Gabbro glacier or sheeted sills? How the lower crust of fast-spreading oceanic ridges is accreted

Based on a newly established profile through the 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 10 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 crust of the Oman ophiolite by systematic outcrop sampling in the Wadi Gideah (Wadi Tayin Block near Ibra), providing the reference frame for the two 400 m long crustal OmanDP drill cores GT1 and GT2, drilled into the same Wadi.

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) 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 located at the top of the gabbroic crust.