Composition of aqueous fluid coexisting with mantle minerals at high pressure and its bearing on the differentiation of the Earth’s mantle

In order to understand the role of aqueous fluid on the differentiation of the mantle, the compositions of aqueous fluids coexisting with mantle minerals were investigated in the system MgO-SiO₂-H₂O at pressures of 3 to 10 GPa and temperatures of 1000 to 1500°C with an MA8-type multianvil apparatus. Phase boundaries between the stability fields of forsterite + aqueous fluid, forsterite + enstatite + aqueous fluid, and enstatite + aqueous fluid were determined by varying the bulk composition at constant temperature and pressure. The composition of aqueous fluid coexisting with forsterite and enstatite can be defined by the intersection of these two phase boundaries. The solubility of silicate components in aqueous fluid coexisting with forsterite and enstatite increases with increasing pressure up to 8 GPa, from about 30 wt% at 3 GPa to about 70 wt% at 8 GPa. It becomes almost constant above 8 GPa. The Mg/Si weight ratio of these aqueous fluids is much higher than at low pressure (0.2 at 1.5 GPa) and almost constant (1.2) at pressures between 3 and 8 GPa. At 10 GPa, it becomes about 1.4. Aqueous fluid migrating upward through the mantle can therefore dissolve large amounts of silicates, leaving modified Mg/Si ratios of residual materials. It is suggested that the chemical stratification of Mg/Si in the Earth may have been formed as a result of aqueous fluid migration.     

Laboratory condensation of refractory dust in protosolar and circumstellar conditions

In order to better understand condensation processes that took place in the solar nebula and to evaluate the effect of kinetics on the condensed matter, we have built an experimental apparatus for studying condensation of multi-elemental refractory gases at high-temperature and low-pressure. The condensation of a Mg-Si-rich gas, with solar interelement ratios of Ca, Al, Mg and Si, and of a Ca-Al-rich gas under a total pressure of ∼4 × 10⁻³ bar at temperatures from 1045 to 1285 °C and for run times of 4-60 min results in direct formation of crystalline oxides or silicates such as corundum, spinel, anorthite, melilite, Al-diopside, forsterite and enstatite. The mineralogy of the condensates, close to that predicted at equilibrium, varies with the duration of an experiment and the temperature of condensation. The chemical reactions between gas and condensates are rapid enough to attain a steady state in less than one hour. The condensation results in chemical fractionation of the gas, i.e. a depletion of the gas in refractory elements at high temperature. Finally, besides revealing the textures of refractory crystals, which condense from a gas of complex chemical composition, this study shows that certain phases, such as spinel, have favored kinetics of condensation. Our experimental results confirm that refractory inclusions in primitive meteorites could have formed by condensation from a hot nebular gas. Similarly, we confirm that crystalline grains can condense at high temperature in the outflows of evolved stars. In both cases, our results indicate that kinetic processes certainly influence grain mineralogy. Kinetic processes must thus be taken into account in modeling the pressure-temperature conditions of circumstellar environments.     

陨石氧同位素组成及其地学意义

介绍了各类陨石氧同位素组成的特点,对陨石氧同位素组成的主要成因观点进行了评述,结合地球的原始物质组成,讨论了陨石氧同位素组成的地球科学意义。     

Mechanism of pyroxene and amphibole weathering—I. Experimental studies of iron-free minerals

