Nebular Condensation Research Articles

Enrichment of moderately volatile elements in first-generation planetesimals of the inner Solar System.

The depletion of moderately volatile elements (MVEs) in terrestrial planets remains poorly understood, with explanations including partial nebular condensation and MVE loss during planetesimal differentiation or collisions. In this study, we use magmatic iron meteorites to reconstruct the MVE inventory of the earliest inner [noncarbonaceous (NC)] and outer [carbonaceous (CC)] Solar System planetesimals. We show that several NC and CC iron meteorite parent bodies (IMPBs) exhibit chondrite-like MVE abundances, indicating that "first-generation" inner Solar System planetesimals were remarkably MVE rich. Consistent with isotopic signatures of MVEs in Earth and Mars, these planetesimals made a substantial contribution to the MVE inventories of terrestrial planets. Variations in MVE abundances among IMPBs, particularly the two volatile-depleted NC and CC IMPBs (IVA and IVB), reflect secondary volatile loss after disruption of their parent bodies. Consequently, MVE depletion in terrestrial planets is more closely linked to the protracted history of MVE loss during planetesimal collisions rather than incomplete condensation or MVE loss during planetesimal differentiation.

Submicron-sized anhydrous crystalline silicates and their relation to amorphous silicate in the matrix of Acfer 094

The study of pristine chondrites provides insight into nebular processes that occurred prior to the accretion of small-sized parent bodies. The interchondrule matrix of the primitive chondrite Acfer 094 is characterized by the presence of submicron-sized anhydrous crystalline aggregates embedded in a silicate groundmass that is mostly amorphous. Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDXS) were employed to investigate the matrix of Acfer 094 and its components.The amorphous silicate groundmass is homogeneous in composition and exhibits a Mg depletion relative to the solar value. It embeds Fe-Ni nanosulfides, whose typical size is a few tens of nanometers. Nanometer-sized fibrous silicates are rare in the groundmass indicating a low degree of aqueous alteration. Submicron-sized Mg-rich crystalline silicates (olivine and pyroxene) occur either as isolated grains or as aggregates. These submicron-sized crystalline silicates occupy around 25 % of the matrix volume. The isolated grains display a wide range of shapes, from rounded to irregularly angular, and could have originated from the fragmentation of type I chondrules or from nebular condensation. The aggregates exhibit variable morphologies and grain sizes (typically a few tens of nm). They are chemically equilibrated, and likely formed by solid-state thermal annealing of amorphous precursors.The Acfer 094 matrix contains a range of components that have undergone varying degrees of thermal modification. A significant proportion of a precursor material (i.e. nebular dust) resembling matrix is likely to have undergone one or more brief and intense thermal events, potentially associated with the process of chondrule formation. These events resulted in the formation of magnesium-rich anhydrous silicates (forsterite and enstatite) at high temperatures that were embedded in the matrix of Acfer 094 as isolated grains and crystalline aggregates.

Grossite-bearing refractory inclusions from reduced CV chondrites: Mineralogical and oxygen isotopic constraints on the parent body alteration history

We report the results of coordinated mineralogical, microstructural, and oxygen isotopic analyses of grossite-bearing refractory inclusions from reduced CV (Vigarano type) chondrites to obtain a more complete picture of secondary parent body alteration processes and conditions. Grossite (CaAl4O7) occurs in cores of nodules in fine-grained Ca,Al-rich inclusions (CAIs) that likely represent aggregates of nebular condensates. In many occurrences, grossite has been partially replaced by hercynite [(Fe,Mg,Zn)Al2O4], which displays complex microstructures and compositions, and magnetite nanoparticles. The alteration of grossite was a crystallographically-controlled, fluid-driven process that occurred via partial dissolution of grossite and subsequent precipitation of hercynite and magnetite during short-lived and low-temperature metasomatic alteration on the CV chondrite parent body. The constituent phases of grossite-bearing CAIs show heterogeneous oxygen isotopic compositions, with grossite and perovskite displaying systematically 16O-depleted compositions (Δ17O= − 12 ‰ to − 1 ‰) relative to uniformly 16O-rich hibonite and spinel (Δ17O= − 25 ‰ to − 21 ‰). Melilite is variably 16O-depleted (Δ17O= − 25 ‰ to − 2 ‰). The observed oxygen isotopic distribution is interpreted as a result of mineralogically controlled oxygen isotopic exchange with an 16O-poor fluid on the CV chondrite parent body. Collectively, the presence of limited fluids played an important role in preferential alteration of grossite to hercynite and magnetite and various degrees of 16O depletion in grossite, perovskite, and melilite during thermal metamorphism. We conclude that, among refractory phases in the inclusions, grossite was the most susceptible to metasomatic reactions with Fe-rich fluids and the second most susceptible, after perovskite, to oxygen isotopic exchange with an 16O-poor fluid during the thermal history of the CV chondrite parent asteroid.

