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[PPS08-15] Oxygen isotopic compositions of dolomites in the Tarda meteorite
Keywords:Tarda meteorite, dolomite, oxygen isotope, aqueous alteration
Carbonates are typical secondary minerals observed in aqueously altered carbonaceous chondrites. The oxygen isotopic compositions of carbonates record fluid evolution in the parent bodies. In aqueously altered CM chondrites, type 1 (T1) calcite [CaCO3], which precipitated in pore spaces, shows higher δ18O and Δ17O values, while type 2 (T2) calcite, which replaced other minerals, shows lower δ18O and Δ17O values (e.g., Vacher et al., 2019). Also, more altered CM chondrites commonly contain dolomite [CaMg(CO3)2], which has O isotopic compositions between T1 and T2 calcites (Tyra et al., 2016; Lindgren et al., 2017).
In this study, we found two types of dolomites in the Tarda C2-ungrouped chondrite: one (T1) precipitated in pore spaces, and the other (T2) replaced chondrules or other minerals (Fig. 1). Tarda is thought to be genetically related to the Tagish Lake meteorite which is presumed to be derived from a D-type asteroid (Marrocchi et al., 2021). Type 2 carbonate has not been reported for these meteorites, and their aqueous alteration processes are poorly constrained. In this work, we present the results of in-situ O-isotope analyses of T1 and T2 dolomites in the Tarda meteorite.
In-situ O-isotope analyses of the dolomites were conducted using secondary ion mass spectrometry (SIMS: CAMECA ims 1280-HR) at Kochi Institute for Core Sample Research, JAMSTEC. A focused Cs+ primary beam (20 kV, 20 pA, ~2 µm) was used. Five analyses of the UW-6220 dolomite standard were performed before and after every ten to fifteen analyses of the Tarda dolomites to determine the SIMS instrumental bias and reproducibility. The external reproducibilities (2SD) were 0.9-3.9‰ for δ18O and 0.7-1.6‰ for Δ17O. The O isotopic compositions of T1 and T2 dolomites were from 29 to 34‰ for δ18O and from −2.6 to 1.0‰ (n = 31, weighted average: −0.64‰) for Δ17O, and from 29 to 33‰ for δ18O and from −2.9 to 1,1‰ (n = 23, weighted average: −0.72‰) for Δ17O, respectively (Fig. 2). An instrumental bias of up to ~10‰, depending on the FeCO3 contents, has been reported for dolomite (Śliwiński et al., 2016). Therefore, we tentatively applied the matrix effect of FeCO3 contents for dolomite described by Śliwiński et al. (2016). However, the O isotopic compositions of the T1 and T2 dolomites are indistinguishable since there is no systematic difference between their FeCO3 contents (Fig. 2). Thus, the T1 and T2 dolomites may have formed when the fluid composition approached equilibrium.
To assess possible difference between formation ages of T1 and T2 dolomites, we will perform Mn-Cr dating for them using Mn-, Cr-, and Fe-bearing dolomite standard materials, produced with a dolomite synthesis method via amorphous carbonate (Sugawara et al., 2022).
In this study, we found two types of dolomites in the Tarda C2-ungrouped chondrite: one (T1) precipitated in pore spaces, and the other (T2) replaced chondrules or other minerals (Fig. 1). Tarda is thought to be genetically related to the Tagish Lake meteorite which is presumed to be derived from a D-type asteroid (Marrocchi et al., 2021). Type 2 carbonate has not been reported for these meteorites, and their aqueous alteration processes are poorly constrained. In this work, we present the results of in-situ O-isotope analyses of T1 and T2 dolomites in the Tarda meteorite.
In-situ O-isotope analyses of the dolomites were conducted using secondary ion mass spectrometry (SIMS: CAMECA ims 1280-HR) at Kochi Institute for Core Sample Research, JAMSTEC. A focused Cs+ primary beam (20 kV, 20 pA, ~2 µm) was used. Five analyses of the UW-6220 dolomite standard were performed before and after every ten to fifteen analyses of the Tarda dolomites to determine the SIMS instrumental bias and reproducibility. The external reproducibilities (2SD) were 0.9-3.9‰ for δ18O and 0.7-1.6‰ for Δ17O. The O isotopic compositions of T1 and T2 dolomites were from 29 to 34‰ for δ18O and from −2.6 to 1.0‰ (n = 31, weighted average: −0.64‰) for Δ17O, and from 29 to 33‰ for δ18O and from −2.9 to 1,1‰ (n = 23, weighted average: −0.72‰) for Δ17O, respectively (Fig. 2). An instrumental bias of up to ~10‰, depending on the FeCO3 contents, has been reported for dolomite (Śliwiński et al., 2016). Therefore, we tentatively applied the matrix effect of FeCO3 contents for dolomite described by Śliwiński et al. (2016). However, the O isotopic compositions of the T1 and T2 dolomites are indistinguishable since there is no systematic difference between their FeCO3 contents (Fig. 2). Thus, the T1 and T2 dolomites may have formed when the fluid composition approached equilibrium.
To assess possible difference between formation ages of T1 and T2 dolomites, we will perform Mn-Cr dating for them using Mn-, Cr-, and Fe-bearing dolomite standard materials, produced with a dolomite synthesis method via amorphous carbonate (Sugawara et al., 2022).