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

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.     

Analytical electron microscopy study of the plessite structure in the Carlton iron meteorite

In order to gain a better understanding of the formation of plessite in iron meteorites, various electron optical techniques were employed to study the range of plessite structures observed in the Carlton fine octahedrite. Compositional and structural studies of twins in clear taenite and the cloudy zone were made. Transmission electron microscopy studies of martensitic and duplex α + γ plessite regions show the presence of γ-taenite rods, 10–200 nm wide, in an α-kamacite matrix. Scanning transmission electron microscope X-ray analyses showed Ni contents in the y rods of $\text{≥43}\text{wt}\text{\%}$ and Ni contents in the a matrix of 3 wt% Ni. The reaction path involves the decomposition of $\text{α}{}_2$ martensite into $\text{α + γ}$ and these reactions occur below 200°C and possibly below 100°C. Apparently the formation of plessite is intimately related to the formation of martensite and the further decomposition of martensite during the cooling history of the meteorite. It is quite probable that the martensite decomposition reaction has occurred in a large number of iron meteorites and is responsible for many of the observed plessite structures.     

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

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.     

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

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).     

A microstructural study of the Tishomingo meteorite

Metallography, electron microscopy and X-ray diffraction techniques were employed to study a fragment of the Tishomingo iron meteorite. The results suggest the following thermal-mechanical history: The fragment was originally a large crystal of taenite (γ). Cooling through the α + γ phase boundary did not result in accompanying precipitation of kamacite (α). Transformation to a martensitic structure initiated between ? 25 and ?65°C. Transformation continued as the temperature fell to ? 75 to ? 115°C, resulting in approx 80% martensite (α′). Subsequent shock deformation and thermal aging processes substantially modified the taenite and martensite microstructures. Twins in the retained taenite phase are attributed to shock deformation at a pressure estimated for a single event at ~170 kbar. The existing complex, altered martensite structure containing both taenite and kamacite (3–15% Ni) particles was apparently the product of both shock deformation and thermal aging processes. The maximum temperature reached during thermal aging is estimated to be less than 400°C, and perhaps below 310°C.     

Studies of an artificially shock-loaded H group chondrite

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.     

The nature of dark-etching rims in meteoritic taenite

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.     

Analytical electron microscope study of eight ataxites

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.     

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

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.     

Metal and associated phases in Bishunpur,a highly unequilibrated ordinary chondrite

It appears that the highly unequilibrated Bishunpur ordinary chondrite preserves phase relations acquired during solar nebular processes to a relatively high degree; metamorphic temperatures may not have exceeded 300–350°C. The major categories of metal are: 3 kinds of metal in the metal matrix, three kinds in chondrule interiors and 2 kinds in chondrule rims. The fine-grained matrix metal is highly variable in composition: the kamacite Co content (7.8 ± 2.0 mg/g) is within the L-group range (6.7–8.2 mg/g) but extends well above and below; its Ni content (38 ± 5 mg/g) is considerably lower than in more equilibrated chondrites and taenite is Ni-rich ( > 450 mg/g) and unzoned. These compositions imply equilibration at very low temperatures of about 300–350°C. It seems unlikely that volume diffusion could account for the observed relatively unzoned phases; a better model involves mass transport by grain boundary diffusion and grain growth at the indicated temperatures. We find no evidence that the matrix was ever at higher temperatures. Large (50–650 μm) polycrystalline metal aggregates consisting of individually zoned crystals are also found in the matrix; they probably represent clusters formed in the solar nebula. A few large (50–250 μm) round monocrystalline grains are also present in the matrix.Metal-bearing chondrules tend to be highly reduced; they contain low-Ni metal that occasionally contains Si and/or Cr. Silicates in these chondrules tend to have low $\text{FeO}\text{(FeO + MgO)}$ ratios. The Si-rich metal grains are never in contact with silicates and are always surrounded by troilite with a poorly characterized Ca, Cr-sulfide at the metal-troilite interface; they appear to be high temperature nebular condensates that avoided oxidation even during the chondrule forming process. Silicon contents drop below our detection limit when the sulfide coating is absent. Much more common in chondrule interiors are Si-free spheroidal metal grains not associated with sulfides. These have Ni and Co contents very similar to the Si-bearing grains, and appear to be oxidized variants of the same material. The third class of chondrule metal is fine ( ~1 μm) dusty grains inside individual olivine grains. These seem to reflect high temperature in situ reduction of FeO from the olivine.The composition of kamacite is different in sulfide-rich and sulfide-poor chondrule rims and in both cases it is dissimilar to the compositions in the chondrule interiors and matrix; this indicates that chondrule rims could not have resulted from reactions with the matrix, but are primary features acquired prior to accretton.     

