Retrieving timescales of oceanic crustal evolution at Oceanic Core Complexes: Insights from diffusion modelling of geochemical profiles in olivine

The building of oceanic crust at Oceanic Core Complexes (OCC) has been described as a complex process involving multiple intrusions of magma over a protracted period of time. The migration of primitive magmas (i.e., Mid Ocean Ridge Basalts, MORBs) can lead to melt-rock interactions during reactive porous flow processes through the lithosphere. The timescales of these reactive processes and the subsequent cooling of the modified crystal matrix remain unconstrained. Diffusion modelling has been widely used to retrieve timescales of magmatic processes. In this study, we use diffusion models to constrain (i) the minimum timescales of melt-rock interactions and (ii) the cooling rates of the gabbroic sequence forming the oceanic crust at an OCC. We chose samples of the most primitive olivine-rich troctolites from the gabbroic sequence sampled in IODP Hole U1309D (Atlantis Massif OCC, Mid-Atlantic Ridge 30°N). Olivine-rich troctolites were interpreted as marking local partial assimilation of mantle intervals into the oceanic crust, and thus allowed to understand the dynamics of mantle assimilation and the formation of slow-spreading oceanic crust at the Atlantis Massif OCC. Olivines in olivine-rich troctolites represent relicts of pre-existing mantle olivine, while clinopyroxenes and plagioclases are crystallized during reactive percolation. Olivine chemical compositions show that olivine-rich troctolites inherit chemical heterogeneity from the mantle precursor. Flat geochemical profiles in olivine indicate complete chemical re-equilibration of olivine crystals with the locally modified percolating melt. Exception is made for most Ca profiles that show lower Ca contents at the olivine rim compared to the relative crystal core, as the result of subsolidus cooling. Three-dimensional (3D) diffusion models at magmatic conditions (T = 1210–1300 °C and P = 2 kbar) reveal that complete chemical re-equilibration of 4 mm-size mantle-derived olivine with percolating MORB-type melts can be attained within durations of less than 300 yr. The Ca-in-olivine geospeedometer reveal that cooling rates from ~1200 °C to ~1050 °C are constant downhole and on average 0.004 °C/yr; they are comparable with lower temperature cooling rates (850 °C–250 °C) estimated at the Atlantis Massif OCC. The minimum timescales from 3D models point to rather fast re-equilibration of olivine. The downhole chemical heterogeneity inherited from the precursor mantle, coupled with the timescales of diffusive re-equilibration suggest that the partial assimilation of the upwelling mantle and its incorporation into the oceanic crust occurred in the time-frame of a single melt input. Our cooling data are consistent with building of the oceanic crust at OCCs controlled by continuous uplift, in turn governed by long-lived detachment faults. The latter contribute to the rapid cooling of the assimilated mantle intervals and the magma bodies. Diffusion models of geochemical profiles in olivine from a single crustal section allow to reconstitute the early magmatic processes leading to mantle assimilation and early crystallization of gabbros, and the cooling history of the oceanic crust at OCC from magmatic conditions to hydrothermalism.