Helium and carbon isotopes in fluorites: implications for mantle carbon contribution in an ancient subduction zone

The concentrations of helium and carbon in fluorite associated with Cretace ous to Neogene (90-13 Ma) granitic magmatism in the Japanese are have been measured. Concentrations of Li, U, Th and Gd were measured to correct for s econdary generated He-3. The CO2/He-3 of fluorites are almost uniform (1.5 x 10(10)-4 x 10(10)) and in fair agreement with the range of present island are volcanic gases. The calculated mantle C contribution in the Mesozoic s ubduction zone appear to have been identical to the present one (7-19%) ind icating that the C flux from the mantle in supra-subduction zone environmen ts has remained fairly constant during the past 70 million years. (C) 2001 Elsevier Science B.V. All rights reserved.