REFRACTORY PHASES

Solar nebula refractory metals (W, Re, Os, Ir, Mo, Ru, Pt, and Rh) with high vaporization temperatures in excess of ~1620K were the first solids to condense as a single alloy near the midplane of the hot, primordial, pre-stellar core outward to ~3 AU during the rapid infall stage (Class 0–I), which began over 4.568 b.y. ago (Liffman et al., 2012). These first metal condensates are now present within sub-micron-sized refractory metal nuggets, which likely formed as precipitates of a silicate liquid during rapid quenching (~ 700–1000°C/40 s) under reducing conditions (Schwander et al., 2015). Thereafter, these metal nuggets might have served as nucleation sites for later phases such as spinel or melilite, minerals that became major constituents of CAIs. Solar refractory metal nuggets also occur as large, isolated opaque assemblages contained in primitive carbonaceous meteorites, initially referred to as Fremdlinge, while presolar refractory metal nuggets have been discovered hosted in presolar graphite grains in primitive meteorites. These presolar phases are thought to be primary condensate assemblages originating as dust ejecta from explosive stellar nucleosynthesis—core-collapse supernovae, neutron star mergers, and low mass (1.5–4 M_⊙) AGB stars.

The solar nebula refractory metal nuggets are thought to have formed relatively fast (~100 years) during an interval of ~100,000 years, most at a temperature of 1440–1616 K and a pressure of 10^–4 bar (100 dyne/cm²); these conditions were created during periods of low accretion by the proto-Sun (Schwander et al., 2011). Even higher-temperature condensates termed "ultra-refractory" are known to have formed above 1700 K (Manga et al., 2022 #6527). These ultra-refractory phases include nearly pure Os metal which condenses at 1832 K at 10^–4 bar, and minerals such as allendeite (Sc4Zr3O12, condensing at 1830 K at 10^–4 bar), baddeleyite (ZrO2), lakargite (CaZrO3), and calzirtite (Ca2Ti5Zr2O16).

Condensation of other refractory phases (oxides and silicates with vaporization temperatures in excess of ~1350 K) followed. In regions of solar composition, mineral condensation is predicted to have progressed in the following sequence: corundum ⇒ hibonite ⇒ grossite ⇒ perovskite ⇒ melilite ⇒ spinel ⇒ diopside ⇒ forsterite ⇒ enstatite. This equilibrium condensation sequence is most closely exemplified in refractory inclusion 31-2 in the CO3.00 chondrite DOM 08006 (Simon et al., 2019). A different sequence was predicted by Manga et al. (2019) based on density functional theory applied to the pyroxene solid-solution series. They contend that the first phase to condense would be cubic perovskite at temperatures of 1682–1637 K, followed by an Al-Ti-rich pyroxene phase (CaTiAl2O6) at ~1670 K. Similarly, Han et al. (2019) analyzed zoned CAIs from Efremovka and TIL 07003/7 and observed that perovskite was the first phase to condense, possibly attesting to rapid cooling beyond the corundum condensation point.

The condensation of these refractory minerals and the formation of calcium–aluminum-rich inclusions (CAIs) occurred episodically during the time interval between ~40 t.y. and ~160 t.y. after the beginning of the rapid infall stage (Pignatale et al., 2019 and references therein). CAI formation was further constrained by Pignatale et al. (2018) to within the first 80 t.y. of the molecular cloud collapse to account for CAI delivery to the outer CC region of the disk by viscous expansion. Conversely, modeling by Morbidelli et al. (2022) as well as Al–Mg dating of primitive, unmelted CAIs by MacPherson et al. (2012) limit the interval of CAI formation to ~20 t.y. or less; the later proposed timing likely reflects reprocessing of CAIs after formation. By analyzing only the Mo isotope variability in CAIs that is associated with the rapid neutron capture process of nucleosynthesis (r-process, quantified by Δ95Mo notation), Brennecka et al. (2020) determined that the timing of CAI formation corresponded to the Class I/II transition stage of protostellar evolution (see schematic diagram below). During this transition stage, infall from the molecular cloud evolved from a higher to lower abundance of r-process material (i.e., higher to lower Δ95Mo).

