FY 1999

Enriched H₂, CH₄, and ³He concentrations in hydrothermal plumes associated with the 1996 Gorda Ridge eruptive event

Kelley, D.S., M.D. Lilley, J.E. Lupton, and E.J. Olson

Deep-Sea Res. Pt. II, 45(12), 2665–2682, doi: 10.1016/S0967-0645(98)00088-5 (1998)

The rapid field response in March and April 1996 to diking-eruptive events on the Northern Gorda Ridge allowed the first detailed co-measurement of ³He, H₂, and CH₄ in plumes associated with a submarine eruption, and examination of the chemical transformation of these volatile species during aging of the plumes. The diking-eruptive event generated extensive event plumes with a low ³He/heat ratio of ~0.4 × 10⁻¹² cm³ STP gm⁻¹ °C⁻¹, while chronic plumes had ³He/heat ratios of 2.1-6.2 × 10⁻¹² cm³ STP gm⁻¹ °C⁻¹. Hydrogen concentrations in the initial event plume (EP96A) reached a maximum value of 47 nM, 235 times that of ocean background concentration (0.2 nM). In contrast, roughly one month later H₂ concentrations, although still elevated, had drastically dropped to 40 times background values. Methane concentrations in both event and chronic plume fluids were also elevated and similar to stable hydrothermal systems in unsedimented ridge environments, reaching a maximum value of 7 nM. The intense rapid interaction at very high temperatures of hydrothermal fluids with the host rock during dike intrusion or with seawater during extrusion of the basalt likely generated high, localized concentrations of H₂. The data indicate, however, that maximum rates of H₂ production during event plume formation could have lasted a few days at most.