Wendy R. Nelson

The role of continental lithosphere metasomes in the production of HIMU-like magmatism on the northeast African and Arabian plates

Intraplate alkaline lavas typically exhibit isotopic characteristics that require a source with long-term isolation from the convecting asthenosphere, such as in the sub-continental lithosphere mantle or a mantle boundary layer. Melting of metasomatically enriched domains, or metasomes, within the lithospheric mantle provides a viable mechanism for generating the geochemical characteristics of intraplate alkaline basalts. The origins and distribution of these metasomes have been attributed to recent enrichment of the lithosphere by a mantle plume or ancient events that occurred during the early evolution of the sub-continental lithosphere mantle. Here, we present a geochemical study of Ethiopian Miocene intraplate alkaline lavas: melts of a lithospheric mantle that was enriched metasomatically during lithospheric stabilization and by recent plume-lithosphere interaction. We find that these lavas have geochemical characteristics consistent with melting of an amphibole-bearing lithospheric-mantle metasome. New Pb and Hf isotope data for these lavas require a HIMU-like source component, similar to other alkaline lavas erupted through the Horn of Africa, Sudan, and Egypt, and adjacent Arabian plate lithospheres. The isotopic characteristics of this component are distinct from the Afar plume mantle source and instead are consistent with the long-term evolution of a lithospheric metasome created during a Neoproterozoic subduction event associated with the Pan-African orogeny. The widespread distribution of easily fusible lithospheric metasomes within the continental lithosphere mantle may facilitate magma generation without the need for substantial lithospheric thinning or elevated mantle potential temperatures. Mantle heterogeneity of this nature has implications for the source origin of HIMU magmas associated with continental lithosphere.

Subduction initiation and ophiolite crust: new insights from IODP drilling

ABSTRACT International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.