Tritium and stable isotopes of magmatic waters

To investigate the isotopic composition and age of water in volcanic gases and magmas, we analyzed samples from 11 active volcanoes ranging in composi tion from tholeiitic basalt to rhyolite: Mount St. Helens (USA), Kilauea (U SA), Pacaya (Guatemala), Galeras (Colombia), Satsuma Iwo-Jima (Japan), Sier ra Negra and Alcedo (Ecuador), Vulcano (Italy), Paricutin (Mexico), Kudryav y (Russia), and White island (New Zealand). Tritium at relatively low level s (0.1-5 T.U.) is found in most emissions from high-temperature volcanic fu maroles sampled, even at discharge temperatures >700 degrees C. Although ma gmatic fluids sampled from these emissions usually contain high CO2, S-tota l, HCl, HF, B, Br, He-3 R/R-A, and low contents of air components, stable i sotope and tritium relations of nearly all such fluids show mixing of magma tic volatiles with relatively young meteoric water (model ages less than or equal to 75 y). Linear delta D/delta(18)O and H-3/delta(18)O mixing trends of these two end-members are invariably detected at are volcanoes. Tritium is also detected in fumarole condensates at hot spot basalt volcanoes, but collecting samples approaching the composition of end-member magmatic flui d is exceedingly difficult, in situ production of H-3, mostly from spontane ous fission of U-238 in magmas is calculated to be <0.001 T.U., except for the most evolved compositions (xhigh U, Th, and Li and low H2O contents). T hese Values are below the detection limit of H-3 by conventional analytical techniques (about 0.01 T.U. at best). We found no conclusive evidence that natural fusion in the Earth produces anomalous amounts of detectable H-3 ( >0.05 T.U.). (C) 2000 Elsevier Science B.V. All rights reserved.