The Tasmantid Seamounts: shallow melting and contamination of an EM1 mantle plume

Primitive basalts (Mg# ∼ 61–70;Ni= 232–322 ppm) dredged from the Tasmantid Seamounts range between incompatible element-rich alkali-olivine basalt and incompatible element-poor tholeiite compositions. They are primary and near-primary magma candidates, consistent with a decrease in depth of magma segregation from ∼ 2.5 GPa (for alkali-olivine basalts) to ∼ 1 GPa (for tholeiites). Variation in geochemical and isotopic data in these basalts indicates mixing involving two distinct mantle sources: (1) an EM1 mantle source dominating the tholeiitic basalts (Ba/Nd 15, 87/Sr 86/Sr 0.7050,ε_Nd < −3.5) and (2) a “long-term depleted” upper mantle source dominating the alkali-olivine basalts (Ba/Nd < 8,⁸⁷Sr/⁸⁶Sr< 0.7035,ε_Nd > +3). This mixing can be explained by the interaction of a deeply derived EM1 mantle plume (or diapirs) with surrounding upper mantle (asthenosphere or oceanic lithosphere). The hot plume core penetrates to shallow levels to generate tholeiitic magmas, whereas the plume margins interact with surrounding upper mantle, resulting in cooler temperatures, smaller degrees of melting, and the formation of alkaline magmas.

The fractionated REE patterns (i.e. Dy/Yb chondrites) of the tholeiitic and transitional basalts imply melting in the presence of residual garnet but are inconsistent with the low-pressure segregation of these magmas. This may be resolved if small melt fractions, generated in the presence of garnet, have been added to the tholeiitic magma source volume. This requires a melt segregation process where small melt fractions migrate from relatively deep levels within the plume and its margins to enrich the plume core at shallow levels. Alternatively, the plume could incorporate incompatible-element enriched oceanic lithosphere, formed through addition of small melt fractions derived from underlying garnet-bearing asthenosphere prior to Tasmantid magmatism.