Isotopic geochemistry of Fernando de Noronha

Abstract

Volcanic and hypabyssal rocks ranging in age from 12 to 3 Ma from the Fernando de Noronha archipelago in the western equatorial Atlantic Ocean can generally be divided into two age-compositional groups that have variable and distinct isotopic compositions. Predominantly older alkali basalts and trachytes are generally characterized by more radiogenic Sr-isotopic ( 87Sr/ 86Sr= 0.70457–0.70485 ) compositions and less radiogenic Nd-isotopic ( 143/Nd 144Nd= 0.51271–0.51281 ) and Pb-isotopic ( 206Pb/ 204Pb= 19.132–19.282 ) compositions relative to the generally younger, more alkaline Si-undersaturated rocks which include nephelinites, ankaratrites, and melilitites ( 87Sr/ 86Sr= 0.70365–0.70418, 143Nd/ 144Nd= 0.51277–0.51290, 206Pb/ 204Pb= 19.317–19.565 ). These variations suggest the influence of at least two separate components in the source(s) of both series. One component is characterized by high Rb/Sr and low μ, possibly derived from delaminated subcontinental lithosphere, whereas the other has high μ and low Rb/Sr similar to the source of St. Helena lavas. A third component is suggested by correlated compositions in the latest alkaline, Si-undersaturated lavas, and this component may be derived from depleted mantle. These isotopic variations in conjunction with the generally increasing degree of alkalinity with time are consistent with the temporal depletion of a low-μ, high Rb/Sr component and increasing contributions from a high-μ component in the sources of the volanic rocks of Fernando de Noronha.