Inclusions In Carbonaceous Chondrites Research Articles

Radiogenic chromium isotope evidence for the earliest planetary volcanism and crust formation in the Solar system

ABSTRACT Erg Chech (EC) 002 is a meteorite with andesitic composition, potentially recording the lava crystallization and crust formation of its parent body. Nucleosynthetic Cr isotope composition (ε54Cr = −0.35 ± 0.06) for EC 002 suggests a non-carbonaceous region of the Solar system, and possibly represents the crustal composition of the brachinite parent body. The 53Mn-to-53Cr decay system shows it crystallized at 4566.6 ± 0.6 Ma, i.e. 0.7 ± 0.6 Ma after Solar system formation (only considering the cogenetic matrix fractions with similar ε54Cr values). This age represents the earliest recorded evidence for planetary melting and volcanism in the Solar system, suggesting that the planetary crust formation occurred very early, only within the first few hundred thousand years of Solar system history. However, the 53Mn–53Cr age does not overlap with 26Al–26Mg dating results, which might indicate that non-carbonaceous achondrites have lower initial 26Al/27Al than the canonical value defined by refractory inclusions in carbonaceous chondrites.

Temperatures of canonical condensation: Constraints from CAIs

The analogy between Al_8/3O₄ – MgAl₂O₄ spinels and AgSbS₂ − PbS galenas is explored. Although the spinel observed in calcium-aluminum-rich inclusions in carbonaceous chondrites (CAIs) is nearly pure MgAl₂O₄ and the galena found in hydrothermal ore deposits is nearly pure PbS, in both occurrences a very strong case may sometimes be made that these phases originally crystallized as solid solutions with substantially higher abundance of these minor components, Al_8/3O₄ in CAI spinel and AgSbS₂ in hydrothermal galena. It is shown that a few retrograde net-transport reactions accompanying Al_8/3O₄ breakdown in Al_8/3O₄ − MgAl₂O₄ spinels can account for several important features of spinel-rich CAIs including reverse zoning of åkermanite−gehlenite melilites, the production of Al-rich diopside and the depletion of anorthite in reaction rims, and the scarcity of grossite despite its previous presence as a high temperature condensate. AgSbS₂ in galena is analogous, because galenas in ore deposits today have an undetectable amount of AgSbS₂ (for example, Knowles,1983), yet, for example, the galena-rich ore deposits from the Coeur d9Alene mining district (ID) were responsible for the largest recorded number of ounces of Ag produced in the world until recently. Accordingly, it is appropriate that the primary Al_8/3O₄ contents of spinels in CAIs are reconstructed following methods analogous to those employed by Sack and others (2002,2005) to reconstruct the primary AgSbS₂ contents of galenas from the Coeur d9Alene ores, if CAIs spinels are to be used to constrain temperatures, and hence pressures, of canonical condensation. Several examples are given to demonstrate the thermal constraints which may be obtained from reconstructed primary spinel Al_8/3O₄ concentrations starting with the Al_8/3O₄ -rich CAI spinels reported by Yurimoto and others (2021) and El Goresy and others (1984). The compositions of spinels which form rims around corundum and corundum-hibonite aggregates in CAIs from the Murchison and Murray chondrites (Makide and others,2013) are then used to prove that Al_8/3O₄ in Al_8/3O₄ − MgAl₂O₄ spinels from CAIs are completely analogous to AgSbS₂ in AgSbS₂ − PbS galenas from hydrothermal ore deposits.

Spectral and Petrographic Properties of Inclusions in Carbonaceous Chondrites and Comparison with In Situ Images from Asteroid Ryugu

Abstract We imaged a set of carbonaceous chondrites from the CM2, CO3, CV3, and CK4 groups using the qualification model of MasCam, the camera on board the asteroid lander MASCOT, which touched down on asteroid Ryugu in 2018 October. A CI1 meteorite was also imaged but excluded from the analysis due to prominent terrestrial weathering. Following the methods used to image the rock on Ryugu, we placed a total of 14 meteorites approximately 20 cm in front of the camera to achieve a spatial resolution of about 0.2 mm per pixel and illuminated the samples with onboard light-emitting diodes of four different colors in the visible wavelength range. We mapped bright and dark inclusions within the meteorites and derived the inclusion brightness relative to the matrix in the red light, the relative spectral slope of each inclusion, the inclusion size frequency distribution and the matrix volume abundance. We find that the meteorite groups overlap within these parameters, but individual samples, as well as individual inclusions, can have deviating values. Terrestrial weathering appears to have no systematic influence on these parameters. Relating our analysis to the inclusions found in the rock on Ryugu, we find that the spectral parameters of Ryugu’s inclusions fit well in the parameter space of the carbonaceous chondrites. Compared with the most common types of carbonaceous chondrites, Ryugu’s rock has larger inclusions (mean diameter: 0.63 ± 0.91 mm) and a higher upper limit to the matrix abundance (92.4 vol%).