The short term (2–40 days) dissolution of enstatite, diopside, and tremolite in aqueous solution at low temperatures (20–60°C) and pH 1–6 has been studied in the laboratory by means of chemical analyses of reacting solutions for Ca²⁺, Mg²⁺, and Si(OH)₄ and by the use of X-ray photoelectron spectroscopy (XPS) for detecting changes in surface chemistry of the minerals. All three minerals were found to release silica at a constant rate (linear kinetics) providing that ultrafine particles, produced by grinding, were removed initially by HF treatment. All three also underwent incongruent dissolution with preferential release of Ca and/or Mg relative to Si from their outermost surfaces. The preferential release of Ca, but not Mg for diopside at pH 6 was found by both XPS and solution chemistry verifying the theoretical prediction of greater mobility of cations located in M₂ structural sites. Loss mainly from M₂ sites also explains the degree of preferential loss of Mg from enstatite at pH 6; similar structural arguments apply to the loss of Ca and Mg from the surface of tremolite. In the case of diopside and tremolite initial incongruency was followed by essentially congruent cation-plus-silica dissolution indicating rapid formation of a constant-thickness, cation-depleted surface layer. Cation depletion at elevated temperature and low pH (~ 1) for enstatite and diopside was much greater than at low temperature and neutral pH, and continued reaction resulted in the formation of a surface precipitate of pure silica as indicated by solubility calculations, XPS analyses, and scanning electron microscopy.From XPS results at pH 6, model calculations indicate a cation-depleted altered surface layer of only a few atoms thickness in all three minerals. Also, lack of shifts in XPS peak energies for Si, Ca, and Mg, along with undersaturation of solutions with respect to all known Mg and Ca silicate minerals, suggest that cation depletion results from the substitution of hydrogen ion for Ca²⁺ and/or Mg²⁺ in a modified silicate structure and not from the precipitation of a new, radically different surface phase. These results, combined with findings of high activation energies for dissolution, a non-linear dependence on a_H⁺ for silica release from enstatite and diopside, and the occurrence of etch pitting, all point to surface chemical reaction and not bulk diffusion (either in solution or through altered surface layers) as the rate controlling mechanism of iron-free pyroxene and amphibole dissolution at earth surface temperatures.     

Growth of multilayered polycrystalline reaction rims in the MgO–SiO<Subscript>2</Subscript> system,part I: experiments

Growth of transport-controlled reaction layers between single crystals of periclase and quartz, and forsterite and quartz was investigated experimentally at 1.5 GPa, 1100°C to 1400°C, 5 min to 72 h under dry and melt-free conditions using a piston-cylinder apparatus. Starting assemblies consisting of Per | Qtz | Fo sandwiches produced polycrystalline double layers of forsterite and enstatite between periclase and quartz, and enstatite single layers between forsterite and quartz. The position of inert Pt-markers initially deposited at the interface of the reactants and inspection of mass balance confirmed that both layer-producing reactions are controlled by MgO diffusion, while SiO₂ is relatively immobile. BSE and TEM imaging revealed thicknesses from 0.6 μm to 14 μm for double layers and from 0 to 6.8 μm for single layers. Both single and double layers displayed non-parabolic growth together with pronounced grain coarsening. Textural evolution and growth rates for each reaction are directly comparable. Forsterite–enstatite double layers are always wider than enstatite single layers, and the growth of enstatite in the double layer is slower than that in the single layer. In double layers, the enstatite/forsterite layer thickness ratio significantly increases with temperature, reflecting different MgO mobilities as temperature varies. Thus, thickness ratios in multilayered reaction zones may contain a record of temperature, but also that of any physico-chemical parameter that modifies the mobilities of the chemical components between the various layers. This potential is largely unexplored in geologically relevant systems, which calls for further experimental studies of multilayered reaction zones.     

Primordial compositions of refractory inclusions

Bulk chemical and O-, Mg- and Si-isotopic compositions were measured for each of 17 Types A and B refractory inclusions from CV3 chondrites. After bulk chemical compositions were corrected for non-representative sampling in the laboratory, the Mg- and Si-isotopic compositions of each inclusion were used to calculate its original chemical composition assuming that the heavy-isotope enrichments of these elements are due to Rayleigh fractionation that accompanied their evaporation from CMAS liquids. The resulting pre-evaporation chemical compositions are consistent with those predicted by equilibrium thermodynamic calculations for high-temperature nebular condensates, but only if different inclusions condensed from nebular regions that ranged in total pressure from 10⁻⁶ to 10⁻¹ bar, regardless of whether they formed in a system of solar composition or in one enriched in dust of ordinary chondrite composition relative to gas by a factor of 10 compared to solar composition. This is similar to the range of total pressures predicted by dynamic models of the solar nebula for regions whose temperatures are in the range of silicate condensation temperatures. Alternatively, if departure from equilibrium condensation and/or non-representative sampling of condensates in the nebula occurred, the inferred range of total pressure could be smaller. Simple kinetic modeling of evaporation successfully reproduces observed chemical compositions of most inclusions from their inferred pre-evaporation compositions, suggesting that closed-system isotopic exchange processes did not have a significant effect on their isotopic compositions. Comparison of pre-evaporation compositions with observed ones indicates that 80% of the enrichment in refractory CaO + Al₂O₃ relative to more volatile MgO + SiO₂ is due to initial condensation and 20% due to subsequent evaporation for both Types A and B inclusions.     

