The cosmic ray produced ³He/21Ne ratio in ultramafic rocks

Abstract

Although production of noble gases in meteorites are well studied since many years, [e.g. Bogard and Cressy, 1973] the first analysis of cosmic ray produced helium in terrestrial rocks were performed only in 1986 by Kurz and by Craig and Poreda. Later, Marti and Craig (1987) reported cosmic‐ray produced neon in summit lavas from Maui. More data on cosmogenic helium and neon were reported by Staudacher and Allègre (1991) on ultramafic nodules from Asia. The absolute production rate of cosmic‐ray produced helium has been determined by Kurz (1986, 1987) in olivine rich basalts from Hawaii with known K‐Ar or 14C ages. The resulting ³Hec production rate is 3.72×10−18cm³STP/g/a. This allows cosmogenic ³He to be used to determine cosmic ray exposure ages of surface samples. For neon, no such absolute production rate has yet been determined. In order to develop a complementary dating method using cosmogenic 21Ne, Staudacher and Allègre (1991) measured the cosmogenic ³He/21Ne ratio in ultramafic nodules. Their value was 1.4, which is significantly smaller than the value of 3.5, obtained in this work.Here we discuss the cosmogenic ³He/21Ne ratio in view of new noble gas data on olivine separates of oceanites from Réunion Island and on ultramafic nodules from Mongolia.