10:45 〜 11:00
[PPS08-07] 無水鉱物の同位体比情報から氷や流体の特徴を読み解く
★招待講演
キーワード：炭素質コンドライト、無水鉱物、二次イオン質量分析計、酸素同位体、水素同位体、硫黄同位体
Isotopic compositions of nominally anhydrous minerals (NAMs) precipitated from fluid (or formed in volatile-rich environment) very likely preserve isotopic signatures of fluid (or volatiles in environment). Although such data are indirect information of isotopic compositions of fluid or volatiles, considering robustness of isotope systematics of NAMs, isotopic records of NAMs can be useful constraints on fluid or volatiles which have not preserved in planetary materials. In this presentation, I would like to introduce some studies to explore isotopic compositions of ice or fluid component based on isotopic signatures of NAMs by SIMS at UW-Madison and Kochi Institute, JAMSTEC.
One example is recognition of distinct Δ17O values between type II (FeO-rich in silicates) chondrules and type I (FeO-poor in silicates & metallic Fe) chondrules in carbonaceous chondrites1,2. Most chondrules in carbonaceous chondrites are type I and their Δ17O values are ∼–5‰. In contrast, type II chondrules in CM, CO, CV chondrites have slightly elevated Δ17O values (∼–2‰). Furthermore, type II chondrules with much higher Δ17O values (∼+1‰) are found in CR, CH chondrites, Tagish Lake (TL) and TL-like meteorites3-6. Similar FeO-rich silicates with Δ17O ∼ +1‰ are found in Comet 81P/Wild samples7. Type II chondrules must have formed in anomalously oxidizing condition in the protoplanetary disk. Since type II chondrules have consistent Δ17O values but diverse FeO/MgO ratios indicating diverse oxygen fugacity, type II chondrules possibly preserve Δ17O values of oxidizing agents (e.g., H2O ice)2,3.
Another example is oxygen isotope systematics of carbonates in carbonaceous chondritic materials. Since carbonates precipitated from fluid during an early stage of hydrothermal activity in chondritic parent body, their isotopic compositions potentially preserve isotopic signatures of primordial fluids. It has been shown that carbonates in CM and CR chondrites show distinct parallel oxygen isotope trend lines (δ17O ∼ 0.67 × δ18O – 5.3 for CM & δ17O ∼ 0.64 × δ18O – 2.5 for CR, respectively)8,9. Our data support existence of these two trends for CM and CR carbonates10-12 and, interestingly, we found that oxygen isotope data of TL carbonates are also distributed on the CR carbonate trend line12. Carbonate data of the Ryugu and CI samples are also close to the CR carbonate trend line13. Slightly elevated Δ17O values of carbonates of CR, CI, TL chondrites and the Ryugu samples relative to CM carbonates by ∼2‰ suggest that accreted ice into their parent bodies had slightly elevated Δ17O values. Since this is consistent with the occurrence of type II chondrules with elevated Δ17O values in CR, CH, TL(-like) chondrites and the comet 81P/Wild, we might recognize radial variation in oxygen isotopic compositions of icy components where these parent bodies accreted in the protoplanetary disk.
At Kochi Institute, we also established a sulfur 4-isotope analysis technique for sulfides, and sulfur and hydrogen 2-isotope analysis techniques for basaltic glass (though not NAM) to study mantle-scale volatile circulations14-17. Isotopic compositions of trace amounts of volatiles would be useful to understand igneous processes of differentiated extra-terrestrial materials. If time permits, I will briefly report recent progress at Kochi Institute.