Condensation of major elements during chondrule formation and its implication to the origin of chondrules

Two glassy refractory Al-rich chondrules in Semarkona (LL3.0), the most primitive unequilibrated ordinary chondrite, provide direct evidence for condensation of Si and Mg on melt droplets during cooling. The chondrules are completely rounded, rich in Ca and Al, and poor in Fe and alkalis. They have extraordinarily abundant glass (70-80 vol%) with a subordinate amount of forsterite as the only crystalline phase that occurs mostly rimming the chondrule edge. The groundmass glass is concentrically zoned in terms of Si with an outward increase, which is overlapped with local heterogeneity of Mg and Al induced by crystallization of forsterite. The outward increase of Si, mostly compensated by Al, cannot be formed solely by crystallization of forsterite from a homogeneous melt in a closed system. Combined with skeletal or dendritic morphology and sector zoning of forsterite, it is suggested that Si condensed onto totally molten droplets (“initial melts”) accompanied by nucleation and rapid growth of forsterite with lowering temperature. The “initial melts”, the compositions of which were estimated from the Ca contents of the first crystallized forsterite, are very similar to Type C CAI but are notably poorer in Mg and Si than the bulk chondrules, indicating condensation of Mg in addition to Si with an atomic ratio of Mg:Si ∼ 3:2. The condensation after the nucleation of forsterite took place below ∼1300 °C under cooling at ∼70 °C/h and amounted to 30 wt% of the current chondrule. This study suggests a model that a short-time and local shock heating event induced melting of Type C CAI and concomitant evaporation of dusts, ferromagnesian chondrules of earlier generation, and their fragments to generate Mg and Si-rich gas, which condensed onto the melt droplets upon cooling accompanying condensation of Type I chondrules.     

Far from equilibrium enstatite dissolution rates in alkaline solutions at earth surface conditions

Far from equilibrium enstatite dissolution rates both open to atmospheric CO₂ and CO₂ purged were measured as a function of solution pH from 8 to 13 in batch reactors at room temperature. Congruent dissolution was observed after an initial period of incongruent dissolution with preferential Si release from the enstatite. Steady-state dissolution rates in open to atmospheric CO₂ conditions decrease with increase in solution pH from 8 to 12 similar to the behavior reported by other investigators. Judging from the pH 13 dissolution rate, rates increase with pH above pH 12. This is thought to occur because of the increase in overall negative surface charges on enstatite as Mg surface sites become negative above pH 12.4, the pH of zero surface charge of MgO.Steady-state dissolution rates of enstatite increase above pH 10 when CO₂ was purged by performing the experiments in a N₂ atmosphere. This suggests inhibition of dissolution rates above pH 10 when experiments were open to the atmosphere. The dissolved carbonate in these solutions becomes dominantly CO₃²⁻ above pH 10.33. It is argued that CO₃²⁻ forms a >Mg₂-CO₃ complex at positively charged Mg surface sites on enstatite, resulting in stabilization of the surface Si-O bonds. Therefore, removal of solution carbonate results in an increase in dissolution rates of enstatite above pH 10. The log rate of CO₂-purged enstatite dissolution in moles per cm² per s as a function of increasing pH above pH 10 is equal to 0.35. This is consistent with the model of silicate mineral dissolution in the absence of surface carbonation in alkaline solutions proposed earlier in the literature.     

Refractory precursor components of Semarkona chondrules and the fractionation of refractory elements among chondrites

Chondrules from the Semarkona (LL3.0) chondrite show refractory and common lithophile fractionation trends similar to those observed among the chondrite groups. It appears that chondrules are mixtures of a small number of pre-existing solid components, and we infer that chondrule precursor materials were related to the nebular components involved in the lithophile element fractionations recognized in ordinary chondrites. Compositional trends among the chondrules can be used to deduce the compositions of these components.We use instrumental neutron activation analysis to measure many (~20) of the lithophile elements in 30 chondrules. The amounts of oxidized iron were calculated from other compositional parameters; concentrations of Si were estimated using mass-balance considerations. The data were corrected for the diluting effects of non-lithophile constituents. Plots of lithophile elements versus a reference refractory element such as Al show that there were two major chondrule silicate precursor components: a refractory, olivine-rich, FeO-free one, and a non-refractory, SiO₂-, FeO-rich one.The refractory component probably forms from olivine-enriched condensates formed above the condensation temperature of enstatite. The non-refractory component must have formed from fine-grained materials that were able to equilibrate down to lower nebular temperatures. Chondrite matrix may have had an origin similar to that of the non-refractory material, and constitutes a third lithophile-bearing component that took part in chondrite fractionation processes. The low abundance of refractories and Mg in ordinary and enstatite chondrites was produced by the loss of materials having a higher refractory-element/Mg ratio than that in the refractory component of chondrules.     

