Evaporation and recondensation of sodium in Semarkona Type II chondrules

We have investigated the Na distributions in Semarkona Type II chondrules by electron microprobe, analyzing olivine and melt inclusions in it, mesostasis and bulk chondrule, to see whether they indicate interactions with an ambient gas during chondrule formation. Sodium concentrations of bulk chondrule liquids, melt inclusions and mesostases can be explained to a first approximation by fractional crystallization of olivine ± pyroxene. The most primitive olivine cores in each chondrule are mostly between Fa₈ and Fa₁₃, with 0.0022–0.0069 ± 0.0013 wt.% Na₂O. Type IIA chondrule olivines have consistently higher Na contents than olivines in Type IIAB chondrules. We used the dependence of olivine–liquid Na partitioning on FeO in olivine as a measure of equilibration. Extreme olivine rim compositions are ～Fa₃₅ and 0.03 wt.% Na₂O and are close to being in equilibrium with the mesostasis glass. Olivine cores compared with the bulk chondrule compositions, particularly in IIA chondrules, show very high apparent D_Na, indicating disequilibrium and suggesting that chondrule initial melts were more Na-rich than present chondrule bulk compositions. The apparent D_Na values correlate with the Na concentrations of the olivine, but not with concentrations in the bulk melt. We use equilibrium D_Na to find the Na content of the true parent liquid and estimate that Type IIA chondrules lost more than half their Na and recondensation was incomplete, whereas Type IIAB chondrules recovered most of theirs in their mesostases.Glass inclusions in olivine have lower Na than expected from fractionation of bulk composition liquids, and mesostases have higher Na than expected in calculated daughter liquids formed by fractional crystallization alone. These observations also require open system behavior of chondrules, specifically evaporation of Na before formation of melt inclusions followed by recondensation of Na in mesostases. Within this record of evaporation followed by recondensation, there is no indication of a stage with zero Na in the chondrules, which is predicted by models for shock wave cooling at canonical nebular pressures, suggesting high P_T.The high Na concentrations in olivine and mesostases indicate very high P_Na while chondrules were molten. This may be explained by local, very high particle densities where Type II chondrules formed. The high P_T, P_Na and number densities of chondrules implied suggest formation in debris clouds after protoplanetary collisions as an alternative to formation after passage of shock waves through large particle-rich clumps in the disk. Encounters of partially molten chondrules should have been frequent in these dense swarms. However, in many ordinary chondrites like Semarkona, “cluster chondrites”, compound chondrules are not abundant but instead chondrules aggregated into clusters. Chondrule melting, cooling and clustering in dense swarms contributed to rapid accretion, possibly after collision, by fallback on the grandparent body and by reaccretion as a new body downrange.     

An experimental study of the formation of metallic iron in chondrules

The abundance of metallic iron is highly variable in different kinds of chondrites. The precise mechanism by which metal fractionation occurred and its place in time relative to chondrule formation are unknown. As metallic iron is abundant in most Type I (FeO-poor) chondrules, determining under what conditions metal could form in chondrules is of great interest. Assuming chondrules were formed from low temperature nebular condensate, we heated an anhydrous CI-like material at 1580°C in conditions similar to those of the canonical nebula (P_H2 = 1.3 × 10⁻⁵ atm). We reproduced many of the characteristics of Type IA and IIA chondrules but none of them contained any iron metal. In these experiments FeO was abundant in charges that were heated for as long as 6 h. At a lower temperature, 1350°C, dendritic/cellular metal crystallized from Fe-FeS melts during the evaporation of S. However, the silicate portion consisted of many relict grains and vesicles, not typical of chondrules.Evaporation experiments conducted at P_H2 = 1 atm and 1565°C produced charges containing metallic iron both as melt droplets and inclusions in olivine, similar to those found in chondrules. Formation of iron in these experiments was primarily the result of desulfurization of FeS. With long heating times Fe° was lost by evaporation. Apart from some reduction of FeO by kerogen to make metal inclusions within olivine grains, reduction of FeO to make Fe° in these charges was not observed.This study shows that under canonical nebular conditions FeS and iron-metal are extremely volatile so that metal-rich Type I chondrules could not form by melting “CI.” Under these conditions FeO is lost predominantly by hydrogen stripping and, due to the relative low abundance of hydrogen at low pressures, remains in the melt for as long as 6 h. Conversely, at higher total pressures (1-atm H₂) iron metal (produced mainly by the desulfurization of troilite) is less volatile and remains in the melt for longer times (at least 6 h). In addition, due to elevated pressures of hydrogen, FeO is stripped away much faster. These results suggest that chondrule formation occurred in environments with elevated pressures relative to the canonical nebula for iron metal to be present.     

