Mantle Plume Mixing Along the Reykjanes Ridge Axis: Lead Isotopic Evidence

Gradients of lead isotopic ratios from basalts erupted along the Reykjanes Ridge and Median Neovolcanic Zone ofIceland confirm mantle plume mixing with the depleted asthenosphere along the ridge axis. Geochemical studies of basalts erupted along the Reykjanes Ridge and its extension over the Median Neovolcanic Zone of Iceland have revealed striking gradients in minor and trace element concentrations (1). These studies and other geophysical and morphological evidence in the area have given strong support to the mantle plume hypothesis (2, 3) and led Schilling (1) to suggest a model consisting of two mantle sources, a mantle plume rising beneath Iceland and a depleted low velocity layer beneath the ridge. Mixing of these two sources at mantle depth was assumed to be the cause of the gradients, particularly in the La/Sm ratio. A subsequent 87Sr/ 86Sr test of the model (4) supported the concept presented by Schilling, but also suggested a less regular mixing along the transitional zone than was apparent from the gradient in the La/Sm ratio. Although challenged on various and debatable grounds (5-8), the binary mantle mixing model remains, we believe, the most important process in producing the gradients of large ionic lithophile (LIL) trace element ratios thus far observed along the Reykjanes Ridge-Iceland profile (1, 9). As further evidence in support of this model, we now present Pb iotopic compositions for 16 Reykjanes Ridge and 2 Iceland basalts, as well as Th, U, and Pb concentrations for 7 of these samples, whose locations are shown in Fig. 1. Results. Table I shows the Pb isotopic data obtained in this study and Table 2 the U, Th, and Pb concentrations (10, 11). All basalts analyzed are very fresh, tholeiitic in composition, and vary from slightly quartz normative to olivine normative (12). Figure 2 shows the 206Pb/204Pb and 208Pb/ 204Pb ratios of these basalts with respect to their distance from the southern tip of Iceland. Data for three samples from Sun and Jahn (13) and one from Welke et al (14), all from the Median Neovolcanic Zone of Iceland, are also plotted in Fig. 2. For comparison, 87Sr/86Sr and La/Sm ratios previously reported by Hart et al. (4) and Schilling (1), respectively, are also shown in Fig. 2. The 206Pb/204Pb profile shows high values over Iceland and low values south of 61°N to the Gibbs fracture zone. In be-