S, O AND SR ISOTOPE SYSTEMATICS OF ACTIVE VENT MATERIALS FROM THE MARIANA BACKARC BASIN SPREADING AXIS AT 18-DEGREES-N

Stable isotope ratios of S, O and Sr have been measured for active vent materials which were first found and sampled in April 1987 from the Mariana backarc spreading axis at 18°N. Chimneys consisted mostly of barite with a lesser proportion of sulfide minerals such as sphalerite, galena, chalcopyrite and pyrite. The δ34S values of sphalerite and galena taken from several chimneys and various parts of a chimney showed a narrow range from 2.1 to 3.1‰, suggesting uniform conditions of fluid chemistry during chimney growth. The sulfur isotopic results imply a contribution of hydrogen sulfide reduced from seawater sulfate in the deep hydrothermal reaction zone, considering that fresh glasses of the Mariana Trough basalts have δ34S = -0.6 ± 0.3‰. Sulfur isotopic compositions of hydrogen sulfide in the high temperature vent fluids (δ34S = 3.6-4.8‰) which are higher than those of the sulfide minerals suggest the secondary addition of hydrogen sulfide partially reduced from entrained seawater SO42- at a basal part of the chimneys. This interpretation is consistent with the δ34S values of barite (21-22‰) that are higher than those of seawater sulfate. The residence time of the entrained SO42- was an order of an hour on a basis of oxygen isotopic disequilibrium of barite. Strontium isotopic variations of barite and vent waters indicated that Sr in barite was mostly derived from the Mariana Trough basalts with a slight contribution from Sr in circulating sea-water, and that 10-20% mixing of seawater with ascending hydrothermal fluids induced precipitation of barite at the sea-floor. © 1990.