Condensate evolution in the solar nebula inferred from combined Cr, Ti, and O isotope analyses of amoeboid olivine aggregates

Refractory inclusions in chondritic meteorites, namely amoeboid olivine aggregates (AOAs) and Ca-Al-rich inclusions (CAIs), are among the first solids to have formed in the solar system. The isotopic composition of CAIs is distinct from bulk meteorites, which either results from extreme processing of presolar carriers in the CAI-forming region, or reflects an inherited heterogeneity from the Sun's parental molecular cloud. Amoeboid olivine aggregates are less refractory than CAIs and provide a record of how the isotopic composition of solid material in the disk may have changed in time and space. However, the isotopic composition of AOAs and how this composition relates to that of CAIs and later-formed solids is unknown. Here, using new O, Ti, and Cr isotopic data for eight AOAs from the Allende CV3 chondrite, we show that CAIs and AOAs share a common isotopic composition, indicating a close genetic link and formation from the same isotopic reservoir. Because AOAs are less refractory than CAIs, this observation is difficult to reconcile with a thermal processing origin of the isotope anomalies. Instead, the common isotopic composition of CAIs and AOAs is readily accounted for in a model in which the isotopic composition of infalling material from the Sun's parental molecular cloud changed over time. In this model, CAIs and AOAs record the isotopic composition of the early infall, while later-formed solids contain a larger fraction of the later, isotopically distinct infall. This model implies that CAIs and AOAs record the isotopic composition of the Sun and suggests that the nucleosynthetic isotope heterogeneity of the solar system is predominantly produced by mixing of solar nebula condensates, which acquired their distinct isotopic compositions as a result of time-varied infall from the protosolar cloud.

Thermodynamics of ultrarefractory condensates: Implications for the high-temperature limit of the inner dust rim of the early solar protoplanetary disk

Ultrarefractory (UR) condensates found in refractory inclusions in chondrites contain records of the early nebular thermochemistry that prevailed in the high-temperature region close to the protosun. Recent reports of the UR phases such as allendeite (Sc4Zr3O12), tazheranite ((Zr,Ti,Ca)O2−x), and kangite ((Sc,Ti,Al,Zr,Mg,Ca,□)2O3) imply, respectively, nebular condensation temperatures and origins higher than and inward of those previously deduced from calcium-aluminum-rich inclusions. However, knowledge gaps on their thermochemistry have precluded a quantitative understanding of temperatures and chemical pathways that led to their origins in the early solar protoplanetary disk. Here we use density functional theory to determine the thermochemistry of these materials for the first time. We find that allendeite is a stable phase under equilibrium conditions with its condensation temperature (1643 K at 10−4 bar) in the same range as that of nominal hibonite (CaAl12O19, 1637 K at 10−4 bar). Among the UR oxides, tetragonal ZrO2 exhibits the highest condensation temperature (1739 K at 10−4 bar) and potentially reveals the high-temperature limits at which solid dust could have survived in the inner region of the disk. In comparison, we find that pure cubic ZrO2 does not form from a cooling gas of solar composition undergoing equilibrium condensation. Similarly, we find that the stoichiometric endmember of kangite, Sc2O3 does not condense under equilibrium conditions, and moreover, the role of Ti and Zr as solutes is crucial to modeling its stability and origins.

Determination of activities and condensation temperatures of GaO1.5 and InO1.5 in anorthite-diopside eutectic melts by Knudsen Effusion Mass Spectrometry

The group 13 elements Ga and In are overabundant in bulk silicate Earth (BSE) when compared to lithophile elements of similar 50% nebular condensation temperature Tc50. To understand whether evaporation from silicate melts provides a more accurate description of volatility during the later stages of planetary accretion, namely, at higher temperatures and oxygen fugacities than in the solar nebula, knowledge of the activities of GaO1.5 and InO1.5 in silicate melts and their stable gaseous species are required. To this end, we doped anorthite-diopside (An-Di) eutectic glasses with ∼1000 and ∼10,000 ppm of Ga and In and determined their equilibrium partial pressures above the silicate liquid by Knudsen Effusion Mass Spectrometry (KEMS) using Ir cells at 1550–1740 K over the log(fO2) range ΔIW+1.5 to ΔIW+2.5 (IW = iron-wüstite buffer). We detect Ga0 and In0 as the dominant vapour species and determine activity coefficients of γ(GaO1.5) = 0.036(6) at 1700 K and of γ(InO1.5) = 0.017(12) at 1674 K. Using these activity coefficients, we calculate partial pressures of Ga and In, together with those of similarly volatile elements, K and Zn and show that their relative volatilities from An-Di eutectic melts are in the in order Ga > K ∼ In > Zn, different from those predicted from their Tc50 under nebular conditions but in line with their relative abundances in the BSE. This substantiates the view that the abundances of volatiles in BSE, such as Ga and In, may have been set by evaporation from silicate melts under oxidising conditions at later stages of planetary accretion. Moreover, chondrules likely never underwent significant evaporation during melting and their volatile-depleted nature is likely inherited from the earliest solid condensates.

