The geochemistry of boron and lithium in mid-ocean ridge hydrothermal vent fluids.

摘要

Examining the chemistry of hydrothermal vent fluids can provide insight into processes occurring below the seafloor along the mid-ocean ridge axis. Boron and lithium in hydrothermal vent fluids were studied because previous results indicated that these elements might be useful tracers of water-rock processes. The B concentration and the Li isotopic signature of hydrothermal fluids from four areas on the mid-ocean ridge were examined in order to better constrain the physical and chemical controls on these elements.; The results for B suggest that water-rock reaction and phase separation are important controls on the B content of hydrothermal fluids. Boron concentrations vary over a relatively narrow range (∼0.7 to 1.5 times the seawater value) and show little variation with time as hydrothermal systems age. Unlike most other elements including Cl and Li, B is not significantly fractionated between the vapor- and liquid-phases during phase separation, resulting in little spatial or temporal variability in B content. While the majority of the fluids sampled have B concentrations greater than the seawater value (∼415 μmol/kg), fluids from the Irina vent at the ultramafic-hosted Logatchev hydrothermal site are unique in that the B is depleted with respect to seawater by ∼28%, likely due to low temperature reaction with serpentinites in the down flow zone.; The results for Li demonstrate that the δ7Li signature of high temperature hydrothermal fluids is remarkably constant with a global average value of 7.5 ± 1.6‰. This contrasts the variability seen for many other chemical parameters in hydrothermal fluids including Li concentration. The δ7Li signature of the hydrothermal fluids is interpreted as an equilibrium value between the fluids and secondary alteration mineral products as previously suggested by Chan et al. [1993]. Very subtle variations in the δ7Li isotopic signature (near the level of the analytical precision) are noted in the fluids sampled immediately after a volcanic eruption, likely resulting from non-equilibrium conditions during this period.; Measurements of Li isotopes in low temperature diffuse flow fluids are consistent with diffuse flow originating from mixing of high temperature (>300°C) or intermediate temperature (∼140–170°C) crustal fluids with ambient seawater. This suggests little low temperature interaction with the rock is occurring and that the fluids have very short (days to months) residence times within the oceanic crust.