The formation of chondrules by open-system melting of nebular condensates

Experiments were conducted under canonical nebular conditions to see whether the chemical compositions of the various chondrule types can be derived from a single CI-like starting material by open-system melting and evaporation. Experimental charges, produced at 1580 °C and P_H2 of 1.31×10⁻⁵ atm over 1 to 18 hours, consisted of only two phases, porphyritic olivine crystals in glass. Sulfur, metallic-iron and alkalis were completely evaporated in the first minutes of the experiments and subsequently the main evaporating liquid oxides were FeO and SiO₂. Olivines from short runs (2-4 hours) have compositions of Fo₈₃-Fo₈₉, as in Type IIA chondrules, while longer experimental runs (12-18 hours) produce ∼Fo₉₉ olivine, similar to Type IA chondrules. The concentration of CaO in both olivine (up to 0.6 wt.%) and glass, and their Mg#, increased with increasing heating duration. Natural chondrules also show increasing CaO with decreasing S, alkalis, FeO and SiO₂. The similarities in bulk chemistry, mineralogy and textures between Type IIA and IA chondrules and the experimental charges demonstrate that these chondrules could have formed by the evaporation of CI precursors. The formation of silica-rich chondrules (IIB and IB) by evaporation requires a more pyroxene-rich precursor.Based on the FeO evaporation rates measured here, Type IIA and IA chondrules, were heated for at least ∼0.5 and ∼3.5 h, respectively, if formed at 1580 °C and P_H2 of 1.31×10⁻⁵ atm. Type II chondrules may have experienced higher cooling-rates and less evaporation than Type I.The experimental charges experienced free evaporation and exhibited heavy isotopic enrichments in silicon, as well as zero concentrations of S, Na and K, which are not observed in natural chondrules. However, experiments on potassium-rich melts at the same pressure but in closed capsules showed less evaporation of K, and less K isotopic mass fractionation, than expected as a function of decreasing cooling rate. Thus the environment in which chondrules formed is as important as the kinetic processes they experienced. If chondrule formation occurred under conditions in which evaporated gases remained in the vicinity of the residual melts, the extent of evaporation would be reduced and back reaction between the gas and the melt could contribute to the suppression of isotopic mass fractionation. Hence chondrule formation could have involved evaporative loss without Rayleigh fractionation. Volatile-rich Type II and volatile-poor Type I chondrules may have formed in domains with high and low chondrule concentrations, and high partial pressures of lithophile elements, respectively.     

Metal and associated phases in Krymka and Chainpur: Nebular formational processes

The highly unequilibrated LL3 chondrites Krymka and Chainpur preserve a relatively unaltered record of formation in the solar nebula in the texture and chemistry of their opaque mineral assemblages. A moderate degree of diversity among these meteorites and Bishunpur is apparently associated with formation under differing conditions.Spheroidal kamacite, some Cr-bearing, is present in chondrule interiors. Fine-grained metal within the Fe-rich opaque matrix of Krymka consists exclusively of taenite and minor tetrataenite; kamacite occurs inside metal-sulfide nodules. These nodules are surrounded by an inner layer of FeO-rich, fine-grained silicate material (FeO/(FeO + MgO) > 80%) and an outer troilite-rich layer, and contain variable amounts of a hydrated Fe-oxide phase. It appears that the nodules were melted, often incompletely, possibly during the chondrule formation process. Some nodule metal is Si- and Cr-bearing, indicating little reaction with nebular H₂O. Nodules are much less common in Chainpur than in Krymka and rare in Bishunpur.Most metal-poor chondrules in Krymka, Bishunpur and Chainpur appear to have formed from precursors that had acquired significant amounts of FeO as a result of reaction with the nebular gas down to low temperatures; metal-rich chondrules seem to have derived from aggregates of coarse, high-temperature Fe-poor silicates. Low Ni concentrations (34–41 mg/g) in chondrule kamacite may largely result from dilution by Fe reduced from the silicates during chondrule formation.The opaque silicate matrix of Krymka is considerably more oxidized than that of Bishunpur and Chainpur, it contains no kamacite and its composition is very uniform. This may either reflect the growth of silicate grains during incipient recrystallization in the matrices of Bishunpur and Chainpur or, more likely, a lower mean grain size of the Krymka matrix components, possibly indicating later formation of the Krymka parent planetesimal.     

Partitioning of iron and magnesium between crystals and partial melts in peridotite upper mantle

The iron-magnesium distribution coefficient, $$K'_D = (X_{\Sigma {\text{FeO}}} /X_{{\text{MgO}}} )^{{\text{olivine}}} (X_{{\text{MgO}}} /X_{\Sigma {\text{FeO}}} )^{{\text{liquid}}} ,$$ has frequently been used as a means of testing whether experimental and natural silicate liquids could have been in equilibrium with olivine of mantle composition. It is shown here that this K′ _D decreases with increasing oxygen fugacity (xxx) for a hydrous partial melt in equilibrium with a natural spinel peridotite assemblage under pressure and temperature conditions corresponding to those of the upper mantle (from 0.52 at the xxx of the iron-wüstite buffer to 0.04 at the xxx of the magnetite-hematite buffer). K′ _D also increases with increasing pressure, with decreasing temperature, and probably with increasing Mg/(Mg+∑ Fe) of the parental peridotite, suggesting that $$K_D = (X_{{\text{FeO}}} /X_{{\text{MgO}}} )^{{\text{olivine}}} (X_{{\text{MgO}}} /X_{{\text{FeO}}} )^{{\text{liquid}}}$$ also increases with increasing pressure and decreasing temperature. Thus, unless these four variables (P, T, xxx, silicate composition) are known for a natural magma, K′ _D and probably K _D are variables, and the Mg/(Mg+∑ Fe) of such a magma cannot be correlated to that of the parent. The K _D determined at 1 atm pressure by Roeder and Emslie has frequently been used to test whether the Mg/(Mg+∑ Fe) ratios of experimentally formed liquids at high pressure in equilibrium with olivine of known Fo content represent the equilibrium Mg/(Mg+Fe²⁺) of this liquid, assuming that ∑Fe=Fe²⁺ and that K′ _D does not vary with P, T, and composition of the system. Published data demonstrate that the oxygen fugacities of the experimental designs employed by different laboratories vary between those of the magnetite-hematite and magnetite-wüstite buffers (6 orders of magnitude), resulting in K′ _D between 0.04 and 0.31 at 1050° C and 15 kbar, for example. Thus, published arguments as to whether the quenched liquids represent equilibrium compositions based on iron-magnesium partitioning are inadequate. The effects of P, T, xxx, and the composition of the starting material must also be considered.     