Cation Ordering in Spinel from Calcium-Aluminum-Rich Inclusions in Carbonaceous Chondrites NWA 2364 and NWA 6991 to Quantify Temperature in the Early Solar System

Abstract Minerals extracted from two calcium-aluminum-rich inclusions, one each from NWA 2364 and NWA 6991 CV3 chondrite meteorites, were examined using micro X-ray diffraction, 27Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) and Triple Quantum (3Q) MAS NMR. In situ examination by micro X-ray diffraction was used to confirm the presence of spinel (MgAl2O4) and to identify co-existing minerals. Aluminum-27 3Q MAS NMR was used to confirm the identity of co-existing minerals by their NMR signature in this two-dimensional experiment and to ensure that their NMR peaks did not overlap with those attributed to spinel. Aluminum-27 MAS NMR was used to quantitatively measure cation ordering between the tetrahedral ([4]Al) and octahedral ([6]Al) sites in the spinel. The measured cation distribution was used to calculate the inversion parameter, x, for each sample: x = 0.077 ± 0.007 for NWA 2364, x = 0.027 ± 0.001 for NWA 6991 (gehlenite-rich fraction), and x = 0.052 ± 0.003 for NWA 6991 (Al-bearing diopside-rich fraction). The measured cation-ordering data were input into six literature calibration curves to estimate the temperature of formation or the most recent equilibration temperature of the spinel. The NWA 2364 sample yielded temperatures between 420 and 707 K and the NWA 6991 sample yielded temperatures between 153 and 615 K, depending on the calibration curve used. These temperatures are lower than expected for nebular condensation temperatures, however, reordering may have occurred during cooling from high temperatures, so these values may be taken to represent temperature minima. The calculated spinel-related temperatures may thus represent equilibration temperatures related to subsequent nebular or CV chondrite parent body processes.

Earth’s volatile depletion trend is consistent with a high-energy Moon-forming impact

The abundance of volatile elements in the silicate Earth relative to primitive chondrites provides an important constraint on the thermochemical evolution of the planet. However, an overabundance of indium relative to elements with similar nebular condensation temperatures is a source of debate. Here we use ab initio molecular dynamics simulations to explore the vaporization behavior of indium from pyrolite melt at conditions of the early magma ocean just after the Moon-forming impact. We then compare this to the vaporization behavior of other minor elements. When considering the volatility of the elements from the magma ocean in the absence of the solar nebula gas, we find that there is no overabundance of indium. On the contrary, there is a slight deficit in the abundance of indium, which is consistent with its moderately siderophile nature. Thus, we propose that a high-energy Moon-forming impact may have had a more significant contribution to volatile depletion than previously believed.

Origin of moderately volatile element depletion on differentiated bodies: Insights from the evaporation of indium from silicate melts

In comparison with the Sun and CI chondrites, moderately volatile elements (MVEs) are depleted in terrestrial planets and other small, rocky differentiated bodies in the inner solar system. The abundances of most MVEs in the bulk silicate Earth (BSE) fall on a trend that defines a near log-linear decrease with their 50% nebular condensation temperature (Tc50). This temperature scale has traditionally been used to infer elemental volatility during planetary formation and accretion, however, indium (In) deviates from this correlation. Despite being a siderophile element that could have been depleted by core formation, In is overabundant for its calculated Tc50 in the BSE, as well as in the silicate portions of other small bodies (e.g., Moon and Vesta). This overabundance of In indicates that Tc50, calculated under nebular conditions, may not be applicable to planetary evaporation that occurs at much higher oxygen fugacity (fO2) and pressure than nominal nebular conditions. Here, we conduct a series of evaporation experiments for basaltic melts to quantify the volatility of In under conditions relevant to planetary evaporation. Our results show that, when using the evaporation temperature (Te1, refers to the temperature at which 1% of element i has evaporated from liquid to gas phase under equilibrium) as the volatility scale, the abundances of volatile elements, including In, of the Moon and Vesta display a progressive depletion with increasing volatility (decreasing Te1). This smooth depletion pattern contrasts with the overabundance of In shown on the Tc50 scale, suggesting that volatile depletion on small bodies occurred under non-nebular environment instead of during nebular condensation. On the other hand, the volatile element composition of the BSE (including In) could be explained by integrating (i) early accreted precursor materials of the proto-Earth that underwent volatile loss under conditions more oxidizing than those of the solar nebula with (ii) late added volatile-rich materials.

