Composition of pore and spring waters from Baby Bare: Global implications of geochemical fluxes from a ridge flank hydrothermal system

摘要

Warm hydrothermal springs were discovered on Baby Bare, which is an isolated basement outcrop on 3.5 Ma-old crust on the eastern flank of the Juan de Fuca Ridge. We have sampled these spring waters from a manned submersible, along with associated sediment pore waters from 48 gravity and piston cores. Systematic variations in the chemical composition of these waters indicate that hydrothermal reactions in basement at moderate temperatures (63 degrees C in uppermost basement at this site) remove Na, K, Li, Rb, Mg, TCO2, alkalinity, and phosphate from the circulating seawater and leach Ca, Sr, Si, B, and Mn from the oceanic crust; and that reactions with the turbidite sediment surrounding Baby Bare remove Na, Li, Mg, Ca, Sr, and sulfate from the pore water while producing ammonium and Si and both producing and consuming phosphate, nitrate, alkalinity, Mn, and Fe. K, Rb, and B are relatively unreactive in the sediment column. These data confirm the earlier inference that sediment pore waters from areas of upwelling can be used to estimate the composition of altered seawater in the underlying basement, even for those elements that are reactive in the sediment column or are affected by sampling artifacts. The composition of altered seawater in basement at Baby Bare is similar to the inferred composition of 58 degrees C formation water from crust nearly twice as old (5.9 Ma) on the southern flank of the Costs Rica Rift. The Baby Bare fluids also exhibit the same directions of net elemental transfer between basalt and seawater as solutions produced in laboratory experiments at a similar temperature, and complement compositional changes from seawater observed in seafloor basalts altered at cool to moderate temperatures. The common parameter among the two ridge flanks and experiments is temperature, suggesting that the residence time of seawater in basement at the two ridge-flank sites is sufficiently long for the solutions to equilibrate with altered basalt. This conclusion is supported by compilations of data from other ridge-flank sites, which show a systematic relationship between inferred basement water composition and temperature. We use the Baby Bare spring waters along with constraints from the riverine flux of Mg to estimate upper limits on the global fluxes of 14 elements at warm ridge-flank sites such as Baby Bare. Maximum calculated fluxes of Mg, Ca, sulfate, B, and K may equal or exceed 25% of the riverine flux, and such sites may represent the missing, high K/Rb sink required for the K budget. Additional fluxes from/to the crust on ridge flanks are expected from cool (<20-25 degrees C) hydrothermal sites. Copyright (C) 2000 Elsevier Science Ltd. [References: 49]