Conditions of condensate rim formation on the surface of lunar regolith particles

Condensate objects observed in the lunar regolith are distinctly separated on the basis of morpho-logical and chemical characteristics into droplets condensed during the expansion of an impact-generated vapor cloud and films condensed on the relatively cold surface of mineral particles. Using the analyses of both condensate forms and experimental data on the evaporation of melt corresponding to a typical lunar highland rock of the gabbro-anorthosite composition from Apollo 16 sample 68415.40, the temperature conditions of vapor condensation during lunar impact events were estimated. The comparison of condensate compositions with the analyses of vapors from the evaporation experiment showed that, compared with the compositions of droplet-type condensates, the condensate rims were formed from a vapor with high contents of refractory CaO and Al₂O₃ and at very different condensation temperatures. The enrichment of vapor in CaO and Al₂O₃ could be attained only at high temperatures of melt evaporation (higher than ∼ 1850°C according to experimental data). The estimated condensation temperatures of droplets are significantly lower, ∼1750–1500°C. Rim-type condensates were produced by vapor quenching on the relatively cold surface of a solid mineral particle, which resulted in almost complete precipitation of all major components of the silicate vapor without fractionation in accordance with their individual volatilities.     

Activities and volatilities of trace components in silicate melts: a novel use of metal–silicate partitioning data

Ian Carmichael spent 45 years thinking about and working on the activities of components in silicate melts and their use to estimate physicochemical conditions at eruption and in the source regions of igneous rocks. These interests, principally in major components such as SiO₂, led us to think about possible ways of determining the complementary activity coefficients of trace components in silicate melts. While investigating the conditions of accretion and differentiation of the Earth, a number of authors have determined the partitioning of trace elements such as Co, Ni, Mo and W between liquid Fe metal and liquid silicate. These data have the potential to provide activity information for a large number of trace components in silicate melts. In order to turn the partitioning measurements into activities, however, we need to know the activity coefficient of FeO, γ_FeO in the silicate. We obtained γ_FeO as a function of melt composition by fitting a simple model to 83 experimental data for which the authors had measured the FeO content of the silicate melt in equilibrium with metal (Fe-bearing alloy) at known f_O2. The compositional dependence of γ_FeO is weak, but, when calculated in the system Diopside–Anorthite–Forsterite, it decreases towards the Forsterite apex. A similar approach for Ni, for which twice as many data are available, leads to similar composition dependence of activity coefficient and confirms the suggestion that γ_NiO/γ_FeO is almost constant over a wide range of silicate melt composition. The activity coefficients for FeO were used in conjunction with measured Mo and W partitioning between Fe-rich metal and silicate melt to estimate activity coefficients for trace MoO₂ and WO₃ dissolved in silicate melt. When combined with data on Mo- and W-saturated silicate melts a strong dependence of activity coefficient is observed. Calculated in the system Diopside–Anorthite–Forsterite, both MoO₂ and WO₃ exhibit similar behaviour to FeO and NiO in that activity coefficients decrease as Forsterite content increases. The effect is much larger for Mo and W, however, γ_MoO2 and γ_WO3 varying by factors of 20 and nearly 100, respectively, in this system. In order to illustrate the potential applications of the metal–silicate partitioning approach to determine the activity coefficients of volatile elements, we used it to determine activity coefficients of PbO, CuO_0.5 and InO_1.5 in a silica-saturated melt at 1,650 °C. We find values of 0.22, 3.5 and 0.02, respectively, indicating a strong dependence on cation charge. The value for CuO_0.5 is in excellent agreement with experimental data of Holzheid and Lodders (Geochim Cosmochim Acta 65:1933–1951, 2001), which shows that the method is viable. When combined with thermodynamic data on the gas species, we find that Pb is the most volatile of the 3 elements under ‘normal’ terrestrial conditions of oxygen fugacity but that In should become the most volatile under strongly reducing conditions such as those of the solar nebula. The oxygen fugacity dependence of volatility has implications for the high relative abundance of In in silicate Earth. We conclude that metal–silicate partitioning experiments are a viable means for determining activities of trace components in silicate melts and are particularly useful if the metal of the element is unstable or volatile at igneous temperatures.     

