THE GEOCHEMICAL CONTROLS ON VENT FLUIDS FROM THE LUCKY STRIKE VENT FIELD, MID-ATLANTIC RIDGE

Hydrothermal vent fluids were collected from the Lucky Strike site at 37 degrees 17'N on the Mid-Atlantic Ridge in both 1993 and 1996. Seven vents were sampled with the DSV Alvin in 1993 and six vents were samp led in 1996 using the ROV Jason during the LUSTRE '96 Cruise. As three of the vents were sampled in both 1993 and in 1996, a time series of vent fluid chemistry is also reported. Measured temperatures ranged fr om 202 to 333 degrees C at the 1618-1726 m depth of the vent field, wh ich is located on Lucky Strike Seamount. These fluids are either equal to or less than the local bottom seawater in chlorinity. While the ra nge in fluid compositions at Lucky Strike is generally within that obs erved elsewhere, the unusual aspects of the fluid chemistries are the relatively high pH and low Fe, Mn, Li and Zn. We attribute this, as we ll as an usually low Sr/Ca ratio, to reaction with a highly altered su bstrate. The high Si and Cu contents suggest a deep, as well as hot, s ource for these fluids. The fluid compositions therefore suggest forma tion by super-critical phase separation at a depth not less than 1300 m below the seafloor, and reaction with a relatively oxic, and previou sly altered, substrate. There is temporal variability in some of the v ent fluid compositions as Li, K, Ca and Fe concentrations have increas ed in some of the vents, as has the Fe/Mn (molar) ratio, although the chlorinities have remained essentially constant from 1993 to 1996. Whi le there is not a simple relationship between vent fluid compositions (or temperatures) and distance from the lava lake at the summit of the seamount, the vent fluids from many of the vents can be shown to be r elated to others, often at distances >200 m. The most southeasterly ve nts (Eiffel Tower and the Marker/Mounds vents) are distinct in chlorin ity and other chemical parameters from the rest of the vents, although closely related to each other within the southeastern area. Similarly all of the vents not in this one area, appear closely related to each other. This suggests one or two source fluids for many of the vents, as is also inferred to be the case at TAG, but which is in contrast to observations on faster spreading ridges. This may suggest inherently different plumbing for hydrothermal systems at slower versus faster sp reading ridges. (C) 1998 Elsevier Science B.V. All rights reserved.