Accretion of fast-spreading oceanic crust: benefits from large-scale sampling in the Oman ophiolite in combination with cores drilled by the ICDP Oman Drilling Project

Based on a newly established profile through fast-spreading oceanic crust of the Oman ophiolite and on cores drilled within the ICDP Oman Drilling Project (OmanDP), we present here the results of 12 years research, focusing on the nature of the magmatic accretion of the deep crust beneath fast-spreading mid-ocean ridges. We established a 5 km long profile through the whole plutonic crust of the Oman ophiolite by systematic outcrop sampling in the Wadi Gideah (Wadi Tayin Block near Ibra), providing the reference frame for the 300 to 400 m long OmanDP drill cores GT1 and GT2 (lower crust, mid-crust), as well as CM1 and CM2 (crust-mantle boundary) drilled into the same area.
The results allow implication on the mechanism of accretion of fast-spreading lower oceanic crust. Depth profiles on bulk rock and mineral compositions, crystallization temperature and microstructures combined with petrological modeling reveal insights into the mode of magmatic formation of fast-spreading lower oceanic crust, implying a hybrid accretion mechanism. The lower 2/3 of the crust (mainly layered gabbros) was formed via the injection of melt sills and in situ crystallization. Here, upward moving fractionated melts mixed with more primitive melts through melt replenishments, resulting in an upward differentiation trend. Since the fraction of crystallization is only small, upmoving melts could easily transport the latent heat produced by deep crystallization upward. The upper third of the gabbroic crust is significantly more differentiated, in accord with a model of downward differentiation of a parental melt originated from the axial melt lens sandwiched between the gabbroic crust and the sheeted dike complex. While the 5 km long profile shed light on the overall magmatic accretion process, the Oman DP drill cores showing ~ 100% recovery allowing high density sampling provide incredible details on the magmatic accretion process. Examples are the identification of individual melt sills from which the layered gabbro section has been formed (drill core GT1) or the detailed observation of olivine accumulation at the base of the crust (drill cores CM1/CM2).