A petrologic and microstructural study of a compact type A calcium‐aluminum‐rich inclusion from the Northwest Africa 5028 CR2 chondrite: Implications for nebular and parent‐body processes

AbstractCompact type A calcium‐aluminum‐rich inclusions (CTA CAIs) are believed to have experienced partial melting that erased all information on their original nebular condensation. To investigate this question, we report new microstructural data on a CTA CAI, composed primarily of melilite, spinel, and perovskite, in the Northwest Africa 5028 CR2 chondrite. The melilite grains contain low (5–10 mole%) åkermanite contents and are not compositionally zoned. Spinel and perovskite each occur as near endmember compositions MgAl2O4 and CaTiO3 and contain minor V and Al, respectively. A continuous rim composed of melilite, spinel, and perovskite, with minor hibonite grains occur around the CAI. We extracted two regions of interest from the interior CAI and two from the rim using focused ion beam techniques for detailed analysis using transmission electron microscopy. Evidence for thermal processing occurs as a perovskite–spinel–spinel triple junction in an interior section and a spinel inclusion within perovskite in a rim section. Evidence for parent‐body alteration occurs in the form of Fe‐rich sheet silicates in the rim, and localized amstallite in the interior of the CAI. While previous work suggested that many CTA CAIs experienced thermal processing in the solar nebula, including partial melting, our data show that signatures of primary condensation can be preserved in the form of more refractory phases contained within less refractory minerals, namely melilite and perovskite grains within spinel, and hibonite grains within perovskite, respectively. The inclusion we report on here has a complex history involving gas‐phase condensation, nebular thermal processing, and parent‐body alteration.

Constraints on the ice composition of carbonaceous chondrites from their magnetic mineralogy

Carbonaceous chondrites experienced varying degrees of aqueous alteration on their parent asteroids, which influenced their mineralogies, textures, and bulk chemical and isotopic compositions. Although this alteration was a crucial event in the history of these meteorites, their various alteration pathways are not well understood. One phase that formed during this alteration was magnetite, and its morphology and abundance vary between and within chondrite groups, providing a means of investigating chondrite aqueous alteration. We measured bulk magnetic properties and first-order reversal curve (FORC) diagrams of CM, CI, CO, and ungrouped C2 chondrites to identify the morphology and size range of magnetite present in these meteorites. We identify two predominant pathways of aqueous alteration among these meteorites that can be distinguished by the resultant morphology of magnetite. In WIS 91600, Tagish Lake, and CI chondrites, magnetite forms predominantly from Fe-sulfides as framboids and stacked plaquettes. In CM and CO chondrites, <0.1 μm single-domain (SD) magnetite and 0.1–5 μm vortex (V) state magnetite formed predominantly via the direct replacement of metal and Fe-sulfides. After ruling out differences in temperature, water:rock ratios, terrestrial weathering effects, and starting mineralogy, we hypothesise that the primary factor controlling the pathway of aqueous alteration was the composition of the ice accreted into each chondrite group's parent body. Nebula condensation sequences predict that the most feasible method of appreciably evolving ice concentrations was the condensation of ammonia, which will have formed a more alkaline hydrous fluid upon melting, leading to fundamentally different conditions that may have caused the formation of different magnetite morphologies. As such, we suggest that WIS 91600, Tagish Lake, and the CI chondrites accreted past the ammonia ice line, supporting a more distal or younger accretion of their parent asteroids.

Variable refractory lithophile element compositions of planetary building blocks: Insights from components of enstatite chondrites

Chondrites are sediments of materials left over from the earliest stage of the solar system history. Based on their undifferentiated nature and less fractionated chemical compositions, chondrites are widely considered to represent the unprocessed building blocks of the terrestrial planets and their embryos. Models of chemical composition of the terrestrial planets generally find chondritic relative abundances of refractory lithophile elements (RLE) in the bulk bodies (“constant RLE ratio rule”), based on limited variations of RLE ratios among chondritic meteorites and the solar photosphere. Here, we show that ratios of RLE, such as Nb/Ta, Zr/Hf, Sm/Nd and Al/Ti, are fractionated from the solar value in chondrules from enstatite chondrites (EC). The fractionated RLE ratios of individual EC chondrules document different chalcophile affinities of RLE under highly reducing environments and a separation of RLE-bearing sulfides from silicates before and/or during chondrule formation. In contrast, the bulk EC have solar-like RLE ratios, indicating that a physical sorting of silicates and sulfides was negligible before and during the accretion of EC parent bodies. Likewise, if the Earth’s accretion was dominated by EC-like materials, as supported by multiple isotope systematics, physical sorting of silicates and sulfides in the accretionary disk did not occur. Alternatively, the Earth’s precursors were high-temperature nebular condensates that formed prior to the precipitation of RLE-bearing sulfides. A lack of Ti depletion in the bulk silicate Earth, combined with similar silicate-sulfide and rutile-melt partitioning behaviors of Nb and Ti, prefers a moderately siderophile behavior of Nb as the origin of the accessible Earth’s Nb depletion. Highly reduced planets that have experienced selective removal or accretion of silicates or metal/sulfide phases, such as Mercury, possibly yield fractionated, non-solar bulk RLE ratios.