The Tucson meteorite

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.     

Subsolidus and melting relations for the join CaCO3-MgCO3 at 10 kbar

Mixtures of pure dry CaCO₃ and MgCO₃ were reacted at 10 kbar in a piston-cylinder apparatus. Solidus and liquidus boundaries were delineated by interpretation of quenched textures. X-ray determined compositions of quenched carbonates are not a reliable guide to the phase relations. The binary melting loop for CaCO₃-MgCO₃ extends from CaCO₃ at 1460°C through a liquidus minimum near 30 wt% MgCO₃ and 1075°C, and it is terminated at the incongruent melting reaction for dolomite solid solution at 1125° C (liquid with 32 wt% MgCO₃) Magnesite solid solution dissociates at 1090°C to produce dolomite + periclase + CO₂, truncating the dolomite-magnesite solvus. The 10 kb liquidus minimum at 1075°C and 30 wt% MgCO₃ occurs at lower temperature and higher $\text{Ca}\text{Mg}$ ratio than the 27 kbar liquidus minimum at 1290°C and 38 wt% MgCO₃. This relationship suggests that the first liquid produced by melting of a carbonate-bearing peridotite has increasing $\text{Mg}\text{Ca}$ ratio with increasing pressure. These phase relations provide part of the framework required to trace paths of crystallization of kimberlite and carbonatite magmas.     

Rutile solubility and titanium coordination in silicate melts

The solubility of rutile has been determined in a series of compositions in the K₂O-Al₂O₃-SiO₂ system ($\text{K}{}^1\text{=}\text{K}{}_2\text{O}\text{(K}{}_2\text{O}$ + Al₂O₃) = 0.38–0.90), and the CaO-Al₂O₃-SiO₂ system ($\text{C}{}^1\text{=}\text{CaO}\text{(CaO + Al}{}_2\text{O}{}_3\text{)}\text{= 0.47–0.59}$). Isothermal results in the KAS system at 1325°C, 1400°C, and 1475°C show rutile solubility to be a strong function of the $\text{K}{}^1$ ratio. For example, at 1475°C the amount of TiO₂ required for rutile saturation varies from 9.5 wt% ($\text{K}{}^1\text{= 0.38}$) to 11.5 wt% ($\text{K}{}^1\text{= 0.48}$) to 41.2 wt% ($\text{K}{}^1\text{= 0.90}$). In the CAS system at 1475°C, rutile solubility is not a strong function of $\text{C}{}^1$. The amount of TiO₂ required for saturation varies from 14 wt% ($\text{C}{}^1\text{= 0.48}$) to 16.2 wt% ($\text{C}{}^1\text{= 0.59}$).The solubility changes in KAS melts are interpreted to be due to the formation of strong complexes between Ti and K⁺ in excess of that needed to charge balance Al³⁺. The suggested stoichiometry of this complex is K₂Ti₂O₅ or K₂Ti₃O₇. In CAS melts, the data suggest that Ca²⁺ in excess of A1³⁺ is not as effective at complexing with Ti as is K⁺. The greater solubility of rutile in CAS melts when $\text{C}{}^1$ is less than 0.54 compared to KAS melts of equal $\text{K}{}^1$ ratio results primarily from competition between Ti and Al for complexing cations (Ca vs. K).TiK_β x-ray emission spectra of KAS glasses ($\text{K}{}^1\text{= 0.43–0.60}$) with 7 mole% added TiO₂, rutile, and Ba₂TiO₄, demonstrate that the average Ti-O bond length in these glasses is equal to that of rutile rather than Ba₂TiO₄, implying that Ti in these compositions is 6-fold rather than 4-fold coordinated. Re-examination of published spectroscopic data in light of these results and the solubility data, suggests that the 6-fold coordination polyhedron of Ti is highly distorted, with at least one Ti-O bond grossly undersatisfied in terms of Pauling's rules.     

