FY 1999

Manganese and iron in hydrothermal plumes resulting from the 1996 Gorda Ridge Event

Massoth, G.J., E.T. Baker, R.A. Feely, J.E. Lupton, R.W. Collier, J.F. Gendron, K.K. Roe, S.M. Maenner, and J.A. Resing

Deep-Sea Res. Pt. II, 45(12), 2683–2712, doi: 10.1016/S0967-0645(98)00089-7 (1998)

We sampled hydrothermal plumes over the N. Gorda Ridge four times between March and August 1996 to document Mn and Fe discharge resulting from a magmatic intrusion/seafloor eruption. Two separate event plumes, EP96A and B, and chronic hydrothermal emissions lasting ~6 months were characterized. Shipboard time-series measurements of an event plume sample were used to calibrate an Fe phase clock useful for estimating sample age up to ~6 days after fluid discharge. Samples collected from EP96A and B had Mn/heat (<0.15 nmol J^–1) and Fe/Mn (>2 mol mol^–1) ratios similar to historical event plume observations. We suggest these "signature" ratio values are generally characteristic of event plumes and hypothesize that Mn and Fe may be supplied to event plumes by different processes: Mn by entrainment of fluids from an extant shallow subseafloor reservoir, and Fe by short-lived, high-temperature water-rock reaction coincident with dike emplacement. Calculations based on the Fe phase clock indicate that the two event plumes were released more than a month apart. The largest event plume, EP96A (~2.3 × 10⁶ M Mn and 13 × 10⁶ M Fe), formed ~7 March soon after seismic activity began. The smaller EP96B (~0.49 × 10⁶ M Mn and 3.5 × 10⁶ M Fe) was not discharged until ~11 April, 3 weeks after the cessation of seismic activity detectable by SOSUS T-phase monitoring. We hypothesize that the subseafloor disturbance that triggered EP96B also resulted in the episodic flushing of a reservoir of chronic-plume-like fluids. Total event plume inventories of Mn and Fe at N. Gorda Ridge are much smaller than those associated with the 1986 event at N. Cleft segment of the Juan de Fuca Ridge, but comparable to event plume inventories at N. Cleft segment in 1987 and CoAxial segment in 1993. Mn/heat values for chronic plumes over the eruption site underlying EP96A evolved from moderate (~0.25 oon-mol J^–1, reflecting probable admixture with event plume formation fluids) to high (~0.7 nmol J^–1, typical of chronic plumes) to low (~0.1 nmol J^–1, similar to diffuse vent fluid values), marking a complete episode of intrusion/eruption-induced hydrothermal discharge.