The formation and alteration history of a forsterite-bearing Type C CAI from Allende: Evidence for a Type B CAI precursor, and implications for fluid-assisted metasomatism on the CV chondrite parent body

Type C CAIs are a rare group of refractory inclusions in carbonaceous chondrites that are compositionally and isotopically distinct from the more commonly observed igneous CAIs (i.e., Type As and Type Bs). We have investigated a forsterite-bearing Type C CAI ALNH-04 from the Allende CV3 chondrite. This CAI has both textural and compositional similarities to some of the Type C CAIs previously reported; however, there are notable differences that imply that ALNH-04 may have formed from a different precursor from other Type C inclusions. Based on the bulk composition of ALNH-04 and the minor element contents of forsterite, we suggest that the forsterite grains were inherited from a Forsterite-bearing Type B CAI (FoB) precursor. The presence of augite on the periphery of ALNH-04 implies a re-melting event that probably occurred in a chondrule-forming region.Another interesting feature of ALNH-04 is the secondary iron-alkali-halogen zoning sequence as manifested by varying proportions of nepheline, sodalite, fayalitic olivine, and sulfides in different regions of the CAI. Nepheline ± sodalite have replaced anorthite in the outer part of the inclusion, giving way to the presence of ubiquitous sodalite with minor nepheline, partially replacing anorthite at grain boundaries and fractures in the interior of the inclusion. Sulfides and Fe-bearing olivine form an iron-rich alteration zone. The textural relationships between nepheline and sodalite show no evidence of a direct replacement relationship between the two phases. Combined with the SEM observations, the microstructures are most consistent with a two-stage fluid alteration process: (1) nepheline replaced anorthite in the outer part of the CAI via a fluid with aNaCl(aq) within the stability range of nepheline; (2) a later-stage fluid, with elevated aNaCl(aq) that could preferentially stabilize sodalite, penetrated further into the CAI interior, replacing anorthite with sodalite. The lack of a nepheline-sodalite replacement relationship indicates that the conditions and fluid chemistry were suitable for nepheline and/or sodalite to be stable. Together with other Fe-rich secondary phases, fayalitic olivine may have recorded an increase in pH and aSiO2(aq) of the fluid. These changes were probably induced by the extensive alteration of the outer part of the CAI by feldspathoids. The observed alteration microstructures are consistent with a coupled dissolution-precipitation alteration mechanism. The fluid alteration was also responsible for the formation of Na- and Ca-rich halos in the matrix surrounding the CAI.We compared ALNH-04 with other CAIs and chondrules showing alkali-halogen-(iron) zoning sequences in Allende, and found that the observed zoning structures are consistent with the two-stage fluid-assisted metasomatic process mentioned above. The different distribution patterns of nepheline and sodalite in plagioclase-rich CAIs, chondrules, and melilite-rich CAIs may be explained by different chemical potential gradients in SiO2 in the fluid. Precipitation of nepheline and sodalite may require a higher SiO2 activity compared to grossular and dmisteinbergite (±secondary anorthite), which controlled the formation location of sodalite during the second fluid alteration event. Fluids with different compositions may be produced by fluid percolation along different directions and pathways, changing convection patterns, or release of water from a differentiated asteroidal interior.

CHONDRITES AND THEIR COMPONENTS: RECORDS OF EARLY SOLAR SYSTEM PROCESSES.