Partial molar volumes of oxide components in silicate liquids

Densities of 21 silicate liquids have been determined from 1,000 ° to 1,600 ° C. The compositions studied contain from two to eight oxide components and have the following ranges in composition (mole %): SiO₂, 35–79%; TiO₂, 4–36%; Al₂O₃, 5–25%; FeO, 11–41%; MgO, 7–28%; CaO, 7–35%; Na₂O, 5–50%; and K₂O, 4–20%. The compositions thus cover the upper range observed in magmas for each oxide. Precision for each determination of liquid density is always better than ±1%.Volumes/gfw (gram formula weight) calculated from the density measurements and the chemical compositions of the analyzed liquids have been combined with data on 96 silicate liquids reported in the literature. From this data set we derive, by using multiple linear regression, partial molar volumes of the components SiO₂, TiO₂, A1₂O₃, FeO, MgO, CaO, Na₂O, and K₂O at five temperatures. The standard deviation of the multiple regression is 1.8% of the molar volumes, which is considered about equal to the total errors due to compositional and instrumental uncertainties.These derived partial molar volumes have been used to calculate volumes/gfw of natural silicate liquids which are found to agree within 1% of the measured values. No compositional dependence of the partial molar volumes can be detected within the error considered to be typical of the measurements. This is further supported by the close agreement between the calculated volumes of CaMgSi₂O₆ and Fe₂SiO₂ liquids derived from the initial slopes of their fusion curves and their heats of fusion, and the volumes obtained by summing the respective partial molar volumes. The experimental data indicate that silicate liquids mix ideally with respect to volume, over the temperature and composition range of this data set.     

Thermodynamic analysis of Fe and Mg partitioning between plagioclase and silicate liquid

 Thermodynamic analysis of Fe- and Mg-bearing plagioclase and silicate liquid was carried out based on reported element partitioning data between plagioclase and silicate liquid in reduced conditions, solution properties of ternary feldspar, standard state properties of plagioclase endmembers and solution properties of multicomponent silicate liquid. Derived mixing properties of Fe- and Mg-bearing plagioclase are in harmony with estimated results from synthetic experiments in the systems CaAl₂Si₂O₈-CaFeSi₃O₈ and CaAl₂Si₂O₈-CaMgSi₃O₈. Based on the determined solution properties of the plagioclase, a computer program to calculate the element partition relationships between Fe- and Mg-bearing plagioclase and multicomponent silicate liquid was developed. The FeO, MgO and MgO/(MgO + FeO) in plagioclase predicted from known liquid compositions and pressure are in agreement with measurements within 0.2 wt%, 0.1 wt% and 0.1 (mol ratio), respectively. The Fe³⁺ content in plagioclase crystallized at high oxygen fugacity can be estimated with this program. The Fe³⁺/total Fe ratio in plagioclase crystallized near the quartz-fayalite-magnetite buffer ranges from 0 to 0.5, which is consistent with previous study on natural plagioclase in submarine basalt. Derived solution properties of the Fe- and Mg-bearing plagioclase are also used to calculate equilibrium composition relationship between olivine and plagioclase. Change of X _Fo in olivine coexisting with plagioclase affects MgO and FeO contents in plagioclase greatly. The present model predicts X _Fo of coexisting olivine from the chemical composition of plagioclase to ±0.1 accuracy at given pressure and temperature. Received: 27 March 1998 / Accepted: 30 September 1999     

Chemistry of glass inclusions in olivines of the CR chondrites Renazzo, Acfer 182, and El Djouf 001

Glass inclusions in olivines of the Renazzo, El Djouf 001, and Acfer 182 CR-type chondrites are chemically divers and can be classified into Al-rich, Al-poor, and Na-rich types. The chemical properties of the glasses are independent of the occurrence of the olivine (isolated or part of an aggregate or chondrule) and its composition. The glasses are silica-saturated (Al-rich) or oversaturated (Al-poor, 24% normative quartz). All glasses have chondritic CaO/Al₂O₃ ratios, unfractionated CI-normalized abundances of refractory trace elements and are depleted in moderately volatile and volatile elements. Thus the glasses are likely to be of a primitive condensate origin whose chemical composition has been established before chondrule formation and accretion, rather then the product of either crystal fractionation from chondrule melts or part melting of chondrules. Rare Na-rich glasses give evidence for elemental exchange between the glass and a vapor phase. Because they have Al₂O₃ contents and trace element abundances very similar to those of the Al-rich glasses, they likely were derived from the latter by Ca exchange (for Na) with the nebula. Elemental exchange reactions also have affected practically all olivines (e.g., exchange of Mg of olivine for Fe²⁺, Mn²⁺, and Cr³⁺). Glasses formed contemporaneously with the host olivine. As the most likely process for growing nonskeletal olivines from a vapor we consider the VLS (vapor-liquid-solid) growth process, or liquid-phase epitaxy. Glasses are the possible remnants of the liquid interface between growing crystal and the vapor. Such liquids can form stably or metastably in regions with enhanced oxygen fugacity as compared to that of a nebula of solar composition.     

