Melt migration under oceanic ridges: Inferences from reactive transport modelling of upper mantle hosted dunites

Dunite bodies within the ophiolitic upper mantle are widely considered to locate pathways for melt transport in the sub-oceanic mantle. As such, the dunites may carry information about the primitive melts Seeding a spreading centre. Analyses by ion probe of clinopyroxene grains from dunites in the mantle section from the Blow Me Down (BMD) and zipper Table Mountain (TM) massifs of the Bay of Islands Ophiolite reveal that BMD dunites fire highly refractory in their trace element composition. Upper TM dunites are much less depleted and probably reflect aggregated melts of variable compositions. A numerical reactive transport scheme was developed to simulate replacive dunite formation from harzburgite along a melt channel, triggered by a silica-undersaturated melt. The basic trace element effects associated with replacive dunite formation are demonstrated Sor combinations of diffusive and advective transport and varying reaction rates. They can explain relatively low NiO concentrations observed in mast dunitic olivines. To model the incompatible trace element chemistry of the BMD dunites, extraction of refractory melt from the depleted host harzburgite must have dominated. Otherwise, during melt infiltration, the reactive melt composition, assumed to be more enriched dominates the dunite chemistry even in the absence of local equilibrium. A preferred model for dunite formation favours short time periods for a channel formation by fracture and flow of melt in dykes. This event produces an initial, centimetre-scale replacive dunite. it may be followed by a main growth stage for dunite formation as a result of reaction of host rock with pockets of trapped melt and porous flow-dominated melt migration within the initial replacive dunite. The time scales involved in the formation of metre-wide dunites are at least several hundred, if not thousands of years. In BMD, a final event associated with compaction of the host harzburgites, related to ceasing flow in the porous channel, caused refractory melt migration into the dunite by compaction of the host.