86th Annual Meeting of the Meteoritical Society 2024 (LPI Contrib. No. 3036)

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Oxygen isotopic compositions of dolomite in Ryugu samples: Constraints on the thermal history of the Ryugu
parent body
W. Fujiya1, T. Ushikubo2, S. Sugawara1, A. Yamaguchi3, K. Fukuda4, M. R. Lee5, K. Terada4, P. A. Bland6 and B. J.
Travis7
1
Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, JAPAN (wataru.fujiya.sci@vc.ibaraki.ac.jp), 2Kochi Institute for Core Sample Research, JAMSTEC, 3National Institute of Polar Research, 4Osaka University, 5Glasgow University, 6Space Science and Technology Centre, Curtin University, 7Planetary Science Institute.
Introduction: Ryugu samples underwent extensive aqueous alteration by which a variety of secondary minerals
formed [1-5]. Dolomite (CaMg(CO3)2), a major carbonate mineral in Ryugu, is an important secondary mineral because their O and C isotopic compositions as well as 53Mn-53Cr ages can constrain the conditions and timing of theaqueous alteration. The O and C isotopic compositions of dolomite in Ryugu suggest that it formed at temperatures
lower than 80 °C [1,2,6,7] when the system was approaching equilibrium [7,8]. The 53Mn-53Cr ages of Ryugu dolomite
imply that their formation spanned ~5 Myr [1,4,6,9,10], although inaccurate standardization of Mn/Cr ratios obtainedusing SIMS may cause systematic uncertainty [11]. Therefore, Ryugu dolomite provides a snapshot of the thermalevolution, i.e., time-temperature relationship, of the Ryugu parent body. However, it is unclear whether Ryugu carbonates formed during prograde alteration when temperature was increasing or retrograde cooling, which is a major
challenge in establishing a realistic thermal history model. In this work, we measured the O isotopic compositions ofdolomite in Ryugu samples A0203 and C0192 to further constrain its formation conditions.
Experimental: The in-situ O-isotope analyses on three and six dolomite grains from A0203 and C0192, respectively, were performed using CAMECA IMS 1280-HR at Kochi Institute, JAMSTEC. Secondary ions of 16O−, 17O−,
and 18O−, produced by a focused Cs+ primary ion beam (~30 pA, 3-4 µm in diameter), were simultaneously detectedwith a Faraday cup (FC) for 16O− signals and two electron multipliers (EMs) for 17O− and 18O−. Instrumental massfractionation was corrected using a series of dolomite–ankerite standards [12]. The uncertainties on δ17O, δ18O, andΔ17O values were typically ±1.12, ±0.86, and ±1.35‰ (2σ), respectively.
Results and discussion: The analyzed dolomite grains have only a small variation in δ 18O values as observed byprevious studies [1,6-8]. The δ18O values of A0203 and C0192 dolomite are at the higher and lower end of thosereported previously and clustered at 33.34 ± 0.91‰ (N = 3; 2SE) and 26.54 ± 0.58‰ (N = 6; 2SE), respectively.
Notably, the Δ17O values of A0203 and C0192 dolomite are also systematically different (−0.94 ± 0.61‰ and 0.46 ±0.36; 2SE). A similar anti-correlation between δ18O and Δ17O values can also be recognized for the previous data [1,68], implying that Ryugu dolomite with higher δ18O values tends to have lower Δ17O values. The Δ17O, and possibly,δ18O values of aqueous fluids are expected to have decreased during progressive aqueous alteration because of the Oisotope exchange between water and anhydrous rock which initially had higher and lower Δ 17O (and δ18O) values[13,14]. Thus, the A0203 dolomite with lower Δ17O values likely precipitated from more evolved fluids that underwent
O-isotope exchange to a greater extent than the C0192 dolomite. On the other hand, the equilibrium O isotopic fractionation between water and dolomite is larger at lower temperatures with the δ18O values of dolomite being higher
than those of water [15]. Therefore, the A0203 dolomite with higher δ 18O values likely precipitated at lower temperatures than the C0192 dolomite. These observations suggest that Ryugu dolomite formed during retrograde cooling.
This argument is in line with the small variation in δ 13C values observed for Ryugu dolomite, which suggests its
formation in higher O fugacity at the later stage of aqueous alteration [8]. A recent study of dolomite crystallization
reported that frequent cycling of a solution between supersaturation and undersaturation can accelerate dolomite
growth [16]. Dynamic fluid flow in the Ryugu parent body might have resulted in this cycling and rapid cooling due
to effective heat transfer. We plan to perform 53Mn-53Cr dating for the A0203 and C0192 dolomite with our dolomite
standards synthesized to estimate Mn/Cr ratios accurately using SIMS [11].
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