AbstractChondrites consist of three major components: refractory inclusions (Ca,Al‐rich inclusions [CAIs] and amoeboid olivine aggregates), chondrules, and matrix. Here, I summarize recent results on the mineralogy, petrology, oxygen, and aluminum‐magnesium isotope systematics of the chondritic components (mainly CAIs in carbonaceous chondrites) and their significance for understanding processes in the protoplanetary disk (PPD) and on chondrite parent asteroids. CAIs are the oldest solids originated in the solar system: their U‐corrected Pb‐Pb absolute age of 4567.3 ± 0.16 Ma is considered to represent time 0 of its evolution. CAIs formed by evaporation, condensation, and aggregation in a gas of approximately solar composition in a hot (ambient temperature >1300 K) disk region exposed to irradiation by solar energetic particles, probably near the protoSun; subsequently, some CAIs were melted in and outside their formation region during transient heating events of still unknown nature. In unmetamorphosed, type 2–3.0 chondrites, CAIs show large variations in the initial 26Al/27Al ratios, from <5 × 10–6 to ~5.25 × 10–5. These variations and the inferred low initial abundance of 60Fe in the PPD suggest late injection of 26Al by a wind from a nearby Wolf–Rayet star into the protosolar molecular cloud core prior to or during its collapse. Although there are multiple generations of CAIs characterized by distinct mineralogies, textures, and isotopic (O, Mg, Ca, Ti, Mo, etc.) compositions, the 26Al heterogeneity in the CAI‐forming region(s) precludes determining the duration of CAIs formation using 26Al‐26Mg systematics. The existence of multiple generations of CAIs and the observed differences in CAI abundances in carbonaceous and noncarbonaceous chondrites may indicate that CAIs were episodically formed and ejected by a disk wind from near the Sun to the outer solar system and then spiraled inward due to gas drag. In type 2–3.0 chondrites, most CAIs surrounded by Wark–Lovering rims have uniform Δ17O (= δ17O−0.52 × δ18O) of ~ −24‰; however, there is a large range of Δ17O (from ~−40 to ~ −5‰) among them, suggesting the coexistence of 16O‐rich (low Δ17O) and 16O‐poor (high Δ17O) gaseous reservoirs at the earliest stages of the PPD evolution. The observed variations in Δ17O of CAIs may be explained if three major O‐bearing species in the solar system (CO, H2O, and silicate dust) had different O‐isotope compositions, with H2O and possibly silicate dust being 16O‐depleted relative to both the Genesis solar wind Δ17O of −28.4 ± 3.6‰ and even more 16O‐enriched CO. Oxygen isotopic compositions of CO and H2O could have resulted from CO self‐shielding in the protosolar molecular cloud (PMC) and the outer PPD. The nature of 16O‐depleted dust at the earliest stages of PPD evolution remains unclear: it could have either been inherited from the PMC or the initially 16O‐rich (solar‐like) MC dust experienced O‐isotope exchange during thermal processing in the PPD. To understand the chemical and isotopic composition of the protosolar MC material and the degree of its thermal processing in PPD, samples of the primordial silicates and ices, which may have survived in the outer solar system, are required. In metamorphosed CO3 and CV3 chondrites, most CAIs exhibit O‐isotope heterogeneity that often appears to be mineralogically controlled: anorthite, melilite, grossite, krotite, perovskite, and Zr‐ and Sc‐rich oxides and silicates are 16O‐depleted relative to corundum, hibonite, spinel, Al,Ti‐diopside, forsterite, and enstatite. In texturally fine‐grained CAIs with grain sizes of ~10–20 μm, this O‐isotope heterogeneity is most likely due to O‐isotope exchange with 16O‐poor (Δ17O ~0‰) aqueous fluids on the CO and CV chondrite parent asteroids. In CO3.1 and CV3.1 chondrites, this process did not affect Al‐Mg isotope systematics of CAIs. In some coarse‐grained igneous CV CAIs, O‐isotope heterogeneity of anorthite, melilite, and igneously zoned Al,Ti‐diopside appears to be consistent with their crystallization from melts of isotopically evolving O‐isotope compositions. These CAIs could have recorded O‐isotope exchange during incomplete melting in nebular gaseous reservoir(s) with different O‐isotope compositions and during aqueous fluid–rock interaction on the CV asteroid.

Isotopic Dichotomy among Meteorites and Its Bearing on the Protoplanetary Disk.

Whole rock Δ17O and nucleosynthetic isotopic variations for chromium, titanium, nickel, and molybdenum in meteorites define two isotopically distinct populations: carbonaceous chondrites (CCs) and some achondrites, pallasites, and irons in one and all other chondrites and differentiated meteorites in the other. Since differentiated bodies accreted 1-3 Myr before the chondrites, the isotopic dichotomy cannot be attributed to temporal variations in the disk. Instead, the two populations were most likely separated in space, plausibly by proto-Jupiter. Formation of CCs outside Jupiter could account for their characteristic chemical and isotopic composition. The abundance of refractory inclusions in CCs can be explained if they were ejected by disk winds from near the Sun to the disk periphery where they spiraled inward due to gas drag. Once proto-Jupiter reached 10-20 M ⊕, its external pressure bump could have prevented millimeter- and centimeter-sized particles from reaching the inner disk. This scenario would account for the enrichment in CCs of refractory inclusions, refractory elements, and water. Chondrules in CCs show wide ranges in Δ17O as they formed in the presence of abundant 16O-rich refractory grains and 16O-poor ice particles. Chondrules in other chondrites (ordinary, E, R, and K groups) show relatively uniform, near-zero Δ17O values as refractory inclusions and ice were much less abundant in the inner solar system. The two populations were plausibly mixed together by the Grand Tack when Jupiter and Saturn migrated inward emptying and then repopulating the asteroid belt with roughly equal masses of planetesimals from inside and outside Jupiter's orbit (S- and C-type asteroids).

