THE COSMIC-RAY PRODUCED HE-3 NE-21 RATIO IN ULTRAMAFIC ROCKS

Although production of noble gases in meteorites are well studied sinc e many years, [e.g. Bogard and Cressy, 19731 the first analysis of cos mic ray produced helium in terrestrial rocks were performed only in 19 86 by Kurz and by Craig and Poreda. Later, Marti and Craig (1987) repo rted cosmic-ray produced neon in summit lavas from Maui. More data on cosmogenic helium and neon were reported by Staudacher and Allegre (19 91) on ultramafic nodules from Asia. The absolute production rate of c osmic-ray produced helium has been determined by Kurz (1986, 1987) in olivine rich basalts from Hawaii with known K-Ar or C-14 ages. The res ulting He-3c production rate is 3.72 x 10(-18) cm3 STP/g/a. This allow s cosmogenic 3He to be used to determine cosmic ray exposure ages of s urface samples. For neon, no such absolute production rate has yet bee n determined. In order to develop a complementary dating method using cosmogenic Ne-21, Staudacher and Allegre (1991) measured the cosmogeni c He-3/Ne-21 ratio in ultramafic nodules. Their value was 1.4, which i s significantly smaller than the value of 3.5, obtained in this work. Here we discuss the cosmogenic He-3/Ne-21 ratio in view of new noble g as data on olivine separates of oceanites from Reunion Island and on u ltramafic nodules from Mongolia.