Looking for the "missing endmember" in South Atlantic Ocean mantle around Ascension Island

Published bathymetric, geophysical and geochemical data suggest the presenc e of a hotspot in the South Atlantic, between the Bode Verde and Ascension fracture zones, in the vicinity of Ascension Island which has been named As cension hotspot. We provide here new isotope and trace element data on Grat tan (a near-ridge seamount) basalts which suggest that this edifice was pro duced by the plume presently feeding the adjacent Mid-Atlantic Ridge (MAR) segment and that it could represent the geochemical "missing end-member" de scribed by Hanan et al. (1986) in the South Atlantic Ocean. Its isotopic si gnature differs slightly from that of Ascension Island and is more radiogen ic in Pb-208. Trace element data are intermediate between those of MORB and characteristic GIB. We suggest that near Grattan seamount there exists a w eak, perhaps intermittent, and slightly chemically heterogeneous plume.