2017 Leonard Medal for Mark Thiemens

2017 Leonard Medal for Mark Thiemens

Corundum–hibonite inclusions and the environments of high temperature processing in the early Solar System

Corundum-bearing Ca–Al-rich inclusions (CAIs) are a rare class of high-temperature condensates from the inner regions of the protoplanetary disk. Their mineralogy is intermediate between isolated corundum grains and CAIs where corundum has been replaced by lower-temperature phases. These inclusions sample a critical transitional period of the inner nebula where both the Sun and protoplanetary disk were rapidly evolving. We conducted O isotopic, Al–Mg chronological, petrographic, and crystallographic studies of four corundum-bearing inclusions in the Murchison CM2 and ALHA 77307 CO3.0 carbonaceous chondrites. Within each inclusion, corundum, hibonite, and spinel have indistinguishable 16O-rich compositions. The O isotopic compositions from all inclusions fall within a narrow range of Δ17O=−22.8±3.6‰ that matches values of most previously studied micrometer-sized corundum grains and mineralogically pristine CAIs. These data indicate that, with few exceptions, the most refractory inclusions in carbonaceous chondrites formed from the same O isotopic reservoir. One CAI from ALHA 77307, ALH-61, exhibits a continuous corundum mantle overlying a hibonite core, opposite the equilibrium condensation sequence at typical nebular pressures and dust/gas ratios. Transmission electron microscopy examination of the hibonite–corundum interface suggests that the corundum condensed on the hibonite and was itself then partially overlain with spinel. Additionally, high dust/gas ratios are interpreted from the W- and Mo-depleted composition of a refractory metal nugget within a second corundum-bearing CAI, ALH-160. Together, these observations show that the primary formation conditions of some corundum-bearing CAIs involved non-equilibrium condensation in environments with elevated dust-gas ratios.The corundum-bearing CAIs studied here have inferred initial 26Al/27Al ratios that fall within the roughly bimodal distribution of values observed in most CAIs. ALH-160 retains no resolvable excess 26Mg while ALH-61 has a well-resolved initial 26Al/27Al ratio of 4.2±0.4×10−5. The presence or absence of live 26Al at the time of CAI formation may record distinct chronology if 26Al was initially homogeneously distributed in the early Solar System. Alternatively, variations in 26Al/27Al ratios may reflect late injection and/or heterogeneous distribution of 26Al. Regardless of which model for 26Al distribution is correct, the data presented here indicate that formation of corundum-bearing CAIs was repeated during multiple heating and non-equilibrium condensation events throughout early Solar System history and within a single oxygen isotopic reservoir.

Ca–Al-rich inclusions in carbonaceous chondrites: the oldest solar system objects

This paper presents a review of recent available data on the first solid condensates of the Solar System, which include refractory CAIs (Ca–Al-rich Inclusions) mostly composed of Ca, Al, Mg, and Ti minerals. A theoretical condensation sequence calculated from thermodynamic data confirmed that CAIs formed as fine-grained aggregates in the protoplanetary disk from an 16О-rich gas of solar composition at temperatures >1300° K and pressures <10–4 bar. Based on the diversity of CAI types, their mineralogical, bulk chemical, and isotopic compositions, it can be concluded that CAIs experienced melting and evaporation, possibly by shock waves, which may have occurred in the protoplanetary disk within a brief time interval. Some CAIs may have experienced multiple events such as melting, evaporation, and recycling back to the disk by means of a bipolar outflow. The CAIs having an absolute age of 4567.30 ± 0.16 Myr are the oldest objects in the Solar System. The study of CAIs revealed two distinct oxygen isotope reservoirs (16О-rich and 16О-poor) and established a chronology of the sequence of processes forming individual CAI components using Mg–Al, Cr–Mn and Pb–Pb isotopic systematics.

