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CAIs in FERMO: an unusual aspect for ordinary chondrites

*ISAC Institute, CNR Research Area, via Gobetti 101, 40126 Bologna, Italy,

CAIs or Calcium-Aluminum-rich inclusions, roughly millimeter-to-centimeter in size, are the oldest objects in the solar system, discovered in the most ancient meteorites. These inclusions are believed to have formed very early in the evolution of the solar system either as solid condensates or as molten droplets. Relatively to planetary materials, CAIs are enriched with the lightest oxygen isotope and are believed to record the oxygen composition of solar nebular gas where they grew. CAIs are millions of years (My) older than more modern objects in the solar system, such as planets, which formed about 30 My after CAIs. These inclusions are thought to have formed far away from Sun and then later fell back into the mid-plane of the solar system.

The constituents of chondritic meteorites — chondrules, refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)], Fe,Ni-metal grains, and fine-grained matrices — are largely crystalline, and were formed from thermal processing of these grains and from condensation of solids from gas.

Fermo is a one-piece stony meteorite of 10,2 kg fallen in the central Italy on September 25, 1996. The relatively low FeO/(FeO + MgO) ratios of olivine and low-Ca pyroxene measured in dark and light clasts cemented by a grey matrix, and petrologic type,  allow the cosmic body  to be classified as genomictic, regolith breccia of chondritic group H, with fragments of petrologic types 3 to 5 (Molin et al., 1997).

Fig.1 – (left) Fermo, a one-piece stony meteorite of 10,2 kg fallen in Central Italy on September 25, 1996, presently hosted at Villa Vitali in Femo; (center) a fragment of the meteorite given in donation by the Municipality of Fermo to the Vatican Observatory during the 64th World Meeting of the Meteoritical Society held in Vatican City in the 10 -14 September 2001 period (right) (in the figure: the Jesuit Guy Consolmagno, the reference point of the congress and the author, one of the organizers of the event.

The composition and texture were shown to exhibit some peculiar characteristics such as: (a) the co-existence of H-group material of distinct types referred to at least three distinct sources representing dark clasts, grey matrix and light clasts, respectively (Cevolani, 1999); (b) indication of shock and re-heating events pointed out by an ordered phase of Ni-rich (>50%) metal (tetrataenite, important to study the low-temperature thermal history of metal particles) present in each of the three areas (Orlicki et al, 2000; Cevolani et al., 2001); (c) the presence of many (12, overall) short-lived cosmogenic radioisotopes including 44Ti and 26Al (Bonino et al., 2001). 26Al, which decays to 26Mg with a half-life of ~0.7 million years (Davis & McKeegan, 2014) can be produced in supernovae jets; Dauphas & Chassidon, 2012). Researchers think that most 26Al was incorporated into the solar nebula at the same time, and this aspect makes 26Al valuable as a chronometer to age date of CAIs and other solar system materials (Dunham, 2018).

In Fermo, collected immediately after the fall on September 1996, the author discovered on april 2018 after cutting in slices  the small block in his possession, the presence of CAIs, found so far only in carbonaceous chondrite, such as Vigarano (the first one to have shown this feature). Ordinary chondrites (CO) like Fermo are the result of recrystallization processes of carbonaceous chondrites (CC) that comes directly from the condensation and aggregation of the constituents of the primitive solar nebula.

Fig.2 -Cutting a sample of Fermo into slices. (top) the head-body with the melting crust and the back-body with the presence of CAIs; (below) two more thin slices of 12.1 gr and 4.4 gr. (center) an impressive CAI in another sample of 65 g.

