16:30 〜 16:45
[PPS06-05] 炭素質コンドライト中の炭酸塩鉱物から制約する小惑星中の氷の組成
キーワード：炭素質コンドライト、炭酸塩鉱物、炭素同位体比
Carbonate minerals in carbonaceous chondrites formed by aqueous alteration in the parent bodies. Previous studies have shown that C isotopic ratios of carbonates in CM chondrites are highly variable among individual grains even in a single meteorite. The reasons for the variability in δ13C values and the origin of the carbon in carbonates remain poorly understood. Here we report C isotopic ratios of individual calcite (CaCO3) grains in the LaPaz Icefield (LAP) 031166 CM chondrite (CM 2.1), and of both calcite and dolomite (CaMg(CO3)2) grains in the Tagish Lake meteorite, measured with Secondary Ion Mass Spectrometry (SIMS). We discuss the origin of the carbon in carbonates and explore a possible link between ice in CMs and Tagish Lake and cometary ice.
An apparent contrast between carbonates in LAP 031166 as well as other CMs analyzed previously and Tagish Lake can be seen: the δ13C values of CM carbonates are highly variable, ranging from ~20 to 80‰, whereas all Tagish Lake carbonates have high δ13C values of ~70‰. To explain these observations, we propose here mixing of at least two C reservoirs with different δ13C values. The candidates of C reservoirs include CO2 and CO accreted as ice, and organic matter. CO2 is the major C-bearing species in cometary ice. If C from organic matter contributed to the carbonate C, organic matter must have been oxidized to CO2 which requires strong oxidants such as peroxide. The upper limit on the amounts of CO2 produced by oxidation of organic matter with peroxide is estimated to be ~0.1 wt%, which is far smaller than the carbonate C abundance of Tagish Lake (~1.3 wt%). Thus, the C reservoir with a high δ13C value is almost certainly CO2 accreted as ice. On the other hand, the C reservoir(s) with a low δ13C value is poorly constrained: both organic matter and CO are possible. The δ13C values of IOM in CMs and Tagish Lake are from -17 to -19‰ and from -13 to -15‰, respectively. The δ13C value of trapped CO in Murchison is approximately -32‰.
Here we assume that the δ13C value of CO2 ice is 80‰. Also, we assume that other C reservoirs (i.e., organic matter or CO) with low δ13C values from -10 to -30‰ contributed to the carbonate C. The fact that Tagish Lake carbonates have homogenous and high δ13C values indicates larger contribution from CO2 ice to the Tagish Lake carbonates than the CM carbonates. The abundance of the carbonate C in Tagish Lake is ~1.3 wt% and the δ13C value of the Tagish Lake carbonates is ~70‰. Thus, from mass balance calculation, CO2 ice accounts for 89-91% of the carbonate C in Tagish Lake. Likewise, the average abundance of the carbonate C in CMs is ~0.17 wt% and the average δ13C value of CM carbonates is ~45‰. For the case of CMs, CO2 ice accounts for 61-68% of the carbonate C. Based on the values above and water contents in CMs and Tagish Lake, we can calculate CO2/H2O mole ratios of ice accreted to the CM and Tagish Lake parent bodies to be ~0.018 and ~0.28, respectively. The CO2/H2O mole ratios of CM and Tagish Lake ice are compared with those of cometary ice. The CO2/H2O ratio of the CM ice is smaller than in any comet observed while the Tagish Lake ice has a CO2/H2O ratio within the range of comets. These observations may suggest a genetic link between D-type asteroids from which Tagish Lake is likely derived, and comets. On the other hand, C-type asteroids from which CM chondrites is likely derived, may have accreted closer to the Sun (and therefore, at higher temperature) than D-type asteroids and comets where CO2 ice partially sublimated.
An apparent contrast between carbonates in LAP 031166 as well as other CMs analyzed previously and Tagish Lake can be seen: the δ13C values of CM carbonates are highly variable, ranging from ~20 to 80‰, whereas all Tagish Lake carbonates have high δ13C values of ~70‰. To explain these observations, we propose here mixing of at least two C reservoirs with different δ13C values. The candidates of C reservoirs include CO2 and CO accreted as ice, and organic matter. CO2 is the major C-bearing species in cometary ice. If C from organic matter contributed to the carbonate C, organic matter must have been oxidized to CO2 which requires strong oxidants such as peroxide. The upper limit on the amounts of CO2 produced by oxidation of organic matter with peroxide is estimated to be ~0.1 wt%, which is far smaller than the carbonate C abundance of Tagish Lake (~1.3 wt%). Thus, the C reservoir with a high δ13C value is almost certainly CO2 accreted as ice. On the other hand, the C reservoir(s) with a low δ13C value is poorly constrained: both organic matter and CO are possible. The δ13C values of IOM in CMs and Tagish Lake are from -17 to -19‰ and from -13 to -15‰, respectively. The δ13C value of trapped CO in Murchison is approximately -32‰.
Here we assume that the δ13C value of CO2 ice is 80‰. Also, we assume that other C reservoirs (i.e., organic matter or CO) with low δ13C values from -10 to -30‰ contributed to the carbonate C. The fact that Tagish Lake carbonates have homogenous and high δ13C values indicates larger contribution from CO2 ice to the Tagish Lake carbonates than the CM carbonates. The abundance of the carbonate C in Tagish Lake is ~1.3 wt% and the δ13C value of the Tagish Lake carbonates is ~70‰. Thus, from mass balance calculation, CO2 ice accounts for 89-91% of the carbonate C in Tagish Lake. Likewise, the average abundance of the carbonate C in CMs is ~0.17 wt% and the average δ13C value of CM carbonates is ~45‰. For the case of CMs, CO2 ice accounts for 61-68% of the carbonate C. Based on the values above and water contents in CMs and Tagish Lake, we can calculate CO2/H2O mole ratios of ice accreted to the CM and Tagish Lake parent bodies to be ~0.018 and ~0.28, respectively. The CO2/H2O mole ratios of CM and Tagish Lake ice are compared with those of cometary ice. The CO2/H2O ratio of the CM ice is smaller than in any comet observed while the Tagish Lake ice has a CO2/H2O ratio within the range of comets. These observations may suggest a genetic link between D-type asteroids from which Tagish Lake is likely derived, and comets. On the other hand, C-type asteroids from which CM chondrites is likely derived, may have accreted closer to the Sun (and therefore, at higher temperature) than D-type asteroids and comets where CO2 ice partially sublimated.