Atomic-scale structure and non-stoichiometry of meteoritic hibonite: A transmission electron microscope study

Abstract Hibonite (CaAl12O19) is a common refractory mineral in Ca-Al-rich inclusions (CAIs) in primitive meteorites. Transmission electron microscope (TEM) studies have identified enigmatic planar defects in different occurrences of hibonite in the Allende meteorite that give rise to strong streaking along c* in electron diffraction patterns. Atomic resolution high-angle annular dark-field (HAADF) imaging and energy-dispersive X-ray (EDX) analyses were used to determine the nature and origin of these planar features. HAADF images of hibonite grains reveal lamellar intergrowths of common 1.6 nm spacing, and less commonly 2.0 and 2.5 nm spacings, interspersed in stoichiometric hibonite showing 1.1 nm (002) spacing. Stoichiometric hibonite consists of alternating Ca-containing (“R”) and spinel-structured (“S”) blocks stacked in a sequence RS. In contrast, the 1.6 nm layers result from a doubled S block such that the stacking sequence is RSS, while in the widest defect observed, the stacking sequence is RSSSS. These intergrowths are epitaxial and have coherent, low-strain boundaries with the host hibonite Meteoritic hibonite shows common Ti and Mg substitution for Al in its structure. Atomic-resolution EDX maps of hibonite grains in the Allende CAI confirm the preferred site occupancy of Mg on tetragonal M3 sites in S blocks and of Ti on trigonal bipyramidal M2 and octahedral M4 sites in R blocks. Mg is highly concentrated, but Ti is absent in the planar defects where wider S blocks show Al-rich compositions compared to stoichiometric MgAl2O4 spinel. Therefore, Mg likely played the major role in the formation and metastability of planar defects in hibonite. Electron energy loss spectroscopy data from the Ti L2,3 edge show the presence of mixed Ti oxidation states with ~15–20% of Ti as Ti3+ in hibonite, suggesting a direct substitution of Ti3+ ↔ Al3+ in hibonite. The remaining ~80–85% of Ti is present as Ti4+ and corresponding EDX analyses are consistent with the well-known coupled substitution 2Al3+ ↔ Ti4+ + Mg2+ being the major mechanism for Ti and Mg substitution in hibonite. The formation of planar defects in hibonite occurred during high-temperature nebular condensation or melting/crystallization processes. The occurrence of non-stoichiometric hibonite in the Allende CAI deviates from the mineral formation sequence predicted from equilibrium condensation models. Overall, our atomic resolution TEM observations signify non-equilibrium, kinetic-controlled crystal growth during the high-temperature formation of refractory solids in the early solar nebula.

Thermochemistry of melilites I. Towards resolving an inconsistency in nebular condensation calculations