Heterogeneous distribution of 26Al at the birth of the Solar System: Evidence from corundum-bearing refractory inclusions in carbonaceous chondrites

We report on the mineralogy, petrology, and in situ oxygen- and magnesium-isotope measurements using secondary ion mass spectrometry of 10 corundum-bearing calcium–aluminum-rich inclusions (CAIs) from the Adelaide (ungrouped), Murray and Murchison (CM) carbonaceous chondrites. We also measured in situ oxygen-isotope compositions of several isolated corundum grains in the matrices of Murray and Murchison. Most of the corundum-bearing objects studied are uniformly 16O-rich [Δ17O values range from −17‰ to −28‰ (2σ=±2.5‰) (Δ17Oavr=−23±5‰)], suggesting that they formed in a 16O-rich gas of approximately solar composition and largely avoided subsequent thermal processing in an 16O-poor gaseous reservoir. There is a large spread of the initial 26Al/27Al ratio [(26Al/27Al)0] in the corundum-bearing CAIs. Two Adelaide CAIs show no resolvable excess of radiogenic 26Mg (δ26Mg∗): the inferred (26Al/27Al)0 are (0.6±2.0)×10−6 and (−0.9±1.2)×10−6, respectively. Slopes of the model 26Al–26Mg isochrons in five CAIs from Murray and Murchison are (4.4±0.2)×10−5, (3.3±0.3)×10−5, (4.1±0.3)×10−5, (3.9±0.4)×10−5, and (4.0±2.0)×10−6, respectively. These values are lower than the canonical (26Al/27Al)0 ratio of (5.23±0.13)×10−5 inferred from the whole-rock magnesium-isotope measurements of the CV CAIs, but similar to the (26Al/27Al)0 ratio of (4.1±0.2)×10−5 in the corundum-bearing CAI F5 from Murray. Five other previously studied corundum-bearing CAIs from Acfer 094 (ungrouped) and CM carbonaceous chondrites showed no resolvable δ26Mg∗.We conclude that the corundum-bearing CAIs, as well as the solar corundum grains from matrices and acid-resistant residues of unequilibrated ordinary and carbonaceous chondrites, recorded heterogeneous distribution of 26Al in the Solar System during an epoch of CAI formation. The 26Al-rich and 26Al-poor corundum-bearing CAIs and solar corundum grains represent different generations of refractory objects formed during this epoch. As a result, its duration cannot be inferred from 26Al–26Mg systematics of CAIs. Oxygen-isotope composition of the protoplanetary disk was probably heterogeneous during this time reflecting either initial differences in oxygen isotopic compositions of the solid and gaseous reservoirs in the early Solar System or rapid isotopic evolution of these reservoirs in the protoplanetary disk with time. We suggest that 26Al was injected into the protosolar molecular cloud core, possibly by a wind from a neighboring massive star or by or a low-mass asymptotic giant branch star, prior to formation of CAIs and refractory grains, and was subsequently homogenized through the protoplanetary disk by radial mixing.

On the thermal history of Saturn’s satellites Titan and Enceladus

The thermal histories of two geologically active satellites of Saturn—Titan and Enceladus—are discussed. During the Cassini mission, it was found that there are both nitrogen-containing compounds—NH3 and N2-and CO2 and CH4 in the water plumes of Enceladus; at that, ammonia is the prevailing form. This may testify that during evolution, the material of the satellite was warmed up to T ∼ 500–600 K, when NH3 (the form of nitrogen capable of being accreted) could only be partly converted into N2. Contrary to Enceladus, the temperature inside Titan probably reached values higher than 800 K or even higher than 1000 K, since the process of the chemical dissociation of ammonia was completely finished on this satellite and its atmosphere contains only molecular nitrogen. While the internal heating of Titan up to high temperatures can be explained by its large mass, the heating source for Enceladus’ interior is far from evident. Such traditional heating sources as the energy of gravitational differentiation and the radiogenic heating due to shortliving 26Al and 60Fe could not be effective. The first one is because of the small size of Enceladus (RE ≈ 250 km), and the inefficiency of the second one is caused by the fact that the satellite was formed not earlier than 8–10 Myr after the formation of calcium and aluminum-enriched inclusions in carbonaceous chondrites (CAI), i.e., after 26Al had completely decayed. In the present paper, we propose other heating mechanisms-the heat of long-living radioactive elements and tidal heat, which could provide the observed chemical composition of the water plumes of Enceladus rather than only the differentiation of its protomatter into the ironstone core and the ice mantle.

