Low temperature phase equilibria in the Fe-Ni and Fe-Ni-P systems: application to the thermal history of metallic phases in meteorites     

A.D. Romig  J.I. Goldstein 《Geochimica et cosmochimica acta》1981,45(7):1187-1197

The solubility limits of the α (kamacite) and γ (taenite) phases in the Fe-Ni and Fe-Ni-P phase diagrams have been measured at low temperatures, 700-300°C. The predicted $\text{α}\text{α}\text{+ γ}$ retrograde solubility below 500°C was demonstrated experimentally for the first time in the Fe-Ni system. The minimum solubility of Ni in γ at the $\text{γ}\text{α}\text{+γ}$ boundary increases with decreasing temperature to as much as 54 wt% at 300°C. The addition of P increases the maximum solubility of Ni in α by as much as 1.6 wt% and decreases the minimum solubility of Ni in γ by as much as 7 wt% at 300°C.The solubility limits of kamacite and taenite were also obtained from heat-treated samples of the Grant and Cape York iron meteorites. The data indicate that in iron meteorites minor and trace elements other than P do not significantly shift the Ni solubility limits of the Fe-Ni and Fe-Ni-P phase diagrams. The measured phase diagrams can be used to explain the Agrell effect and the differences in maximum Ni content of taenite among irons and chondrites. The formation of plessite and the influence of the measured solubility limits on the cooling rate simulation method are also considered.  相似文献   

Studies of an artificially shock-loaded H group chondrite     

D.W Sears  J.R Ashworth  C.P Broadbent  A.W.R Bevan 《Geochimica et cosmochimica acta》1984,48(2):343-360

Five samples of the naturally unshocked Kernouve (H6) meteorite were artificially shock-loaded to pressures of 70, 165, 270, and 390 kbar and the silicates and metal examined optically, by scanning and transmission electron microscopy and by thermoluminescence (TL). Olivine deformation is closely comparable to that in naturally shocked meteorites, producing dislocations with Burgers vector [001]. At pressures of ?165 kbar, these are formed in well-defined slip planes. At 270 kbar, olivine develops optical mosaicism, has high dislocation densities throughout and is also highly fractured. Recovery, due to heating is minimal. In orthopyroxene, the deformation mechanism changes, from the clino-inversion to unit-dislocation slip, between 70 and 165 kbar. In diopside, (001) and (100) twinning was produced. Plagioclase is inferred to have been progressively converted to maskelynite, but some is still present in 270 kbar sample.The microhardness of the kamacite in the samples increases with shock pressure. The α → ? transformation pressure in the kamacite is 30–40 kbar higher than observed for iron meteorites. Annealed kamacite displays incipient polycrystallinity and α-martensite and taenite sometimes contains slip lines. Troilite acquired cracks, undulose extinction, twins, polycrystallinity and finally melted as the shock pressure increased.At pressures over 200 kbar there was a systematic decrease in the natural TL and the TL sensitivity. Detailed considerations of changes in the natural $\text{TL}\text{TL}$ sensitivity ratio for various regions of the TL glow curve suggest that two processes were effective during shock; thermal drainage of electron traps and a reduction in the effective trap density. It is suggested that the latter process associated with the vitrification of feldspar, the TL phosphor.An additional sample was subjected to a shock pulse which was “spiked” instead of square. Very distinctive changes were apparent; thermal effects are conspicuous and with widespread annealing (~600–800°C) of metal and sulfide. Glassy, opaque veins were produced which are analogous to the black veins in shock-lithified gas-rich meteorites. Anomalous low-temperature TL was induced, suggesting that a new or modified phase or mineral has become the dominant TL phosphor.  相似文献   

A major revision of iron meteorite cooling rates—An experimental study of the growth of the Widmanstätten pattern     

