APPENDIX

PART V

REFRACTORY PHASES

CONTINUE TO
[PART I] Chondrites
[PART II] Achondrites
[PART III] Irons
[PART IV] Stony-Irons
[PART VI] Trends for Classification
[APPENDECTOMY]

Solar nebula refractory metal (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 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 within primitive carbonaceous meteorites, initially referred to as Fremdlinge, while presolar refractory metal nuggets have been discovered hosted in presolar graphite grains within primitive meteorites, thought to be primary condensate assemblages after Type II supernovae and low-metallicity AGB stars (Croat et al., 2013). The solar nebula refractory metal nuggets are thought to have formed relatively fast (~100 years) during an interval of ~100,000 years, at a temperature of 1440–1616K and a pressure of 100 dyne/cm²; these conditions were created during periods of low accretion by the proto-Sun (Schwander et al., 2011).

Condensation of other refractory phases (oxides and silicates with vaporization temperatures in excess of ~1350K) followed. In regions of solar composition, mineral condensation progressed in the following sequence: corundum ⇒ hibonite ⇒ perovskite ⇒ grossite ⇒ melilite ⇒ spinel ⇒ diopside ⇒ forsterite ⇒ anorthite.

standby for phase realtions photo standby for phase realtions photo
Condensation phase relations at various temperatures, pressures, and gas:dust ratios
Photo credit: Ebel, D.S. (2006), Condensaton of rocky material in astrophysical environments.
In Meteorites and the Early Solar System II, D. Lauretta et al., editors.
University of Arizona in Tucson. pp. 253–277, + four color plates (after plates 1 and 4).

The condensation of these refractory minerals and the formation of calcium–aluminum-rich inclusions (CAIs) occurred during the time interval between 70 t.y. and 2.5 m.y. after the beginning of the rapid infall stage (Mishra and Chaussidon, 2011). During this time interval, radiogenic 26Al was introduced into the solar nebula by one or more supernovae (Ciesla and Yang, 2010).

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 ≈ Δ18O ~ –60‰ to –40‰) 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 4,568.5 (±0.3) m.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 4,568.2 m.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. Others 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 bipolar outflows (x-wind) or 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. 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 ultimately coalesced with newly forming chondrules to form the nascent chondritic planetesimals. 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 reconnectiion 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.

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.

standby for nwa 2086 cai photo
NWA 2086, CV3R, 46 g end section with large (the largest known?) CAI
Photo courtesy of Dr. Martin Horejsi
See the complete story of this CAI as published in Meteorite Times Magazine—The Accretion Desk.

COARSE-GRAINED CAIs

  • Type A Inclusions:

    • Compact

      • refractory elements (proto-CAIs) initially crystallized from a melt following evaporation of nebular dust and gas in the earliest stages of solar system evolution

      • inclusions are composed of an aggregate of compositionally similar proto-CAIs that were rapidly coagulated

      • rounded inclusions contain coarse-grained melilite (Åk 1–80), spinel, perovskite, and the pyroxenes fassaite (Al–Ti-diopside) and rhönite

      • minerals typically 16O-rich

    • Fluffy

      • unmelted but have experienced varying degrees of recrystallization

      • aggregates of refractory elements (proto-CAIs) formed by gas–solid condensation from a solar composition gas

      • irregular shape

      • contain melilite (Åk 0–36), V-rich spinel, hibonite, and perovskite (the latter sometimes with associated pyroxene)

      • melilite may be altered to secondary phases such as andradite after accretion within a planetesimal

      • minerals typically 16O-rich

  • Type B Inclusions:

    1. crystallized from partially molten droplets in which some evaporation occurred
    2. unique to CV chondrites; formed in the CV region by clumping of type A CAIs, incorporation of dust, and melting (Rubin, 2011)
    3. generally depleted in Si and Mg from evaporation compared to type A CAIs
    4. less refractory than type A CAIs
    5. many experienced complex alteration histories
    6. large sizes (~5–25 mm) due to continued incorporation of forsterite dust and multiple melting events
    7. contain melilite (Åk174), fassaite (sometimes with associated Fremdlinge (now known as opaque assemblages, which are refractory metal-rich objects formed as early nebular condensates), spinel (including palisade bodies* and framboids**), anorthite, perovskite, and forsterite

    8. *palisade bodies formed within partially molten CAIs as a result of the filling of spinel-lined, early-formed vesicles by a melt phase, while the host was solid enough to preserve the shape; **framboids are off-center, near-polar sections of palisade bodies

    9. three divisions of type B inclusions have been adopted forming a continuum:
    • B1

      • inclusions are zoned, consisting of melilite and spinel, with the cpx fassaite along with glass existing at their interface; the fassaite and glass are residues after rapid evaporation of Mg and Si from the primary melt

