Evidence against water delivery by impacts within 10 million years of planetesimal formation

• Published in: Earth and planetary science letters 2024-09, Vol.642, p.118860, Article 118860
• Main Authors: Rider-Stokes, B.G., Stephant, A., Anand, M., Franchi, I.A., Zhao, X., White, L.F., Yamaguchi, A., Greenwood, R.C., Jackson, S.L.
• Format: Article
• Language: English
• Subjects: Angrite; Impacts; NanoSIMS; Volatile delivery; Water
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2024.118860

•The angrite parent body (APB) was one of the most hydrated differentiated bodies in the early inner solar system.•No evidence of impacts adding to the H2O contents of planetismals in the early inner solar system.•The average δD value for the angrites closely overlaps with acapulcoite-lodranites and eucrites.•A pervasive single source of water existed in the early inner solar system. The quenched (rapidly-cooled) angrite meteorites, which formed in the inner Solar System, record large-scale planetary mixing in the first few Ma of Solar System history, and therefore, provide a unique opportunity to investigate the role of impacts in terms of water addition to the growing planetesimals. Here we investigate the H isotopic composition and H2O abundance of relict olivine grains that survived impact melting within Asuka (A) 12,209 and compare them with impact melt-produced groundmass fractions using in-situ nanoscale secondary ion mass spectrometry (NanoSIMS). These analyses test if the angrite parent body (APB) acquired a CC-like H isotopic composition before early large-scale impact mixing and/or acquired volatiles by subsequent impact(s). Furthermore, we analyse the H isotopic composition and H2O abundance of later-forming plutonic (NWA 4801), intermediate (NWA 10,463) and dunitic (NWA 8535) angrite meteorites to assess the role of impacts, in terms of volatile delivery, during the first 50 Ma of the inner Solar System history. The H isotopic composition of most quenched angrites appears to be affected by degassing. Consequently, we opt to use the weighted average δD of pyroxenes and olivines in the plutonic angrite, NWA 4801, to estimate the original composition of the APB (-235 ± 113 ‰ 1σ, n = 18), in agreement with recent studies on the hydrogen isotopic signatures of mineral-hosted melt inclusions in D'Orbigny and Sahara 99,555. Additionally, we use the H2O abundances of NWA 4801 pyroxene (7.9 ± 1 µg/g 2σ) and olivine (6.1 ± 0.6 µg/g 2σ) to estimate the lower (85 to 110 µg/g) and upper (519 to 1089 µg/g) limits of the primitive APB mantle H2O content, implying that the APB was one of the most hydrated bodies in the early inner Solar System. The similarity of δD/H2O systematics in the relict olivine grains and groundmass olivine within A 12,209 argues against water delivery through impacts in the early inner Solar System. Overall, the non-carbonaceous reservoir in the inner Solar System appears to retain a single source of water, which isotopically resembl