observation, and noble-gas isotope analysis. Isotopic ratios of He and Ne indicate that the noble gases in these MMs are mostly of solar wind (SW). Based on the release patterns of SW 4He, which should reflect the degree of heating during atmospheric entry, the seven MMs were classified into three types including two least heated, three moderately heated, and two severely heated MMs. The heating degrees are well correlated to their mineralogical features determined by TEM observation. One of the least heated MMs is composed of phyllosilicates, whereas the other consists of anhydrous minerals within which solar flare tracks were observed. The two severely heated MMs show clear evidence of atmospheric heating such as partial melt of the uppermost surface layer in one and abundant patches of dendritic magnetite and Si-rich glass within an olivine grain in the other. It is noteworthy that a moderately heated MM composed of a single crystal of olivine has a 3He/4He ratio of 8.44 × 10−4, which is higher than the SW value of 4.64 × 10−4, but does not show a cosmogenic 21Ne signature such as 20Ne/21Ne/22Ne = 12.83/0.0284/1. The isotopic compositions of He and Ne in this sample cannot be explained by mixing of a galactic cosmic ray (GCR)-produced component and SW gases. The high 3He/4He ratio without cosmogenic 21Ne signature likely indicates the presence of a 3He-enriched component derived from solar energetic particles., This work was supported by Grant-in-Aid for Scientific Research (S) (No. 22224010, PI: H. Nagahara) and partly by a Grant-in-Aid for Young Scientists (A) (No. 23684046, PI: R. Okazaki).

Corundum is predicted to be the most abundant refractory circumstellar dust and has been suggested as a candidate emitting the 13 μm feature in infrared spectra of oxygen-rich asymptotic giant branch stars. Crystalline dust has morphologies reflecting the anisotropic crystal structure and formation conditions or processes and may emit characteristic infrared features. In this study, evaporation and condensation experiments of corundum along different crystallographic orientations were performed in vacuum to determine key kinetic parameters controlling morphological anisotropy: evaporation and condensation coefficients (αe and αc), the degrees of kinetic hindrance on evaporation and condensation. Plausible shapes of corundum condensates were estimated from the experimental results to evaluate the infrared features of corundum condensates. The evaporation coefficients of corundum are 0.02–0.2 at 1600–1785°C, which increase with temperature and show notable anisotropy. The order of αe along the crystallographic c, a, and m axes is α ≫ α > α irrespective of temperature. The obtained condensation coefficients along the c, a, and m axes at 1575°C and a supersaturation ratio of ∼4 are 0.04–0.06, 0.06–0.08, and 0.1–0.2, respectively. The morphology of condensed circumstellar corundum expected from our experiments is oblate and slightly flattened to the c axis and is consistent with the fact that no presolar corundum with eccentric shapes has been found. The mass absorption coefficient of oblate corundum slightly flattened to the c axis shows a peak at 13 μm without any accompanying strong peaks. These results strongly indicate that corundum condensed anisotropically in circumstellar environments is a carrier of the unidentified 13 μm feature around oxygen-rich evolved stars.

Metallic iron is one of the most abundant condensing materials in systems of solar abundance. Because metallic iron is responsible for the continuum opacity of dust particles, it has a large contribution to the thermal structure of circumstellar environments and hence to dust evolution itself. In order to understand the formation processes of metallic iron in circumstellar environments, condensation and evaporation kinetics of metallic iron were studied experimentally. Metallic iron condenses at the maximum rate with the condensation coefficient (a parameter ranging from 0 to 1 to represent kinetic hindrance for surface reaction) of unity under high supersaturation conditions, and evaporates nearly ideally (evaporation coefficient of unity) in vacuum. On the other hand, evaporation of metallic iron takes place with more kinetic hindrance in the presence of metallic iron vapor. It is also found that metallic iron atoms nucleate heterogeneously on Al2O3. Metallic iron does not necessarily condense homogeneously in circumstellar environments, but might condense through heterogeneous nucleation on pre-existing dust. Metallic iron formation proceeds with little kinetic hindrance for highly unequilibrated conditions, but the effects of kinetic hindrance may appear for evaporation and condensation occurring near equilibrium with a timescale of months to years in protoplanetary disks.

A petrological study has been conducted on ALH-77307 (CO3) chondrite. Chondrules are classified into magnesian type and iron-rich type. The magnesian chondrules contain olivine and clinoenstatite with a small amount of groundmass, whereas the iron-rich chondrules contain only olivine as phenocrysts in the groundmass. Inclusions show gradual changes in crystallinity, grain size of constituents, texture and shape. The difference among the inclusions is considered to be due to the difference of heating temperature of the precursor finegrained materials. Intensive heating of the inclusions might have formed the magnesian chondrules. Isolated olivine grains are also classified into magnesian and iron-rich types, which are thought to have been derived from the magnesian and iron-rich chondrules, respectively. Genetical relations of chondrules, inclusions and isolated olivine grains are discussed by the process of condensation, successive heating, break up, and accreation.

