Is Detroit Seamount a "Hawaiian" Volcano?

Detroit Seamount, near the northern terminus of the Emperor seamounts may be an early manifestation of the Hawaiian hotspot. Surprisingly, lavas recovered from Detroit seamount by ODP Leg 145 have incompatible element abundances and isotopic ratios (Sr and Nd) more similar to MORB than shield lavas forming Hawaiian islands. The age of oceanic lithosphere at the time of Emperor seamount formation decreases northwards; hence the depleted nature of these ~81 Ma Detroit seamount lavas has been proposed to reflect: (a) an extreme case of plume-spreading ridge interaction whereby seamount lavas are dominated by components derived from MORB-related asthenoshere and lithosphere (Keller et al., 2000) or (b) enhanced melting of refractory parts of the Hawaiian plume as a result of plume ascent to lower pressures beneath thin lithosphere (Regelous et al., in press). Other possible explanations are that Detroit seamount is unrelated to the Hawaiian hotspot or that the geochemical characteristics of the Hawaiian plume have varied. Drilling at Site 1203 Leg 197 penetrated 457 m of 18 basalt flow units and 12 volcaniclastic interbeds. The upper flows are tholeiitic basalt, but lower flows are vesicular and thick flows of alkalic basalt. Tholeiitic basalt overlying alkalic basalt is unlike the Hawaiian sequence where alkalic postshield-stage lavas erupt after tholeiitic shield-stage lavas, but it is characteristic of the initial submarine volcanism forming Hawaiian shields. We have not completed acquisition of Sr, Nd and Pb isotopic data, but our compositional data show that neither tholeiitic nor alkalic lavas at Site 1203 are similar to Hawaiian shield and postshield lavas. Although not as depleted in incompatible elements as MORB-like lavas from Leg 145 Site 884 on the eastern flank, Site 1203 tholeiitic lavas have incompatible element abundance intermediate between MORB and Hawaiian tholeiites. The alkalic lavas at Site 1203 have especially surprising geochemical characteristics. At given MgO content the abundance of relatively immobile incompatible elements are similar to those of Mauna Kea postshield alkalic basalt, but abundance of Sr and Ba are buffered at 220-260 ppm and 50-70 ppm, respectively. Some alkalic units also have anomalously low Ti/Zr (<80). Relatively compatible behavior of Sr, Ba and Ti is a characteristic of melts in equilibrium with phlogopite. Although phlogopite may be a residual phase during generation of highly alkalic rejuvenated-stage lavas, it is not a residual phase for late-shield or post-shield lavas. However, residual phlogopite has been inferred for submarine alkalic lavas erupted during the initial growth of Kilauea Volcano (Sisson et al., 2002). The broad northern summit region of Detroit Seamount has been sampled by 4 closely spaced holes (883E and F, 1204A and B). Relative to tholeiitic basalt from the flanks (Sites 884 and 1203), summit lavas have lower SiO₂ and higher abundance of incompatible elements; higher pressure of melt segregation and lower extent of melting are inferred. In summary, Detroit Seamount lavas are less enriched in highly incompatible elements than lavas forming Hawaiian shields. For (La/Yb)_PM the island extremes are ~2.8 for Mauna Loa and 4.6 for Kilauea whereas Detroit Seamount lavas range from a MORB-like <1 at Site 884 to 1.2-2.1 in summit lavas (Sites 883 and 1204) to 2.2-2.5 in alkalic basalt (Site 1203). Although it is premature to endorse any of the proposed hypotheses for explaining these differences, an important result is that the evidence for residual phlogopite during segregation of alkalic lavas at Site 1203 is consistent with shallow melt segregation, and inconsistent with a relatively large extent of plume melting promoted by a longer melting column.