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 of 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 similar to 150 Ma followed by an episode of "Nb-enriched" magmatism with trace-element enrichments exceeding E-MORB during intraplate rifting similar to 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 found 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. The oceanic crust forms approximately two-thirds of the Earth's surface and is continuously being formed at mid-ocean ridges. Due to the difficulties involved in accessing oceanic crust directly, especially the deeper stratigraphic levels, the magmatic processes involved in forming the oceanic crust remain poorly understood. Ophiolites, fragments of oceanic lithosphere that were emplaced onto the continent, can offer valuable insights to geologists, but since most ophiolites are thought to have originated in marginal oceanic basins or at the initiation of subduction, a direct comparison remains problematic. In this study, we circumvent this problem by investigating the Masirah Ophiolite, a rare example of a "true" mid-ocean ridge ophiolite. By combining field observations, geochemistry and geochronology at a level of detail not possible for present-day oceanic crust, we reconstruct the magmatic evolution of the Masirah paleoridge. We present a more accurate formation age for Masirah and find that magmatism was compositionally variable throughout crustal accretion, with later off-axis melts becoming more enriched in incompatible trace elements. We propose that the large geochemical variations observed at Masirah may be commonplace at modern mid-ocean ridges, but that these are often masked due to the typically higher volumes of magmatism that enhance mixing and homogenization. Two ophiolite nappes exposed on Masirah were formed by ocean crustal accretion at 135 and 131 Ma respectively On-axis magmatism shows varying degrees of trace element enrichment and depletion, and is followed by Nb-enriched "near-axis" magmatism Similar processes may be common along modern mid-ocean ridges but masked by the homogenizing effect of an enhanced "crustal filter"