Calcium-aluminum-rich inclusions were found to be tens of micrometers to centimeter-sized irregularly shaped or rounded objects composed mostly of oxides and silicates of Ca, Al, Ti, and Mg, such as corundum (Al2O3), hibonite (CaAl12O19), grossite (CaAl4O7), perovskite (CaTiO3), spinel (MgAl2O4), melilite (solid solution of Ca2MgSi2O7 and Ca2Al2SiO7), and anorthite (CaAl2Si2O8). Evaporation and condensation appear to have been the dominant processes during formation of refractory inclusions. Subsequently, it is suspected that some CAIs experienced melting to various degrees and crystallization over timescales of days under highly reducing (solar nebula) conditions

The supernovae are prodigious forges of 26Al (and other radioactive elements) in the processes of internal fusion of hydrogen and especially of helium. We expect that within these CAIs the isotopic ratios 26Al /27Al are quite different to the canonical ones of the solar-terrestrial relationships.

The stable isotopes of CAIs refer to the abundance of 16O oxygen. The individual minerals present in these inclusions have lower 17O /16O and 18O /16O ratios than the terrestrial ones. The largest abundances of 16O (4-5%) are usually observed in the spinel mineral (MgAl2O4). Since type II supernovae (those produced by the collapse of evolved massive stars) produce large quantities of 16O, it seemed natural to assume that the CAIs with their abnormal 16O abundance were formed in one or more supernovae.

Fig.3- X-ray Chandra image of supernova residues G292.0 + 1.8, known to contain large amounts of oxygen. These supernovae are one of the primary sources of heavy elements (that is, everything except hydrogen and helium). The image shows a field of debris about 20,000 light years in rapid expansion that contains, together with the oxygen (yellow and orange), other elements such as magnesium (green) and the silicon and sulfur (blue) that were forged in the star before it exploded

The small isotopic anomalies observed in CAIs clearly represent a residue of the nucleosynthesis processes of the presolar supernova, but the same inclusions observed in the carbonaceous chondrites do not seem to be formed in the supernovae, even if from these the constitutive elements of the CAIs derive their origin (Kawasaki et al., 2017).
After residence in the solar nebula, CAIs became incorporated into chondritic parent bodies, which are strongly related to CC. This must have happened up to a few million years later, as indicated by chondrule ages. Chondrules formed before accretion of chondrite parent-body  and are younger than CAIs. Isotopic analyses of components in pristine chondrites using short-lived nuclide chronometers, 207Pb-206Pb dating, and oxygen isotopes aided by laboratory and theoretical studies of chondrites and differentiated meteorites have provided key constraints on the processes that shaped the early solar system (Scott, 2007).  207Pb-206Pb  ages indicate that CAIs, the oldest known solar system objects, formed 4567.2±0.6 My ago and this is often referred to as the age of the solar system. Times of for CAIs (<0.3My), chondrules (1-3 My), and for early asteroidal differentiation (≥ 3My) are comparable to time scales estimated from astronomical observations of low-mass young stars (Kita et al., 2005).

The discovery of CAIs in CO like Fermo (we found that it isn’t a unique case in the scenario of CO, as shown in Fig.4) is an important aspect because it allows us to speculate that the formation of some CO has been practically contemporary to that of CC, the oldest of our planetary system. The existence of CAIs in CO brings new light on the formation processes of the constituents inside the primitive molecular cloud, effectively reducing the transition times from the phase of CC to that of CO.

Fig4 Similarities between the internal structures of two ordinary chondrites (CO) that include CAIs in their oxidized matrix. (left) a fragment of NWA10790, a CO of class L5 (L, low iron content) collected in Morocco in 2014. (right) a fragment of Fermo, a CO of class H3-5 (H, high content of iron) (courtesy, Romano Serra)

Moreover, CAIs do not appear to be just only an important feature of the CC and of some CO such as Fermo, but they are present also in comets (this is an absolute novelty) as it was recently shown by the analysis of  particles collected from refractory material of Wild2 (Joswiaki, 2017). The latter finding seems to support the hypothesis that the most ancient material of the presolar nebula moved from the innermost to the outer part of the solar system, in contradiction with the belief that the CAIs were confined to a small number of internal bodies in our solar system.
The finding may lead to a greater understanding of how our solar system and possibly other solar systems formed and evolved.


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