David Wark, 1939-2005

David Wark, 1939-2005

Chemical and isotopic fractionation during the evaporation of the FeO-MgO-SiO 2-CaO-Al 2O 3-TiO 2 rare earth element melt system

A synthetic material with solar elemental proportions of iron, magnesium, silicon, titanium, calcium, and aluminum oxides and doped with rare earth elements was evaporated in a vacuum furnace at 1800 and 2000°C for different durations to study its chemical and isotopic evolution during the evaporation process. It is demonstrated that kinetic evaporation of solar composition material can produce residues of calcium-, aluminum-rich inclusion bulk chemistry. The evaporation sequence of the main constituents in this solar composition material is iron > silicon ≈ magnesium > titanium. Calcium and aluminum remain unevaporated after evaporation of 95% of the solar composition material. The chemical fractionation between the gas and condensed phase is a function not only of temperature and pressure, but also of the bulk chemical composition of the condensed phase. During the evaporation process, cerium is almost as volatile as iron. The 2,000-fold cerium depletion found in some refractory inclusions in carbonaceous chondrites was reproduced in the evaporation experiment and can be readily explained as a result of evaporation of preexisting meteoritic material. Kinetic isotopic fractionation of magnesium, oxygen, and silicon follows the Rayleigh distillation law during the laboratory evaporation of synthetic solar composition material. This implies that the residue is well mixed during the evaporation process and that the evaporation kinetic processes (both chemical and isotopic) are surface reaction-controlled. The isotopic mass fractionation factors are lower than those predicted from theoretical calculations by using the square root of mass ratios of likely evaporating species. Thus, the surface reaction is more complicated than decomposition into single gas species of each element.

Source regions and timescales for the delivery of water to the Earth

Abstract—In the primordial solar system, the most plausible sources of the water accreted by the Earth were in the outer asteroid belt, in the giant planet regions, and in the Kuiper Belt. We investigate the implications on the origin of Earth's water of dynamical models of primordial evolution of solar system bodies and check them with respect to chemical constraints. We find that it is plausible that the Earth accreted water all along its formation, from the early phases when the solar nebula was still present to the late stages of gas‐free sweepup of scattered planetesimals. Asteroids and the comets from the Jupiter‐Saturn region were the first water deliverers, when the Earth was less than half its present mass. The bulk of the water presently on Earth was carried by a few planetary embryos, originally formed in the outer asteroid belt and accreted by the Earth at the final stage of its formation. Finally, a late veneer, accounting for at most 10% of the present water mass, occurred due to comets from the Uranus‐Neptune region and from the Kuiper Belt. The net result of accretion from these several reservoirs is that the water on Earth had essentially the D/H ratio typical of the water condensed in the outer asteroid belt. This is in agreement with the observation that the D/H ratio in the oceans is very close to the mean value of the D/H ratio of the water inclusions in carbonaceous chondrites.

The effect of H 2O gas on volatilities of planet-forming major elements: I. Experimental determination of thermodynamic properties of Ca-, Al-, and Si-hydroxide gas molecules and its application to the solar nebula

Vapor pressures of Ca(OH) 2(g) and Al(OH) 3(g) were measured using the transpiration technique, employing the reactions CaO(s) + H 2O(g) ⇌ Ca(OH) 2(g); 2CaAl 2O 4(s) + H 2O(g) ⇌ CaAL 4O 7(s) + Ca(OH) 2(g); and CaAl 4O 7(s) +3H 2O(g) ⇌ CaAl 2O 4(s) + 2Al(OH) 3(g). From these vapor pressures, enthalpy and entropy changes in the above reactions were determined for the average experimental temperature (1650 K). This, combined with literature thermodynamic data, gives the heats of formation and entropies of the hydroxide gas species at the standard state (1 atm and 298.15 K): ΔH 0 = −615.0 ± 7.1 (2 σ) kJ/ mol and S 0 = 284.5 ± 7.9 (2 σ) J/ mol · K for Ca( OH) 2( g), and ΔAH 0 = −1004.2 ± 15.0 (2 σ) kJ/ mol and S 0 = 313.8 ± 7.9 (2 σ) J/ mol · K for Al( OH) 3( g). The Si-tetrahydroxide molecule, Si(OH) 4, was produced in the reaction SiO 2(s) + 2H 2O(g) ⇌ Si(OH) 4(g) by application of the same technique. This is the first report of a stable Si-hydroxide gas species. Its heat of formation and entropy at 1600 K and 1 atm was determined to be −1342.7 ± 2.7 (2 σ) kJ/ mol and 592.5 ± 1.0 (2 σ) J/ mol · K, respectively. Using these values for the gas species investigated and literature thermodynamic data for other gas species, “volatilities” (the total of vapor pressures of gas species containing the element in question, over the pure condensed oxide ofthat element) of the five major planet-forming elements Fe, Mg, Si, Ca, and Al were calculated for a range of physical and chemical conditions ( P, T, bulk system composition) appropriate to the solar nebula. The results indicate 1. (1) volatilities of all the elements investigated are significantly enhanced if the formation of hydroxide species by reaction with H 2O(g) is taken into account, compared with previous calculations which neglected these species; 2. (2) generally, hydroxide gas species predominate at relatively low temperatures and/or in an oxidizing atmosphere, while monatomic and undersaturated-oxidized gas species are most abundant at high temperatures and/or in a reducing atmosphere; 3. (3) hydroxide species of Ca and Al are relatively stable even at high temperatures compared to those of Si, Mg, and Fe; 4. (4) these effects explain how Ca and Al could have evaporated from Ca,Alrich inclusions in carbonaceous chondrites while Si, Mg, and Fe condensed onto them during the preaccretion alteration of CAIs.

