Cosmochemical Study of Halogens Based on Halogen-derived Noble Gas Isotopes

我们在Yamato-74191和阿连德（CV 3）球粒陨石的CAIs中观察到非常高浓度的Cl、Br和I。由于卤素通常被认为是在太阳星云中产生固态凝聚物时的挥发性元素，因此在CAIs中产生高浓度卤素的机制很难理解。这是由于缺乏有关卤素的宇宙化学行为的知识。对阿连德陨石进行了激光微探针惰性气体分析，其空间分辨率约为150μm。高浓度的NH 4<129>+显示出与用EPMA分析测定的氯浓度的良好相关性。在<80>具有高Δ Kr/Δ Kr比的测量部分中，Δ Kr/Δ <84>Kr和Δ <128>Kr<132>/Δ Kr比也高<129><132>。在<80>空间中，<128>^ Br和^ I的中子俘获分别产生<79>了^ Kr和^<127>Kr，这些结果表明，当水蚀变作用在CAI内部产生方钠石时，卤素从外部通过CAI的无序边界进入CAI内部。由于我们在周围的基质材料中没有发现卤素衍生的惰性气体，因此CAIs中的富卤素物质一定是在太阳星云中产生的，而不是嵌入基质后在母体中产生的。富<129>钠长石物质中高浓度的^I表明，水蚀变一定发生在太阳系的极早期阶段，当时仍有大量的^<129>I存在。

We have observed very high concentrations of Cl, Br and I in Yamato-74191 and in CAIs of Allende(CV3) chondrite. Because halogens are generally thought to have behaved as volatile elements when solid condensates were produced in solar nebula, mechanism to produce the observed high concentrations of halogens in CAIs is difficult to understand. This is due to a lack of knowledge about the cosmochemical behavior of halogens. Laser microprobe noble gas analysis was applied to Allende meteorite traversing the CAI and matrix with a space resolution of about 150μm. The high concentrations of ^<129>Xe show good correlation with the chlorine concentrations determined with EPMA analysis. ^<80>Kr/^<84>Kr and ^<128>Xe/^<132>Xe ratios are also high in the measured portions with high ^<129>Xe/^<132>Xe ratios. The ^<80>Kr and ^<128>Xe can be produced by neutron capture on ^<79>Br and ^<127>I, respectively, in space.The present results show that halogens have penetrated into the CAIs from outside through the disordered boundary of CAI, when aqueous alteration produced sodalite inside the CAIs. Since we could not find halogen-derived noble gases in surrounding matrix materials, the halogen-rich materials in CAIs must have been produced in the solar nebular, and not in parent body after embedded in matrices. The high concentrations of ^<129>Xe in sodalite-rich materials indicate that the aqueous alteration must occur at the very early stage of solar system when abundant ^<129>I was still alive.