A thermodynamic model is formulated for (Ca,Na)₂(Mg,Fe²⁺,Al,Fe³⁺)^T1 (Al,Fe³⁺,Si<math xmlns="http://www.w3.org/1998/Math/MathML" id="u1" display="inline" overflow="scroll"><mrow><msubsup><mo>)</mo><mn>2</mn><mrow><mtext mathvariant="bold">T2</mtext></mrow></msubsup></mrow></math>O₇ melilites. It employs the compositional vertices: åkermanite (Ca₂MgSi₂O₇, 1), gehlenite (Ca₂Al₂SiO₇, 2), iron åkermanite (Ca₂Fe²⁺Si₂O₇, 3), ferrigehlenite (Ca₂<math xmlns="http://www.w3.org/1998/Math/MathML" id="u2" display="inline" overflow="scroll"><mrow><msubsup><mtext>Fe</mtext><mrow><mn>2</mn></mrow><mrow><mn>3</mn><mo> </mo><mo>+</mo><mo> </mo></mrow></msubsup></mrow></math>SiO₇, 4), sodium melilite (NaCaAlSi₂O₇, 5), and the convergent ordering variables: <i>s</i> = <math xmlns="http://www.w3.org/1998/Math/MathML" id="u3" display="inline" overflow="scroll"><mrow><msubsup><mi>X</mi><mrow><msup><mrow><mtext mathvariant="bold">Al</mtext></mrow><mrow><mn>3</mn><mo> </mo><mo>+</mo><mo> </mo></mrow></msup></mrow><mrow><mtext mathvariant="bold">T2a</mtext></mrow></msubsup></mrow></math> – <math xmlns="http://www.w3.org/1998/Math/MathML" id="u4" display="inline" overflow="scroll"><mrow><msubsup><mi>X</mi><mrow><msup><mrow><mtext mathvariant="bold">Al</mtext></mrow><mrow><mtext mathvariant="bold">3+</mtext></mrow></msup></mrow><mrow><mtext mathvariant="bold">T2b</mtext></mrow></msubsup></mrow></math> and <i>t</i> = <math xmlns="http://www.w3.org/1998/Math/MathML" id="u5" display="inline" overflow="scroll"><mrow><msubsup><mi>X</mi><mrow><msup><mrow><mtext mathvariant="bold">Fe</mtext></mrow><mrow><mn mathvariant="bold">3</mn><mo> </mo><mo>+</mo><mo> </mo></mrow></msup></mrow><mrow><mtext mathvariant="bold">T2a</mtext></mrow></msubsup></mrow></math> – <math xmlns="http://www.w3.org/1998/Math/MathML" id="u6" display="inline" overflow="scroll"><mrow><msubsup><mi>X</mi><mrow><msup><mrow><mtext mathvariant="bold">Fe</mtext></mrow><mrow><mn mathvariant="bold">3</mn><mo> </mo><mo>+</mo><mo> </mo></mrow></msup></mrow><mrow><mtext mathvariant="bold">T2b</mtext></mrow></msubsup></mrow></math> to describe the distribution of Al³⁺, Fe³⁺ and Si⁴⁺ between T2 subsites T2a and T2b. It is calibrated for åkermanite–gehlenite melilites based on the calorimetric data of Charlu and others (1981), the assumption that the synthetic samples of Charlu and others approached “equilibrium” states of Al-Si tetrahedral ordering at 970 K, and analogy with the Al₂(MgSi)_{− 1} substitution in CaMgSi₂O₆ – CaMg_1/2<math xmlns="http://www.w3.org/1998/Math/MathML" id="u9" display="inline" overflow="scroll"><mrow><msub><mtext>Ti</mtext><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msub></mrow></math>AlSiO₆ – CaAl₂SiO₆ fassaites (for example, Sack and Ghiorso, 2017). In this model gehlenite has a disordered Al-Si distribution on T2 sites above 1443 K (1170 °C), consistent with the crystallographic data on c/a ratios of lattice parameters as a function of annealing temperature (Woodhead and Waldbaum, 1974) and the high-temperature heat capacities inferred from drop calorimetric data (Pankratz and Kelley,1964). However, above this critical temperature a partially ordered Al-Si distribution persists between T2a and T2b sites in åkermanite – gehlenite solid solutions with intermediate <i>X</i>₂ (for example, 0.19 &lt; <math xmlns="http://www.w3.org/1998/Math/MathML" id="u10" display="inline" overflow="scroll"><mrow><msub><mi>X</mi><mn>2</mn></msub></mrow></math> &lt; 0.89 at 1573 K). To a first approximation activity-composition relations of the gehlenite component approximate those of ideal mixing (that is, <i>a_i</i> = <i>X_i</i>), particularly in gehlenite-rich compositions, but those of åkermanite component display pronounced temperature dependence in intermediate compositions. Enthalpies of formation of åkermanite and gehlenite from the elements at 298.15 K, <math xmlns="http://www.w3.org/1998/Math/MathML" id="u11" display="inline" overflow="scroll"><mrow><mi>Δ</mi><msubsup><mover accent="true"><mi>H</mi><mo>¯</mo></mover><mrow><mi>f</mi><mo> </mo><mn>298.15</mn></mrow><mrow><mi mathvariant="normal">o</mi><mo> </mo><mtext mathvariant="bold">AK</mtext></mrow></msubsup></mrow></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" id="u12" display="inline" overflow="scroll"><mrow><mo>Δ</mo><msubsup><mover accent="true"><mi>H</mi><mo>¯</mo></mover><mrow><mi>f</mi><mo> </mo><mn>298.15</mn></mrow><mrow><mi mathvariant="normal">o</mi><mo> </mo><mtext mathvariant="bold">GEHL</mtext></mrow></msubsup></mrow></math>, consistent with the experimental brackets on decarbonation equilibria of Walter (1963), Hoschek (1974), and Shmulovich (1974), the thermodynamic model for åkermanite-gehlenite melilites developed here, the thermodynamic properties of the other phases in these reactions tabulated by Berman (1988), and the revised estimates for <math xmlns="http://www.w3.org/1998/Math/MathML" id="u13" display="inline" overflow="scroll"><mrow><msub><mover accent="true"><mi>C</mi><mo>¯</mo></mover><mi>p</mi></msub></mrow></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" id="u14" display="inline" overflow="scroll"><mrow><msubsup><mover accent="true"><mi>S</mi><mo>¯</mo></mover><mrow><mn>298.15</mn></mrow><mi mathvariant="normal">o</mi></msubsup></mrow></math> of diopside of Sack and Ghiorso (2017), are roughly 1 and 3 (kJ/gfw) more positive than those estimated by Berman (1988). More positive standard enthalpies of formation of both endmembers, together with a decrease in the vibrational heat capacity of gehlenite and less negative deviations from ideal mixing compared with previous calibrations, all contribute to reducing the stability of melilites in this model. Together these effects will decrease the predicted temperature of condensation of melilite from nebular vapors, bringing calculated temperatures of melilite condensation into closer alignment with those of MgAl₂O₄ spinel than the 80 to 100 K separating their appearances in previous calculations (for example, Yoneda and Grossman, 1995; Petaev and Wood,1998; Ebel and Grossman,2000). These effects, together with a possible increase in spinel stability due to non-negligible solubility of Al_8/3O₄ alumina component, may allow equilibrium models to match the observed condensation sequence of spinel before melilite in calcium-aluminum inclusions (CAIs) in carbonaceous chondrites, without the need to invoke kinetic effects.

