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第2日　5月20日（木）　10:40～11:00　C会場（Zoom）

2C-O8-1040　　PDF

二次イオン質量分析計（SIMS）を用いた炭素質コンドライト中の炭酸塩鉱物の⁵³Mn-⁵³Cr年代測定および炭素・酸素同位体比測定

(茨城大院理工)
^o藤谷渉・ 菅原慎吾・ 小木曽綸

Secondary Ion Mass Spectrometry (SIMS) has widely been applied to geochemistry and cosmochemistry. We have utilized SIMS to analyze carbonate minerals in carbonaceous chondrites (CCs) to try to understand the evolution of water-rich asteroids and water-rock interaction (aqueous alteration) in such asteroids.
Recently, we analyzed oxygen and carbon isotopic compositions of carbonates the Tagish Lake meteorite. We found that the carbonates in Tagish Lake are consistently enriched in ¹³C by ~7 % compared to terrestrial carbonates. The observed ¹³C-rich compositions are similar to that of 67P/Churyumov-Gerasimenko, a Jupiter-family comet. In addition, the H₂O/CO₂ ratio of Tagish Lake is consistent with those of comets. Thus, we concluded that the Tagish Lake parent asteroid formed in the cold, outer solar system but was transported to the current orbit in the inner solar system owing to the gravity of giant planets.
We also measured ⁵³Mn-⁵³Cr radiometric ages of carbonates in CCs. We used a Mn- and Cr-bearing carbonate as a standard material to minimize an analytical bias caused by the matrix effect. The obtained formation ages of carbonates are 4,563 Myr ago, indicating that the aqueous alteration in CCs was induced by ²⁶Al decay (half-life: 0.7 Myr). We are still trying to produce a carbonate standard with more homogeneous Mn and Cr concentrations and small amounts of Fe to acquire more accurate ⁵³Mn-⁵³Cr ages.