The surface chemistry of multi-oxide silicates

The surface chemistry of natural wollastonite, diopside, enstatite, forsterite, and albite in aqueous solutions was characterized using both electrokinetic techniques and surface titrations performed for 20 min in batch reactors. Titrations performed in such reactors allow determination of both proton consumption and metal release from the mineral surface as a function of pH. The compositions, based on aqueous solution analysis, of all investigated surfaces vary dramatically with solution pH. Ca and Mg are preferentially released from the surfaces of all investigated divalent metal silicates at pH less than ∼8.5-10 but preferentially retained relative to silica at higher pH. As such, the surfaces of these minerals are Si-rich and divalent metal poor except in strongly alkaline solutions. The preferential removal of divalent cations from these surfaces is coupled to proton consumption. The number of protons consumed by the preferential removal of each divalent cation is pH independent but depends on the identity of the mineral; ∼1.5 protons are consumed by the preferential removal of each Ca atom from wollastonite, ∼3 protons are consumed by the preferential removal of each Mg or Ca atom from diopside or enstatite, and ∼4 protons are consumed by the preferential removal of each Mg from forsterite. These observations are interpreted to stem from the creation of additional ‘internal’ adsorption sites by the preferential removal of divalent metal cations which can be coupled to the condensation of partially detached Si. Similarly, Na and Al are preferentially removed from the albite surface at 2 > pH > 11; mass balance calculations suggest that three protons are consumed by the preferential removal of each Al atom from this surface over this entire pH range. Electrokinetic measurements on fresh mineral powders yield an isoelectric point (pH_IEP) 2.6, 4.4, 3.0, 4.5, and <1, for wollastonite, diopside, enstatite, forsterite, and albite, respectively, consistent with the predominance of SiO₂ in the surface layer of all of these multi-oxide silicates at acidic pH. Taken together, these observations suggest fundamental differences between the surface chemistry of simple versus multi-oxide minerals including (1) a dependency of the number and identity of multi-oxide silicate surface sites on the aqueous solution composition, and (2) the dominant role of metal-proton exchange reactions on the reactivity of multi-oxide mineral surfaces including their dissolution rate variation with aqueous solution composition.     

Can enstatite meteorites form from a nebula of solar composition?

Doubts have been expressed as to whether solar matter is sufficiently reducing to explain the minerals occurring in the enstatite chondrites and in the enstatite achondrites. Thermodynamic calculations on the stabilities of TiN, Si₂N₂O, CaS and silicon-bearing iron metal show that these substances can form under equilibrium conditions from a nebula of solar composition provided that the total pressure exceeds ~ 1 atm and that thermodynamic equilibria are frozen in at near-formation temperatures.     

Rates of grain boundary diffusion through enstatite and forsterite reaction rims

 The growth rates of enstatite rims produced by reaction of Fo₉₂ and SiO₂ were determined at 250–1500 MPa and 900–1100°C for a wide range of water contents. Growth rates were also determined for forsterite rims between MgO and Mg₂Si₂O₆ and between MgO and SiO₂. Rim growth rates are parabolic indicating diffusion-controlled growth of the polycrystalline rims which are composed of ˜ 2 μm diameter grains. Rim growth rates were used to calculate the product of the grain boundary diffusion coefficient (D'_A) times the effective grain boundary thickness (δ) assuming in turn that MgO, SiO₂, and Mg₂Si₋₁ are the diffusing components (coupled diffusion of a cation and oxygen or interdiffusion of Mg and Si). The values for D'_MgOδ, D', and D' for enstatite at 1000°C and 700 MPa confining pressure with about 0.1 wt %  water are about five times larger than the corresponding D'_Aδ values for samples initially vacuum dried at 250°C. Most of the increase in D'_Aδ occurs with the first 0.1 wt %  water. The activation energy for diffusion through the enstatite rims (1100–950°C) is 162 ± 30 kJ/mole. The diffusion rate through enstatite rims is essentially unchanged for confining pressures from 210–1400 MPa, but the nucleation rate is greatly reduced at low confining pressure (for  ≤ 1.0 wt % water present) and limits the conditions at which rim growth can be measured. The corresponding values for D'_Aδ through forsterite rims are essentially identical for the two forsterite-producing reactions when 0.1 wt % water is added and similar to the D'_Aδ values for enstatite at the same conditions. The D'_Aδ values for forsterite are ˜ 28 times larger for samples starting with 0.1 wt %  water compared to samples that were first vacuum dried. Thus water enhances these grain boundary diffusion rates by a factor of 5–30 depending on the mineralogy, but the total range in D'_Aδ is only slightly more than an order of magnitude for as wide a range of water contents as expected for most crustal conditions. Received: 1 July 1995 / Accepted: 1 August 1996     

