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

We sampled hydrothermal plumes over the N. Gorda Ridge four times between M arch and August 1996 to document Mn and Fe discharge resulting from a magma tic intrusion/seafloor eruption. Two separate event plumes, EP96A and B, an d chronic hydrothermal emissions lasting similar to 6 months were character ized. Shipboard time-series measurements of an event plume sample were used to calibrate an Fe phase clock useful for estimating sample age up to simi lar to 6 days after fluid discharge. Samples collected from EP96A. and B ha d 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 va lues are generally characteristic of event plumes and hypothesize that Mn a nd Fe may be supplied to event plumes by different processes: Mn by entrain ment of fluids from an extant shallow subseafloor reservoir, and Fe by shor t-lived, high-temperature water-rock reaction coincident with dike emplacem ent. Calculations based on the Fe phase clock indicate that the two event p lumes were released more than a month apart. The largest event plume, EP96A (similar to 2.3 x 10(6) M Mn and 13 x 10(6) M Fe), formed similar to 7 Mar ch soon after seismic activity began. The smaller EP96B (similar to 0.49 x 10(6) M Mn and 3.5 x 10(6) M Fe) was not discharged until similar to 11 Apr il, 3 weeks after the cessation of seismic activity detectable by SOSUS T-p hase monitoring. We hypothesize that the subseafloor disturbance that trigg ered 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. Clef t segment of the Juan de Fuca Ridge, but comparable to event plume inventor ies 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 mo derate (similar to 0.25 oon-mol J(-1), reflecting probable admixture with e vent plume formation fluids) to high (similar to 0.7 nmol J(-1), typical of chronic plumes) to low (similar to 0.1 nmol J-(1,) similar to diffuse vent fluid values), marking a complete episode of intrusion/eruption-induced hy drothermal discharge. (C) 1999 Elsevier Science Ltd. All rights reserved.