The rates of volatilization of Na from liquid spheres of chondrule compositions have been determined as functions of time, temperature, partial pressure of oxygen, and sizes of the spheres. The Na2O content in the sphere is uniform in each run. but it decreases with time of the run, indicating that the rate of diffusion of Na in the liquid is greater than that of volatilization, and that the latter is the rate-controlling process. The rate of sodium volatilization becomes greater with increasing temperature and with decreasing PO2 and size of the spheres. The relation of the Na2O content in the liquid sphere with time and its size indicate that the amount of Na2O volatilized from the liquid spheres within unit time is proportional to the surface area of the spheres and the concentration of Na2O in the liquid. From these relations, the rate of volatilization of sodium can be obtained at constant temperature and Po2. The rate of volatilization of sodium satisfies the Arrhenius relation within the temperature range from about 1450–1600 C at 10−9,2 atm pO2; the activation energy for the sodium volatilization is approximately 100 kcal-mole−1. The rate is also approximately proportional to pO2−14 within the range of pO2 from 10−10.2 to 10−5.0 atm at about 1500° C. Based on the present results and the Na2O contents in chondrules. it is suggested that they experienced an instant heating with maximum temperature of 1400–2200° C followed by an immediate cooling.

In order to understand the origin of iron-rich olivine in the matrices of type 3 ordinary chondrites, the reaction of metallic iron and enstatite, with and without forsterite and SiO2, has been experimentally reproduced at temperatures between 1150° and 800°C and PO2 between 10−11 and 10−16 atm (between the IQF and MW buffers). The olivine produced ranges from Fo58 to Fo34 and this composition does not change significantly with temperature and time of the runs. The magnesian olivine which forms does become more magnesian with increasing forsterite/enstatite ratio of the starting materials. Iron-rich olivine (Fo< 35) cannot be formed by the reaction of enstatite and metallic iron, with or without forsterite as starting materials but it can be formed in the presence of free silica. The composition of olivine becomes more iron-rich with increasing silica/enstatite ratio. The compositional range of olivine formed from each mixture is 25–30 mole% Fo regardless of the temperature, composition, mineral assemblage, and run duration. From these experimental results, two possibilities suggested for the origin of the iron-rich olivine in the matrices of type 3 ordinary chondrites: (1) free silica must have been present if the iron-rich olivine was formed by solid-state reactions under oxidizing condition in the solar nebula; (2) reaction of silicon-rich gas with metallic iron took place under oxidizing condition in the solar nebula. Though it is difficult to define which alternative was dominant, the formation of free silica or silicon-rich gas may be a result of fractional condensation. This is possible if there is a reaction relation between forsterite and gas to produce enstatite. The suggested fractional condensation is supported by the fact that the compositions of the fine-grained matrices of type 3 ordinary chondrites are more silica-rich than the bulk compositions of the chondrites. Though it is not known whether such conditions were established all over the nebula or locally in the nebula, both fractionation and more oxidizing conditions than the average solar nebula are required for the formation of matrix olivine.

Petrologic studies were made on the fine-grained matrices of type 3 ordinary chondrites of the lowest petrologic subtype. The matrix minerals, in order of abundance, are olivine (Fo99 to Fo9), enstatite or bronzite, augite or subcalcic augite, albite, Fe-Ni metal, troilite, magnetite, spinel (MgAl2O4), chromite, and calcite. Fe- and Mg-rich fluffy particles and albite-like particles are also major constituents. The chemical compositions of olivine and pyroxenes vary within and among the chondrites and are in gross disequilibrium, showing that the matrix materials were hardly heated after their formation. Textural relationships indicate that magnesian olivine was formed after Ca-pyroxene, followed by intermediate to iron-rich olivine. Intermediate olivine was formed from enstatite and metallic iron under relatively oxidizing conditions. The observations indicate that matrices of chondrites are neither the fragments of chondrules nor the precursors of chondrules. They were mostly the products of condensation and reaction among solids and/or between solids and the ambient gas mostly at low temperatures, and thus they contain records of primitive processes in the nebula. In order to explain the presence of olivines more iron-rich than Fo50, the presence of free SiO2 or a high activity of SiO2 in the gas is necessary, which was not shown in previous thermochemical calculations. Mineral assemblages of matrix minerals of chondrites of different chemical groups differ systematically according to oxidation state of the parental meteorites, indicating that they were formed at different oxygen fugacities. The rims of chondrules, and surrounding matrix materials, must have accreted onto chondrules during turbulent movements of the nebula.