Shape and porosity of refractory inclusions in CV3 chondrites: A micro‐computed tomography (µCT) study

AbstractRefractory calcium‐aluminum‐rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) in chondritic meteorites are the earliest solids of our solar system, bearing the information of nebular condensation as well as accretion and asteroidal shock and metasomatism processes. While the compositions of refractory inclusions have been intensely studied for ~50 years, their physical properties such as shape and porosity are poorly constrained. Here, we present a microcomputed tomography (µCT) study on 16 refractory inclusions of condensate origin in five CV3 chondrites. We find that they are prolate or triaxial in shape with very rough morphologies. The CAIs have nodular textures and are thought to form by agglomerating individual nodules via collision‐induced bouncing and/or fragmentation, where the nodules were grown by gas–solid reactions during condensation. On the parent body, refractory inclusions from the CVR meteorite Leoville experienced intense shocks that led to the flattening of their shapes and lowering of their porosities. High‐temperature metasomatism in CVOxA meteorites and low‐temperature metasomatism in CVOxB meteorites do not seem to have large effects on the porosities of their refractory inclusions, which have similar ranges and pore‐size distributions. Instead, we infer that their pores are mostly inherited from the gas–solid condensation and subsequent agglomeration processes. The porosities of CAIs are higher than those of AOAs, which is mainly due to the high‐temperature sintering process of AOAs.

Heating events in the nascent solar system recorded by rare earth element isotopic fractionation in refractory inclusions.

Equilibrium condensation of solar gas is often invoked to explain the abundance of refractory elements in planets and meteorites. This is partly motivated, by the observation that the depletions in both the least and most refractory rare earth elements (REEs) in meteoritic group II calcium-aluminum-rich inclusions (CAIs) can be reproduced by thermodynamic models of solar nebula condensation. We measured the isotopic compositions of Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb in eight CAIs to test this scenario. Contrary to expectation for equilibrium condensation, we find light isotope enrichment for the most refractory REEs and more subdued isotopic variations for the least refractory REEs. This suggests that group II CAIs formed by a two-stage process involving fast evaporation of preexisting materials, followed by near-equilibrium recondensation. The calculated time scales are consistent with heating in events akin to FU Orionis- or EX Lupi-type outbursts of eruptive pre-main-sequence stars.

Evidence for post-nebula volatilisation in an exo-planetary body

The loss and gain of volatile elements during planet formation is key for setting their subsequent climate, geodynamics, and habitability. Two broad regimes of volatile element transport in and out of planetary building blocks have been identified: that occurring when the nebula is still present, and that occurring after it has dissipated. Evidence for volatile element loss in planetary bodies after the dissipation of the solar nebula is found in the high Mn to Na abundance ratio of Mars, the Moon, and many of the solar system's minor bodies. This volatile loss is expected to occur when the bodies are heated by planetary collisions and short-lived radionuclides, and enter a global magma ocean stage early in their history. The bulk composition of exo-planetary bodies can be determined by observing white dwarfs which have accreted planetary material. The abundances of Na, Mn, and Mg have been measured for the accreting material in four polluted white dwarf systems. Whilst the Mn/Na abundances of three white dwarf systems are consistent with the fractionations expected during nebula condensation, the high Mn/Na abundance ratio of GD362 means that it is not (>3σ). We find that heating of the planetary system orbiting GD362 during the star's giant branch evolution is insufficient to produce such a high Mn/Na. We, therefore, propose that volatile loss occurred in a manner analogous to that of the solar system bodies, either due to impacts shortly after their formation or from heating by short-lived radionuclides. We present potential evidence for a magma ocean stage on the exo-planetary body which currently pollutes the atmosphere of GD362.

