Growth mechanism and additional constraints on FeNi metal condensation in the solar nebula

Chemically zoned FeNi metal grains in the metal-rich chondrites QUE 94411 and Hammadah al Hamra 237 formed by gas-solid condensation in the temperature range from similar to1500 to 1400 K during highly energetic thermal events in the solar nebula. We observe a linear correlation between the apparent diameter of the zoned FeNi metal grains and their inferred condensation temperature interval, which indicates that the grain growth rate was essentially constant. This lends strong support for a kinetic "hit-and-stick" growth model that yields growth timescales of similar to20-85 hours and gas cooling rates of similar to1-2 K h(-1) for six representative zoned metal grains studied in QUE 94411. In the core regions of the zoned metal grains the Ni concentration is systematically lower than the thermodynamically predicted values, suggesting that solid-state diffusion played an important role in shaping the zoning profiles. Combined with existing data, our observations provide a set of constraints on the physics and chemistry of large-scale, high-temperature processes in the earliest solar nebula, which present astrophysicists with profound challenges

Primitive FeNi metal grains in CH carbonaceous chondrites formed by condensation from a gas of solar composition

Some FeNi metal grains, similar to 150 mu m in apparent diameter, in CH carbonaceous chondrites are concentrically zoned in Ni (similar to 5-10 wt%), Co (0.2-0.4 wt%), and Cr (0.3-0.8 wt%); Silicon is present at the similar to 0.1 wt% level. These observations are consistent with predicted gas-solid condensation from a gas of solar composition at temperatures of similar to 1370-1270 K and total pressure of similar to 10(-4) bar. Estimates of FeNi metal grain growth and cooling rates in this temperature range are consistent with brief and localized thermal episodes in the solar nebula. Compositionally similar FeNi metal grains have also been reported in CR and Bencubbin-like chondrites. Because FeNi metal is highly susceptible to secondary alteration (i.e., metamorphism, melting, oxidation), the observed FeNi metal condensates in CH, Bencubbin-like, and CR chondrites indicate that these meteorites experienced no thermal processing after their lithification and thus are among the most primitive meteorites in our collections

The ZONMET thermodynamic and kinetic model of metal condensation

The ZONMET model of metal condensation is a FORTRAN computer code that calculates condensation with partial isolation-type equilibrium partitioning of the 19 most abundant elements among 203 gaseous and 488 condensed phases and growth in the nebula of a zoned metal grain by condensation from the nebular gas accompanied by diffusional redistribution of Ni, Co, and Cr. Of five input parameters of the ZONMET model (chemical composition of the system expressed as the dust/gas [D/G] ratio, nebular pressure [P-tot], isolation degree [xi], cooling rate (CR), and seed size), only two-the D/G ratio and the CR of the nebular source region of a zoned Fe,Ni grain-are important in determining the grain radius and Ni, Co, and Cr zoning profiles. We found no evidence for the supercooling during condensation of Fe,Ni metal that is predicted by the homogeneous nucleation theory. The model allows estimates to be made of physicochemical parameters in the CH chondrite nebular source regions

Large-scale thermal events in the solar nebula: Evidence from Fe,Ni metal grains in primitive meteorites

Chemical zoning patterns in some iron, nickel metal grains from CH carbonaceous chondrites imply formation at temperatures from 1370 to 1270 kelvin by condensation from a solar nebular gas cooling at a rare of similar to 0.2 kelvin per hour. This cooling rate requires a Large-scale thermal event in the nebula, in contrast to the localized, transient heating events inferred for chondrule formation. In our model, mass accretion through the protoplanetary disk caused Large-scale evaporation of precursor dust near its midplane inside of a few astronomical units. Gas convectively moved from the midplane to cooler regions above it, and the metal grains condensed in these parcels of rising gas

Ferrous silicate spherules with euhedral iron-nickel metal grains from CH carbonaceous chondrites: Evidence for supercooling and condensation under oxidizing conditions

The CH carbonaceous chondrites contain a population of ferrous (Fe/(Fe + Mg) approximate to 0.1-0.4) silicate spherules (chondrules), about 15-30 mum in apparent diameter, composed of cryptocrystalline olivine-pyroxene normative material, +/-SiO2-rich glass, and rounded-to-euhedral Fe,Ni metal grains. The silicate portions of the spherules are highly depleted in refractory lithophile elements (CaO, Al2O3, and TiO2 Fe-met) during melting of metal-silicate solid precursors. Rather, we suggest that this is a condensation signature of the precursors formed under oxidizing conditions. Each metal grain is compositionally uniform, but there are significant intergrain compositional variations: about 8-18 wt% Ni, <0.09 wt% Cr, and a sub-solar Co/Ni ratio. The precursor materials of these spherules were thus characterized by extreme elemental fractionations, which have not been observed in chondritic materials before. Particularly striking is the fractionation of Ni and Co in the rounded-to-euhedral metal grains, which has resulted in a Co/Ni ratio significantly below solar. The liquidus temperatures of the euhedral Fe,Ni metal grains are lower than those of the coexisting ferrous silicates, and we infer that the former crystallized in supercooled silicate melts. The metal grains are compositionally metastable; they are not decomposed into taenite and kamacite, which suggests fast postcrystallization cooling at temperatures below 970 K and lack of subsequent prolonged thermal metamorphism at temperatures above 400-500 K

Shock melts in QUE 94411, Hammadah al Hamra 237, and Bencubbin: Remains of the missing matrix?

We have studied the CB carbonaceous chondrites Queen Alexandra Range (QUE) 94411, Hammadah al Hamra (HH) 237, and Bencubbin with an emphasis on the petrographical and mineralogical effects of the shock processing that these meteorite assemblages have undergone. Iron-nickel metal and chondrule silicates are the main components in these meteorites. These high-temperature components are held together by shock melts consisting of droplets of dendritically intergrown Fe,Ni-metal/sulfide embedded in silicate glass, which is substantially more FeO-rich (3040 wt%) than the chondrule silicates (FeO < 5 wt%). Fine-grained matrix material, which is a major component in most other chondrite classes, is extremely scarce in QUE 94411 and HH 237, and has not been observed in Bencubbin. This material occurs as rare, hydrated matrix lumps with major and minor element abundances roughly similar to the ferrous silicate shock melts (and Cl). We infer that hydrated, fine-grained material, compositionally similar to these matrix lumps, was originally present between the Fe,Ni-metal grains and chondrules, but was preferentially shock melted. Other shock-related features in QUE 94411, HH 237, and Bencubbin include an alignment and occasionally strong plastic deformation of metal and chondrule fragments