A geochemical transect across a heterogeneous mantle upwelling: Implications for the evolution of the Madeira hotspot in space and time

Young (≤3Ma) lavas from volcanic centers along a 75-km N–S transect from northern Madeira Island to a submarine volcanic field 55km south of Madeira exhibit distinct spatial geochemical variations. From south to north along this transect, there is a general decrease in Fe(8) (total Fe as FeO normalized to MgO=8wt.%), K(8), Pb isotope ratios and Zr/Hf and an increase in Al(8) and Nd isotope ratios, reflecting an increasing contribution of depleted source components to the north. The south to north spatial variations mimic the ca. 6million year temporal geochemical evolution on Madeira with the isotopic composition of the volcanic rocks becoming progressively more depleted through time. The temporal and spatial variation in chemical composition of lavas is consistent with melt extraction from a heterogeneous blob of upwelling mantle consisting of enriched garnet pyroxenite/eclogite material (recycled hydrothermally altered oceanic crust) in a depleted ultramafic/peridotitic matrix (recycled lower oceanic crust and/or lithospheric mantle). The geochemical transect provides new insights into the spatial heterogeneity of the Madeira plume pulse and confirms models for the temporal evolution of ocean island volcanoes.