Preferential dissolution of SiO<Subscript>2</Subscript> from enstatite to H<Subscript>2</Subscript> fluid under high pressure and temperature

Stability and phase relations of coexisting enstatite and H₂ fluid were investigated in the pressure and temperature regions of 3.1–13.9 GPa and 1500–2000 K using laser-heated diamond-anvil cells. XRD measurements showed decomposition of enstatite upon heating to form forsterite, periclase, and coesite/stishovite. In the recovered samples, SiO₂ grains were found at the margin of the heating hot spot, suggesting that the SiO₂ component dissolved in the H₂ fluid during heating, then precipitated when its solubility decreased with decreasing temperature. Raman and infrared spectra of the coexisting fluid phase revealed that SiH₄ and H₂O molecules formed through the reaction between dissolved SiO₂ and H₂. In contrast, forsterite and periclase crystals were found within the hot spot, which were assumed to have replaced the initial orthoenstatite crystals without dissolution. Preferential dissolution of SiO₂ components of enstatite in H₂ fluid, as well as that observed in the forsterite H₂ system and the quartz H₂ system, implies that H₂-rich fluid enhances Mg/Si fractionation between the fluid and solid phases of mantle minerals.     

CaO-MgO-Al2o3-SiO2 liquids: chemical and isotopic effects of Mg and Si evaporation in a closed system of solar composition

A method is shown for calculating vapor pressures over a CMAS droplet in a gas of any composition. It is applied to the problem of the evolution of the chemical and Mg and Si isotopic composition of a completely molten droplet having the composition of a likely refractory inclusion precursor during its evaporation into the complementary, i.e. modified solar, gas from which it originally condensed, a more realistic model than previous calculations in which the ambient gas is pure H_2(g). Because the loss rate of Mg is greater than that of Si, the vapor pressure of Mg_(g) falls and its ambient pressure rises faster than those of SiO_(g) during isothermal evaporation, causing the flux of Mg_(g) to approach zero faster and MgO to approach its equilibrium concentration sooner than SiO₂. As time passes, δ²⁵Mg and δ²⁹Si increase in the droplet and decrease in the ambient gas. The net flux of each isotope crossing the droplet/gas interface is the difference between its outgoing and incoming flux. δ²⁵Mg and δ²⁹Si of this instantaneous gas become higher, first overtaking their values in the ambient gas, causing them to increase with time, and later overtaking their values in the droplet itself, causing them to decrease with time, ultimately reaching their equilibrium values. If the system is cooling during evaporation and if mass transfer ceases at the solidus temperature, 1500 K, final MgO and SiO₂ contents of the droplet are slightly higher in modified solar gas than in pure H_2(g), and the difference increases with decreasing cooling rate and increasing ambient pressure. During cooling under some conditions, net fluxes of evaporating species become negative, causing reversal of the evaporation process into a condensation process, an increase in the MgO and/or SiO₂ content of the droplet with time, and an increase in their final concentrations with increasing ambient pressure and/or dust/gas ratio. At cooling rates <∼3 K/h, closed-system evaporation at P^tot ∼ 10⁻³ bar in a modified solar gas, or at lower pressure in systems with enhanced dust/gas ratio, can yield the same δ²⁵Mg in a residual CMAS droplet for vastly different evaporated fractions of Mg. The δ²⁵Mg of a refractory residue may thus be insufficient to determine the extent of Mg loss from its precursor. Evaporation of Mg into an Mg-bearing ambient gas causes δ²⁶Mg and δ²⁵Mg of the residual droplet to fall below values expected from Rayleigh fractionation for the amount of ²⁴Mg evaporated, with the degree of departure increasing with increasing fraction evaporated and ambient pressure of Mg. δ²⁶Mg and δ²⁵Mg do not depart proportionately from Rayleigh fractionation curves, with δ²⁵Mg being less than expected on the basis of δ²⁶Mg by up to ∼1.2‰. Such departures from Rayleigh fractionation could be used in principle to distinguish heavily from lightly evaporated residues with the same δ²⁵Mg.     