The precise age of CAI formation at 4567.30 (±16) m.y. as determined by Amelin et al. (2002, 2010) and Connelly et al. (2012) is typically used as the measure for the Solar System's time-0. The age of the Solar System was refined by Desch et al. (2021 #6231, 2022 #2567, 2022 [parts I and II]) utilizing a suite of achondrites and chondrules (in which the isotopic systems closed simultaneously) in a more precise examination of the Al–Mg and Pb–Pb (and other) chronometric systems. From their two-part paper on statistical chronometry, the relative Solar System time-0 (t_SS)—defined as the time when the solar nebula had the canonical ratio (26Al/27Al) of 5.23 × 10^–5—was determined to be 4568.67 (±0.16) m.y. in part I, or 4568.65 (±0.10) m.y. in part II, the latter age based also on data from the Mn–Cr, Hf–W, and I–Xe isotopic systems. They attribute any discordance in ages between the Al–Mg system (older) and the Pb–Pb system (younger) to transient heating events that reset only the Pb–Pb chronometer. By using relative ages to date meteorites instead of using anchors to attain U-corrected Pb–Pb absolute ages, Desch et al. (2022) demonstrated a significant improvement in the associated uncertainties involved, achieving a reduction from ±9 m.y. to ±0.3–0.5 m.y. Their work substantiates concordancy in the formation ages of these achondrites and chondrules with respect to each of the applied chronometers, and bolsters the case for homogeneity of the short-lived radionuclides 26Al, 53Mn, 182Hf, and 129I in the early protoplanetary disk.

Employing MC-ICP-MS technology, Ku et al. (2022) obtained high precision 41Ca–41K chronometry and established a new Allende bulk CAI isochron. They determined that the initial ratios in the protoplanetary disk for 26Al/27Al (5.23 [±0.13] × 10^–5) and 41Ca/40Ca (2.00 [±0.52] × 10^–8) were both enriched by about three orders of magnitude over that of the protosolar molecular cloud (1.1 × 10^–8 and 4 × 10^–11, respectively). Their results attest to a late injection of short-lived radionuclides into the protosolar molecular cloud from a likely common stellar source (26Al is synthesized during the core H-burning phase when 25Mg gains a proton); this event may be associated with the initial collapse of the molecular cloud leading to formation of the Solar System. The type of stellar object involved depends on the exact timing of the radionuclide injection, and given that Type II supernovae are excluded as a potential source due to incompatible 60Fe systematics, the injection is constrained to between 0.6 m.y. (wind from a 60 M_⊙ Wolf–Rayet star) and 1.1 m.y. (ejecta from a 1.5–2 M_⊙ AGB star) before CAIs (see diagram below).

Martinet et al. (2022) calculated the contribution of 26Al ejected through stellar winds of very massive stars (300+ M_⊙), increases of which are correlated with increasing mass, metallicity, and rotation. Very massive, rotating, high-metallicity stars could account for a significant increase in 26Al—at least a 400% increase in the galactic production rate.

Gaches et al. (2020) concluded that an external source for the 26Al enrichment in the protostellar disk would necessarily involve many unlikely chance occurrences. Instead, they presented an internal source model whereby enrichment of 26Al in the inner disk was attained through irradiation of the disk surface by high-energy (MeV) cosmic ray protons which had been accelerated in accretion shocks. The 26Al production efficiency is generally greatest with a lower accretion rate (≤10^–8 M_⊙/yr) and a weaker protostellar magnetic field strength (thus a smaller magnetosonic radius favorable to cosmic ray acceleration). The production of 26Al was coeval with that of CAIs as the gas cooled below 1500 K during the protostellar Class I/II stage.

During this Class I/II transition stage of protostellar evolution, radiogenic 26Al would have been injected into the solar protoplanetary disk by one or more supernovae (Ciesla and Yang, 2010), or what is now considered more plausible, by a nearby Wolf-Rayet star. A number of independent chronometric studies (e.g., Budde et al., 2018) have demonstrated a concordance in Al–Mg and Hf–W ages for CV CAIs, CV chondrules, CR chondrules, and angrites (D'Orbigny and Sahara 99555), which attests to a homogeneous distribution of 26Al during the first ~4–5 m.y. of solar system history. A homogeneous distribution of 26Al was also supported by Desch et al. (2022 #2567 in their CAI dating analysis mentioned above, as well as in their chapter for Protostars and Planets VII (2022) titled 'Short-Lived Radionuclides in Meteorites and the Sun's Birth Environment'. In the latter work, they concluded that virtually all of the short-lived radionuclides present in meteorites were homogeneously distributed in the molecular cloud prior to the start of the Solar System, the abundances of which were the result of ongoing stellar nucleosynthesis within the Galactic host spiral arm.