Magmatic evolution of the material of the Earth’s lower mantle: Stishovite paradox and origin of superdeep diamonds (Experiments at 24–26 GPa)

The ultrabasic–basic magmatic evolution of the lower mantle material includes important physicochemical phenomena, such as the stishovite paradox and the genesis of superdeep diamonds. Stishovite SiO₂ and periclase–wüstite solid solutions, (MgO · FeO)_ss, associate paradoxically in primary inclusions of superdeep lower mantle diamonds. Under the conditions of the Earth’s crust and upper mantle, such oxide assemblages are chemically impossible (forbidden), because the oxides MgO and FeO and SiO₂ react to produce intermediate silicate compounds, enstatite and ferrosilite. Experimental and physicochemical investigations of melting phase relations in the MgO–FeO–SiO₂–CaSiO₃ system at 24 GPa revealed a peritectic mechanism of the stishovite paradox, (Mg, Fe)SiO₃ (bridgmanite) + L = SiO₂ + (Mg, Fe)O during the ultrabasic–basic magmatic evolution of the primitive oxide–silicate lower mantle material. Experiments at 26 GPa with oxide–silicate–carbonate–carbon melts, parental for diamonds and primary inclusions in them, demonstrated the equilibrium formation of superdeep diamonds in association with ultrabasic, (Mg, Fe)SiO₃ (bridgmanite) + (MgO · FeO)_ss (ferropericlase), and basic minerals, (FeO · MgO)_ss (magnesiowüstite) + SiO₂ (stishovite). This leads to the conclusion that a peritectic mechanism, similar to that responsible for the stishovite paradox in the pristine lower mantle material, operates also in the parental media of superdeep diamonds. Thus, this mechanism promotes both the ultrabasic–basic evolution of primitive oxide–silicate magmas in the lower mantle and oxide–silicate–carbonate melts parental for superdeep diamonds and their paradoxical primary inclusions.     

Experimental investigation of H2O and Cl− solubilities in F-enriched silicate liquids; implications for volatile saturation of topaz rhyolite magmas

To interpret the degassing of F-bearing felsic magmas, the solubilities of H₂O, NaCl, and KCl in topaz rhyolite liquids have been investigated experimentally at 2000, 500, and ≈1 bar and 700° to 975 °C. Chloride solubility in these liquids increases with decreasing H₂O activity, increasing pressure, increasing F content of the liquid from 0.2 to 1.2 wt% F, and increasing the molar ratio of ((Al + Na + Ca + Mg)/Si). Small quantities of Cl⁻ exert a strong influence on the exsolution of magmatic volatile phases (MVPs) from F-bearing topaz rhyolite melts at shallow crustal pressures. Water- and chloride-bearing volatile phases, such as vapor, brine, or fluid, exsolve from F-enriched silicate liquids containing as little as 1 wt% H₂O and 0.2 to 0.6 wt% Cl at 2000 bar compared with 5 to 6 wt% H₂O required for volatile phase exsolution in chloride-free liquids. The maximum solubility of Cl⁻ in H₂O-poor silicate liquids at 500 and 2000 bar is not related to the maximum solubility of H₂O in chloride-poor liquids by simple linear and negative relationships; there are strong positive deviations from ideality in the activities of each volatile in both the silicate liquid and the MVP(s). Plots of H₂O versus Cl⁻ in rhyolite liquids, for experiments conducted at 500 bar and 910°–930 °C, show a distinct 90° break-in-slope pattern that is indicative of coexisting vapor and brine under closed-system conditions. The presence of two MVPs buffers the H₂O and Cl⁻ concentrations of the silicate liquids. Comparison of these experimentally-determined volatile solubilities with the pre-eruptive H₂O and Cl⁻ concentrations of five North American topaz and tin rhyolite melts, determined from melt inclusion compositions, provides evidence for the exsolution of MVPs from felsic magmas. One of these, the Cerro el Lobo magma, appears to have exsolved alkali chloride-bearing vapor plus brine or a single supercritical fluid phase prior to entrapment of the melt inclusions and prior to eruption. Received: 6 November 1995 / Accepted: 29 January 1998     

The compressibility of silicate liquids containing Fe2O3 and the effect of composition,temperature, oxygen fugacity and pressure on their redox states

Ultrasonic longitudinal acoustic velocities in oxidized silicate liquids indicate that the pressure derivative of the partial-molar volume of Fe₂O₃ is the same in iron-rich alkali-, alkaline earth- and natural silicate melt compositions at 1 bar. The dV/dP for multicomponent silicate liquids can be expressed as a linear combination of partial-molar constants plus a positive excess term for Na₂O−Al₂O₃ mixing. Partial-molar properties for FeO and Fe₂O₃ components allow extension of the empirical expression of Sack et al. (1980) to permit the calculation of Fe-redox equilibrium in a natural silicate liquid as a function of composition, temperature, fo₂ and pressure; a more formal thermodynamic expression is presented in the Appendix. The predicted equilibrium fo₂ of natural silicate melts, of fixed oxygen content, closely parallels that defined by the metastable assemblage fayalite+magnetite+β-quartz (FMQ), in pressure-temperature space. A silicate melt initially equilibrated at 3 GPa and FMQ, will remain within approximately 0.5 log₁₀ units of FMQ during its closed-system ascent. Thus, for magmas closed to oxygen, iron-redox equilibrium in crystal-poor pristine glassy lavas represents an excellent probe of the relative oxidation state of their source regions.     

