Abstract
Our knowledge of melt distribution in the lower crust and upper mantle at oceanic fast spreading centers is very limited. Evidence of melt accumulation, sometimes away from the axis, has been imaged and interpreted at the Moho of the East Pacific Rise; but the detailed structures of these deep magma lenses remains much more difficult to unveil than that of the shallow axial melt lens at the top of the plutonic crust. Ophiolites offer on-land sections of oceanic lithosphere that can complement marine geological and geophysical observations. We present results of a geological survey of the Moho transition zone at a paleospreading center in the Oman ophiolite. We find that the thickness of this dunite-rich horizontal layer increases from a few meters at the axis to hundreds of meters 6 km away from the axis, and is reduced to a few meters 2-3 km further away. The base of the Moho transition zone contains dunite and very depleted harzburgite with isotropic plagioclase and clinopyroxene impregnations, and stockwork-like magmatic breccia, indicative of episodic high melt fractions. We conclude that the melt-free dunite horizontal layer may stop the progression of ascending melt; this lead to melt accumulation within the uppermost harzburgite beneath the Moho transition zone and forms the isotropic impregnations. As the melt dissolves the harzburgite orhtopyroxene, and is flushed to the top of the MTZ though the breccia, it leaves new dunite at the base of the Moho transition zone. Repetition of this process renders the Moho transition zone thicker as it moves away from the ridge axis, until it leaves the main area of mantle melt delivery. Then, tectonic thinning and intrusion of parts of the MTZ into the lower crust reduce the MTZ thickness. These processes seem coherent with several marine geophysical observations.