Formation of dunites within the Moho transition of the Samail Ophiolite: A study using drill cores obtained by of the Oman Drilling Project

Here we investigate the genesis of the Moho transition zone (MTZ), sandwiched between mantle and crustal rocks of the Samail ophiolite, through a petrological and geochemical study of cores taken by the Oman Drilling Project (Oman DP). The MTZ drilled, comprises a 150 m-thick dunite horizon that is practically pure dunite in the upper 90 m but cut by several gabbroic intrusions in the lower part. The dunite is nearly completely altered to serpentinite, with only Cr-spinel as the relict primary phase. We interpret the entire MTZ as having formed through reactive interactions between MORB melts and shallow mantle peridotite. The key process is the dissolution of orthopyroxene and concomitant precipitation of olivine during porous flow of ascending MORB melts.

In the lower part of the MTZ, evolution trends in Cr-spinel chemistry are absent, precluding crystal fractionation processes. Instead, the presence of relict clino- and orthopyroxene in the serpentinized dunites, exhibiting characteristic mantle deformation, associated with elevated “spoon”-shaped bulk REE patterns, are interpreted as signatures of replacive dunites formed by MORB/harzburgite interaction. In contrast, relict spinels of the upper pure dunite zone at the top of the MTZ show systematic compositional variations with depth, interpreted to have resulted from differentiation of a primitive MORB melt. Although spinel compositions are well correlated with incompatible element concentrations, including Ti and V, there is very poor or non-existent correlation of compatible elements, such as Ni and Cr. These are diagnostic signatures of “reactive fractionation” according to Collier and Kelemen (2010), where compatible element concentrations are buffered by diffusive interaction with the surrounding harzburgite, whereas the decreasing magma mass leads to the accumulation of incompatible elements in the remaining melt.

This upper dunite zone could correspond to the “missing cumulates”, necessary to model the evolution of the Oman bulk crust, which shows a too-evolved composition, assuming it was formed from a typical primitive parental MORB.