SR-ND-PB GEOCHEMICAL MORPHOLOGY BETWEEN 10-DEGREES-N AND 17-DEGREES-N ON THE MID-ATLANTIC RIDGE - A NEW MORB ISOTOPE SIGNATURE

Basalts dredged along the Mid-Atlantic Ridge axis between 10-degrees-N and 17-degrees-N have been studied for their trace element characteristics [1]. To give complementary information on mantle source history and magma genesis, these samples have been analysed for their Sr-Nd-Pb isotopic compositions. There is a good correlation between the structure of the ridge axis which shows a topographic anomaly centered around 14-degrees-N and hygromagmaphile element ratios such as Rb/Sr, (Nb/Zr)N or Sm/Nd as well as isotopic ratios plotted as a function of latitude. The samples coming from the 14-degrees-N topographic high show new MORB Sr-Nd isotopic characteristics which pictured in a classical mantle array diagram, put their representative points close to HIMU sources of ocean islands such as St. Helena, Tubuai and Mangaia. The 14-degrees-N mantle source presents geochemical characteristics which indicate mantle differentiation processes and a mantle history that are more distinct than so far envisaged from typical MORB data. Pb data indicates that the 14-degrees-N mantle source cannot be the result of binary mixing between a depleted mantle and a HIMU-type source. Rather, the enriched endmember could itself be a mixture of Walvis-like and HIMU-like materials. The geochemical observations presented favour the model of an incipient ridge-centered plume, in agreement with [1].