Comparative study of degassing between Mars and Earth bade on noble gases in Martian meteorites

大气和地球内部的稀有气体同位素组成表明，地球内部存在早期脱气作用，形成大气。与此相反，从维京海盗号着陆器和火星陨石上观测到的火星惰性气体分布与地球上的完全不同。为了详细证实这一点，我们使用高灵敏度质谱仪和中子辐照样品测量了许多火星陨石。主要研究结果如下：1.在Dhofar 378 shergottite中富集的Ne显示火星大气的^<20>Ne/^<22>Ne比值为7.3±0.1。这个数值表明火星大气层严重流失。NWA 2737型硅质岩的^<40>Ar/^<36>Ar和^Ar<129>/^<132>Ar值远高于硅质岩。这意味着惰性气体同位素在火星内部的不均匀分布。火星走廊可能没有发生均匀的脱气。MIL 03346 nakhlite有两个阶段，含有大量元素分馏的火星大气。火星上形成的风化产物可能是分馏惰性气体的载体。大多数shergottite的最终成分之一被认为是火星内部的惰性气体，正如在Chassigny观察到的那样。我们对橄榄石和沙漠砂风化产物的惰性气体数据强烈表明，该成分不是火星起源的，而是分馏的地球大气。SaU 060辉绿云母的Ar-Ar年龄在700-1400℃为1.0- 1.8Ga，在较高温度为3- 4Ga。较老的年龄可能代表火星地壳的年龄。

Noble gas isotopic compositions in atmosphere and interior of the Earth suggest early degassing from interior to form atmosphere. Contrary to this, noble gas distributions of Mars observed from the Viking lander and Martian meteorites are completely different from those in the Earth. To confirm this in detail, we measured many Martian meteorites using high sensitive mass spectrometers and neutron irradiated samples. Some of the results are followings :1. Enriched Ne in Dhofar 378 shergottite shows ^<20>Ne/^<22>Ne ratio of 7.3±0.1 for Martian atmosphere. The value suggests a severe loss of Martian atmosphere.2. NWA2737 chassignite shows much higher ^<40>Ar/^<36>Ar and ^<129>Xe/^<132>Xe than those for Chassigny. This means heterogeneous distribution of noble gas isotopes in the interior of Mars. Uniform degassing might not have occurred from the Martian interior.3. MIL 03346 nakhlite have two phases containing heavily elementally fractionated Martian atmosphere. Weathering products formed on Mars are probable carriers for the fractionated noble gases.4. One of the end components observed for most shergottites is thought to be a Martian interior noble gas as is observed in Chassigny. Our noble gas data for weathering products of olivine and desert sands strongly indicate that the component is not Martian origin but fractionated Earth atmosphere.5. Ar-Ar ages for SaU 060 shergottite are 1.0-1.8 Ga at the temperatures of 700-1400℃ and 3-4 Ga at the higher temperatures. The older age might represent the age of Martian crust.