Trace element and REE composition of a low-temperature ridge-flank hydrothermal spring

Abstract

Warm (25°C) hydrothermal springs have been sampled on Baby Bare, a basaltic outcrop on 3.5-Ma-old crust ∼100-km east of the Endeavor Segment of the Juan de Fuca Ridge. The source for these springs is a 62 to 64°C formation water that has cooled conductively as it ascends to feed the springs. This water originated as bottom seawater that probably descended into basement ∼52 km to the southwest at another, much larger outcrop called Grizzly Bare. As this seawater flows towards Baby Bare, it is heated and altered by reactions within basaltic basement and by diffusive fluxes to and from the overlying sediment. Concentrations of Mn, Co, Ni, Zn, Cd, and Mo in the spring waters are greater than in bottom seawater, indicating that the oceanic crust is a source for these elements to the oceans. At least a portion of this increase probably results from the redox cycling of Mn in sedimentary sources near the basement interface that produces a diffusive flux to basement formation waters. Additional removal of Mo and inputs of the other five elements to two of the three springs are observed locally near sites of venting, where density gradients can form shallow circulation cells within the sediment and diffusive exchange occurs. Concentrations of Cu, U, V, Y, and the rare earth elements (REEs, excluding Ce) in these samples are less than in bottom seawater, indicating that the oceanic crust is a net sink for these elements in this environment. Copper is probably removed into newly formed carbonate and/or sulfide phases. Removal of the oxyanions U and V is consistent with a net removal of phosphate demonstrated previously for ridge-flank hydrothermal systems. Similarly, removal of Y and the REEs is associated with carbonate, phosphate-rich, and oxide phases. Calculated maximum global chemical fluxes from “warm” ridge-flank hydrothermal systems such as Baby Bare are insignificant relative to riverine fluxes for these elements, except possibly for Mn and Mo. The impact on global geochemical budgets for these elements from lower temperature (<25°C) alteration on ridge flanks is still unknown, but it may well be larger than for warm ridge flanks.