Chemical equilibrium calculations for bulk silicate earth material at high temperatures

The chemical equilibrium distribution of 69 elements between gas and melt is modeled for bulk silicate Earth (BSE) material over a wide P – T range (1000–4500 K, 10⁻⁶–10² bar). The upper pressure end of this range may occur during lunar formation in the aftermath of a Giant Impact on the proto-Earth. The lower pressures may occur during evaporation from molten silicates on achondritic parent bodies. The virial equation of state shows silicate vapor behaves ideally in the P - T range studied. The BSE melt is modeled as a non-ideal solution and the effects of different activity coefficients and ideal solution are studied. The results presented are 50% condensation temperatures, major gas species of each element, and the pressure and temperature dependent oxygen fugacity (fO₂) of dry and wet BSE material. The dry BSE model has no water because it excludes hydrogen; it also excludes the volatile elements (C, N, F, Cl, Br, I, S, Se, Te). The wet BSE model has water because it includes hydrogen; it also includes the other volatiles. Some key conclusions include the following: (1) much higher condensation temperatures in silicate vapor than in solar composition gas at the same total pressure due to the higher metallicity and higher oxygen fugacity of silicate vapor (cf. Fegley et al. 2020), (2) a different condensation sequence in silicate vapor than in solar composition gas, (3) good agreement between different activity coefficient models except for the alkali elements, which show the largest differences between models, (4) agreement, where overlap exists, with prior published silicate vapor condensation calculations (e.g., Canup et al. 2015, Lock et al. 2018, Wang et al. 2019), (5) condensation of Re, Mo, W, Ru, Os oxides instead of metals over the entire P – T range, (6) a stability field for Ni-rich metal as reported by Lock et al. (2018), (7) agreement between ideal solution (from this work and from Lock et al. 2018) and real solution condensation temperatures for elements with minor deviations from ideality in the oxide melt, (8) similar 50% condensation temperatures, within a few degrees, in the dry and wet BSE models for the major elements Al, Ca, Fe, Mg, Si, and the minor elements Co, Cr, Li, Mn, Ti, V, and (9) much lower 50% condensation temperatures for elements such as B, Cu, K, Na, Pb, Rb, which form halide, hydroxide, sulfide, selenide, telluride and oxyhalide gases. The latter results are preliminary because the solubilities and activities of volatile elements in silicate melts are not well known, but must be considered for the correct equilibrium distribution, 50% condensation temperatures and mass balance of halide (F, Cl, Br, I), hydrogen, sulfur, selenium and tellurium bearing species between silicate melt and vapor.     

Microchondrule-bearing clast in the Piancaldoli LL3 meteorite: a new kind of type 3 chondrite and its relevance to the history of chondrules

In the Piancaldoli LL3 chondrite, we found a mm-sized clast containing ~100 chondrules 0.2–64 μm in apparent diameter (much smaller than any previously reported) that are all of the same textural type (radial pyroxene; FS_1–17). This clast, like other type 3 chondrites, has a fine-grained Ferich opaque silicate matrix, sharply defined chondrules, abundant low-Ca clinopyroxene and minor troilite and Si- and Cr-bearing metallic Fe,Ni. However, the very high modal matrix abundance (63 ± 8 vol. %), unique characteristics of the chondrules, and absence of microscopically-observable olivine indicate that the clast is a new kind of type 3 chondrite. Most chondrules have FeO-rich edges, and chondrule size is inversely correlated with chondrule-core FeO concentration (the first reported correlation of chondrule size and composition). Chondrules acquired Fe by diffusion from Fe-rich matrix material during mild metamorphism, possibly before final consolidation of the rock. Microchondrules (those chondrules ? 100 μm in diameter) are also abundant in another new kind of type 3 chondrite clast in the Rio Negro L chondrite regolith breccia. In other type 3 chondrite groups, microchondrule abundance appears to be anticorrelated with mean chondrule size, viz. 0.02–0.04 vol. % in H and CO chondrites and ?0.006 vol. % in L, LL, and CV chondrites.Microchondrules probably formed by the same process that formed normal-sized droplet chondrules: melting of pre-existing dustballs. Because most compound chondrules in the clast and other type 3 chondrites formed by collisions between chondrules of the same textural type, we suggest that dust grains were mineralogically sorted in the nebula before aggregating into dustballs. The sizes of compound chondrules and chondrule craters, which resulted from collisions of similarly-sized chondrules while they were plastic, indicate that size-sorting (of dustballs) occurred before chondrule formation, probably by aerodynamic processes in the nebula. We predict that other kinds of type 3 chondrites exist which contain chondrule abundances, size-ranges and proportions of textural types different from known chondrite groups.     