      • coarser-grained than B2 inclusions

      • crystallized rapidly from early, homogeneous melts

      • composition of melilite varies (Åk30–65 in the core, 20–35 in the rim)

      • melilite has variable O-isotopic compositions

    • B2

      • inclusions are unzoned and lack melilite-rich mantles

      • melilite is zoned with a compositional range of Åk4590

      • more silica-rich compositions than B1

      • higher anorthite/gehlenite ratios than B1

      • crystallized slowly from evolved, isolated melt pockets

    • B3 (forsterite-bearing)

      • uncommon inclusion type (found only in CV and CB chondrites) that contains forsteritic olivine (Fo93100, up to 45 vol%)

      • precursor material was AOA-like

      • spinel, forsterite, and pyroxene cores are 16O-rich; melilite and anorthite mantles are 16O-depleted

      • cores crystallized from partial to complete melts; mantles have an evaporative origin

      • less refractory than other CAIs due to Mg and SiO evaporation

      • melilite in zoned inclusions may have variable compositions (Åk12100)

      • melilite has a compositional range of Åk690 and is commonly altered to secondary phases such as nepheline

  • Type C Inclusions:
    • crystallized from partially molten droplets possibly related to fine-grained, spinel-rich CAI precursor material, with the addition of altered CAI Type B inclusions

    • high volatile element contents suggest melting under high pressures or high dust/gas ratios

    • coarse-grained with diverse textures and mineralogies

    • melilite has wide ranges, but typically Åk4050

    • contain abundant anorthite (38–60 vol%), along with fassaite and spinel, but are deficient in forsterite

    • minerals are typically 16O-poor due to O-isotopic exchange during melting and assimilation of dust in an 16O-poor reservoir of the nebula, or through isotopic exchange on the parent asteroid

    • CV3 (Allende) exhibits asteroidal isotopic exchange during metasomatic alteration of melilite to secondary phases such as grossular, nepheline, sodalite, hedenbergite, and andradite

    • considered to be a precursor component in the condensation origin of Al-rich chondrules (those containing >10 wt% Al2O3)

FINE-GRAINED, SPINEL-RICH CAIs:

  • rimmed, concentrically-zoned structure
  • nebular condensate origin with a multi-stage formation history
  • composed primarily of spinel at their cores and mantles of melilite, along with Al-diopside or fassaite, anorthite, nepheline, and salite
  • melilite mostly altered to secondary phases such as andradite and grossular

AMOEBOID OLIVINE AGGREGATES (AOAs)

  • olivine-rich objects present in most grouped and ungrouped carbonaceous chondrites, and have been found in an LL3.0 ordinary chondrite
  • the least refractory fine-grained inclusions
  • irregularly-shaped, mm- to cm-sized, porous or compact, sintered and annealed aggregates of high-temperature (~1200–1384K), 16O-rich, solar nebular condensates
  • originated by fractional condensation or fractional vaporization in an 16O-rich reservoir
  • rapid cooling occurred at an estimated rate of >0.02K/hr at a nebular pressure of 0.0001 bar
  • porous AOA olivines contain low CaO and high MnO and CrO concentrations indicative of slow accretion and rapid cooling of nebular forsterite through disequilibrium condensation
  • compact AOA olivines contain higher CaO and lower MnO and CrO concentrations indicative of reheating of porous AOAs, or rapid accretion and slow cooling of nebular forsterite
  • compact AOAs likely formed closer to the Sun than porous AOAs
  • likely formed in the same region as CAIs but experienced cooler condensation temperatures (Fagan et al., 2004)
  • alternatively, but not strongly supported, formation occurred from rapidly cooled igneous melts (Wasson et al., 2004)
  • composed primarily of forsteritic olivine and a refractory component composed of the high-Ca pyroxene Al-Ti-diopside, along with anorthite, spinel, CAIs, and rare melilite; secondary nepheline, sodalite, and other phases may be present as a result of aqueous alteration processes on the parent body (Fagan et al., 2003)
  • FeNi-metal is rare, indicating rapid extraction from the condensation site following forsterite condensation
  • AOA olivine was a precursor to chondrule olivine; AOAs may provide a genetic link between CAIs and low-FeO type-I chondrules via metasomatic processing (Krot et al., 2004) in which low-Ca pyroxene shells have accreted and melting has occurred within less 16O-rich, chondrule-forming nebular regions (Krot et al., 2005; Ruzicka et al., 2011)
  • may be associated with formation of ordinary, rumuruti, and enstatite chondrite groups through removal of varying amounts of AOA fractionate from an initial CI-like composition

Excellent images of AOAs can be seen in John Kashuba's article 'Amoeboid Olivine Aggregates' published in the November 2015 issue of Meteorite Times Magazine.

