Hiroko Nagahara

Kinetic and equilibrium mass-dependent isotope fractionation laws in nature and their geochemical and cosmochemical significance

Abstract 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

A short duration of chondrule formation in the solar nebula: evidence from 26Al in Semarkona ferromagnesian chondrules

Abstract 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.

Simulation of space weathering of planet-forming materials: Nanosecond pulse laser irradiation and proton implantation on olivine and pyroxene samples

For the purpose of simulating the surface alteration process called “space weathering”, experiments of pulse laser irradiation, proton implantation, and laser irradiation to proton implanted samples were performed and reflectance spectra of altered materials were measured. To simulate the impact heating by micrometeorite bombardments, we made a new apparatus using a pulse laser whose pulse duration is 6–8 nanoseconds, comparable with a timescale of micrometeorite impacts. We find that the degree of space weathering, i.e., change of reflectance spectrum should depend on mineral composition. Laser irradiation onto olivine produces the largest reduction of albedo and the highest reddening of reflectance spectrum. In general, variation of olivine spectra is much larger than that of pyroxenes. Depths of absorption bands do not change in the scaled spectra. The olivine spectrum after the laser irradiation can match spectra of some olivine asteroids within a subtype of S-type asteroids. Comparison of Vesta spectrum with altered pyroxene spectra suggests that Vesta surface would be relatively older than olivine asteroids. We also investigate the influence of solar wind proton and pyroxene FeO content. The proton implantation causes small changes in olivine and enstatite spectra. Implanted protons do not influence spectral change by the laser irradiation: the laser irradiation and the proton implantation do not produce multiplicative but additive changes on the reflectance spectra. FeO content of pyroxenes does not relate to the degree of reflectance change.

Volatilization of sodium from silicate melt spheres and its application to the formation of chondrules

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.

Matrices of type 3 ordinary chondrites—primitive nebular records

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.

Evaporation of forsterite in H2 gas

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 (<10 μm in size) could be less than 1 h at 1700 °C. The experimental results were further applied to examine the possibility of isotopic fractionation for forsterite grains in the solar nebula. By evaluating the competition between evaporation from surface and elemental diffusion in forsterite, it is shown that forsterite grains could have isotopically fractionated to be heavier only for Mg, but not for Si and O.

Experimental reproduction of textures of chondrules

The textures of chondrules have been reproduced by crystallizing melts of three different compositions at 1 atm with cooling rates ranging from 400 to 20°C/min under 10−9 to 10−12 atmPO2. A porphyritic olivine texture has been formed from a melt of olivine-rich composition (SiO2 = 45 wt.%), a barred-olivine texture from melt of intermediate composition (SiO2 = 47 wt.%), and radial-olivine texture from melt of pyroxene-rich composition (SiO2 = 57 wt.%). The cooling rate for producing barred olivine is most restricted; the rate ranges from 120 to 50°C/min. Other textures can be formed with wider ranges of cooling rate. The results of the experiments indicate that some of the major types of textures of chondrules can be formed with cooling rate of about 100°C/min. With this cooling rate, the texture varies depending on the composition of melt.

Non-Rayleigh oxygen isotope fractionation by mineral evaporation: theory and experiments in the system SiO2

Experiments demonstrate that partial evaporation of solid silica at 1600–1700°C and low pressure (10−9 bar) results in enrichment of 18O/16O and 17O/16O in solid products. Evaporative residues formed in H2 or N2 gas at higher pressures (>10−5 bar) exhibit limited or negligible heavy isotope enrichment. The degree of enrichment is controlled by kinetic fractionation at the ablating grain surfaces, the rate of sublimation, and the efficacy of oxygen self diffusion in the solid. Observed isotopic effects are consistent with numerical simulations, confirming that vaporization of solid silicate and oxide minerals is a viable cause for non-Rayleigh fractionation of 16O, 17O, and 18O. Experiment and theory suggest that partial melting during evaporation is not required a priori to explain mass-dependent variations in oxygen isotope ratios in primitive meteoritical materials. Experimental determinations of the rates of ablation of appropriate minerals are required to evaluate the meteoritical data.

Chemical and isotopic fractionations by evaporation and their cosmochemical implications

Abstract 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.

Isotopic fractionation as a probe of heating processes in the solar nebula

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<η(ω0+1)−1, where no isotopic mass fractionation is expected irrespective of Pe values. Spherical grains quickly develop larger isotopic fractionation and more distinct isotopic zoning than cylinder and platy grains having the same size. Surface roughness within the order of the grain size quickly disappears except for steady jagged surface developed from pits or grooves, and does not significantly affect the degree of isotopic fractionation. The model predicts that magnesium and oxygen isotopes in forsterite are in either Rayleigh or intermediate fractionation regime over a wide range of initial grain size and temperature in the solar nebula. The absence of Mg isotopic mass fractionation in forsterite in chondrites suggests that the dust enrichment factor is much larger than unity and that the expansion factor is smaller than η−1.