Metal in CR chondrites

In section many low-FeO CR chondrules are surrounded by rings of metal; this metal-cladding seems to have formed during chondrule melting events as films of metal that wetted the surface. Electron microprobe studies show that in each ring the metal is very uniform in composition, consistent with efficient mixing during formation of the metal film. In contrast the mean Ni contents of 13 different rings vary by up to a factor of 2. There is no FeS associated with ring metal. Ring metal Co is positively correlated with Ni but the Co/Ni ratio seems to decrease with increasing Ni. We observed a weak negative correlation between ring metal Ni and the fayalite content of the host olivine. Coarse interior metal has higher Ni contents than that in the surrounding rings. At any specific chondrule location, smaller grains tend to have lower Ni contents than larger grains. These trends in Ni seem to reflect two processes: (1) The mean Ni content of metal (and easily reduced sulfides or oxides) in chondrule precursor materials seems to have decreased with the passage of time; on average, the metal in earlier-formed chondrules had higher Ni contents than the metal in later-formed chondrules. (2) Some oxidized Fe was reduced during chondrule formation leading to lower Ni contents in small grains compared to large grains; prior to reduction the Fe was in FeS or in FeO in accessible (fine-grained) sites. We suggest that the compositional evolution of nebular solids was responsible for the interchondrule variations whereas reduction of minor amounts of FeS or FeO was responsible for the size-related small variations in Ni content. We suggest that, during chondrule formation events, CR chondrules experienced relatively long thermal pulses that were responsible for the thorough loss of FeS and the common granoblastic texture observed in low-FeO chondrules. The preservation of the structures of internal rings shows, however, that even though high temperatures occurred in the secondary chondrule, temperatures in the centers of large (>20 μm) metal and silicate grains in the primary chondrule did not get high enough to cause appreciable melting.     

Sulfur isotope composition of putative primary troilite in chondrules from Bishunpur and Semarkona

The sulfur isotopic compositions of putative primary troilite grains within 15 ferromagnesian chondrules (10 FeO-poor and 5 FeO-rich chondrules) in the least metamorphosed ordinary chondrites, Bishunpur and Semarkona, have been measured by ion microprobe. Some troilite grains are located inside metal spherules within chondrules. Since such an occurrence is unlikely to be formed by secondary sulfidization processes in the solar nebula or on parent bodies, those troilites are most likely primary, having survived chondrule-forming high-temperature events. If they are primary, they may be the residues of evaporation at high temperatures during chondrule formation and may have recorded mass-dependent isotopic fractionations. However, the supposed primary troilites measured in this study do not show any significant sulfur isotopic fractionations (<1 ‰/amu) relative to large troilite grains in matrix. Among other chondrule troilites that we measured, only one (BI-CH22) apparently has a small excess of heavy isotopes (2.7 ± 1.4 ‰/amu) consistent with isotopic fractionation during evaporation. All other grains have isotopic fractionations of <1 ‰/amu. Because sulfur is so volatile that evaporation during chondrule formation is probably inevitable, non-Rayleigh evaporation most likely explains the lack of isotopic fractionation in putative primary troilite inside chondrules. Evaporation through the surrounding silicate melt would have suppressed the isotopic fractionation after silicate dust grains melted. At lower temperatures below extensive melting of silicates, a heating rate of >10⁴-10⁶ K/h would be required to avoid a large degree of sulfur isotopic fractionation in the chondrule precursors. This heating rate may provide a new constraint on the chondrule formation processes.     

Melting relationships in the system Fe-Feo at high pressures: Implications for the composition and formation of the earth's core

A reconnaissance investigation has been carried out on melting relationships in the system Fe-FeO at pressures up to 25 GPa and temperatures up to 2200° C using an MA-8 apparatus. Limited studies were also made of the Co-CoO and Ni-NiO systems. In the system FeFeO, the rapid exsolution of FeO from liquids during quenching causes some difficulties in interpretation of textures and phase relationships. The Co-CoO and Ni-NiO systems are more tractable experimentally and provide useful analogues to the Fe-FeO system. It was found that the broad field of liquid immiscibility present at ambient pressure in the Co-CoO system had disappeared at 18 GPa, 2200° C and that the system displayed complete miscibility between molten Co and CoO, analogous to the behaviour of the Ni-NiO system at ambient pressure. The phase diagram of the system Fe-FeO at 16 GPa and from 1600–2200° C was constructed from interpretations based on the textures of quenched run products. The solubility of FeO in molten iron is considerably enhanced by high pressures. At 16 GPa, the Fe-FeO eutectic contains about 10–15 mol percent FeO and the eutectic temperature in this iron-rich region of the system occurs at 1700±25° C, some 350° C below the melting point of pure iron at the same pressure. The solubility of FeO in molten Fe increases rapidly as temperature increases from 1700 to 2200° C. A relatively small liquid immiscibility field is present above 1900° C but is believed to be eliminated above 2200° C. This inference is supported by thermodynamic calculations on the positions of key phase boundaries. A single run carried out on an Fe₅₀ FeO₅₀ composition at 25 GPa and 2200° C demonstrated extensive and probably complete miscibility between Fe and FeO liquids under these conditions. The melting point of iron is decreased considerably by solution of FeO at high pressures; moreover, the melting point gradient (dP/dT) of the Fe-FeO eutectic is much smaller than that of pure iron and is also smaller than that of mantle pyrolite under the P, T conditions studied. These characteristics make it possible for melting of metal phase and segregation of the core to proceed within the Earth under conditions where most of the mantle remains below solidus temperatures. Under these conditions, the core would inevitably contain a large proportion of dissolved FeO. It is concluded therefore, that oxygen is likely to be the principal light element in the core. The inner core may not be composed of pure iron, as often proposed. Instead, it may consist of a crystalline oxide solid solution (Ni, Fe)₂O.     

