Oxygen isotopic composition of dolomite in Ryugu: New insights into the thermal history of theRyugu parent body.
Wataru Fujiya1, Takayuki Ushikubo2, Shingo Sugawara1, Akira Yamaguchi3, Kohei Fukuda4, Martin R. Lee5, Kentaro Terada4,Phillip A. Bland6 and Bryan J. Travis7
1
Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, JAPAN (wataru.fujiya.sci@vc.ibaraki.ac.jp),2
Kochi Institute for Core Sample Research, JAMSTEC, 3National Institute for Polar Research, 4Osaka University, 5Glasgow
University, 6Space Science and Technology Centre, Curtin University, 7Planetary Science Institute
Introduction: Ryugu samples underwent extensive aqueous alteration, and a variety of secondary minerals such as
phyllosilicate, oxide, sulfide, and carbonate formed during water-rock interaction [1-5]. Among the secondary minerals,
carbonates are of particular interest because their O and C isotopic compositions reflect the conditions of aqueous alteration,and because they can be dated using the 53Mn-53Cr systematics. Previous studies of the O and C isotopic compositions ofdolomite (CaMg(CO3)2) in Ryugu have suggested that it formed at temperatures lower than 80 °C [1,2,6,8] when the system
was approaching equilibrium [7,8]. The 53Mn-53Cr dating of Ryugu carbonate suggests that their formation may have spanned
as much as 5 Myr [1,6]. Therefore, Ryugu carbonate potentially helps us decipher the thermal history of the Ryugu parentbody. However, it is unclear whether Ryugu carbonate formed during prograde alteration when temperature was increasing or
during retrograde cooling, which makes it difficult to establish a realistic thermal history model. In this work, we investigatedthe O isotopic compositions of dolomite in Ryugu samples A0203 and C0192 to further constrain its formation conditions.
Also, we will measure the O isotopic compositions of magnetite in these samples to estimate the O isotopic equilibrium
temperatures between dolomite and magnetite.
Experimental: Prior to isotope work, the microstructures of dolomite grains in samples A0203 and C0192 were characterizedby Transmission Electron Microscopy (TEM) using a FEI T20 operated at 200 kV. The in-situ O-isotope analyses on two andsix dolomite grains from A0203 and C0192, respectively, were performed using CAMECA IMS 1280-HR [9]. Negative ionsof 16O−, 17O−, and 18O−, produced by a focused Cs+ primary ion beam (~30 pA, 3-4 µm in diameter), were simultaneously
detected with 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 [10]. The uncertainties on δ17O, δ18O, and Δ17O
values were typically ±1.12, ±0.86, and ±1.35‰ (2σ), respectively.
Results and discussion: TEM showed that dolomite grains in A0203 and C0192 differ in their crystal size and habit, withthose in A0203 typically being 1-2 m size euhedral crystals whereas C0192 dolomite is coarser and anhedral. The analyzed
dolomite grains have only a small variation in δ18O values as observed by previous studies [1,6-8]. The δ18O values of A0203
and C0192 dolomite are at the higher and lower end of those reported previously and clustered at 32.91 ± 0.47‰ (N = 6; 2SE)
and 26.54 ± 0.58‰ (N = 2; 2SE), respectively. Notably, the Δ17O values of A0203 and C0192 dolomite are also systematicallydifferent (−1.13 ± 0.82‰ and 0.46 ± 0.36; 2SE). A similar anti-correlation between δ18O and Δ17O values can also berecognized for the previous data, implying that Ryugu dolomite with higher δ18O values tends to have lower Δ17O values [6-8].
The Δ17O, and possibly, δ18O values of aqueous fluids are expected to have decreased during progressive aqueous alteration
because of the O-isotope exchange between water and anhydrous rock which initially had higher and lower Δ17O (and δ18O)
values [11,12]. 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[13]. Therefore, the A0203 dolomite with higher δ18O values likely precipitated at lower temperatures than the C0192 dolomite.
These observations may suggest that Ryugu dolomite formed during retrograde cooling. This argument is in line with the smallvariation in δ13C values observed for Ryugu dolomite, which suggests its formation in higher O fugacity at the later stage of
aqueous alteration [7]. For further information regarding the formation conditions of dolomite, we plan to measure the 53Mn53
Cr ages of dolomite in A0203 and C0192.
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