C. Narayan  J.I. Goldstein 《Geochimica et cosmochimica acta》1985,49(2):397-410

In this study kamacite was experimentally grown in taenite grains of Fe-Ni-P alloys containing between 5 and 10 wt% Ni and 0 and 1.0 wt% P. Both isothermal heat treatments and non-isothermal heat treatments at cooling rates of 2 to 5°C/day were carried out. Analytical electron microscopy was used to examine the orientation and chemical composition of the kamacite and the surrounding taenite matrix. The kamacite so produced is spindle or rod shaped and has a Widmanstätten pattern orientation. The presence of heterogeneous sites such as phosphides is necessary for the nucleation of the intergranular kamacite. During kamacite growth both Ni and P partition between kamacite and taenite with chemical equilibrium at the two phase interface. The growth kinetics are limited by the diffusion of Ni in taenite. Additional diffusion experiments showed that the volume diffusion coefficient of Ni in taenite is raised by a factor of 10 at 750°C in the presence of only 0.15 wt% P.A numerical model to simulate the growth of kamacite in Fe-Ni-P alloys, based on our experimental results, was developed and applied to estimate the cooling rates of the iron meteorites. The cooling rates predicted by the new model are two orders of magnitude greater than those of previous studies. For example the cooling rates of chemical groups I, IIIAB and IVA are 400–4000°C/10⁶years, 150–1400°C/ 10⁶ years and 750–6000°C/10⁶years respectively. Previous models gave 1–4°C/10⁶ years, 1–10°C/10⁶ years and 3–200°C/10⁶ years. Such fast cooling rates can be interpreted to indicate that meteorite parent bodies need only be a few kilometers in diameter or that iron meteorites can be formed near the surface of larger asteroidal bodies.  相似文献   

Analytical electron microscopy study of the plessite structure in four IIICD iron meteorites     

L.S. Lin  J.I. Goldstein  D.B. Williams 《Geochimica et cosmochimica acta》1979,43(5):725-737

Electron optical techniques were employed to investigate the plessite structure and composition of four IIICD fine octahedrites. These meteorites have a similar thermal history and differences in plessite structure can be ascribed to varying bulk Ni content and/or localized differences in carbon content. Microdiffraction patterns from regions as small as 20 nm dia. were obtained for the first time from plessite structures. It was established that transformation twins in clear taenite I have the conventional fcc twin relationship, individual kamacite and taenite cells in the cloudy zone have the Kurdjumov-Sachs orientation and fine γ rods in the decomposed martensite zone display both the Nishiyama and Kurdjumov-Sachs relation with the matrix-α. All the IIICD irons contain cloudy zone and martensitic plessite. Except for Dayton, martensitic plessite shows further decomposition into α + λ at low temperatures. Using STEM X-ray microanalysis with a spatial resolution of ~ 50 nm, Ni composition profiles in taenite from all the IIICD irons showed a maximum of ~48 wt% Ni. The structural and compositional data indicate that plessite formation occurs at quite low temperatures (~ 200–300°C) during the cooling history of the IIICD irons.  相似文献   

Composition of the metal phases in ordinary chondrites: implications regarding classification and metamorphism     

Firooz Afiattalab  John T. Wasson 《Geochimica et cosmochimica acta》1980,44(3):431-446

EMP determinations of Fe, Co and Ni in the metal phases of ordinary chondrites confirm the report of Sears and Axon that kamacite Co contents show restricted, nonoverlapping ranges in the three groups; ranges are 3.3–4.8 mg/g in H, 6.7–8.2 mg/g in L and 15–110 mg/g in LL. Experimental data by Widge and Goldstein show that the Ni concentration of the $\text{α}\text{(α + γ)}$ boundary increases with increasing Co concentration: unexpectedly, we find lower kamacite Ni concentrations in unequilibrated LL chondrites (44–55 mg/g) than in H and L chondrites (57–69 mg/g). We infer that, at temperatures below 550° C increasing Co causes a decrease in the equilibrium kamacite Ni concentration of an α-γ system. Although some evidence indicates that the equilibrated L chondrites Barratta, Knyahinya and Shaw have siderophile concentrations lower than the normal L-group range, they have kamacite and taenite Co concentrations in the L-group range.Metal-phase studies of petrologic type-3 ordinary chondrites having highly unequilibrated silicates showed a wide range in the degree of matrix kamacite equilibration ranging from nearly equilibrated in Mezö-Madaras to highly unequilibrated in Bishunpur, Ngawi and Semarkona. Kamacite in chondrule interiors is highly unequilibrated in all 9 chondrites, and in each setting taenite data are consistent with the expectation that it should be less equilibrated than kamacite. Our kamacite Co data confirm that Sharps is H and Hallingeberg. Khohar and Mezö-Madaras are L chondrites. Chainpur and Parnallee have kamacite Co concentrations between the L and LL ranges: we present evidence indicating that they are truly intermediate, i.e. neither L nor LL. Highly unequilibrated Ngawi is either LL or, less likely, still more oxidized. Bishunpur and Semarkona have mean kamacite Co concentrations in the H range but too unequilibrated to be used for classification. The highly heterogeneous compositions of the metal in Bishunpur, Ngawi and Semarkona indicate that their metal partially preserves properties established during nebular processes. Most of the taenite in these chondrites has high Ni contents (>470 mg/g) and is essentially unzoned; much of the kamacite is polycrystalline with crystals ?5μm across. Metamorphism causes tiny grains to disappear, increases the grain size of both kamacite and taenite, tends to equilibrate metallic minerals and, during cooling, can produce zoned taenite.A petrologic type-5 clast in the Ngawi LL3 chondrite has 3 coexisting metal phases, clear taenite (540 mg/g Ni, 21 mg/g Co), kamacite (30 mg/g Ni, 120 mg/g Co) and a phase tentatively identified as ordered FeCo (8.5 mg/g Ni, 370 mg/g Co).  相似文献   