Isotopic variations in the rock-forming elements in meteorites

Variations in isotopic abundances of the major rock-forming elements can be used as tracers for chemical processes in the solar nebula, and may also provide links to the presolar cloud from which the solar nebula was derived. Emphasis in this paper is placed on the correlation of isotopic variations between pairs of elements, both for mass-dependent fractionation effects and for nucleosynthetic effects. Variations in oxygen isotope abundances, which are ubiquitous in all Solar System matter, are decoupled from those in other elements, probably because of the effect of a large oxygen reservoir in the nebular gas. Among the metallic rock-forming elements (Mg, Si, Ca, Ti, Cr, Fe, Ni) isotopic variations are small to immeasurable in ordinary chondrites and achondrites. Large variations are observed in refractory inclusions in carbonaceous chondrites in the elements Mg, Si, Ca and Ti. Fractionation effects result from evaporation and condensation at high temperatures. The dominant nucleosynthetic effects are seen as excesses and deficiencies of the neutron-rich isotopes in the region of the iron abundance peak: 48 Ca, 49 Ti, 50 Ti, 54 Cr, 64 Ni. These effects result from mixing in different proportions of the products of different regions of a supernova. The rock-forming elements also show isotopic variations due to extinct radioactivities: 26 Mg, 53 Cr and 41 K. A local source is likely, as is heterogeneous distribution within the solar nebula.

Enthalpies of formation of CaAl 4O 7 and CaAl 12O 19 (hibonite) by high temperature, alkali borate solution calorimetry

The enthalpies of formation of CaAl 4O 7 and CaAl 12O 19 (hibonite), by alkali borate solution calorimetry at 1063 K are: ΔH 0 f,CaAl 4O 7 , (1063 K; from oxides) = −[25.6 ± 4.7] kj g.f.w. and ΔH 0 f,CaAl 12O 19 (1063 K; from oxides) = −[33.0 ± 9.7] kj g.f.w. Using experimental enthalpy data for CaAl 4O 7 and estimated values for CaAl 12O 19 , the standard enthalpies of formation of these compounds from the elements at 298 K are: ΔH 0 f,CaAl 4O 7 (298 K; from elements) = −4,007 ± 5.2 kj g.f.w. and ΔH 0 f,CaAl 12O 19 (298 K; from elements) = −10,722 ± 12 kj g.f.w. Comparison with high-temperature galvanic cell data for the Gibbs energy of formation of CaAl 12O 19 allows a calculation of the standard entropy of hibonite, S 0 hibonite (1100 K) = 1280.2 J K g.f.w. This value is only about 2% larger than the oxide sum. Hence it is inferred that the standard entropy of hibonite at 298 K is probably only slightly larger than the oxide sum value of 343.7 J K g.f.w. The present data were used to extrapolate Kumar and Kay's (1985) data for the Gibbs energies of formation of hibonite and CaAl 4O 7 to the temperature range 1500–1700 K. These data were then used in equilibrium thermodynamic calculations of the condensation of a gas of solar composition. Contrary to calculations of Kornacki and Fegley (1984), who used thermodynamic data of Allibert et al. (1981) for calcium aluminates, our results show no stability field for CaAl 4O 7 in a gas of solar composition at 10 −3 to 10 −5 atm total pressure. At 10 −3 atm pressure, hibonite forms by reaction of corundum with the gas at 1725 K, begins to react with the gas to form gehlenite at 1607 K and disappears completely in a reaction to form spinel at 1494 K. The absence of CaAl 4O 7 from hibonite-, spinel-rich inclusions in carbonaceous chondrites cannot be used as an argument against a condensation origin for these objects.

Plagioclase-rich inclusions in carbonaceous chondrite meteorites: Liquid condensates?

