Phosphorus zonation in olivine reveals the long-lasting history of basalt crystal cargo

Phosphorus and aluminum are preferentially incorporated in olivine during rapid crystal growth (skeletal morphologies). Their zonation therefore records the early growth event, while the morphological evidence gets erased during subsequent crystal maturation. Here we focus on plutonic and volcanic olivine grains from slow-spreading ocean ridges, using P & Al chemical maps obtained via EPMA. Our study reveals the existence of a plutonic signal based on the decoupling of these elements due to their difference in diffusion rates (D_P < D_Al) within the olivine crystal. As the zonation in aluminum fades over time, the zonation in phosphorus persists. The coexistence, or lack thereof, of the skeletal enrichments of aluminum and phosphorus in olivine serves as an indicator of the environment in which the crystal resided. Using this criterion, we show that most of the olivine grains from the lavas studied here are inherited from the disruption of a long-lasting mushy domain as they record a chemical zonation that is similar to that of plutonic crystals. Lava thus represents a minestrone-like igneous assemblage of various melts and crystals inherited from several molten domains within the crustal plumbing system. The use of bulk-rock compositions to discuss mantle processes thus appears delusional. The crystal cargo derived from the disaggregation of mush zones provides a unique access to processes occurring in the deep active mushy reservoirs, which can usually only be reached by studying plutonic rocks (gabbros) after mush solidification.