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.     

Gel synthesis of magnesium silicates: A 29Si magic angle spinning NMR study

The formation of the magnesium silicate minerals forsterite, enstatite, and roedderite by heating of amorphous “protosilicate” gels precipitated from aqueous solution has been studied by ²⁹Si MAS nmr. Gentle drying of the hydrogels at 110° C gives materials with broad nmr signals that do not differ appreciably with preparation conditions, but the minerals formed by heating at 750° C or higher are greatly dependent on the precipitation and washing conditions of the original gel. The rare mineral roedderite, best known from studies of unequilibrated enstatite chondrite meteorites, becomes a major species along with forsterite when the hydrogels are washed with sodium hydroxide solution before drying and heating to 750° C.     

Presolar diamond, silicon carbide, and graphite in carbonaceous chondrites: implications for thermal processing in the solar nebula

We have determined abundances of presolar diamond, silicon carbide, graphite, and Xe-P1 (Q-Xe) in eight carbonaceous chondrites by measuring the abundances of noble gas tracers in acid residues. The meteorites studied were Murchison (CM2), Murray (CM2), Renazzo (CR2), ALHA77307 (CO3.0), Colony (CO3.0), Mokoia (CV3_ox), Axtell (CV3_ox), and Acfer 214 (CH). These data and data obtained previously by Huss and Lewis (1995) provide the first reasonably comprehensive database of presolar-grain abundances in carbonaceous chondrites. Evidence is presented for a currently unrecognized Ne-E(H) carrier in CI and CM2 chondrites.After accounting for parent-body metamorphism, abundances and characteristics of presolar components still show large variations across the classes of carbonaceous chondrites. These variations correlate with the bulk compositions of the host meteorites and imply that the same thermal processing that was responsible for generating the compositional differences between the various chondrite groups also modified the initial presolar-grain assemblages. The CI chondrites and CM2 matrix have the least fractionated bulk compositions relative to the sun and the highest abundances of most types of presolar material, particularly the most fragile types, and thus are probably most representative of the material inherited from the sun's parent molecular cloud. The other classes can be understood as the products of various degrees of heating of bulk molecular cloud material in the solar nebula, removing the volatile elements and destroying the most fragile presolar components, followed by chondrule formation, metal-silicate fractionation in some cases, further nebula processing in some cases, accretion, and parent body processing. If the bulk compositions and the characteristics of the presolar-grain assemblages in various chondrite classes reflect the same processes, as seems likely, then differential condensation from a nebula of solar composition is ruled out as the mechanism for producing the chondrite classes. Presolar grains would have been destroyed if the nebula had been completely vaporized. Our analysis shows that carbonaceous chondrites reflect all stages of nebular processing and thus are no more closely related to one another than they are to ordinary and enstatite chondrites.     

Experimental determination of magnesia and silica solubilities in graphite-saturated and redox-buffered high-pressure COH fluids in equilibrium with forsterite + enstatite and magnesite + enstatite

We experimentally investigated the dissolution of forsterite, enstatite and magnesite in graphite-saturated COH fluids, synthesized using a rocking piston cylinder apparatus at pressures from 1.0 to 2.1 GPa and temperatures from 700 to 1200 °C. Synthetic forsterite, enstatite, and nearly pure natural magnesite were used as starting materials. Redox conditions were buffered by Ni–NiO–H₂O (ΔFMQ = ??0.21 to ??1.01), employing a double-capsule setting. Fluids, binary H₂O–CO₂ mixtures at the P, T, and fO₂ conditions investigated, were generated from graphite, oxalic acid anhydrous (H₂C₂O₄) and water. Their dissolved solute loads were analyzed through an improved version of the cryogenic technique, which takes into account the complexities associated with the presence of CO₂-bearing fluids. The experimental data show that forsterite?+?enstatite solubility in H₂O–CO₂ fluids is higher compared to pure water, both in terms of dissolved silica (mSiO₂?=?1.24 mol/kg_H2O versus mSiO₂?=?0.22 mol/kg_H2O at P?=?1 GPa, T?=?800 °C) and magnesia (mMgO?=?1.08 mol/kg_H2O versus mMgO?=?0.28 mol/kg_H2O) probably due to the formation of organic C–Mg–Si complexes. Our experimental results show that at low temperature conditions, a graphite-saturated H₂O–CO₂ fluid interacting with a simplified model mantle composition, characterized by low MgO/SiO₂ ratios, would lead to the formation of significant amounts of enstatite if solute concentrations are equal, while at higher temperatures these fluid, characterized by MgO/SiO₂ ratios comparable with that of olivine, would be less effective in metasomatizing the surrounding rocks. However, the molality of COH fluids increases with pressure and temperature, and quintuplicates with respect to the carbon-free aqueous fluids. Therefore, the amount of fluid required to metasomatize the mantle decreases in the presence of carbon at high P–T conditions. COH fluids are thus effective carriers of C, Mg and Si in the mantle wedge up to the shallowest level of the upper mantle.     