Plagioclase-rich inclusions in carbonaceous chondrites are of two types: (1) mm/cm-sized Ca-Al-rich inclusions (CAIs) with 30–60 vol.% anorthite and up to 35 vol.% each of Ti-Al-pyroxene (Tpx), melilite and spinel, and (2) smaller (<3 mm) CA chondrules similar to those found in ordinary chondrites, containing more sodic plagioclase, pyroxene, olivine and spinel. The CAIs, which were formerly named “Type I” and thought to be “Intermediate” between Type A and Type B CAIs, are shown to be an independent type, renamed “Type C”, with distinctive chemical and mineralogical compositions. Type C CAIs have Group I or Group II trace element patterns, low initial 26Al 27Al ratios of ~5 × 10 −6, typical ak 35–55 melilite, Tpx with 2–12% TiO 2, and opaque grains (OsIrRuMoPt-alloy nuggets and fremdlinge) smaller than 10 μm. The textures, sequence of crystallization, and concentric zonation indicate that they crystallized from melt droplets. The texture of the anorthite ranges from coarse laths to a fine-grained groundmass, reflecting increasing retardation of its nucleation. The few CAIs with the coarse laths cooled slowly (2°–5°/h?) from below the liquidus, but the majority cooled faster from completely molten material. As the elemental abundances in Type C's are dominated by volatility, the melts were not formed by planetary (magmatic) processes. Evaporative residues and remelted solid condensates are also ruled out because the compositions of naturally occurring, experimentally produced, and thermodynamically calculated residues and solid condensates differ from those of Type C CAIs. Type C CAIs have compositions similar to those calculated for liquid condensates under high solar gas pressure (~ 1 atm?) or in dust-rich environments, and are probably liquid condensates. The few Type C CAIs that crystallized from incompletely molten materials may have been reheated liquid condensates or mixed liquid solid condensates.

Evidence for oxidizing conditions in the solar nebula from Mo and W depletions in refractory inclusions in carbonaceous chondrites

Depletions of Mo and W relative to other refractory metals of similar volatility (Re, Os, Ir, Ru, Pt) are common in a suite of 16 Ca,Al-rich inclusions (CAIs) from 5 carbonaceous chondrites. Twelve of the 16 CAIs from Allende, Grosnaja, Leoville, and Ornans show Mo depletions; six of these 12 inclusions also show W depletions. The one CAI analyzed from the Essebi chondrite shows no depletions. The Mo and W depletions have a very characteristic pattern with Mo always more depleted than W. The same Mo and W depletion pattern occurs in calculated refractory metal alloy compositions formed at oxygen fugacities 10 3 to 10 4 times greater than the canonical solar nebula oxygen fugacity. We conclude that Mo and W depletions are common in CAIs from carbonaceous chondrites and that the depletions result from high temperature oxidation. The oxidation may have occurred during the evaporation of primitive dust into CAIs but is also consistent with condensation at high oxygen fugacities. No potential alternative processes appear to be capable of producing the observed Mo and W depletion patterns.

The identification of Group II inclusions in carbonaceous chondrites by electron probe microanalysis of perovskite

Group II Ca,Al-rich inclusions (CAIs), which are relatively depleted in super-refractory lithophile trace elements (SRE) (e.g., Er, Lu, Sc, Zr, Y, Hf), formed from nebular or interstellar reservoirs depleted in SRE-enriched dust; their unusual chemistry preserves a record of widespread (but incompletely-understood) fractionation processes in the primitive solar nebula. We demonstrate that because Group II inclusions have anomalously high Ti/SRE ratios they can be identified by electron probe microanalysis (EPMA) of perovskite—a host for Ti and several SRE. Perovskites in Group II Allende CAIs generally contain < 0.10 wt.% ZrO 2, while perovskites in Group I, III, V, and VI Allende CAIs generally contain > 0.25 wt.% ZrO 2. The Y content of perovskite also can be used to identify Group II inclusions by this technique. Inclusions with Zr,Y-poor perovskite in principle need not be depleted in SRE if they contain other phases suitably enriched in SRE, but an analysis of SRE carriers in Allende inclusions and bulk compositions of CAIs shows that in practice this factor does not invalidate the EPMA technique for identifying Group II CAIs. EPMA of perovskite in eight Allende inclusions demonstrates that ∼ 75% of “fine-grained” CAIs (i.e., spinel-rich complex CAIs) are Group II inclusions, in agreement with their abundance as determined by bulk trace element analyses of 24 CAIs. A spinel-rich nodule in a rimmed olivine aggregate contains Zr,Y-poor perovskites, evidence that similar nodules (which occur in ∼ 25% of olivine-rich Allende inclusions) are the Group II REE carrier in olivine-rich inclusions that have highly fractionated trace element patterns (i.e., “fractionated AOAs”).