FUN CAIs (Fractionated and Unknown Nuclear isotope anomalies)

  • rare type of inclusion present in some carbonaceous chondrite groups
  • large isotopic anomalies are present for O, Mg, and Si, while nonlinear isotopic anomalies exist for Ca, Sr, Ba, Nd, Sm, Ti, and Cr
  • anomalies resulted from the mixing of components from normal nucleosynthetic processes (e.g., the r-process in Type Ia SN, type II SN) in unusual proportions
  • subsequently subjected to mass fractionation processes (e.g., Rayleigh distillation), possibly within gaseous protoplanets
  • volatility-fractionated REE patterns
  • abundant spinel and large isotopic fractionations may indicate a higher temperature origin
  • magnesium in the inclusions is isotopically heavy
  • lack the 26Mg excess that is present in other CAIs, suggesting they formed very early, prior to 26Al incorporation into solar nebula
  • they were segregated quickly from the region of solar flare irradiation to preserve evidence of the composition of the pristine protosolar molecular cloud
  • the group includes some mass fractionated hibonite inclusions with or without nucleosynthetic anomalies

µCAIs (Bland et al., 2007)

  • a distinct population of µm-sized CAI inclusions; not fragments of larger CAIs
  • consist of corundum cores with complete Al, Ca-containing rims
  • O-isotopic compositions are unique
CAIs are also classified based on REE abundances into Groups I–VI. For example, Group I formed from an essentially unfractionated nebular gas, and Group II formed by condensation of a fractionated nebular gas depleted in an ultra-refractory component (e.g., fine-grained CAIs).

PLACs (platy hibonite crystals)

  • rare type of inclusion (60–110 µm) present in CM carbonaceous chondrite groups
  • gas–solid nebular condensate which lacks the 26Mg excess present in CAIs
  • represent some of the first solids that formed in the solar nebula following the low-velocity impact of a stellar shock front with the protosolar cloud, triggering its collapse
  • formed prior to the incorporation and/or homogenization of freshly synthesized short-lived nuclides like 26Al
  • formed rapidly over a span of 10,000–100,000 years
  • formed prior to CV CAIs having the "canonical" 26Al/27Al initial ratio (5.2 [±0.2] × 10–5); nuclides would not be manifest in CAIs until a few 100,000 years later
  • large nucleosynthetic anomalies are present for Ca, Ti, and Si, which were implanted by interstellar dust grains
  • volatility-fractionated REE patterns
  • the short-lived nuclide 41Ca is absent
  • formed in a highly 16O-enriched reservoir
  • plots more broadly spread about the CCAM (carbonaceous chondrite anhydrous mineral) line relative to SHIBs on an oxygen three-isotope diagram (see below)

standby for shib oxygen-three diagram
Diagram credit: Kööp et al., GCA, vol. 184, pg. 164 (2016)

SHIBs (spinel–hibonite spherules)

  • rare type of hibonite grain (50–100 µm) present in CM carbonaceous chondrite group
  • show evidence of in situ decay of 26Al
  • early condensates that formed rapidly over a span of 10,000–100,000 years
  • most likely formed by late reprocessing ~100,000–700,000 years after CV CAI formation
  • formed at lower temperatures than PLACs, in a region already depleted in the most refractory REEs
  • O-isotopes more similar to CAIs in CR chondrites and Acfer 094 than to PLACs
  • formed in a homogeneous, highly 16O-enriched reservoir
  • define a tight cluster along the CCAM (carbonaceous chondrite anhydrous mineral) and PCM (primitive chondrule mixing) lines on an oxygen three-isotope diagram (see below)

standby for shib oxygen-three diagram
Diagram credit: Kööp et al., GCA, vol. 184, pg. 161 (2016)

Selected References:

Planetary Materials, Reviews in Mineralogy, J.J. Papike (editor), vol. 36, (1998)

How the Type B1 CAIs Got Their Melilite Mantles, Richter et al., LPSC XXXIII, #1901 (2002)

Oxygen isotopic compositions and origins of calcium–aluminum-rich inclusions and chondrules, E. Scott and A. Krot, MAPS, vol. 36, #10 (2001)

Early solar system events and timescales, G. Lugmair and A. Shukolyukov, MAPS, vol. 36, #8 (2001)

The formation of rims on calcium–aluminum-rich inclusions: Step I–Flash heating, D. Wark and W. Boynton, MAPS, vol. 36, #8 (2001)

Precursors of Type C inclusions—Evidences from the new kind of anorthite–spinel-rich inclusions in the Ningqiang carbonaceous chondrite, Y. Lin and M. Kimura, LPSC XXVIII, #1067 (1997)

