Crystal fractionation in a CUmulate Gabbro, Mid‐Atlantic Ridge, 26°N

Abstract

Gabbros dredged from the Mid‐Atlantic Ridge near 26°N have distinct cumulate textures and display phase, rhythmic, and cryptic layering. The gabbros have a tholeiitic iron enrichment trend and a concomitant increase in the incompatible element abundances. The gabbroic phases have large variations in both the Fo content of olivine (64–82) and the An content of the plagioclase (50–80) and the smaller but significant variations in En content of clinopyroxene and orthopyroxene (42–46 and 68–74, respectively). Increasing Fo contents of olivine correlate with increasing iron enrichment and increasing modal abundance of cumulate pyroxene. These chemical variations and observed textures are consistent with a model involving crystal fractionation. Fractionation models linking crystallization of phases in the gabbros to chemical variations in nearby basalts are tested. A model involving clinopyroxene crystallization can, mathematically, produce the chemical variations found in any one dredge site, but none of the basalts contain clinopyroxene, and hence the model is rejected. The observed gabbroic fractionation cannot produce the magmatic source for more evolved basalts; therefore it is likely that basaltic magmas are not greatly influenced by shallow magma chamber processes either by crystal fractionation or by magma mixing with evolved gabbroic magmas during transport to the surface. The chemical variations observed between dredge sites cannot be successfully modeled through fractionation involving any combination of the phases observed in the basalts or the gabbros.