Mid-ocean ridge magmas are produced by decompression melting of upwelling mantle beneath the ridge axis. These melts subsequently undergo variable degrees of fractional crystallization to form the oceanic crust, however, where within the crust and/or upper mantle crystallization occurs is a matter of debate. In this study, we provide direct geochemical evidence for the depths of crystallization beneath ridge axes using vapor-saturation pressures derived from volatile concentrations in olivine-hosted melt inclusions. These depths are used in conjunction with major and trace element concentrations of melt inclusions to compare crustal accretion processes on the fast-spreading East Pacific Rise and intermediate-spreading Juan de Fuca Ridge.
We found that while the majority of crystallization at both ridges occurs within the shallow seismically-imaged melt lens, over 25% of the melt inclusions have crystallization pressures consistent with formation in the lower oceanic crust. Further, major and trace element concentrations indicate that melts entering the base of the crust are homogenized, suggesting efficient mixing in the mantle prior to crystallization.
Wanless, V.D., Shaw, A.M., 2012. Lower crustal crystallization and melt evolution at mid-ocean ridges. Nature Geoscience 5, 651–655. doi:10.1038/ngeo1552