Derivation of a heterogeneous lithic fragment in the Bovedy L-group chondrite from impact-melted porphyritic chondrules     

Alan E Rubin  Klaus Keil  G.Jeffrey Taylor  M.-S Ma  R.A Schmitt  D.D Bogard 《Geochimica et cosmochimica acta》1981,45(11):2213-2228

The Bovedy L-group chondrite contains a light-colored poikilitic lithic fragment with olivine, low-Ca pyroxene and kamacite compositions characteristic of porphyritic chondrules from unequilibrated ordinary chondrites. Its texture, compositional similarities to porphyritic chondrules, and low Na₂O, K₂O and P₂O₅ content indicate that the fragment represents a solidified, slightly fractionated impact melt formed from a source that was rich in porphyritic chondrules. The fragment is heterogeneous, with a progressive increase in the bulk $\text{MgO}\text{FeO}$ ratio and in MgO content of olivines and low-Ca pyroxenes across its length. ${}^{39}\text{Ar}{}^{40}\text{Ar}$ analyses of the fragment and host indicate that the meteorite experienced extensive degassing due to reheating. The approximate age of 0.5–0.94 Byr dates the reheating event and not the formation of the lithic fragment or the Bovedy breccia. This reheating event renders the fragment's and host's metallographic cooling rate of ~ 5 C/Myr (through 500°C) imprecise. However, the absence of martensite and the presence of kamacite. zoned taenite and tetrataenite in the fragment and host are consistent with such slow cooling through 500°C. This cooling rate must have resulted from burial of the fragment-host assemblage beneath insulating material on the Bovedy parent body. If the thermal diffusivity (κ) of this overburden was approximately comparable to that of the lunar regolith (10^?4cm²/sec), then the fragment was buried at a depth ≌ 6.5 km; if K = 10^?2 cm²/sec (similar to chondritic material), then the fragment was buried at a depth ?65 km.  相似文献   

Analytical electron microscope study of eight ataxites     

P.M. Novotny  J.I. Goldstein  D.B. Williams 《Geochimica et cosmochimica acta》1982,46(12):2461-2469

Optical and electron optical (SEM, TEM, AEM) techniques were employed to investigate the fine structure of eight ataxite-iron meteorites. Structural studies indicated that the ataxites can be divided into two groups; a Widmanstätten decomposition group and a martensite decomposition group. The Widmanstätten decomposition group has a Type I plessite microstructure and the central taenite regions contain highly dislocated lath martensite. The steep M shaped Ni gradients in the taenite are consistent with the fast cooling rates, ≥500°C/my, observed for this group. The martensite decomposition group has a Type III plessite microstructure and contains all the chemical group IVB ataxites. The maximum taenite Ni contents vary from 47.5 to 52.7 wt% and are consistent with slow cooling to low temperatures ≤350°C at cooling rates ≤25°C/my. Ordered FeNi and the cloudy border structure were not observed in any of the ataxites. Modest reheating to ≤350°C may have been responsible for the lack of these structures.  相似文献   

The Tucson meteorite     

Gary T. Miyake  Joseph I. Goldstein 《Geochimica et cosmochimica acta》1974,38(8):1201-1212

