Chondrule-like objects and a Ca-Al-rich inclusion from comets or comet-like icy bodies
Takaaki Noguchi, Daisuke Nakashima, Takayuki Ushikubo, Wataru Fujiya, Noriaki Ohashi, John P. Bradley, Tomoki Nakamura, Noriko T. Kita, Peter Hoppe, Hidemi Ishibashi, Makoto Kimura, Naoya Imae
Geochimica et Cosmochimica Acta
Available online 6 August 2024
“Chondrules and Ca-Al-rich inclusions (CAIs) have been considered characteristic constituents of chondritic meteorites, although the outward transportation of CAIs has been theoretically pointed out. Stardust samples recovered by the Stardust mission from the 81P/Wild2 comet contained chondrule-like objects (CLOs) and refractory inclusions that include CAIs and amoeboid olivine aggregates (AOAs). However, it was not proven that the CLOs, AOAs, and CAIs coexist with fine-grained materials equivalent to chondritic porous interplanetary dust particles (CP IDPs) containing abundant glass with embedded metal and sulfides (GEMS). Here we report on two type II CLOs, containing <90 Mg# in ferromagnesian silicates, enclosed in GEMS-rich CP Antarctic micrometeorites (AMMs) (CP IDPs that reached the surface of the Earth) and one igneous object rich in kosmochloric (Ko-rich: NaCrSi2O6-rich) high-Ca pyroxene and Fe-bearing olivine (KOOL) that is enclosed in a CP IDP. KOOL grains have also been found in Stardust samples and CP IDPs. These three igneous objects are embedded in fine-grained matrices that do not show any evidence of aqueous alteration. The low Mg# and elevated Δ17O of olivine and pyroxene in these CLOs and the KOOL grain are consistent with previously studied CLOs from comet 81P/Wild 2 and a giant cluster IDP. These results support the view that CP IDP- and CP AMM-like materials constitute samples from comets or comet-like icy bodies. The CLOs were formed in oxidizing environment beyond the snow line and then transferred to the comet-forming region. In contrast, a spinel-hibonite (SHIB) fragment found in an AMM experienced aqueous alteration of its rim. The SHIB fragment contains ultrarefractory oxides and refractory metal nuggets and has a 26Mg excess like typical meteoritic CAIs. The mineralogy of the fine-grained matrix is very similar to CP IDPs and CP AMMs. However, because “GEMS” in the matrix lacks Fe-Ni metal and amorphous silicate in it contains Fe, it is clear that the matrix weakly experienced aqueous alteration. Olivine / (Olivine + low-Ca pyroxene) ratios in the matrices of the four samples range from 0.4 to 0.6, which are comparable with those of anhydrous CP IDPs and CP MMs (around 0.5), and those of P- and D-type asteroids and Jupiter-family comets (around 0.5).”
