APPENDIX

PART V

CAIs

Presolar refractory phases (oxides and silicates) with high vaporization temperatures began to condense near the midplane of the hot primordial nebula during the rapid infall stage which began over 4,568 m.y. ago, progressing in the following sequence: corundum => hibonite => perovskite => grossite => melilite => spinel => diopside => forsterite => anorthite. Many of these refractory minerals survived (perhaps within planetesimals) as primary condensates of a dust-enhanced (10 × solar gas composition which was 16O-rich: Δ17O ≤ –20‰) nebular gas, having properties of variable but low pressure, variable but high temperature, and a reduced environment (conditions attested to by volatility fractionated REEs). Other refractory minerals are evaporative residues from one or more episodes of rapid heating and slow cooling of this dust. This process continued over a time period as short as 100,000 years (Krot et al., 2009), which is datable by the Hf–W and U–Pb systems to 4,568.3 (±0.7) 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 from Allende CAIs. 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 subsequent evaporation of more volatile elements (e.g. Mg, Si) (Grossman et al., 2008). Evidence for an early, instantaneous, impact-generated origin for CAIs has been presented by Sanders (2008).

Following formation near the proto-Sun, many of these Ca,Al-rich 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). 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. Finally, they coalesced with newly forming chondrules to constitute the nascent chondritic planetesimals. These Ca,Al-rich inclusions (CAIs) are particularly abundant in the CV-group of carbonaceous chondrites, but they also occur in the other carbonaceous chondrite groups, in K chondrites, in ordinary and enstatite chondrites, and have been identified in comet samples from NASA's STARDUST mission (81P/Wild-2).

Wark–Lovering monomineralic rims commonly occur on most all CAI types, reflecting episodic flash heating events in the solar nebula. These energetic events are often cited as being associated with magnetic reconnectiion flares. These events resulted in the volatilization of Mg, Si, and Ca from the outermost layer of the CAI, then 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 CAI. 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, CV3, 46 g end section with large CAI
Photo courtesy of Dr. Martin Horejsi

COARSE-GRAINED CAIs

  • Type A Inclusions:

    • Compact

      • refractory elements which crystallized from a melt following evaporation of nebular dust and gas; portions experienced shock metamorphism

      • rounded shape

      • 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

      • formed by gas–solid condensation from a solar composition gas

      • irregular shape

      • contain melilite (Åk 0–36), V-rich spinel, perovskite, and hibonite

      • 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. generally have higher Si and Mg contents than type A CAIs

    3. unique to CV chondrites

    4. many experienced complex alteration histories, including multiple melting and alteration events

    5. inclusions range from ~5–25 mm in size

    6. contain melilite (Åk 1–74), fassaite (sometimes with associated ‘Fremdlinge’, now known as opaque assemblages, which are refractory metal-rich objects formed by either low-temperature alteration on the parent body or nebular condensation), spinel (including palisade bodies* and framboids**), anorthite, perovskite, and forsterite

      7. *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

    7. 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 (Åk 30–65 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 Åk 45–90

      • 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 that contains forsteritic olivine (up to 45 vol%)

      • intermediate between (or hybrids of) type B CAIs and Al-rich chondrules

      • also contain spinel, melilite, and anorthite

      • less refractory than B1 or B2

      • crystallized from melts

      • melilite in zoned inclusions may have variable compositions (Åk 12–100)

      • melilite 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 Åk 40–50

    • 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 all 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, 16O-rich, solar nebular condensates
  • rapid cooling occurred at an estimated rate of >0.02°K/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, formation occurred from rapidly cooled igneous melts (Wasson et al., 2004)
  • composed primarily of forsteritic olivine (with some replacement by low-Ca pyroxene) and a refractory component composed of the high-Ca pyroxene Al-diopside, along with anorthite, spinel, 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
  • 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 16O-poor, chondrule-forming nebular regions (Krot et al., 2005)

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

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 present in CM carbonaceous chondrite groups
  • large nucleosynthetic anomalies are present for Ca, Ti, and Si
  • volatility-fractionated REE patterns
  • lack the 26Mg excess that is present in other CAIs, suggesting they formed very early, prior to incorporation and/or homogenization of 26Al in solar nebula, and prior to or contemporaneously with CAIs having the canonical 26Al/27Al ratio
  • the short-lived nuclide 41Ca is absent
  • represent some of the first solids that formed in the solar nebula prior to the injection of freshly synthesized short-lived nuclides following the low-velocity impact of a stellar shock front with the protosolar cloud, triggering its collapse; nuclides would be manifest in CAIs a few 100,000 years after the collision

Selected References:

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

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

Oxygen isotopic compositions and origins of calcium–aluminum-rich inclusions and chondrules, E. Scott and A. Krot, MAPS, vol. 36, no. 10, 2001

Early solar system events and timescales, G. Lugmair and A. Shukolyukov, MAPS, vol. 36, no. 8, 2001

The formation of rims on calcium–aluminum-rich inclusions: Step I–Flash heating, D. Wark and W. Boynton, MAPS, vol. 36, no. 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, 1997, #1067

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

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, no. 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, no. 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, no. 5, 2001

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

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, no. 2, 2001

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

Formation of Chondritic refractory inclusions: the astrophysical setting, J. Wood, GCA, vol. 68, no. 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, 2007, #1755

Type C Ca,Al-rich inclusions from Allende: Evidence for multistage formation, A. Krot et al., GCA, vol. 71, no. 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, no. 7/8, 2007

Primordial compositions of refractory inclusions, L. Grossman et al., GCA, 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, no. 4, 2009

Origin and Chronology of Chondritic Components: A Review, A. Krot et al., GCA, 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


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

© 1997–2009 by David Weir