Spreading Rate-Dependent Variations in Crystallization Along the Global Mid-Ocean Ridge System

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 CO₂ content of 50–85 ppm for the depleted upper mantle feeding the global mid-ocean ridge system.