Stable hydrogen isotopes in iron oxides—isotope effects associated with the dehydration of a natural goethite

The dehydration of a natural goethite to hematite is accompanied by a systematic hydrogen isotope fractionation. Closed system dehydration at, and below, 250°C results in a significantly greater degree of isotopic fractionation than does open system dehydration. This relationship is apparently reversed at 300°C. Both processes produce a progressive decrease in the $\text{D}\text{H}$ ratio of the mineral hydrogen with increasing degree of dehydration. At temperatures of 160°C to 250°C the closed system mineralvapor fractionation factor is independent of temperature, while above 250°C, it varies strongly with temperature. The mineral-vapor fractionation factor associated with open system dehydration appears to be independent of temperature over the interval 160°C to 300°C. The closed system $\text{D}\text{H}$ fractionation suggests that natural goethite undergoing dehydration in the presence of water can isotopically exchange with that water.CO₂ loss from goethite during dehydration is correlated with the loss of H₂O. The CO₃ is thought to be present in carbonates which exist as impurities in the goethite. Loss of both H₂O and CO₂ appears to be diffusion-controlled.     

The chemistry of geothermal waters in Iceland. III. Chemical geothermometry in geothermal investigations

New data from geothermal wells in Iceland have permitted empirical calibration of the chalcedony and $\text{Na}\text{K}$ geothermometers in the range of 25–180°C and 25–250°C respectively. The temperature functions are:

$\text{t°C=}\text{1112}\text{4.91?}\text{log SiO}{}_2\text{?273.15}$

$\text{t°C=}\text{933}\text{0.993+}\text{log Na/K}\text{?273.15}$

Concentrations are expressed in ppm. These temperature functions correspond well with the chalcedony solubility data of Fournier (1973) and the thermodynamic data for low-albite/microcline/solution equilibria of Heloeson (1969).A new CO₂ geothermometer is proposed which is considered to be useful in estimating underground temperatures in fumarolic geothermal fields. Its application involves analysis of CO₂ concentrations in the fumarole steam. The temperature function which applies in the range 180?300°C is: logCO₂ = 37.43 + 73192/T- 11829· 10₃/T² + 0.18923T- 86.187·logT where T is in °K and CO₂ in moles per kg of steam.     

Experimental hydrogen isotope studies—I. Systematics of hydrogen isotope fractionation in the systems epidote-H2O,zoisite-H2O and AlO(OH)-H2O

$\text{H}\text{D}$ Fractionation factors between epidote minerals and water, and between the AlO(OH) dimorphs boehmite and diaspore and water, have been determined between 150 and 650°C. Small water mineral ratios were used to minimise the effect of incongruent dissolution of epidote minerals. Waters were extracted and analysed directly by puncturing capsules under vacuum. Hydrogen diffusion effects were eliminated by using thick-walled capsules.$\text{H}\text{D}$ Exchange rates are very fast between epidote and water (and between boehmite and water), complete exchange taking only minutes above 450°C but several months at 250°C. Exchange between zoisite and water (and between diaspore and water) is very much slower, and an interpolation method was necessary to determine fractionation factors at 450 and below.For the temperature range 300–650°C, the $\text{H}\text{D}$ equilibrium fractionation factor (α^e) between epidote and water is independent of temperature and Fe content of the epidote, and is given by 1000 In α_epidote-H2O^e = ?35.9 ± 2.5, while below 300°C 1000 In , with a ‘cross-over’ estimated to occur at around 185°C. By contrast, zoisite-water fractionations fit the relationship 1000 In .All studied minerals have hydrogen bonding. Fractionations are consistent with the general relationship: the shorter the O-H -- O bridge, the more depleted is the mineral in D.On account of rapid exchange rates, natural epidotes probably acquired their H-isotope compositions at or below 200°C, where fractionations are near or above 0%.; this is in accord with the observation that natural epidotes tend to concentrate D relative to other coexisting hydrous minerals.     

Carbonate equilibria in hydrothermal systems: First ionization of carbonic acid in NaCl media to 300°C

The ionization quotients of aqueous carbon dioxide (carbonic acid) have been precisely determined in NaCl media to 5 m and from 50° to 300°C using potentiometric apparatus previously developed at Oak Ridge National Laboratory. The pressure coefficient was also determined to 250°C in the same media. These results have been combined with selected information in the literature and modeled in two ways to arrive at the best fits and to derive the thermodynamic parameters for the ionization reaction, including the equilibrium constant, activity coefficient quotients, and pressure coefficients. The variation with temperature of the two fundamental quantities $\text{Δ}\text{V}\text{?}{}^{\mathrm{o}}$ and $\text{Δ}\text{C}\text{?}{}^{\mathrm{o}}{}_{\mathrm{p}}$ were examined along the saturation vapor pressure curve and at constant density. The results demonstrated again that for reactions with minimal electrostriction changes the magnitudes and variations of $\text{Δ}\text{C}\text{?}{}^{\mathrm{o}}{}_{\mathrm{p}}$ and $\text{Δ}\text{V}\text{?}{}^{\mathrm{o}}$ with temperature are small and, in addition, $\text{Δ}\text{C}\text{?}{}_{\mathrm{p}}$ and $\text{Δ}\text{V}\text{?}$ are approximately independent of salt concentration.The results have also been applied to an examination of the solubility of calcite as a function of pH (in a given NaCl medium) for the neutral to acidic region both for systems with fixed CO₂ pressure and systems where the calcium ion concentration equals the concentration of carbon. The pH of saturated solutions of calcite with P_CO2 of 12 bars increases from 5.1 to 5.5 between 100° and 300°C.     

