10:15 〜 10:30
[PPS08-16] CM, CRコンドライトとTagish Lake隕石の炭酸塩の炭素、酸素同位体比組成
キーワード：タギッシュレイク隕石、CRコンドライト、炭酸塩、酸素同位体、炭素同位体、二次イオン質量分析計
Carbonate is one of the major secondary products of hydrothermal activity in chondritic parent bodies. Since it precipitated from fluid during an early stage of hydrothermal activity, the isotopic composition of carbonate is useful to understand the isotopic composition of primordial fluids. Recently, consistently high δ13C values (>50‰) were recognized in the Tagish Lake meteorite and CR chondrites [1,2], suggesting the isotopic compositions of fluids in parent bodies of these chondrites were different from those in CM chondrite parent bodies. In this study, we performed O and C isotope measurements of carbonate in Tagish Lake (C2-ungrouped) and Aguas Zarcas (CM2) to understand the relationship of O and C isotopic systematics of CM and CR chondrites and Tagish Lake. Although it is known that Aguas Zarcas consists of multiple lithologies, the exposed surface of Aguas Zarcas (~5mm in size) consists of a CM clast. All studied carbonate grains which were large enough to analyze (>10 μm) were Ca-rich (typically (Ca0.99Fe0.01)CO3).
Carbon and oxygen isotope ratios of Ca-rich carbonates were measured with a CAMECA IMS 1280-HR at the Kochi Institute, JAMSTEC. Analytical conditions were similar to those for the previous CR carbonate measurements [2]. The δ13C values of Tagish Lake carbonates are consistently high (80 to 90‰). In contrast, the δ13C values of Aguas Zarcas carbonates are variable (20 to 60‰). The δ18O values of carbonates in both Tagish Lake and Aguas Zarcas are similar (+32 to 37‰), however, their Δ17O values are distinct (1.6±1.3‰ for TL vs. -1.0 ±1.5‰ for AZ, 2 SD). All Aguas Zarcas carbonate data are on the CM calcite trend line [3]. In contrast, Tagish Lake carbonate data are on the CR calcite trend line [4].
High δ13C carbonates (here tentatively defined as δ13C>40‰) are recognized in CM as well as Tagish Lake and CR chondrites, which are consistent with previous studies (up to ~80‰) [e.g., 1, 5, 6]. There is no apparent difference in major element composition and texture between the high and low δ13C carbonates in Aguas Zarcas. Although δ18O values of the high δ13C carbonates (~33‰) tend to be lower than those of the low δ13C carbonates (~35‰) in Aguas Zarcas, the temperature effect of equilibrium isotopic fractionation between fluid and carbonate seems unlikely to explain the C and O isotopic distributions of Ca-rich carbonates. We consider that the high δ13C carbonates represent a contribution from high δ13C source materials such as CO2, CO, and highly volatile organic matter [cf. 1, 7].
Carbonates in Tagish Lake and CR carbonates lie on the same O isotope trend line [2, 4, this study]. Oxygen isotope ratios of dolomite in CI chondrites and returned samples from asteroid Ryugu are also consistent with this trend [8]. Since the parent bodies of CR chondrites and Tagish Lake, and also asteroid Ryugu, are considered to have accreted further from the Sun than CM parent bodies [e.g., 1, 9, 10], the occurrence of carbonates with slightly higher Δ17O values (by ~2‰ relative to CM carbonates) suggests accreted ice (mainly H2O and possibly a few % of CO and CO2) had slightly elevated Δ17O values.
[1] Fujiya W. et al. (2019) Nat. Astron., 3, 910-915.
[2] Ushikubo T. et al. (2022) LPSC 2022, Abstract #1321.
[3] Lindgren P. et al. (2017) GCA, 204, 240-251.
[4] Jilly-Rehak C. E. et al. (2018) GCA, 222, 230-252.
[5] Vacher L. G. (2017) GCA, 213, 271-290.
[6] Telus M. et al. (2019) GCA, 260, 275-291.
[7] Altwegg K. et al. (2020) MNRAS, 498, 5855-5962.
[8] Yokoyama T. et al. (2022) Science, first release.
[9] Alexander C. M. O’D. et al. (2010) GCA, 74, 4417-4437.
[10] Hopp T. et al. (2022) Sci. Adv., 8, eadd8141.
Carbon and oxygen isotope ratios of Ca-rich carbonates were measured with a CAMECA IMS 1280-HR at the Kochi Institute, JAMSTEC. Analytical conditions were similar to those for the previous CR carbonate measurements [2]. The δ13C values of Tagish Lake carbonates are consistently high (80 to 90‰). In contrast, the δ13C values of Aguas Zarcas carbonates are variable (20 to 60‰). The δ18O values of carbonates in both Tagish Lake and Aguas Zarcas are similar (+32 to 37‰), however, their Δ17O values are distinct (1.6±1.3‰ for TL vs. -1.0 ±1.5‰ for AZ, 2 SD). All Aguas Zarcas carbonate data are on the CM calcite trend line [3]. In contrast, Tagish Lake carbonate data are on the CR calcite trend line [4].
High δ13C carbonates (here tentatively defined as δ13C>40‰) are recognized in CM as well as Tagish Lake and CR chondrites, which are consistent with previous studies (up to ~80‰) [e.g., 1, 5, 6]. There is no apparent difference in major element composition and texture between the high and low δ13C carbonates in Aguas Zarcas. Although δ18O values of the high δ13C carbonates (~33‰) tend to be lower than those of the low δ13C carbonates (~35‰) in Aguas Zarcas, the temperature effect of equilibrium isotopic fractionation between fluid and carbonate seems unlikely to explain the C and O isotopic distributions of Ca-rich carbonates. We consider that the high δ13C carbonates represent a contribution from high δ13C source materials such as CO2, CO, and highly volatile organic matter [cf. 1, 7].
Carbonates in Tagish Lake and CR carbonates lie on the same O isotope trend line [2, 4, this study]. Oxygen isotope ratios of dolomite in CI chondrites and returned samples from asteroid Ryugu are also consistent with this trend [8]. Since the parent bodies of CR chondrites and Tagish Lake, and also asteroid Ryugu, are considered to have accreted further from the Sun than CM parent bodies [e.g., 1, 9, 10], the occurrence of carbonates with slightly higher Δ17O values (by ~2‰ relative to CM carbonates) suggests accreted ice (mainly H2O and possibly a few % of CO and CO2) had slightly elevated Δ17O values.
[1] Fujiya W. et al. (2019) Nat. Astron., 3, 910-915.
[2] Ushikubo T. et al. (2022) LPSC 2022, Abstract #1321.
[3] Lindgren P. et al. (2017) GCA, 204, 240-251.
[4] Jilly-Rehak C. E. et al. (2018) GCA, 222, 230-252.
[5] Vacher L. G. (2017) GCA, 213, 271-290.
[6] Telus M. et al. (2019) GCA, 260, 275-291.
[7] Altwegg K. et al. (2020) MNRAS, 498, 5855-5962.
[8] Yokoyama T. et al. (2022) Science, first release.
[9] Alexander C. M. O’D. et al. (2010) GCA, 74, 4417-4437.
[10] Hopp T. et al. (2022) Sci. Adv., 8, eadd8141.