High-temperature, off-axis, hydrothermal circulation?

We have recently undertaken an investigation of focussed fluid flow in the plutonic section of the Oman ophiolite [1]. Zones of focussed fluid flow are ~10-50 m wide and have been extensively altered by high- temperature (approximately 300-700°C) fluid flow. Strontium isotope analyses show that the fluid was in Sr- isotopic equilibrium with the fluid in the sheeted dike complex across this temperature range requiring large fluid fluxes. Calculated cooling rates in the wall rocks around these zones, based on the down-temperature diffusion of calcium from olivine to clinopyroxene [2], indicates enhanced cooling rates adjacent to the focussed fluid flow zones. We interpret these regions as paleo-channels for off-axis (approximately 5-20 km) focussed hydrothermal fluid flow. Thermal models are permissible of greater fluid and heat fluxes through a lower crustal hydrothermal system than through the axial upper crustal system, although smaller fluxes are likely based on other arguments. If the channels that carry this fluid flow are also focussed in the upper crust (lavas and dikes) this could lead to significant high-temperature off-axis venting at fast-spreading mid-ocean ridges. Two important implications of off-axis high-temperature venting would be: (a) estimates of the chemical fluxes into the oceans would have to be revised because the chemistry of these fluids would differ from that at known black smokers. This is because fluids flowing through channels become chemically rock-buffered at smaller fluid fluxes than those flowing pervasively through a rock mass, and (b) vent fauna dispersal could occur over a wider region than the narrow axial high. [1] Coogan LA, Howard KA, Gillis KM, Bickle MJ, Chapman HJ, Boyce AJ, Jenkin GRT, Wilson RN (2006) Chemical and thermal constraints on focussed fluid flow in the lower oceanic crust. Am. J. Sci. [2] Coogan L. A., Jenkin G. R. T., and Wilson R. N. (2002) Constraining the cooling rate of the lower oceanic crust: a new approach applied to the Oman ophiolite. Earth Planet. Sci. Lett. 199, 127-146.