Incorporation of 16O-rich anhydrous silicates in the protolith of highly hydrated asteroid Ryugu

Abstract

The abundant phyllosilicate and carbonate minerals characterizing most of the returned particles from asteroid Ryugu suggest a history of extensive aqueous alteration on its parent body1,2, similar to the rare, mineralogically altered, but chemically primitive CI (Ivuna-type) chondrite meteorites. Particle C0009 differs mineralogically from other Ryugu particles examined to date by containing anhydrous silicates at a level of ~0.5 vol% (Ito et al. submitted), and thus can help shed light on the original materials that constituted Ryugu's protolith. In-situ oxygen isotope measurements of the most Mg-rich olivine and pyroxene in C0009 reveal two populations of ∆17O: −25‰ to −15‰ and −8‰ to −3‰, correlating well with the silicate morphologies similar to those seen in amoeboid olivine aggregates (AOAs)3-7 and chondrule phenocrysts8,9, respectively, abundant in less aqueously altered carbonaceous chondrites. This result represents the rst discovery of olivine with ∆17O close to the solar value10 in either a CI chondrite or an asteroid of CIchondrite characteristics and provides strong evidence that AOAs and Mg-rich chondrules accreted into Ryugu's protolith.Our data also raise the possibility that the protoliths of CI and other carbonaceous chondrites incorporated similar anhydrous silicates.

Oxygen isotopes of anhydrous primary minerals show kinship between asteroid Ryugu and comet 81P/Wild2

“…Some 16 O-rich (Δ 17 O of ~−20‰) olivine and low-Ca pyroxene grains were observed in CI chondrites and Ryugu samples ( Fig. 1, F and G ), possibly related to AOAs ( 7 – 9 ). Here, we report O isotopic compositions of primary minerals (olivine, low-Ca pyroxene, and spinel) measured in situ in polished sections of Ryugu and Ivuna, and we identify the first unambiguous AOA object in CI chondrites.…”

“…The morphology and chemical compositions of olivine grains in Ryugu ( 2 ) and CI chondrites ( 4 , 6 ) are consistent with originating as chondrule phenocrysts. A porous olivine-diopside object in Ryugu ( 2 , 9 ) could be genetically related to amoeboid olivine aggregates (AOAs), a common type of refractory inclusions observed in chondrites ( 10 ).…”

Oxygen isotope evidence from Ryugu samples for early water delivery to Earth by CI chondrites

“…1a,b). Anhydrous silicates (olivine and pyroxene) have not been observed in any of these four particles, but rare examples have been identified in other Ryugu particles 8,9 . Phyllosilicates comprise a serpentine-saponite intergrowth and have bulk compositions that fully overlap with those found in CIs 7 .…”

“…CI chondrites, and CI-related asteroids such as Ryugu, are highly altered (Fig. 1), having experienced extensive parent body hydrothermal processing, such that only traces remain of their original silicate mineralogy [7][8][9]15,18,19 . Despite this, CIs are chemically the most primitive CC group, with a bulk composition close to that of the solar photosphere for most elements [18][19][20] .…”

“…5). In contrast, the final fluids on the CI/Ryugu parent body/bodies would have evolved to more 16 O-rich values, due to protracted exchange with 16 O-rich solids 8,9 . There is little evidence that such extensive levels of aqueous alteration have taken place within cometary nuclei, with particles from Comet 81P/Wild 2 sampled during the Stardust mission dominated by a 16 O-rich, high temperature assemblage 49 .…”