Exposed ocean crust on Masirah Island, SE Oman: crustal accretion and melt evolution at a slow-spreading (Jurassic) mid-ocean ridge

Conventional layer-cake models for ocean crustal structure cannot explain recent observations from slower spreading-rate mid-ocean ridges (MOR). We now recognise that detachment faults assist plate spreading and the igneous crust may be heterogeneous, thin or even absent. Mantle lithosphere directly beneath the axis potentially exerts a strong influence on the mechanisms of generation, transport, storage and modification of basaltic melt en-route to the seafloor. Deconvolving the relative importance of these different processes and assessing their effects on erupted MORB, the most abundant magma type on Earth, is a first-order problem, yet is almost intractable because of the difficulty of accessing full crustal sections in the modern oceans. Ophiolites, such as Semail (N Oman), are of questionable benefit, having formed by spreading above subduction zones and therefore in an environment where mantle melting and melt evolution is affected by hydrous fluids.

Masirah Island, off the SE coast of Oman (Fig. 1), is near-unique in being an exhumed fragment of oceanic lithosphere from a genuine open-ocean MOR, hence offering a ground-breaking opportunity to bring significant new insights into the structure of slow-spreading lithosphere and the integrated processes of MORB melt production and modification, from mantle to seafloor. Unrelated to the much better-known Semail ophiolite, Masirah formed at a (slow-spreading) MOR in the proto-Indian Ocean during rifting of Gondwanaland in the Jurassic and was emplaced onto the SE Arabian margin in the Cretaceous-Palaeocene. Background investigations in the 1970s-90s showed the island to be formed of two thrust slices, each containing a complete oceanic crustal sequence characterised by a remarkably thin (~0.5km) plutonic crust and heterogeneous normal- to enriched-MORB basalt compositions (Peters & Mercolli, 1998). No modern or in-depth study focusing on mantle melting and lithospheric accretion processes has however been made.

In this project we seek specifically to understand:
(a) the magma plumbing system and resulting oceanic crustal structure of Masirah; (b) where isotopically-diverse mantle melts homogenise en-route to the seafloor; (c) the extent to which ascending melts interact with mantle lithosphere; and (d) how much reactive porous flow, rather than fractional crystallisation, in gabbro modifies MORB compositions (Lissenberg et al., 2013) - which the student will address via extensive field work followed by an integrated major-, trace-element and isotope study of minerals and bulk-rock samples from the Masirah mantle, plutonic section and sheeted dykes/lavas.