Noble gases in solar-gas-rich and solar-gas-free polymict breccias

ポリミクト角礫岩中の希ガス

大澤 崇人  ; 長尾 敬介*

Osawa, Takahito; Nagao, Keisuke*

ポリミクト角礫岩に対する希ガス同位体分析を行った。18試料中5個がレゴリスブレッチアと判定され、うち4つがHコンドライトであった。この結果はレゴリスの厚みと関係するものと思われる。レゴリスブレッチアが系統的に長い宇宙線照射年代を持つという事実は確認されず、母天体での照射が数千万年程度であることを示していた。

Polymict breccias are useful for research of solar activity because some preserve plenty of solar noble gases implanted during asteroidal formation processes. In this study, noble gas compositions of Antarctic and non-Antarctic polymict breccias were determined using laser gas-extraction and stepwise pyrolysis techniques. Of the polymict breccias measured in this work, 5 of 18 were identified as gas-rich meteorites (regolith breccias) and 4 of those 5 are H chondrites. The high population of gas-rich H chondrites compared with L and LL chondrites was presumably related to the depth of regolith formed on each parent body. It is notable that the major part of polymict breccias did not have solar noble gases. Noble gas analyses with stepwise heating method were done for 11 polymict breccias. Gas-rich meteorites have high concentrations of solar-derived He and Ne, which were released at relatively low temperature steps. Cosmogenic nuclides were comparatively dominant at high temperature steps. Five sources determined the observed Ar isotopic compositions. The components were: atmospheric, radiogenic, solar wind (SW), solar energetic particles (SEP), and cosmogenic. In contrast, Ne isotopic compositions of most regolith breccias con be explained simply by three-component mixing, such as SW, SEP, and cosmogenic. Indications of primordial trapped components were observed only in Willard (b), in which carbonaceous chondrite clasts were discovered previously. Cosmic-ray exposure ages were calculated from excess He, Ne, and Ar. Regolith breccias did not have systematically longer ages than gas-poor samples, indicating that the parent body exposure ages of the meteorites on the order of tens of millions of years at most.