Volatiles in mid-ocean ridge environments

Abstract

This paper focuses on the evolution of volatiles and hydrothermal fluids in high-temperature plutonic and shallow-crustal rocks from mid-ocean ridge environments. The thermal and compositional evolution of fluids preserved in samples from the ocean basins and from ophiolite suites reflects complicated histories involving multiple magma and hydrothermal fluid pulses during cooling and transport of the crust away from the zone of crustal accretion. Analyses of primary fluid inclusions in plutonic samples show that with progressive melt fractionation, magmatic fluids evolve from CO 2 -rich vapors, to immiscible (CO 2 + H 2 O)-rich vapors, and finally to metal-rich, CO 2 + H 2 O + NaCl brines that are exsolved at temperatures in excess of 700 °C. In more water-rich systems, such as backarc environments, the last fluids to be exsolved from the melts may involve direct exsolution of 30-60 wt% NaCl brines. Paths of magmatic fluid evolution and compositions may vary significantly in different spreading environments. Magmatic fluids in gabbroic samples from the Southwest Indian Ridge show a continuum between CO 2 -rich fluids and (CO 2 + CH 4 + H 2 O)-rich fluids; the latter contain 30-50 mol% CO 2 and 43 mol% CH 4 . Phase equilibria and isotopic data show that the (CO 2 + CH 4 + H 2 O)-rich fluids reflect Rayleigh distillation of evolved magmatic CO 2 , subsequent closed-system respeciation, and attendant graphite precipitation at temperatures of ∼500-800 °C. In all spreading systems studied to date, the transition from magmatic to hydrothermal seawater-dominated conditions in plutonic rocks is marked by circulation of fluids at temperatures >400 °C, with salinities from 400 °C conduits for systems venting on the seafloor. Plutonic samples from the Mid-Atlantic Ridge and the Southwest Indian Ridge indicate that 400 °C circulation systems may also involve (CH 4 + H 2 O ′ H 2 )-rich fluids formed during high-temperature seawater and rock interactions within the plutonic sequences. In zones of low magmatic supply, fluid migration along deeply penetrating fault systems may intensify hydration of deep-seated mafic and/or mantle material and lead to significant CH 4 (+ H 2 + hydrocarbon) generation. The processes by which these volatiles are transported to the overlying ocean are complex. Some of these volatiles may be released during diking-eruptive events in the course of which there are catastrophic and voluminous releases of buoyant hydrothermal plumes enriched in H 2 , 3 He, and CH 4 above ocean-water background levels. The degassing of volatiles during the eruption and their release during high-temperature interaction of seawater and dike extrusive material may provide energy for subseafloor biotopes and subsequent microbial blooms.