Terrestrial noble gas isotopes –problems to be solved

Abstract

Since the early study on terrestrial noble gases in 1970s, we have accumulated a lot of knowledges about their distribution and characteristics in the Earth's interior. Noble gas isotopes have unique characteristics such as chemical inertness, incompatible properties, fast mobility of He and so on. By utilizing such characteristics, they work nowadays as an important tool to reveal the state and processes which occur on the Earth. However, there still remain many unsolved problems regarding them. Since noble gases easily degas when magmatic processes occur near the surface, their abundances are quite variable and atmospheric components easily affect noble gases in a magma. Hence, to get primary signatures of a magma, sample selection becomes an important issue to minimize the secondary effect. He and Ne isotopes in the Earth's interior have been revealed to show the solar-type properties. Further, He-Ne isotope sytematics between MORB and OIB sources might imply the difference in He/Ne ratios among OIB and/or MORB sources. In the Earth's interior, ⁴⁰Ar/³⁶Ar ratios are generally regarded to be high, at least more than 1,000. However, lower values are often observed in mantle xenoliths and volcanic rocks. They are generally explained by the secondary contamination or the effect of recycled materials, but there remain other cases which require additional explanations. Distribution of excess ¹²⁹Xe should be also examined in more detail. As long as available data are concerned, primordial noble gas isotopes heavier than Ar in the Earth's interior seem to have no difference from the atmospheric values, but He and Ne isotopes are different. Such information requires some additional processes to form the terrestrial noble gas signatures to the components observed in extraterrestrial materials such as solar wind or meteorites. Such problems should be solved to understand the Earth's evolution and its state.