A Comprehensive Study of Pristine, Fine-grained, Spinel-rich Inclusions from the Leoville and Efremovka CV3 Chondrites, I: Petrology, MacPherson et al., LPSC XXXIII, #1526 (2002)

Making Calcium–Aluminum-rich Inclusions and Chondrules near the Young Sun by Flares, F. Shu et al., MAPS, vol. 35, suppl. (2000)

On the Remelting of Type B Calcium–Aluminum-rich Inclusions, H. Connolly and D. Burnett, MAPS, vol. 35, suppl. (2000)

TEM study of compact Type A Ca,Al-rich inclusions from CV3 chondrites: Clues to their origin, A. Greshake et al., MAPS, vol. 33, #1 (1998)

In situ formation of palisade bodies in Ca,Al-rich refractory inclusions, S. Simon and L. Grossman, Meteoritics, vol. 32 (1997)

The origin of type C inclusions from carbonaceous chondrites, J. Beckett and L. Grossman, EPSL, vol. 89, #1 (1988)

Mineralogy and petrography of amoeboid olivine aggregates from the reduced CV3 chondrites Efremovka, Leoville and Vigarano: Products of nebular condensation, accretion and annealing, M. Komatsu et al., MAPS, vol. 36, #5 (2001)

Insights into the Formation of Type B2 Refractory Inclusions, S. Simon and L. Grossman, LPSC XXXIV, #1796 (2003)

The identification of meteorite inclusions with isotope anomalies, D. Papanastassiou and C. Brigham, Astrophysical Journal, vol. 338 (1989)

The origin of the 'FUN' anomalies and the high temperature inclusions in the Allende meteorite, G. Consolmagno and A. Cameron, Moon and the Planets, vol. 23 (1980)

Isotopic Heterogeneity and Correlated Isotope Fractionation in Purple FUN Inclusions, C. Brigham et al., LPSC Abstracts, vol 19 (1988)

On the origin of the Ca–Ti–Cr isotopic anomalies in the inclusion EK-1-4-1 of the Allende-meteorite, K. Kratz et al., Memorie della Società Astronomica Italiana, vol. 72, #2 (2001)

Type C CAIs: New Insights Into Early Solar System Processes, A. Krot et al., 67th Annual Meteoritical Society Meeting, #5042 (2004)

Formation of Chondritic refractory inclusions: the astrophysical setting, J. Wood, GCA, vol. 68, #19 (2004)

Evaporation of cmas-liquids under reducing conditions: constraints on the formation of type B1 CAIs, A. Davis et al., NIPR International Symposium (2003)

TEM/SEM Evidence for residual melt inclusions in type B1 CAIs, J. Paque et al., LPSC XXXVIII, #1755 (2007)

Type C Ca,Al-rich inclusions from Allende: Evidence for multistage formation, A. Krot et al., GCA, vol. 71, #18 (2007)

The White Angel: A unique wollastonite-bearing, mass-fractionated refractory inclusion from the Leoville CV3 carbonaceous chondrite, C. Cailet Komorowski et al., MAPS, vol. 42, #7/8 (2007)

Primordial compositions of refractory inclusions, L. Grossman et al., GCA, vol. 72 (2008)

Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism, A. Krot et al., GCA, vol. 72 (2008)

Nebular history of amoeboid olivine aggregates, N. Sugiura et al., MAPS, vol. 44, #4 (2009)

Origin and Chronology of Chondritic Components: A Review, A. Krot et al., GCA, vol. 73 (2009)

Refractory Phases in Primitive Meteorites Devoid of 26Al and 41Ca: Representative Samples of First Solar System Solids?, S. Sahijpal and J. Goswami, The Astrophysical Journal, vol. 509 (1998)

Isotopic records in CM hibonites: Implications for timescales of mixing of isotope reservoirs in the solar nebula, Liu et al., GCA, vol. 73 (2009)

Amoeboid olivine aggregates (AOAs) in the Efremovka, Leoville, and Vigarano (CV3) chondrites: A record of condensate evolution in the solar nebula, Ruzicka et al., GCA, vol. 79 (2012)

Forsterite-bearing type B refractory inclusions from CV3 chondrites: From aggregates to volatilized melt droplets, Bullock et al., MAPS, vol. 47, #12 (2012)

New constraints on the relationship between 26Al and oxygen, calcium, and titanium isotopic variation in the early Solar System from a multielement isotopic study of spinel-hibonite inclusions, Kööp et al., GCA, vol. 184 (2016)

PSRD article by G. Jeffrey Taylor: "Dating Transient Heating Events in the Solar Protoplanetary Disk", November 16, 2012


CONTINUE TO
[PART I] Chondrites
[PART II] Achondrites
[PART III] Irons
[PART IV] Stony-Irons
[PART VI] Trends for Classification
[APPENDECTOMY]


© 1997–2017 by David Weir