The origin of metal clasts in the Bencubbin meteoritic breccia

Bencubbin is a breccia containing metal and silicate clasts, along with occasional chondritic fragments. The breccia is cemented together by a small amount of shock-melted metal-silicate matrix. There is no evidence, however, for complete melting of either metal or silicate clasts after their incorporation within the breccia. The main metal phase occurs as rounded and angular clasts of Fe-Ni. Each clast is chemically homogeneous, but systematic chemical variations between clasts are observed with Ni concentrations varying from ? 5.3 wt% in some clasts up to 7.5 wt% in others. Cobalt concentrations vary between clasts from 0.25 to 0.35 wt% and are positively correlated with Ni. The Co and Ni concentrations are consistent with the metal condensation path (pressure ? 10^-3 atm) predicted by Grossman and Olsen (1974, Geochim. Cosmochim. Acta38, 173–187). The P vs Ni concentrations are consistent with the metal condensation path (pressure ? 10^?4atm) predicted by Wai et al. (1978, Lunar and Planetary Science IX, pp. 1193–1195). Thus we believe that Bencubbin metal clasts may record chemical information imparted during condensation of the metal from the nebula. Chromium concentrations in Bencubbin metal (0.05–0.30 wt%) greatly exceed concentrations observed in iron meteorites as predicted by Grossman and Olsen (1974, Geochim. Cosmochim. Acta38, 173–187) for metal condensates from the solar nebula. The Cr-Ni trend in Bencubbin, however, is positively correlated with Ni, in contrast to the predicted condensation trend. This unexpected correlation may be the result of subsequent redistribution of Cr between metal and micron-sized troilite blebs. The incorporation of these troilite blebs within the metal clasts is difficult to explain in terms of low temperature (? 700 K) condensation of troilite. The possible explanations for the presence of the troilite may or may not be consistent with an unaltered primitive composition for the metal clasts. High temperature equilibrium condensation of Bencubbin metal, however, is also supported by the low Ga and Ge contents reported by Kallemeynet al. (1978, Geochim. Cosmochim. Acta42, 507–515). Three of the metal clasts were found to contain ?2.3wt% Si in alloy with the metal. The compositions of these clasts are consistent with equilibrium condensation at a pressure of ? 1 atm from a gas of cosmic composition. The Si-rich clasts could also have condensed at lower pressures from a gas with a fractionated $\text{C}\text{O}$ ratio relative to cosmic abundances.     

Origin of rapidly solidified metal-troilite grains in chondrites and iron meteorites

Inclusions of troilite and metallic Fe,Ni 0.2–4 mm in size with a dendritic or cellular texture were observed in 12 ordinary chondrites. Cooling rates in the interval 1400?950°C calculated from the spacing of secondary dendrite arms or cell widths and published experimental data range from 10^?7 to 10⁴°C/sec. In 8 of these chondrites, which are breccias containing some normal slow-cooled metal grains, the inclusions solidified before they were incorporated into the breccias. Their cooling rates of 1–300 °C/sec indicate cooling by radiation, or by conduction in contact with cold silicate or hot silicate volumes only 6–40 mm in size. This is quantitative evidence that these inclusions and their associated clasts were melted on the surface of a parent body (by impact), and were not formed at depth from an internally derived melt. In Ramsdorf, Rose City and Shaw, which show extensive reheating to ? 1000°C, Fe-FeS textures in melted areas are coarser and indicate cooling rates of 10^?1 to 10^?4°C/sec during solidification. This metal may have solidified inside hot silicate volumes that were 10–300 cm in size. As Shaw and Rose City are breccias of unmelted and melted material, their melted metal did not necessarily cool through 1000°C within a few m of the surface. Shock-melted, fine-grained, irregular intergrowths of metal and troilite formed in situ in many irons and some chondrites by rapid solidification at cooling rates of ? 10⁵°C/sec. Their kamacite and taenite compositions may result from annealing at ~250°C of metallic glass or exceedingly fine-grained quench products.