Vapor pressures and evaporation coefficients for melts of ferromagnesian chondrule-like compositions

To determine evaporation coefficients for the major gaseous species that evaporate from silicate melts, the Hertz-Knudsen equation was used to model the compositions of residues of chondrule analogs produced by evaporation in vacuum by Hashimoto [Hashimoto A. (1983) Evaporation metamorphism in the early solar nebula-evaporation experiments on the melt FeO-MgO-SiO₂-CaO-Al₂O₃ and chemical fractionations of primitive materials. Geochem. J. 17, 111-145] and Wang et al. [Wang J., Davis A. M., Clayton R. N., Mayeda T. K., Hashimoto A. (2001) Chemical and isotopic fractionation during the evaporation of the FeO-MgO-SiO₂-CaO-Al₂O₃-TiO₂ rare earth element melt system. Geochim. Cosmochim. Acta 65, 479-494], in vacuum and in H₂ by Yu et al. [Yu Y., Hewins R. H., Alexander C. M. O’D., Wang J. (2003) Experimental study of evaporation and isotopic mass fractionation of potassium in silicate melts. Geochim. Cosmochim. Acta 67, 773-786], and in H₂ by Cohen et al. [Cohen B. A., Hewins R. H., Alexander C. M. O’D. (2004) The formation of chondrules by open-system melting of nebular condensates. Geochim. Cosmochim. Acta 68, 1661-1675]. Vapor pressures were calculated using the thermodynamic model of Ghiorso and Sack [Ghiorso M. S., Sack R. O. (1995) Chemical mass transfer in magmatic processes IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid-solid equilibria in magmatic systems at elevated temperatures and pressures. Contrib. Mineral. Petrol. 119, 197-212], except for the late, FeO-free stages of the Wang et al. (2001) and Cohen et al. (2004) experiments, where the CMAS activity model of Berman [Berman R. G. (1983) A thermodynamic model for multicomponent melts, with application to the system CaO-MgO-Al₂O₃-SiO₂. Ph.D. thesis, University of British Columbia] was used. From these vapor pressures, evaporation coefficients (α) were obtained that give the best fits to the time variation of the residue compositions. Evaporation coefficients derived for Fe_(g), Mg_(g), and SiO_(g) from the Hashimoto (1983) experiments are similar to those found by Alexander [Alexander C. M. O’D. (2004) Erratum. Meteoritics Planet. Sci. 39, 163] in his EQR treatment of the same data and also adequately describe the FeO-bearing stages of the Wang et al. (2001) experiments. From the Yu et al. (2003) experiments at 1723 K, α_Na = 0.26 ± 0.05, and α_K = 0.13 ± 0.02 in vacuum, and α_Na = 0.042 ± 0.020, andα_K = 0.017 ± 0.002 in 9 × 10⁻⁵ bar H₂. In the FeO-free stages of the Wang et al. (2001) experiments, α_Mg and α_SiO are significantly different from their respective values in the FeO-bearing portions of the same experiments and from the vacuum values obtained at the same temperature by Richter [Richter F. M., Davis A. M., Ebel D. S., Hashimoto A. (2002) Elemental and isotopic fractionation of Type B calcium-, aluminum-rich inclusions: experiments, theoretical considerations, and constraints on their thermal evolution. Geochim. Cosmochim. Acta 66, 521-540] for CMAS compositions much lower in MgO. When corrected for temperature, the values of α_Mg and α_SiO that best describe the FeO-free stages of the Wang et al. (2001) experiments also adequately describe the FeO-free stage of the Cohen et al. (2004) H₂ experiments, but α_Fe that best describes the FeO-bearing stage of the latter experiment differs significantly from the temperature-corrected value derived from the Hashimoto (1983) vacuum data.     

Thermochemistry of sulfide liquids. II. Associated solution model for sulfide liquids in the system O-S-Fe

We present a new formulation to describe the thermodynamics of liquids in the system O-S-Fe. The model is based on an associated regular solution formulation. According to this model, liquids in the O-S-Fe ternary are made up of an equilibrium solution of the six melt species S, Fe, FeO, FeO_1.5, FeS and FeOS. The model presented here represents oxygen and sulfur fugacities as well as phase equilibria with stoichiometric solid phases better than models from the literature on O-Fe and S-Fe binaries. Furthermore, this model represents a substantial improvement on the model of Kress (1997), which is the only other thermodynamic model available in the ternary system. Asymmetric regular solution parameters are required along the FeO join in order to reproduce experimental data with the chosen list of species. Symmetric regular solution parameters are required along the Fe-S binary. Mixing between any of the species considered and FeOS close to ideal. The associated solution model presented here will serve as a more solid foundation for future models in O-S-Fe- Ni-Cu liquids. Efficient and robust strategies for calculating equilibrium speciation and estimating model parameters are presented. Received: 15 June 1999 / Accepted: 5 February 2000