Al‐Mg isotopic study of spinel‐rich fine‐grained CAIs

AbstractHigh‐precision SIMS analyses of seven spinel‐rich fine‐grained CAIs from the CV3 chondrites Efremovka, Kaba, and Vigarano and the LL3.0 chondrite Semarkona, two known to have volatility‐fractionated Group II REE patterns (likely condensates) and three more probably Group II, all give initial 26Al/27Al (26Al/27Al0) close to the solar system initial value of 5.2 × 10–5. Our data thus provide no evidence for multiple condensation events in the early solar system. Other work by Ushikubo et al. (2017) and Kawasaki et al. (2019a, 2019b) did find some similar inclusions whose values are significantly lower than the solar system initial value. Because the latter data are too few to indicate whether or not the lower ratios cluster or rather spread out over a range, more work will be required to determine if nebular condensation occurred following only a few major, punctuated thermal events or rather happened more frequently. Regardless, it is now clear that CAIs having the Group II REE pattern formed very early in solar system history, contemporaneous with the formation of CAIs having unfractionated REE patterns. Whatever the mechanism that formed Group II, it occurred under conditions that caused spinel to condense prior to (at a higher temperature than) melilite. This is contrary to the predictions of equilibrium nebular condensation.

Oxygen-isotope heterogeneity in the Northwest Africa 3358 (H3.1) refractory inclusions − Fluid-assisted isotopic exchange on the H-chondrite parent body

The nature of oxygen-isotope heterogeneity in refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)] from weakly metamorphosed chondrites is one of the outstanding problems in cosmochemistry. To obtain insights into possible processes resulting in O-isotope heterogeneity of refractory inclusions, we investigated the mineralogy, petrology, and oxygen isotopic compositions of six CAIs and two AOAs and aqueously formed fayalite grains within the matrix of the H3.1 chondrite Northwest Africa (NWA) 3358. Most of the refractory inclusions studied appear to be unmolten solar nebula condensates; some may have experienced partial melting and/or high-temperature annealing. The NWA 3358 refractory inclusions nearly completely avoided metasomatic alteration on the H-chondrite parent body: nepheline grains replacing anorthite and/or melilite are either very minor or absent. Five out of eight refractory inclusions studied have heterogeneous O-isotope composition: Δ17O ranges from ∼− 25‰ to ∼3.5 ± 2‰ (2σ). This O-isotope heterogeneity appears to be mineralogically controlled with melilite and anorthite being systematically 16O-depleted compared to hibonite, spinel, Al,Ti-diopside, and forsterite all having similar solar-like Δ17O of ∼−24 ± 2‰. In contrast to NWA 3358 refractory inclusions, the previously studied AOAs and a fine-grained CAI from the LL3.00 chondrite Semarkona have uniform Δ17O of ∼−25‰ (Itoh et al., 2007; McKeegan et al., 1998). Because the mineralogically-controlled O-isotope heterogeneity in refractory inclusions from ordinary chondrites appears to correlate with petrologic type of a host meteorite experienced by aqueous alteration, we suggest O-isotope exchange in NWA 3358 CAIs and AOAs resulted from aqueous fluid-rock interaction on the H-chondrite parent asteroids. This is supported by the presence of 16O-depleted anorthite (Δ17O ∼ 3.5 ± 2‰) and aqueously formed fayalite similar depleted in 16O (Δ17O ∼ 4 ± 2‰). The Δ17O of NWA 3358 fayalite is comparable to that of magnetite and fayalite in Semarkona and other weakly metamorphosed L3 and LL3 chondrites (Choi et al., 1998; Doyle et al., 2015) suggesting similar Δ17O of aqueous fluids on the H, L, and LL chondrite parent asteroids.

In situ analysis of platinum group elements in equilibrated ordinary chondrite kamacite and taenite

AbstractPlatinum group element (PGE) concentrations have been determined in situ in ordinary chondrite kamacite and taenite grains via laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). Results demonstrate that PGE concentrations in ordinary chondrite metal (kamacite and taenite) are similar among the three ordinary chondrite groups, in contrast to previous bulk metal studies in which PGE concentrations vary in the order H &lt; L &lt; LL. PGE concentrations are higher in taenite than kamacite, consistent with preferential PGE partitioning into taenite. PGE concentrations vary between and within metal grains, although average concentrations in kamacite broadly agree with results from bulk studies. The variability of PGE concentrations in metal decreases with increasing petrologic type; however, variability is still evident in most type six ordinary chondrites, suggesting that equilibration of PGEs does not occur between metal grains, but rather within individual metal grains via self‐diffusion during metamorphism. The constant average PGE concentrations within metal grains across different ordinary chondrite groups are consistent with the formation of metal via nebular condensation prior to the accretion of ordinary chondrite parent bodies. Post‐condensation effects, including heating during chondrule‐formation events, may have affected some element ratios, but have not significantly affected average metal PGE concentrations.