Relationship between D-MORB and E-MORB magmatism during crustal accretion at mid-ocean ridges: evidence from the Masirah ophiolite (Oman)

Enriched mid-ocean ridge basalts (E-MORB) commonly erupt at mid-ocean ridges (MOR) and seamounts, but their relationship to ‘depleted’ MORB (D-MORB) and the processes controlling their magmatic evolution at MORs are not fully understood, hence raising more general questions about magma generation in the mantle. We here explore this conundrum through an investigation into the Masirah ophiolite (southeast Oman), a near-unique ‘true’ MOR ophiolite. Unlike most (e.g., Tethyan) ophiolites, it was not affected by subduction and is therefore potentially able to provide valuable geological insights into the magmatic evolution of a full section of oceanic crust. Previous work has shown that the igneous crust at Masirah was thin (1.5-2.0 km) and constructed from both D- and E-MORB magmas, concluding that it formed at a slow-spreading ridge at ~150 Ma followed by an episode of ‘Nb-enriched’ magmatism with trace-element enrichments exceeding E-MORB during intraplate rifting ~20 Ma later. We reinvestigate the geology of Masirah and present new field observations, geochemical data and high-precision U-Pb ages to constrain the magmatic history of seafloor spreading and off-axis magmatism. We find that D-MORB and E-MORB magmatism at Masirah was synchronous and overlapped in both composition and time with the Nb-enriched magmatism (no older than 135 Ma). Both types of magmatism were therefore integral in the formation of the Masirah ocean crust. The relationship between D-MORB and E-MORB magmatism described here may be applicable to modern MORs more broadly, but is especially prominent at Masirah due to reduced magmatism and hence a weaker crustal filter.