Tucson is an unusual iron meteorite which contains highly reduced silicate inclusions and elemental silicon and chromium in solution. The metal matrix of Tucson was found to be chemically uniform, suggesting that homogenization has occurred at elevated temperatures. The microstructure of the metal consists of plessite and thin ribbons of kamacite. Nickel and phosphorus concentrations indicate that kamacite nucleated along prior taenite grain boundaries at ~650°C, and grew upon cooling to 500°C. Kamacite growth calculations show that Tucson cooled at ~1°C/1000 yr, a rate which corresponds to a depth of burial at the center of a 15 km radius parent body or closer to the surface of parent bodies of larger sizes. The shapes of the Tucson irons, and the presence and distribution of silicate inclusions in the Fe-Ni matrix appear to be a result of a solidification process.  相似文献   

The nature of dark-etching rims in meteoritic taenite     

Edward R.D. Scott 《Geochimica et cosmochimica acta》1973,37(10):2283-2294

Taenite fields when etched develop a cloudy brown rim with approximate compositional limits of 25 and 40 per cent Ni. In iron meteorites this cloudy zone is only a few microns wide, with a sharp, high-Ni edge about 1 μm from the kamaciteinterface and a diffuse edge several microns from the central plessite. It is always present in irons unless the meteorite has been cosmically or terrestrially reheated.X-Ray and electron diffraction of grains scratched from exceptionally large areas of cloudy taenite in the mesosiderite Estherville show that this etching zone contains a fine exsolution of kamacite. Electron microscopy reveals a cellular structure with kamacite walls surrounding taenite volumes about 1000 Å in diameter; about one-third of the total volume is kamacite. Electron diffraction from a thin foil of Tazewell indicates that for several microns the cloudy border consists of a single crystal of kamacite interpenetrating a single crystal of taenite.Detailed electron-probe investigations of taenite in Estherville show that there is a step in the M-shaped Ni profile at the sharp, high-Ni edge of the cloudy region, the Ni dropping suddenly from approximately 45 to 42 per cent. It is proposed that exsolution in the cloudy region effectively froze in the Ni profile at that temperature. On subsequent cooling only the clear outer taenite continued to equilibrate with the kamacite matrix producing the kink in the M profile.Cloudy taenite is therefore a variety of plessite differing from the usual varieties in that it forms at lower temperatures in areas much richer in Ni, and the morphology is not crystallographically oriented. Its absence can provide a sensitive indication of reheating.  相似文献   

Upper temperature tolerance of spot,Leiostomus xanthurus,from the cape Fear River estuary,North Carolina     

Ronald G. Hodson  Robert G. Fechhelm  Robert J. Monroe 《Estuaries and Coasts》1981,4(4):345-356

The temperature tolerance and resistance times of postlarval (<25 mm SL) and small juvenile spot,Leiostomus xanthurus, from the Cape Fear Estuary, North Carolina were tested in the laboratory. Critical thermal maximum techniques were used to determine first equilibrium loss (FEL) and critical thermal maximum (CTM) end points and thermal shock methods were used to determine 96-h upper incipient lethal temperatures (LT₅₀). Acclimation temperatures ranged from 10 to 35°C and acclimation salinities were 10, 20 and 30‰. A quadratics model was fit to the CTM and FEL data; r² values were 0.924 and 0.928 respectively. Acclimation salinity, estimated weight, acclimation salinity by acclimation temperature interaction and acclimation temperature by estimated weight interaction were the significant components of the CTM model. Predicted CTM values ranged from 30°C at 10 °C and 30‰ acclimation to just over 40°C at 30 °C and 30‰ acclimation. Acclimation temperature, acclimation temperature squared, estimated weight and acclimation temperatures by estimated weight interaction were the significant components of the FEL model. Predicted FEL values ranged from around 28°C at 10°C and 10‰ acclimation to about 39°C at 30°C and 30‰ acclimation. The 96-h LT₅₀ values of spot acclimated to 20‰ increased linearly with acclimation temperature to 25°C. From about 25 to 35°C, LT₅₀ values increased very little with acclimation temperature. The ultimate upper incipient lethal temperature of postlarval and small juvenile spot was estimated at 35.2°C. Increased salinity increased resistance time but decreased LT₅₀ estimates. Thermal shock tests were better for predicting the effects of thermal addition than were CTM tests.  相似文献