At a heliocentric distance corresponding to the location at which CV- and CK-group carbonaceous chondrites were formed, highly porous, refractory, mm-to-cm-sized dustballs became concentrated with CAIs of similar mass forming 16O-rich accretionary rims (Rubin, 2011). Many of these refractory minerals, specifically those which formed within the initial 160,000 years (commensurate with the lifetime of class 0 protostars), survived (perhaps within planetesimals) as primary condensates of a dust-enhanced (10 × solar gas composition which was 16O-rich: Δ17O ~ –25‰ to –20‰; Simon et al., 2019) nebular gas having properties of variable but low pressure, variable but high temperature, and a reduced environment (conditions attested by volatility fractionated REEs). Episodic melting of these CAIs occurred over the next 300,000 years, with some experiencing remelting in the chondrule-forming region at least 900,000 years after initial formation (MacPherson et al., 2010).

Some of the earliest refractory phases (hibonite-bearing) formed prior to the incorporation and mixing of 26Al into the solar nebula and are present in CM chondrites (no resolvable 26Mg excesses; e.g., platy-crystals [PLACs] and blue aggregates [BAGs]), while others do show evidence of in situ 26Al decay (e.g., spinel–hibonite spherules [SHIBs] and CAIs). CAIs are likely the direct condensates or evaporative residues formed from one or more episodes of rapid heating and slow cooling of precursor dust. This process continued over a time span as short as 20–100 t.y. (consistent with an FU-Orionis outburst; see below) (Krot et al., 2009; Wurm and Haack, 2009 and references therein), which is datable by the Hf–W and U–Pb systems to 4.5685 (±0.0003) b.y. ago relative to the angrites D'Orbigny, NWA 4590, and NWA 4801 (Burkhardt et al., 2008; Nyquist et al., 2009). This age is in agreement with that obtained by Pb–Pb and Al–Mg dating methods for a CAI from the CV3 NWA 2364 of 4.5682 b.y. (Bouvier and Wadhwa, 2010). It was calculated that initial nebular condensation processes account for 80% of the refractory element enrichment (e.g. Ca, Al) in type A and type B CAIs, while 20% is due to the subsequent evaporation of more volatile elements (e.g. Mg, Si) (Grossman et al., 2008).

Evidence for an early, instantaneous, impact-generated shock wave origin for CAIs (and chondrules) around large planetesimals has been presented by Sanders (2008) and by Hood and Weidenschilling (2011). This would include an origin associated with bow shocks produced by planetesimal interactions with Jupiter, or more plausibly, an origin within shock zones associated with impact-generated vapor-melt plumes from high-velocity collisions of large planetesimals. Chondrules were melted and slowly cooled within the dusty zone of large planetesimals on a timescale that overlaps the formation of CAIs and which continued for ~2.6 m.y. (commensurate with the lifetime of class 1–3 protostars). During that period, some CAIs were transported radially outward into the accretion zones of chondrite parent bodies. If employing the planetesimal nebular shock model, any delay between the formation of CAIs and chondrules may be explained as the time required for the completion of Jupiter's formation.

Following their formation near the proto-Sun, many of these refractory minerals were transported radially outward by turbulent diffusion mechanisms to chondritic accretionary regions. Calculations show that CAIs measuring up to 1 cm in size, representing a total mass of 0.005 Earth masses, could have been easily transported to the asteroid belt at a distance of ~3 AU, and some may have exceeded 10 AU. Here the condensation sequence was arrested, and these minerals remained stable against gas drag and the accretionary influence of the Sun for at least 1 m.y. Eventually, they rained down to the nebular midplane and coalesced with newly forming chondrules to form the nascent chondritic planetesimals.