1: Ushikubo+ 2012, GCA, 90, 242
2: Tenner+ 2018, In Chondrules, Cambridge Univ. Press
3: Tenner+ 2015, GCA, 148, 228
4: Nakashima+ 2020, GCA, 290, 180
5: Yamanobe+ 2018, Polar Sci., 15, 29
6: Ushikubo & Kimura 2021, GCA, 293, 328
7: Defouilloy+ 2017, EPSL, 465, 145
8: Lindgren+ 2017, GCA, 204, 240
9: Jilly-Rehak+ 2018, GCA, 222, 230
10: Fujiya+ 2019, Nat. Astron., 3, 910
11: Fujiya+ 2020, GCA, 274, 246
12: Ushikubo+ 2023, LPSC 2023, #1907
13: Fujiya+ 2023, Nat. Geosci., 16, 675
14: Shimizu+ 2019, Geochem. J., 53, 195
15: Kuritani+ 2021, Sci. Rep., 11, 18755
16: Kawaguchi+ 2022, J. Petrol., 63, 1
17: Shimizu & Ushikubo 2024, This meeting, session S-GC32
One example is recognition of distinct Δ17O values between type II (FeO-rich in silicates) chondrules and type I (FeO-poor in silicates & metallic Fe) chondrules in carbonaceous chondrites1,2. Most chondrules in carbonaceous chondrites are type I and their Δ17O values are ∼–5‰. In contrast, type II chondrules in CM, CO, CV chondrites have slightly elevated Δ17O values (∼–2‰). Furthermore, type II chondrules with much higher Δ17O values (∼+1‰) are found in CR, CH chondrites, Tagish Lake (TL) and TL-like meteorites3-6. Similar FeO-rich silicates with Δ17O ∼ +1‰ are found in Comet 81P/Wild samples7. Type II chondrules must have formed in anomalously oxidizing condition in the protoplanetary disk. Since type II chondrules have consistent Δ17O values but diverse FeO/MgO ratios indicating diverse oxygen fugacity, type II chondrules possibly preserve Δ17O values of oxidizing agents (e.g., H2O ice)2,3.
Another example is oxygen isotope systematics of carbonates in carbonaceous chondritic materials. Since carbonates precipitated from fluid during an early stage of hydrothermal activity in chondritic parent body, their isotopic compositions potentially preserve isotopic signatures of primordial fluids. It has been shown that carbonates in CM and CR chondrites show distinct parallel oxygen isotope trend lines (δ17O ∼ 0.67 × δ18O – 5.3 for CM & δ17O ∼ 0.64 × δ18O – 2.5 for CR, respectively)8,9. Our data support existence of these two trends for CM and CR carbonates10-12 and, interestingly, we found that oxygen isotope data of TL carbonates are also distributed on the CR carbonate trend line12. Carbonate data of the Ryugu and CI samples are also close to the CR carbonate trend line13. Slightly elevated Δ17O values of carbonates of CR, CI, TL chondrites and the Ryugu samples relative to CM carbonates by ∼2‰ suggest that accreted ice into their parent bodies had slightly elevated Δ17O values. Since this is consistent with the occurrence of type II chondrules with elevated Δ17O values in CR, CH, TL(-like) chondrites and the comet 81P/Wild, we might recognize radial variation in oxygen isotopic compositions of icy components where these parent bodies accreted in the protoplanetary disk.
At Kochi Institute, we also established a sulfur 4-isotope analysis technique for sulfides, and sulfur and hydrogen 2-isotope analysis techniques for basaltic glass (though not NAM) to study mantle-scale volatile circulations14-17. Isotopic compositions of trace amounts of volatiles would be useful to understand igneous processes of differentiated extra-terrestrial materials. If time permits, I will briefly report recent progress at Kochi Institute.
1: Ushikubo+ 2012, GCA, 90, 242
2: Tenner+ 2018, In Chondrules, Cambridge Univ. Press
3: Tenner+ 2015, GCA, 148, 228
4: Nakashima+ 2020, GCA, 290, 180
5: Yamanobe+ 2018, Polar Sci., 15, 29
6: Ushikubo & Kimura 2021, GCA, 293, 328
7: Defouilloy+ 2017, EPSL, 465, 145
8: Lindgren+ 2017, GCA, 204, 240
9: Jilly-Rehak+ 2018, GCA, 222, 230
10: Fujiya+ 2019, Nat. Astron., 3, 910
11: Fujiya+ 2020, GCA, 274, 246
12: Ushikubo+ 2023, LPSC 2023, #1907
13: Fujiya+ 2023, Nat. Geosci., 16, 675
14: Shimizu+ 2019, Geochem. J., 53, 195
15: Kuritani+ 2021, Sci. Rep., 11, 18755
16: Kawaguchi+ 2022, J. Petrol., 63, 1
17: Shimizu & Ushikubo 2024, This meeting, session S-GC32