The Detroit volcanic province and the early evolution of the Hawaiian mantle plume

Detroit Seamount, located near the northern end of the Emperor Seamounts within the Hawaiian-Emperor Chain (HEC), is thought to have formed ∼81–76 Ma when an active spreading center was located near the Hawaiian mantle plume. The origin of the isotopically depleted signatures of basaltic lavas from Detroit is controversial, requiring either entrained ambient upper mantle due to plume-ridge interaction or enhanced melting of an intrinsic depleted component under young, thin lithosphere. Here, we present new major and trace element abundances for drilled tholeiitic basalts from Ocean Drilling Program Sites 884, 1203, and 1204 to re-evaluate the origin of Detroit and its mantle source. Our results show that previously unstudied Site 884 lavas have the most depleted trace element signatures known from the HEC and represent melting of the ambient upper mantle. Seismic reflection data shows that Site 884 penetrated a volcanic platform to the east of Detroit. This platform and adjacent, likely coeval, seamounts further to the east are unlike modern Hawaiian volcanoes and, together with Detroit, form a “Detroit Volcanic Province” that is potentially analogous to the Galápagos Archipelago. Actual basalts from Detroit (shield lavas from Sites 883, 1203, and 1204) are derived from an intrinsic depleted source within the Hawaiian plume that is (1) chemically and isotopically similar to rare high-CaO tholeiitic shield basalts from Maunakea and (2) characterized by high Zr/Sm and Hf/Gd ratios similar to all Hawaiian shield lavas (and unlike those from Site 884). Quantitative modeling indicates that the mantle sources of Detroit and all Hawaiian shields contain ancient oceanic lithosphere, including both recycled oceanic crust (ROC) and the underlying recycled mantle lithosphere (RML). Detroit lavas were formed by the same degree of partial melting as Hawaiian shield lavas, which is inconsistent with the requirement of enhanced melting to preferentially sample an intrinsic depleted plume component. Instead, the Detroit source contained a smaller amount of strongly dehydrated ROC and a larger amount of RML related to the preferential upwelling of these materials during the earliest known portion of the HEC. Today, a similar combination of RML and strongly dehydrated ROC is observed only during pre-shield and postshield volcanism due to a concentric zonation of the modern Hawaiian plume.