Other CAIs may have been confined to the protoplanetary disk embedded in the center of spiral arms (Haghighipour and Boss, 2003), or possibly transported to cooler heliocentric regions (2–5 AU) by photophoresis—a force created in response to an ~100-fold increase in the Sun's luminosity during an FU-Orionis outburst resulting from an enhanced accretion rate within ~1 AU of the central star (Wurm and Haack, 2009). When cm-sized CAIs are fully illuminated in an optically thin region of the disk, they are transported vertically and radially outward along the surface of the (flared) protoplanetary disk following a temperature gradient from hot to cold.

Another hypothesis under consideration attributes the outward transport of CAIs to bipolar outflows (x-wind and/or disk winds). However, this magnetocentrifugal transport mechanism was thoughtfully investigated by Desch et al. (2021 #2663) and Herbst et al. (2021 #6272) and demonstrated to be invalid for all but the smallest CAIs. Their data, based on gravitational and other constraints, show that only CAIs with radii <1 µm can be lofted and accelerated long enough to reach the outer regions of the Solar nebula. At the same time, particles ≪1 µm may exceed escape velocity or remain bound to the outflow gas.

Very ancient ages have been measured for some iron meteorites and for the ungrouped basaltic meteorite Asuka 881394, with the latter having an age of 4.56675 (±0.00031) b.y. based on the 238U/235U ratio of 137.88, or 4.56557 (±0.00055) b.y. based on the newly refined 238U/235U ratio of 137.768 (±0.038) (Amelin et al., 2014; Wimpenny et al., 2013; Koefoed et al., 2015). These similar ancient ages indicate that their respective parent bodies accreted contemporaneously with CAI formation (Wadhwa et al., 2009).

CAIs are particularly abundant in the CV-group of carbonaceous chondrites, but they also occur in many other carbonaceous chondrite groups, in K chondrites, in ordinary chondrites, and in enstatite chondrites, and they have been identified in comet samples (81P/Wild-2) from NASA's STARDUST mission. Wark–Lovering monomineralic rims commonly occur on most all CAI types, providing evidence of episodic flash heating events in a more oxidizing environment of the solar nebula; these energetic events are often cited as being associated with magnetic reconnection flares. These events resulted in volatilization of Mg, Si, and Ca from the outermost layer of CAIs, followed by the diffusion of elements (possibly derived from accetionary forsterite dust which is texturally and mineralogically similar to AOA forsterite) back onto the surface of the CAIs. Alternatively, the formation of Wark–Lovering rims may be attributed to relatively slow evaporation from solid CAIs. Additional information on CAI formation can be found on the Allende page.

Ebert et al. (2018) investigated the Ti isotope systematics of CAIs and Na–Al-rich chondrules from ordinary and CO chondrites. CAIs from both of these chondrite groups show the presence of nucleosynthetic anomalies, such as an average excess in ε50Ti of ~9. On the other hand, although Na–Al-rich chondrules in the CO chondrites studied have a ε50Ti excess, those from the ordinary chondrites do not. Given that Na–Al-rich chondrules from both CO and ordinary chondrites are considered to have incorporated refractory components (~30–80% CAIs and/or AOAs), they reason that the refractory material precursor to ordinary chondrules did not have a ε50Ti excess, and thus was different from the refractory material admixed to form CO chondrules. They contend that this difference may be due to the specific formation region of the respective precursor refractory material: either in the non-carbonaceous (NC) region within the inner Solar System (ordinary chondrites), or in the carbonaceous (CC) region beyond Jupiter (CO chondrites). They propose that the rare CAIs with ε50Ti excesses which are present in ordinary chondrites could represent the smaller-sized CAIs (<150 µm) that were able to migrate across Jupiter's gap in the protoplanetary disk (see diagram below).

CAIs were originally grouped as 'coarse-grained' and 'fine-grained' inclusions (Grossman, 1975). However, continued studies have led to further refinement in their classification, with an emphasis on those from the CV group. Coarse-grained CAIs have been classified into three main groups (A, B, and C) based primarily on the proportions of fassaite and melilite, the latter corresponding to the series with Ca-rich end member åkermanite and Al-rich end member gehlenite. Other characteristic phases include spinel, hibonite, perovskite, and anorthite.

COARSE-GRAINED CAIs