Thermal metamorphism of primitive meteorites—XL The enstatite meteorites: origin and evolution of a parent body

We report neutron activation data for Ag, As, Bi, Cd, Co, Cs, Cu, Ga, In, Rb, Se, Te, Tl and Zn in samples of Abee heated at temperatures of 1000–1400°C in a low-pressure environment (initially ~ 10^?5 atm H₂) and in 9 enstatite achondrites (aubrites) and the silicate portion of the unique stony-iron, Mt Egerton. Trace element losses in heated Abee progress with temperature, the lowest retention being 2.4 × 10^?6 of initial contents. These data indicate trace element loss above 1000°C via diffusion-controlled processes having apparent activation energies of 8–55 kcal/mol ; only Co exhibits a significantly higher energy. These trace element data and those for aubrites, Mt Egerton and E4–6 chondrites, and mineralogic and isotopic evidence link all enstatite meteorites to a common parent body. Volatile, mobile elements vary inversely with cobalt content in aubrites and Mt Egerton but directly in E4–6 chondrites; this is inconsistent with all genetic models positing fractionation of such elements during nebular condensation and accretion. However, the data are consistent with the idea that aubrites and Mt. Egerton reflect fractional crystallization of a magma produced from enstatite chondrite-like parent material (probably E6) and late introduction of chalcophiles and mobile elements transported by FeS-Fe eutectic from an E4–6 region experiencing open-system metamorphism. As suggested earlier, the only primary process that affected enstatite meteorites involved fractionation of non-volatile lithophiles from sulfides and metal during condensation and accretion of chondritic parent material from the nebula. If, as seems likely, volatile/mobile elements reflect secondary processes, they can only be used to establish alteration conditions within the enstatite parent body and not to estimate temperatures during primary nebular condensation and accretion.     

The role of carbon and oxygen in cosmic gases: some applications to the chemistry and mineralogy of enstatite chondrites

A new condensation sequence appears if the $\text{C}\text{O}$ ratio in a gas of otherwise solar composition is increased by less than a factor of two. As the ratio increases from the solar value of 0.6 to ? 1 the gas becomes extremely reduced, the condensation temperatures of silicates and oxides are depressed markedly ～ 400 K and a new suite of refractory minerals appears: AIN, CaS, MgS, SiC, TiN, graphite, Si₂N₂O and probably metastable (Fe,Ni)³C. Many of these minerals are unique to enstatite chondrites and may be analogues of the refractory silicates and oxides found in more oxidized meteorites such as Allende.The change in chemistry is related to the stability of CO, the most stable C or O compound at high T. Since the elements occur in a 1:1 ratio in CO, only the element which is in excess is free to form other compounds. But as T decreases CO reacts with H₂ to form graphite, CH₄ or other hydrocarbons thereby freeing O to form H₂O. If equilibrium is maintained oxides and silicates form at about 1000 K ($\text{C}\text{O}\text{> 1}$, P_τ = 10^?4atm) as products of reactions among the carbides, nitrides, sulfides and the gas. The possibility that equilibrium was not maintained among the C-bearing species was also investigated. If either graphite or CH₄ does not form as predicted the stability fields of the reduced minerals expands to lower temperatures. If neither graphite nor CH₄ form as predicted, CO remains stable and the nebular gas is highly reduced at all temperatures.Enstatite chondrites appear to have originated in a region of the nebula where the $\text{C}\text{O}$ ratio was somewhat higher than the solar value. Various fractionation mechanisms are considered. An interesting possibility is that graphite, which is quite refractory under a wide range of conditions, survived the collapse of the solar nebula.