Element redistribution during hydrothermal alteration of rhyolite in an active geothermal system: Yellowstone drill cores Y-7 and Y-8 (original) (raw)

1986, Geochimica et Cosmochimica Acta

Hydrothermal alteration of the 0.54 Ma Biscuit Basin rhyolite flow in Yellowstone drill cores Y-7 and Y-8 has involved significant changes in Ti-normalized whole rock concentrations of Li, Mg, Ca, Na, K, Mn, Rb, Sr, Sb, Cs, Ba, and U, whereas such changes have generally not occurred for Al, Fe, SC, Co, Y, Zr, REE, Hf, Ta, and Th. The relatively mobile elements have been redistributed over distances from 2-5 cm to more than lo-100 m, and are associated mainly with zeolites and clays, in thoroughly altered samples; average whole rock changes in Ti-normalized mass range from about OS-17 g/Kg (9-491) for major elements and 0.1-280 mg/Kg (19-426OW) for minor and trace elements. Compositional changes correlate with hydrothermal mineralogy. The relatively immobile elements have been redistributed over a distance of at least lo-100 pm but less than 2-5 cm, and are associated mainly with clays, in thoroughly altered samples. Addition of Si02 ranges from 1 lo-890 g/Kg in samples where porosity has been thoroughly sealed by silica deposition. Thermal water is not in oxygen isotopic equilibrium with any primary or secondary phases analyzed except calcite. Modest depletions of "0 (l-2960) due to exchange with thermal water apparently have occurred in Y-8 plagioclase and obsidian. Oxygen isotope ratios in obsidian-replacing smectite and in veinlet celadonite and &cristobalite suggest that these phases precipitated from water locally enriched in "0 (up to -810 heavier than present thermal water). The minimum integrated water/rock mass ratio in Y-7 and Y-8 is lo'-10" (assuming advective transport).