Oxygen isotope thermometry and composition of aqueous fluids within the parent body of asteroid (101955) Bennu

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Abstract
We report on the petrography and oxygen isotopic compositions from in situ analyses of carbonates (Ca‐carbonate, dolomite, and Fe‐rich magnesite), magnetite, and apatite in particles from asteroid Bennu. Using oxygen isotope thermometry of Ca‐carbonate, dolomite, and magnetite, we estimated crystallization temperatures and the oxygen isotopic compositions of the aqueous fluids from which they precipitated. The Ca‐carbonate–magnetite pairs formed at lower temperatures (−5 to 20°C) than the dolomite–magnetite pairs (5–89°C); the uncertainties are ±10–16°C. The inferred fluid compositions for Ca‐carbonate–magnetite pairs are δ 18 O = 0.6‰–9‰ and Δ 17 O = 1.8‰–2.7‰, whereas for dolomite–magnetite pairs, they are δ 18 O = −2.2‰ to 7.6‰ and Δ 17 O = −0.1‰ to 0.9‰. On a three‐isotope oxygen diagram (δ 17 O vs. δ 18 O), 11 out of 13 of our inferred aqueous fluid compositions plot along a single line with a slope of ~0.8, potentially indicating precipitation from a single isotopically evolving fluid. Two dolomite–magnetite pairs appear to be outliers and may have formed from fluids with different oxygen isotopic compositions. The oxygen isotope data presented herein overlap with those reported from samples of asteroid Ryugu, supporting a possible close genetic relationship between these asteroids.
Funding Information
  • National Aeronautics and Space Administration (NNH09ZDA007O)
  • National Aeronautics and Space Administration (NNM10AA11C)

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