Spreading rate‐dependent variations in crystallization along the global mid‐ocean ridge system

Abstract We investigate crustal accretion at mid‐ocean ridges by combining crystallization pressures calculated from major element contents in mid‐ocean ridge basalt (MORB) glasses and vapor‐saturation pressures from melt inclusions and MORB glasses. Specifically, we use established major element barometers and pressures estimated from 192 fractional crystallization trends to calculate crystallization pressures from >9000 MORB glasses across the global range of mid‐ocean ridge spreading rates. Additionally, we estimate vapor‐saturation pressures from >400 MORB glasses from PETDB and >400 olivine‐hosted melt inclusions compiled from five ridges with variable spreading rates. Both major element and vapor‐saturation pressures increase and become more variable with decreasing spreading rate. Vapor saturation pressures indicate that crystallization occurs in the lower crust and upper mantle at all ridges, even when a melt lens is present. We suggest that the broad peaks in major element crystallization pressures at all spreading rates reflects significant crystallization of on and off‐axis magmas along the base of a sloping lithosphere. Combining our observations with ridge thermal models we show that crystallization occurs over a range of pressures at all ridges, but it is enhanced at thermal/rheologic boundaries, such as the melt lens and the base of the lithosphere. Finally, we suggest that the remarkable similarity in the maximum vapor‐saturation pressures (∼3 kbars) recorded in melt inclusions from a wide range of spreading rates reflects a relatively uniform CO 2 content of 50–85 ppm for the depleted upper mantle feeding the global mid‐ocean ridge system.